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Dielectric properties of soils in microwave region

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DIELECTRIC PROPERTIES OF SOILS
IN MICROWAVE REGION
A THESIS
SUBMITTED TO
THE GUJARAT UNIVERSITY, AHMEDABAD
FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY
(SCIENCE)
BY
Deepak H. Gadani
UNDER THE SUPERVISION OF
Dr. A. D. Vyas
RETD. PROFESSOR & HEAD
DEPARTMENT OF PHYSICS
GUJARAT UNIVERSITY
(NAAC ACCREDITED B++)
AHMEDABAD -380 009
GUJARAT, INDIA
SEPTEMBER - 2010
T 3242
ProQuest Number: 3735004
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CERTIFICATE
This is to certify that the thesis entitled “Dielectric
properties of soils in Microwave region” submitted for the
degree of Doctor of Philosophy to the Gujarat University,
Ahmedabad, India is a record of original investigations carried
out by Mr. Deepak H. Gadani in the department of Physics of
this University. This work has not been submitted for any other
degree.
(Dr. A. D. Vyas)
Research Supervisor
Head,’
N «*SC
Department of Physics
Gujarat University, Ahmedabad
HEAD
DEPARTMENT o f p h y s i c s
SCHOOL OF SCIENCES
GUJARAT UNIVERSITY
AHMEDABAD-380Q09.
D E C L A R A T IO N
I here by declare that the thesis entitled “Dielectric properties of soils in
Microwave region” submitted for the degree of Doctor of Philosophy is
not substantially the same as the one which has already been submitted
for a degree or diploma of this or any other university or examining body
in India or in any other country.
Deepak H. Gadani
ACKNOWLEDGEMENT
I would like to dedicate this thesis to my guide Dr. A. D. Vyas, retired Professor and
Head, Department of Physics, Gujarat University, for his inspiring guidance, support
and constant encouragement during the entire period of investigation and preparation
of this thesis. I am also thankful to Mrs. A. D. Vyas for her kind, motherly approach
during my every visit to their family.
I thank my great father Late. Shri. Hargovind Nagardas Gadani (Retired Professor,
Accountancy), and my mother Smt. Pushpaben H. Gadani, who gave me the birth,
brain, and blessings.
I can not forget the great help, patience, mutual understanding, moral support and
time sparing of my angel wife Abhilasha, and my fairy daughter Namrata during my
Ph. D. work in progress; my absentia in evenings, vacations, and during my journey to
various conferences. Without their support I could not complete the Ph. D. work.
I am also thankful to my dear friend Dr. V. A. Rana (Associate Professor, Department
of Physics, Gujarat University, Ahmedabad) for his unconditional thorough support
and valuable suggestions during my Ph. D. work throughout my journey till this thesis
writing. I also thank Mrs V. A. Rana for sparing my friend Dr. V. A. Rana when ever
required. I also acknowledge my dear friend Shri Ashwinbhai Prajapati (Assistant
Professor, Physics Department, Sayaji Rao Gayakwad University, Baroda) for his
unconditional support during the Ph. D. work.
I am obliged to Dr. S.P. Bhatnagar (Head & Professor, Department of Physics,
Bhavnagar University, Bhavnagar) for encouraging for the measurement of dielectric
properties of soil samples using Vector Network Analyzer in his department. Even 1
can not forget the good hospitality o f Mrs. S. P. Bhatnagar.
The positive help and support of Dr. P. N. Gajjar (Head & Professor, Department of
Physics, Gujarat University, Ahmedabad) is truly acknowledged. How can I forget
Dr. V. B. Gohel (Former Head & Professor, Department of Physics, Gujarat
University, Ahmedabad) who continuously inspired and supported my research work?
I also thank our Vice Chancellor Dr. Parimalbhai Trivedi for the positive support.
Infrastrustural facility availed under DST-FIST (Level-1) in the Department, used by
me during the course o f this work is gratefully acknowledged.
I am thankful to Dr. P. R. Vyas (Associate Professor, Department of Physics, Gujarat
University, Ahmedabad) for his worm support and good wishes during my research
work. The continuous inspiration for doing Ph. D. after joining as a lecturer in Physics
in the college was provided by Dr. R. S. Shrivastava, Dr. A. R. Jani, and Dr. K. N.
Joshipura (Department of Physics, Sardar Patel University, Vallabh Vidhyanagar).
I am also thankful to my other co-workers of microwave laboratory, Dr. Atul Trivedi,
Dr. V. M. Vashistha, Shri. D. G. Trivedi, Chintan Trivedi, Thomas and Hament
Chabue for their kind support when ever required.
I thank Smt. Anita Gharekhan (I/C Principal, C. U. Shah Science College) for support.
Further I am thankful to my dear friends Dr. K. M. Chauhan (I/C Head, Physics
Department, C. U. Shah Science College), Smt. Nipa J. Bhatt, Shri. Sandeep Trivedi,
Smt. D. R. Trivedi, and Shri. P. H. Suthar for their kind support in the college and
positive feed back during my Ph. D. work. Thanks to staff members A. N. Oza, N. P.
Patel, P. L. Patel, B. M. Pandya, P.H.Patel, and D.K.Patel. I thank Shri. P. V^Shah
Sahib (former Head, Physics Department, C. U. Shah Science College^'asTWeftT
Shri. N. R. Shah Saheb and Dr. V. Prakash (former principals of my college) for good
wishes for my Ph. D. work.
I acknowledge the true help of the staff of Department of Physics, Gujarat University,
Ahmedabad. The technical help of Shri. M. S. Bhatt Saheb will always be
remembered. Also thanks to Smt. Hersaben Shah, Shri. Kiritbhai Parmar, Shri.
Devendrabhai, Bharatbahi, Bachubhai, Vitthalbhai and Harishbhai. Further I thank the
administrative staff of my college Akhibhai, Jentibhai, Ashwin, Rathodbhai, and
Yogeshbhai Parekh.
Thanks to the administrative staff of Gujarat University Shri. Bhavsarbhai and Shri.
Kandarpbhai for their positive technical guidance.
I also acknowledge the help o f Shri. Himmatbhai, Ragghubhai Thakor, Ghushabhai
Aadroja, Pankaj Trivedi, and Babubhai Gadani for collecting soil samples from
different locations of Gujarat state during my research work.
I cannot forget the unconditional support of the family of Shri. Hasmukhbhai Rathod
including Purvi Chauhan for their worm support to my family during my research
work particularly in my absentia during conferences.
In the last I thank all who directly or indirectly supported me during my Ph. D. work.
Deepak H. Gadani
INDEX
CHAPTER
DESCRIPTION
PAGE NO.
INTRODUCTION
1-9
REFERENCES
THEORETICAL BACKGROUND
II
10-26
2.1
INTRODUCTION
10
2.2.
DIELECTRIC CONSTANT
11
2.3.
PROPAGATION OF ELECTROMAGNETIC WAVES
15
2.4.
DIELECTRIC POLARIZATION OF MATERIAL
16
2.4.1
ORIENTATION (DIPOLAR) POLARIZATION
16
2.4.2. ELECTRONIC AND ATOMIC POLARIZATION
19
2.5.
FREQUENCY RESPONSE OF DIELECTRIC MECHANISMS
19
2 .6.
INTERFACIAL OR SPACE CHARGE POLARIZATION
21
2.7.
RESONANT FREQUENCY
22
28
RELAXATION TIME
22
2.9.
DEBYE RELATION
24
2. 10.
COLE-COLE DIAGRAM
25
2. 11.
IONIC CONDUCTIVITY
27
REFERENCES
28
..
III
PHYSICAL PROPERTIES AND CLASSIFICATION
29-41
OF SOILS OF GUJARAT
3.1
INTRODUCTION
29
3.2.
CLASSIFICATION OF SOILS
31
3.3.
TEXTURE STRUCTURE OF SOILS
32
3.3.1
POROSITY
33
3.4.
SOILS OF GUJARAT AND THEIR TEXTURE STRUCTURE
34
REFERENCES
41
IV
EXPERIMENTAL METHODS FOR DETERMINATION
42-67
OF COMPLEX PERMITTIVITY
4.1.
INTRODUCTION
42
4.2
FREQUENCY DOMAIN TECHNIQUE
42
4.2.1 TECHNIQUES FOR THE MEASUREMENT OF COMPLEX
42
PERMITTIVITY USING MICROWAVE BENCH SET UP
AT SINGLE FREQUENCY
4.2.1.1
THEORY OF THE TWO POINT METHOD
44
FOR MEASURING COMPLEX PERMITTIVITY
INVOLVING THE SOLUTION OF A TRANSCENDENTAL
EQUATION
4.2.1.2
EXPERIMENTAL SET UP FOR ESTIMATION
46
OF COMPLEX PERMITTIVITY EMPLOYING TWO
POINT METHOD
4.2.2
SIMULTANEOUS MULTI FREQUENCY
53
MEASUREMENT TECHNIQUES
4.2.2.1
MEASUREMENT OF COMPLEX PERMITTIVITY
53
USING VNA
4.2.2.2
THEORY OF DE-IMBEDDING OF SAMPLE
54
IMPEDANCE AND MODELING OF
COAX TERMINATION
4.2.2.3
4.2.3
DESIGN OF COAXIAL PROBE
LOW FREQUENCY MEASUREMENTS OF COMPLEX
59
61
PERMITTIVITY
4.2.3.1
4.2.3.2
DESIGN OF COAXIAL CAPACITOR
THEORY
REFERENCES
V
DIELECTRIC PROPERTIES OF DIFFERENT TYPE
61
62
66-67
68-119
OF SOILS WITH MOISTURE CONTENT AT RADIO
AND MICROWAVE FREQUENCIES
5.1.
INTRODUCTION
68
5.2
EXPERIMENTAL
73
5.2.1
SAMPLE COLLECTION AND PREPARATION
73
5.2.2
EXPERIMENTAL TECHNIQUE FOR
75
MEASUREMENT OF COMPLEX PERMITTIVITY
OF SOILS AT X-BAND AND C-BAND MICROWAVE
FREQUENCIES
5.2.3
RESULTS AND DISCUSSION
76
5.2.4
EXPERIMENTAL TECHNIQUE FOR THE
79
MEASUREMENT OF COMPLEX PERMITTIVITY
OF SOILS AT RADIO AND LOWER
MICROWAVE FREQUENCIES AND RESULTS
5.3
CALCULATION OF EMISSIVITY
83
5.4
CALCULATION OF THE DIELECTRIC CONSTANT e ’
84
AND THE DIELECTRIC LOSS e ” OF THE SOILS USING
THE HALLIKAINEN ETAL. MODEL, AND WANG
AND SCHMUGGE MODEL
(A)
HALLIKAINEN ETAL. MODEL
84
(B)
WANG AND SCHMUGGE MODEL
85
(C)
COMPARISON OF MEASURED VALUES OF
88
THE DIELECTRIC CONSTANT e ’ AND THE
DIELECTRIC LOSS e ” OF THE SOILS WITH
THE VALUES CALCULATED USING WANG
AND SCHMUGGE MODEL, AND THE HALLIKAINEN
ETAL. MODEL
REFERENCES
VI
117-119
EFFECT OF SALINITY ON DIELECTRIC
120-141
PROPERTIES OF SOILS
6.1
INTRODUCTION
6.2
MATERIALS AND METHODS
6.3
RESULTS AND DISCUSSION
124
REFERENCES
141
120
U
\•
( ' l i b r a r y ) -<
V.
123
Vn
DIELECTRIC PROPERTIES OF WET AND
142-181
FERTILIZED SOILS AT RADIO AND
MICROWAVE FREQUENCIES
VIII
7.1
INTRODUCTION
142
7.2
SAMPLE PREPARATION
145
7.3
EXPERIMENTAL SET UP
148
7.4
STANDARDIZATION OF CAPACITOR
148
7.5
RESULTS AND DISCUSSION
153
REFERENCES
181
SUMMARY
182-186
CHAPTER I
INTRODUCTION
Frequency dependent dielectric spectroscopy is often used to characterize materials
The application of electric field on dielectric material causes both the movement of
charge carriers and alignment of dipolar molecules. When the electric field is
removed, the molecules try to reorient back to a more stable arrangement, with some
time lag depending on the characteristic of the material. This time lag of reorientation
or relaxation varies as a function of frequency for alternating fields 1.
The complex permittivity (relative to permittivity of vacuum) e* o f a material
consists of a real component <=’ called dielectric constant and an imaginary
component e ” called dielectric loss, represented by e* - e ' - j e".
The imaginary component e ” includes both the relaxation and electrical conductivity
contributions, as
e = e fi +
a’.dc
ev
0)
•
Where, CTdc = direct current electrical conductivity in Sm'1,
f = frequency in Hz,
gr”
= relaxation contribution to e ”, and
/y
\ yv
/
fo ( library) <
€ v = absolute dielectric constant of vacuum = 8.854 x 10‘12 F m '1.
The study of variation of complex permittivity of the soils with moisture content is
very much useful for the interpretation of the data obtained by remote sensors.
Considerable work has been done in this area in different countries. Njoku 2 measured
the complex dielectric constant of sand as a function of moisture contents in the range
from 0 to 30% by volume at frequencies of 0.679, 1.0, 3.0, 8.52, 14.0 and 20.0 GHz.
Hoekstra and Delaney 3 measured the complex dielectric constant of soils as a
function of water content and temperature over the frequency range from 108 to 2.6 x
1010 Hz. Robinson et al. 4 measured the complex permittivity o f soils ranging from
1
sand to clay using a Surface Capacitance Insertion Probe (SCIP) and time domain
reflectometer (TDR) for various moisture contents in the soils. Peplinski et al. 5
measured the dielectric constant e' and dielectric loss e" o f four soil types for
various moisture contents in the frequency range between 0.3 GHz and 1.3 GHz.
India, being a large country, attempt has been made to study the dielectric properties
o f soils of different regions of India (with moisture content) by some workers 2-13. A.
D. Vyas 6 measured the values o f dielectric constant and dielectric loss o f sand and
sandy loam for various moisture contents at X-band microwave frequency. Calla et al.
7 measured the complex dielectric constant of loamy sand for various moisture
contents by weight, in the frequency range from 2 GHz to 20 GHz. Ghosh et al. 8
measured the dielectric constant and dielectric loss o f dry and wet soils at 14.89 GHz.
Mishra and Behari 9 measured the complex dielectric constant o f sandy loam soil for
various moisture contents at S (3.0 and 3.5 GHz), C (5.5 and 6.5 GHz), and X-band
(9.0 and 9.4 GHz) microwave frequencies. Paneholi and Khameshra10 measured the
complex permittivity o f some Rajasthan soils (sand, sandy loam, sandy clay loam and
clay) for various moisture contents at 7.114 GHz microwave frequency using the
infinite sample method. Chaudhary , and Shinde 11 measured the dielectric constant
and dielectric loss o f soils of various texture structure, for various moisture contents,
at X-band microwave frequency of 9.65 GHz employing the infinite sample method.
Sengwa et al. 12,13 measured the dielectric constant and loss tangent o f shale, sandy
sandstone, calcareous sandstone, and some minerals like clay, siliceous earth and
fuller’s earth at the microwave frequency o f 10.1 GHz.
Microwaves can penetrate deep in the soil in comparison to visible and infrared
radiations, and the microwave sensors can operate in all weather conditions 14. Hence
microwave remote sensing technique to estimate the moisture content in the soil has
gained considerable attention. The technique involves either measurement of
emissivity o f soils using radiometers, or back scattering coefficient using an active
sensor. Both the emissivity and back scattering coefficient depend on the complex
dielectric constant o f the soils, which is considerably affected by the moisture content
in the soil.
2
The objective of the present work is to study the effect of water, saline water and
fertilizer content on the dielectric properties of soils o f Gujarat at radio and
microwave frequencies. For this the soil samples were collected from different
regions o f Gujarat state referring to the soil map o f G ujarat15,16. The measurements
for the estimation of complex dielectric constant o f the dry and wet soils at radio and
microwave frequencies were carried out in the laboratory conditions at Department of
Physics, Gujarat University, Ahmedabad. The measurements using Vector Network
Analyzer operating in the frequency range from 300 KHz to 3 GHz have been carried
out at Department o f Physics, Bhavnagar University, Bhavnagar, Gujarat
Chapter II deals with the theoretical aspect o f the dielectric constant e ’ and dielectric
loss e ” o f material. The theoretical background o f the effect o f electric field on polar
and non polar molecules as well as different types o f polarizations occurring in
material, have been discussed. The dependence o f complex dielectric constant on
frequency can be described by the Debye or Cole-Cole models for one relaxation as 1
€*(/) =
(2)
and for two relaxations as
where
e s = low frequency limit o f e',
€» = high frequency limit o f e '
fr = relaxation frequency, and
a = exponent that describes the spread of the relaxation peak.
For the Debye relaxation a = 0, and hence the spread is small.
3
The unconsolidated mineral and organic material on the immediate surface o f the
earth which is subjected to and influenced by genetic and environmental factors of
parent material, climate, macro- and micro organisms, and topography, all factors
acting over a period o f time to produce the soil, which differs from the material from
which it is derived in many physical chemical, biological and morphological
characteristic properties 17. The classification and textural composition of soils have
been explained based on the U.S. Department of Agriculture and the International Soil
Science Society, in chapter III.
The technique for the estimation of dielectric constant e ' and dielectric loss e ” of the
dry and wet soils at 9.5 GHz and 5.65 GHz microwave frequencies, using X and Cband microwave bench set up, employing two-point method 18 have been discussed in
chapter IV. Further the technique for the determination o f complex permittivity from
the measurements o f the reflection coefficient of a simple sample cell consisting o f a
coaxial semi-rigid cable terminated by the material under test using vector network
analyzer 19,20 have been used to measure the dielectric constant and dielectric loss of
dry and wet soils in the frequency range from 30 MHz to 1.23 GHz. The empirical
values o f dielectric constant and dielectric loss of dry and wet soils were calculated at
the C and X-band microwave frequencies using Wang and Schmugge model 21 and
the Hallikainen et al. model 22 have also been discussed in this chapter. The
measurement procedure for the dielectric constant and dielectric loss o f a sample
using a self designed coaxial capacitor and a precision LCR meter in the frequency
range from 10 KHz to 2 MHz is explained in the chapter IV along with calibration
technique.
In chapter V, the estimated values o f dielectric constant e ' and the dielectric loss e ”
of the soils for various moisture contents of distilled water measured using microwave
bench set up operating at 9.5 GHz and 5.65 GHz have been reported. The measured
values of complex dielectric constant of dry and wet soils using VNA are also
reported in chapter V, for the frequency range between 30 MHz and 1.23 GHz.
Further, the measured values of complex dielectric constant (e ’, e ”) o f the soils for
various moisture contents in the soils are compared with the values calculated using
4
the Wang and Schmugge model 21 and the Hallikainen et al. model 22 at X- and Cband microwave frequencies of 9.5 GHz and 5.65 GHz, respectively.
In chapter VI, the measured values o f dielectric constant e ’ and dielectric loss e ” of
the soils for various moisture contents of saline water solutions o f 10,000 ppm and
30.000 ppm are reported at 5.65 GHz C-band microwave frequency, as well as for the
frequency range between 100 MHz and 1.6 GHz. The results o f VNA measurement
are also shown at spot frequencies o f 0.21 GHz, 0.5 GHz, 1.01 GHz and 1.4 GHz,
showing variation of the dielectric constant and dielectric loss o f the soil for various
moisture contents o f distilled water and saline water solutions o f 10,000 ppm and
30.000 ppm in the soil. From the measured values o f the dielectric constant e ’ and the
dielectric loss e ” o f the Gandhinager sandy loam soil, at 0.21 GHz, 0.5 GHz, 1.01
GHz, 1.4 GHz and at 5.65 GHz microwave frequencies, the emissivity values o f the
soil for normal incidence were calculated and represented graphically.
The comparison o f measured values of dielectric constant € ’ and the dielectric loss
€ ” o f water with the values calculated using the Stogryn equations 24, for salinity
levels o f 10,000 ppm and 30,000 ppm in the frequency range from 100 MHz to 1.5
GHz are also shown in the chapter.
The measured values o f dielectric constant e ' and the dielectric loss e ” o f the
Gandhinagar district sandy loam soil for various moisture contents o f distilled water
measured using a newly designed coaxial capacitor attached with a precision LCR
meter operating in the frequency range from 10 kHz to 2 MHz are reported in chapter
VII. Further the values o f loss tan 8, real conductivity o r ’ , and imaginary conductivity
cr” for the given frequency range are calculated from the measured values of e* and
e ”. The variation of dc (ohmic) conductivity with moisture content in the sandy loam
soil of Gandhinagar district have been shown for various moisture contents in the soil.
The dielectric constant e ’ and dielectric loss e ” o f the wet fertilized soils (Sandy
loam soil o f Gandhinagar district, and Sandy soil o f Paianpur district) for various
concentrations o f different fertilizers [Sulftsefeof Potash (SOP), and Zinc Chelate] in
the frequency range from 10 kHz to 2 MHz has been measured using the precision
5
LCR meter. The values o f loss tan 5, as well as real and imaginary conductivity o f the
wet fertilized soils for various concentrations o f different fertilizers in the frequency
range from 10 kHz to 2 MHz have been calculated from the measured values o f e ’
and e ”. Finally the variation o f dielectric constant and dielectric loss of the wet
fertilized soils for various concentrations of different fertilizers at spot frequencies of
0.5 GHz, 1.0 GHz and 1.5 GHz are measured using VNA, and the results are reported
in the chapter VII.
Chapter VIII discuss the concluding remarks o f the work presented in the thesis. The
future plan of work to be carried out is also discussed in the chapter.
6
REFERENCES:
1.
Logsdon S. D., “Soil Dielectric Spectra from Vector Network Analyzer Data”,
Soil Sci. Soc. Am Journal, 69 (2005), 983.
2.
Njoku E. G., and Kong J., “Theory for Passive Microwave Remote Sensing of
Near-Surface Soil Moisture”, Journal o f Geophysical Research, 82/2 (1977)
3108.
3.
Hoekstra P. & Delaney A., “Dielectric Properties o f Soils at UHF and
Microwave Frequencies”, Journal o f Geophysical Res., 79/11 (1974)1699.
4.
Robinson D. A., Kelleners T. J., Cooper J. D., Gardner C.M.K., Wilson P.,
Lebron I., and Logsdon S., “Evaluation o f a Capacitance Probe Frequency
Response Model Accounting for Electrical Conductivity: Comparison with
TDR and Network Analyzer Measurements”, Vadose Zone Journal, 4
November (2005) 992.
5.
Peplinski Neil R., Ulaby F. T., Dobson M. C., “Dielectric Properties of Soils
in the 0.3-1.3 GHz Range”, IEEE Trans. Geosci. & Remote Sens., 33/3 May
(1995)803.
6.
Vyas A. D., “Complex Permittivity of Sand & Sandy Loam Soils at
Microwave Frequency,” Indian J. Radio & Space Physics, 2 (1982)169.
7.
Calla O. P. N, Borah M. C., Vashishtha P., Mishra R., Bhattacharya A. &
Purohit S. P., “Study o f the Properties o f dry and wet loamy sand soil at
microwave frequencies”, Indian J. o f Radio and Space Physics, 28 (1999)109.
8.
Ghosh A., Behari J. & Pyne S., “Dielectric parameters o f dry and wet soils at
14.89 GHz”, Indian J o f Radio and Space Physics, 27 (1998) 130.
9.
Shankar M. U. & Behari J . , “In-situ measurement o f Dielectric Parameter of
Soil at Microwave Frequencies,” Journal o f the Indian Society o f Remote
Sensing, 28/1 (2000) 1.
10.
Pancholi K. C., Khameshra S. M., “Complex dielectric permittivity of some
Rajasthan soils at 7.114 GHz”, Indian J. o f Radio and Space Physics”, 23
(1994)201.
11.
Chaudhary H. C., and Shinde V. J., “Dielectric study o f moisture laden soils at
X-band microwave frequency”, International Journal o f Physical Sciences,
Vol. 3(3), March (2008) 075.
7
12.
Sengwa R. J,, Soni A., Ram B., “Dielectric behaviour of shale and calcareous
sandstone o f Jodhpur region”, Indian J. Radio and Space Physics, Vol. 33,
October (2004) 329.
13.
Sengwa R. J., and Soni A., “Dielectric properties o f some minerals o f western
Rajasthan”, Indian J. Radio and Space Physics, Vol. 37, February (2008) 57.
14.
Ulaby F.T., Moore R. K. and Fung A. K., Microwave Remote Sensing (Active
and Passive), vol. (I, II, III). Artech House Inc. 1981,1982, 1986.
15.
Joshi S., Agriculture in Gujarat, Progress and Potential, Helios Enprint Ltd.
16.
Biswas, Yadav, and Maheshwari, Soils o f India and Their Management,
Published by “The Fertilizer Association of India”, New Delhi (1985).
17.
Miller R W, Gardiner D T, “Soils in our Environment”, 8th edition, PrenticeHall Publications, Upper Saddle River, NJ, (1998).
18.
Sucher, M. and Fox J., Handbook o f microwave measurements, (1963) 504.
19.
Yan-Zhen Wei and Sridhar S., “Technique for measuring the frequencydependent complex dielectric constants of liquids up to 20 GHz”,
Rev.Sci.Instrum., 60/9 September (1989) 3041.
20.
Yan-Zhen Wei and Sridhar S., “Radiation-Corrected Open-Ended Coax Line
Technique for dielectric Measurements o f Liquids up to 20 GHz”, IEEE
Trans. Microwave Theory and Techniques, 39/3 March (1991) 526.
21.
Wang J. R., and Schmugge T. J., “An empirical model for the complex
dielectric permitivity of soils as a function o f water content,” IEEE
Transactions on Geoscience and Remote sensing, 18/4 (1980) 288.
22.
Hallikainen M. T., Ulaby F. T., Dobson M. C., El-Rays M. A., and Lin-Kun
Wu, “Microwave Dielectric Behaviour o f wet Soil-part 1: Empirical Models
and Experimental Observations,” IEEE Trans. Geosci. Remote Sensing, 23/1
(1985)25.
23.
Caila O. P. N., Ranjan V., Bohra C., Naik G. L, Hasan Waseem, and Bali H.
S., “Estimation o f dielectric constant of soil from the given texture at
8
microwave frequency”, Indian Journal o f Radio and Space Physics, 33 June
(2004) 196.
24.
Stogryn A., Equations for calculating the dielectric constant o f Saline Water,
IEEE Trans, on Microwave Theory and Techniques, (1971) 733.
9
CHAPTER II
T heoretical B ackground
2.1.
Introduction
The electrical characteristic of every material is dependent on its dielectric properties.
Measurements of these dielectric properties can provide valuable information of
material characteristics, design parameters for many electronic applications, and
remote sensing applications. The information is also useful in the area o f industrial
microwave processing o f food, rubber, plastic and ceramics.
The electrical properties of material are mainly the permittivity and permeability.
Resistivity (conductivity) is also another important electrical property o f the material.
The permittivity and permeability o f a material are not constant, but can change with
frequency, temperature, orientation, mixture, pressure and molecular structure o f the
material.
In general the molecules o f a substance are divided into two categories:
i.
Polar molecules, and
ii.
Non-polar molecules.
Molecules having a center of charge symmetry and zero dipole moment are known as
non-polar molecules, while those not having a center o f charge symmetry having
permanent dipole moment are known as polar molecules.
When a material having non-polar atoms is subjected to an electric field it gets
polarized due to the displacement o f the center o f charge of electrons relative to the
nucleus in each atom, causing electronic polarization (Pe). When a material containing
non-polar molecules is subjected to an electric field, the displacement of atomic
nuclei relative to one other induces dipole moments in the molecules called atomic
polarization (Pa). These two polarizations together constitute a distortion polarization
(Pd).
10
The permanent dipole moments of the polar molecules in a material are randomly
oriented in absence o f electric field. When the material having polar molecules is
placed in electric field, the permanent dipole moments of these molecules get oriented
along the direction of the electric field, called Orientational polarization (P0). The
alignment of the permanent dipole along the field increases as the electric field
strength increases.
Thus the total polarization of a dielectric material is
P r = Pe + Pa + Po = Pd + Po
.....(1)
The polarizability is a measure o f resistance offered by the atom or molecule to the
displacement of the center of charges, or orientation of its dipole moment in the
electric field. When the polar dielectric is subjected to an electric field, the individual
dipoles experience torques which tend to align them with the field. If the field is very
strong, all dipoles to get aligned completely and the polarization may achieve the
saturation value. In general, for the applied field, the polarization of polar dielectric is
very small in comparison to its saturated value. It decreases with increase in
temperature. This deviation from saturated value is due to the thermal vibrations of
the molecules, which produces random dipole orientations.
In general, the total polarizability of dielectric material is
<xT = a e + cia + a 0
.....(2)
The polar molecules have greater value of permittivity than non-polar molecules
because of an additional amount of polarization due to orientation. The Orientational
polarization falls of rapidly with rising temperature, therefore the permittivity o f polar
materials falls more rapidly with rising temperature than that o f non-polar materials.
2.2.
D ielectric C onstant
A material is classified as a “dielectric” if it has the ability to store energy when
external electric field is applied. The theory o f dielectric was begun by Faraday and
then subsequently developed by Maxwell. In 1837, MichaelFarad<^did experiments to
11
investigate the effect o f filling the space between the plates o f capacitor with
dielectric material5. If a DC voltage is applied across a parallel plate capacitor, more
charge is stored when a dielectric material is between the plates than if no material (a
vacuum) is between the plates. The dielectric material increases the storage capacity
of the capacitor by neutralizing the charges at the electrodes up to certain extent
(depending on its dielectric properties), which ordinarily would contribute to the
external field. The capacitance with the dielectric material is related to the dielectric
constant.
If a DC voltage V is applied across a parallel plate capacitor (Figure 2.1), more charge
is stored when a dielectric material is placed between the plates than if no material (a
vacuum) is there between the plates.
Figure 2.1: Application o f DC voltage across the plates of a parallel plate capacitor
For the figure 2.1,
C0 =
A
t
C - C0 er'
/. k '= e ’=Where k ’ - e r ' is the dielectric constant of the material
Co = Capacitance without material (vacuum) = — and
C = Capacitance with dielectric material = —
V
If an Ac sinusoidal voltage V e ja>t is applied across the same capacitor (Figure 2.2),
the resulting current will be made up o f a charging current
ICharge
and a loss current
12
Iloss that is related to the dielectric constant. The losses in the material can be
represented as a conductance (G) in parallel with a capacitance (C).
For the applied voltage across the capacitor, the current
^
"^Cftarge
^Loss
= jo)VC0k'+VG
If G = a>C0k" then
/ = jG)VC0k'+V(ok''
/ = jmVC 0 k '+ ( - jf Vo) C0k"
I = V(ja)C0)(k'-jk")
I = V(jo)C0)k *
where k* = k'-jk"
(3)
The complex dielectric constant k* consists o f a real part k' which represents the
storage and an imaginary part k" which represents the loss.
13
The complex dielectric constant k * of a material is equivalent to relative permittivity
er ,
i.e. the permittivity e* relative to free space e 0.
*
= e /-J
er
(4)
Here s r - complex relative permittivity, and
e 0= Permittivity in free space = 8.854 x 10'12 Farad/m
The real part of permittivity e / is a measure o f how much energy from an external
electric field is stored in a material. The imaginary part of permittivity <sr " is called
the loss factor and is a measure o f how dissipative or lossy a material is to an external
electric field. The imaginary part o f permittivity e r " is always greater than zero. The
loss factor includes the effects of both dielectric loss and conductivity.
When complex permittivity is drawn as a simple vector diagram (Figure 2.3), the real
and imaginary components ( e r ' , e r " ) are 90° out of phase. The vector sum e r * forms
an angle 5 with the real axis e r'. The relative “lossiness” of a material is the ratio of
the energy lost to the energy stored.
(5)
14
tan 5 - Energy lost per cycle / Energy stored per cycle
tan 5 = D = 1/Q
where tan 8 = loss tangent, tan delta, tangent loss, etc.
D = dissipation factor, and
Q = quality factor.
2.3.
Propagation of E lectromagnetic waves
The electromagnetic waves travel with a speed of light (c = 3 x 108 m/s) in free space
with electric and magnetic fields oscillating perpendicular to each other and
perpendicular to the direction of propagation of the waves. This electromagnetic wave
can propagate through free space, or through materials. There exist electromagnetic
waves of various wavelength or frequency (Figure 2.4).
(300km) (300m)
10°
103
106
(30cm) (300umi (3Q0nm)
ID9
1012
I01i
1018
_L
1 11
1 1
i ii
11
;11
11
1 |
1 r ""Irr
IT"
Lf
Mtcfo»vave
IR
I
(MHD CM3
RF
<(H ?1
UV
III
mm-Wave
I (Ml
V X-Ray
Figure 2.4: Electromagnetic Spectrum2
Many aspects of wave propagation are dependent on permittivity and permeability of
a material. Since the impedance of the wave in the material Z is different (lower) from
the free space impedance q (or
Z0),an
impedance mismatch occurs at the boundary
causing part of the energy to be reflected from the material and the rest of energy to
be transmitted through the material. Once in the slab, the wave velocity v, is slower
than the speed of light c. The wavelength Zd is shorter than the wavelength A.0 in free
space according to the equations given below. Since the material will always have
some loss, there will be attenuation or insertion loss. For simplicity the mismatch on
15
the second border is not considered. If the material is lossy, there will be insertion loss
or attenuation through the material (Figure 2.5).
n
TEM
o rZ 0
i= nl^
«=> w w v
<=> w w v
Im p e d a n c e lo w e r
W a v e le n g th s h o rte r
V e lo c ity s lo w e r
M a g n itu d e a tte n u a te d
Figure 2.5: Reflected and transmitted signals, and corresponding equations1.
2.4.
Dielectric Polarization of material
A material may have several dielectric polarization effects that contribute to its
overall permittivity. On application of an electric field to a dielectric material the
opposite electric charges are displaced. These charges become polarized to
compensate for the electric field such that the positive and negative charges move in
opposite directions. The maximum polarization Po occurs at zero frequency. Because
of inertia of the charge carriers, polarization process requires time, therefore the
polarization P is in general not in-phase with the electric field E. A frequencydependent complex permittivity k = k'~ jk" is used to capture both amplitude and
phase information:
P =k e 0E
The spectra o f polarization mechanisms have the form o f relaxation or resonance. If
the charge is moved against a restoring force a resonance spectrum occurs. The three
types of polarizations are explained as follows:
2.4.1
Orientation (dipolar) polarization
A molecule is formed when atoms combine to share one or more o f theirs electrons.
This rearrangement o f electrons may cause an imbalance in charge distribution
16
creating a permanent dipole moment. These dipole moments are oriented in a random
manner in the absence o f an electric field so that there is no polarization. The electric
field E will exercise torque ro n the electric dipole, and the dipole will rotate to align
with the electric field and orientation polarization occurs (Figure 2.6). If the field
changes the direction, the torque will also change its direction.
Figure 2.6: Dipole rotation in electric field
17
The linear O-C-O arrangement in C 0 2 results in a non-polar molecule. The resulting
dipole moment of a molecule depends on the shape of the molecule and direction of
applied electric field. The dipole moment of each bond is added vectorially, to obtain
resultant po.arization (Figure-2.7 a-c). Orientational polarization exhibits relaxation at
microwave frequencies.
The friction accompanying the orientation of the dipole will contribute to the
dielectric losses. The dipole rotation causes a variation in both e r' and e " at the
relaxation frequency which usually occurs in the microwave region for some organic
18
liquids. As shown in figure (2.7-d), water (H 2 O ) exhibits a strong orientation
polarization.
2.4 .2.
E lectronic and atomic polarization
Electronic polarization occurs in atoms, when the electron cloud is moved out of the
equilibrium trajectory by an applied steady or alternating electric field (Figure 2.8).
The electronic polarization is resonant near ultraviolet frequencies
. Atomic
polarization occurs when adjacent positive and negative ions “stretch” under an
applied electric field (Figure 2.9). For many dry solids, these are the dominant
polarization mechanisms at microwave frequencies, although the actual resonance
occurs at a much higher frequency.
Figure 2.8: Electronic Polarization of atom
2.5.
Frequency response of dielectric mechanisms:
Each of the three types of polarizations is a function of the frequency of the applied
field. When the frequency of the applied field is sufficiently low, all types of
19
polarizations can reach the value they would have at steady field equal to the
instantaneous value o f alternating field. But as the frequency increases, the
polarization no longer has time to reach its steady peak value. First the orientation
polarization is affected. This type of polarization takes a time of the order of 10‘12 to
10'10 sec to reach equilibrium value in liquid and solids with moderately small
molecules. Hence, at the normal temperature when the applied field has a frequency
of 1010 to 1012 Hz, orientation polarization fails to reach its equilibrium value and
contribute less and less to the total polarization as frequency increases further. In
figure (2.10) the Orientational (rotational) polarization for a particular molecule is
shown to decrease after 109 Hz and has minimum at about 10n Hz.
At the microscopic level, several dielectric mechanisms contribute to dielectric
behavior. Dipole orientation and ionic conduction interact strongly at microwave
frequencies. Water molecules, which are permanent dipoles, rotate to follow an
alternating electric field. These mechanisms are quite lossy - which explains why
food heats in a microwave oven. Atomic and electronic mechanisms are relatively
weak, and usually constant over the microwave region. Each dielectric mechanism
has a characteristic “cutoff frequency.” As frequency increases, the slow mechanisms
drop out in turn, leaving the faster ones to contribute to e r’. The loss factor ( e r") will
correspondingly peak at each critical frequency. The magnitude and “cutoff
20
frequency” o f each mechanism is unique for different materials. Water has a strong
dipolar effect at low frequencies - but its dielectric constant rolls off dramatically
around 22 GHz. Teflon, on the other hand, has no dipolar mechanisms and its
permittivity is remarkably constant well into the millimeter-wave region.
2.6.
I nterfacial o r space charge polarization
Electronic, atomic, and orientation polarization occur when charges are locally bound
in atoms, molecules, or structures of solids or liquids. Some of the charge carriers
may migrate over a distance through the material when a low frequency electric field
is applied. Interfacial or space charge polarization occurs when the motion o f these
migrating charges is impeded. The charges can become trapped within the interfaces
o f a material. Motion may also be impeded when charges cannot be freely discharged
or replaced at the electrodes. The field distortion caused by the accumulation of these
charges increases the overall capacitance of a material which increases e r ' .
Mixtures of materials with electrically conducting regions that are not in contact with
each other (separated by non-conducting regions) exhibit the Maxwell-Wagner effect
at low frequencies. If the charge layers are thin and much smaller than the particle
dimensions, the charge responds independently o f the charge on nearby particles. At
low frequencies the charges have time to accumulate at the borders o f the conducting
regions causing e / to increase. At higher frequencies the charges do not have time to
accumulate and polarization does not occur since the charge displacement is small
compared to the dimensions of the conducting region. As the frequency increases, e r '
decreases and the losses exhibit the same 1/f slope as normal ionic conductivity1.
Many other dielectric mechanisms may occur in low frequency region causing a
significant variation in permittivity. For example, colloidal suspension occurs if the
charge layer is on the same order of thickness or larger than the particle dimensions.
The Maxwell-Wagner effect is no longer applicable since the response is now affected
by the charge distribution o f adjacent particles.
21
In absence of restoring forces, e.g. when they vanish by diffusion or if the system is
severely damped, the material exhibits a relaxation spectrum representing molecular,
spatial, and double layer polarizations.
2.7.
Resonant frequency
A resonant effect is usually associated with electronic or atomic polarization. In the
infrared and visible light regions the inertia of the orbiting electrons must be taken
into account. Atoms can be modeled as oscillators with a damping effect similar to a
mechanical spring and mass system (Figure 2.7). The amplitude o f the oscillations
will be small for any frequency other than the resonant frequency. Far below
resonance, the electronic and atomic mechanisms contribute only a small constant
amount to e / and are almost lossless. The resonant frequency is identified by a
resonant response in e / and a peak of maximum absorption in e r". Above the
resonance, the contribution from these mechanisms disappears.
2.8.
Relaxation time
A relaxation effect is usually associated with orientation polarization. Relaxation time
x is a measure of the mobility of the molecules (dipoles) that exist in a material. It is
the time required for a displaced system aligned in an electric field to return to 1/e of
its random equilibrium value (or the time required for dipoles to become oriented in
an electric field). Liquid and solid materials have molecules that are in a condensed
state with limited freedom to move when an electric field is applied. Constant
collisions cause internal friction so that the molecules turn slowly and exponentially
approach the final state of orientation polarization with relaxation time constant x.
When the field is switched off, the sequence is reversed and random distribution is
restored with the same time constant. The relaxation frequency fc is inversely related
to relaxation time:
(Oc
At frequencies below relaxation the alternating electric field is slow enough that the
dipoles are able to keep pace with the field variations. Because the polarization is able
22
to develop fully, the loss ( e r") is directly proportional to the frequency (Figure 2.11).
As the frequency increases, e r" continues to increase but the storage ( e / ) begins to
decrease due to the phase lag between the dipole alignment and the electric field.
Above the relaxation frequency both er" and e / drop off as the electric field is too
fast to influence the dipole rotation and the orientation polarization disappears1.
Figure 2.11: Debye relaxation of water at 30° C '.
In calculating the above curves the static (DC) value o f the dielectric constant is
considered as e s = 76.47, the optical (infinite frequency) value o f the dielectric
constant
4.9 and the relaxation time x = 7.2 ps.
The dipole moment o f the molecule of a substance influences its permittivity and
measurement o f permittivity can be used to calculate the dipole moment. At low
frequency electromagnetic field o f moderate intensity, all types o f polarizations attain
equilibrium with the applied field in an isotropic polar material and the permittivity of
the material is called static permittivity e 0. For investigating the molecular structure
and for the study o f high frequency dielectric behavior, the static permittivity is very
much useful. Various theories have been developed to express the relation between
permittivity and dipole moment.
23
2.9.
D ebye relation
Materials that exhibit a single relaxation time constant can be modeled by the Debye
relation 3, which appears as a characteristic response in permittivity as a function o f
frequency as shown in figure (2.11). e r' is constant above and below the relaxation
with the transition occurring near the relaxation frequency (22 GHz). Further, e r" is
small above and below relaxation and peaks in the transition region at the relaxation
frequency.
An alternating field causes a reorientation o f the dipoles which is met by opposing
effects such as thermal agitation and molecular interactions. This behavior is captured
by the Debye Equation
€
+gs ~
*
e«
1+ j( O T
(7)
which can be decomposed into
€
—
€„ + -
1+ ( m r y
(8)
1 + (art)2
where 0 represents the angular frequency and t is a characteristic constant that
describes the relaxation time. € s and e® are the static and infinite frequency dielectric
constants, respectively and the magnitude o f the dispersion can be expressed as
A e = e s - Soo.
(10)
It is known, however, from a considerable amount o f experimental data that the
dispersion processes o f many liquids and solids cannot be accurately described by the
Debye Equation.
24
2.10.
COLE-COLE DIAGRAM
The complex permittivity may also be represented on a Cole-Cole diagram by plotting
the imaginary part ( er") on the vertical axis and the real part ( e r') on the horizontal
axis with frequency as the independent parameter (Figure 2.12). A material that has a
single relaxation frequency as exhibited by the Debye relation will appear as a
semicircle with its center lying on the horizontal e r" = 0 axis and the peak of the loss
factor occurring at 1/t. A material with multiple relaxation frequencies will be a
semicircle (symmetric distribution) or an arc (nonsymmetrical distribution) with its
center lying below the horizontal e r"= 0 axis. The curve in figure (2.12) is a half
circle with its center on the x-axis and its radius. The maximum imaginary part o f the
dielectric constant e 'rmax will be equal to the radius. The frequency moves counter
clockwise on the curve.
er "
The dispersive behavior displays a broadening throughout the frequency band of
interest and can be more accurately described using a modified form o f the Debye
Equation, known as the Cole-Cole Equation 4.
4
-•
-e.)
U - ( j 0 T ) l~a
.( 11)
25
Where a is a constant having values lying between 0 and 1 and is called distribution
parameter. It is a measure of the width of distribution. Rationalizing this expression
and using
/
= exp
jn (l-a )
(12)
2
the real and imaginary parts of the complex dielectric constant as a function of
e'= em+-
___ 1
'
(e s - e «,) 1+ {on)l~a sin
11>-*
frequency can be decomposed as
v2
y
1 + 2 {wry “ sin —a n + { m )
2
j
(Gs e =■
1+ 2(a?r)
2(1- a
(13)
)
cos| ^ a n ''
1
sin —a n + ( cot) 2(1—or)
2
j
..... (14)
When a is equal to 0, then the Cole-Cole Equation equates to the former Debye
Equation.
Yet when a has values greater than 0, the dispersion region is broader and the
maximum value o f e ” is decreased.
It is often necessary for conductive materials to include a conductivity term in the
Cole-Cole expression, leading to
c n _
~ e j0 )T n
1+ ((DT)l~a
]
O'
jco e 0
(15)
This term accounts for ionic conduction, or any type o f charge carrying conduction
that dominates the dielectric loss in the lower frequency spectrum. When obtaining
26
dielectric permittivity measurements in the frequency domain, this allows the loss to
be decomposed relatively easily into the conduction and polarization mechanisms.
Multiple Cole-Cole dispersions can often be seen and described by
e —€
.y
A
T l + t/aw,,)1"*
<J
H h
-
.(16)
j & e0
where relaxations inherent by the unique mechanisms within the material can each be
described by the separate terms in the equation. This becomes convenient when
materials are measured over broad frequency bands where multiple orientational
polarizations must be captured.
2.11.
Ionic conductivity
The measured loss of material can be expressed as a function o f both dielectric loss
( e r(#") and conductivity (a).
e r”= e , / +
a
(17)
At low frequencies, the overall conductivity can be made up o f many different
conduction mechanisms, but the ionic conductivity is most prevalent in moist
materials. e r" is dominated by the influence o f electrolytic conduction caused by free
ions which exist in the presence o f a solvent (usually water). Ionic conductivity only
introduces losses into a material. At low frequencies the effect o f ionic conductivity is
inversely proportional to frequency and appears as a slope o f € r" against (1/f) graph.
27
References:
1.
Agilent, Basics o f Measuring the Dielectric Properties o f Materials,
Application Note, Agilent Technologies.
2.
Loupy A., Microwaves in Organic Synthesis, Second edition. WILEY-VCH
Verlag GmbH & Co. KGaA, Weinheim ISBN: WILEY-VCH Verlag GmbH &
Co. KGaA, Weinheim (2006).
3.
Debye P., Polar Molecules. New York, NY: The Chemical Catalog Company,
(1929).
4.
Cole K. S., and Cole R. H., “Dispersion and absorption in dielectrics-I,
Alternating current characteristics”, J. Chem. Phys , 9 (1941) 341.
5.
Tayal D. C., Electricity and Magnetism, Himalaya Publishers, (1988) Edition.
6.
Nussberg Mathis, Soil moisture determination with TDR: Single-rod probes
and profile reconstruction algorithms, A dissertation submitted to the SWISS
FEDERAL INSTITUTE OF TECHNOLOGY Z "URICH for the degree of
DOCTOR OF TECHNICAL SCIENCES (2005).
28
Chapter IH
PHYSICAL PROPERTIES AND CLASSIFICATION OF SOILS OF
GUJARAT
3.1.
Introduction:
Soil is a vital component in any ecosystem, in fact, our very existence depend on the
6-12 inches underneath our feet. The main functions o f soils can be summarized in
five major key points
1-
1:
It is the natural medium for plant growth. It provides essential nutrient for
plant growth, and supports plant roots.
2-
Soil is the environmental sieve that controls the fate of contaminants, and
directs water in the various pathways o f the hydrologic cycle.
3-
Soil is a natural recycling medium. Dead plants tissue and dead bodies of
animal and people are decomposed into their basic elements; subsequently, re­
assimilated into the life cycle.
4-
Soil is a living habitat for microorganisms, insects, reptiles, and small
mammals.
5-
Soil is an engineering medium for human built ecosystem.
The properties o f soil are mainly classified into three groups as shown in the
diagram.
Properties of Soil
Physical
Colour Texture
Grain
size
Chemical
•
Salinity
E ectrical
Conductivity
Alkaline
or
Acidic
Dielectric
constant
29
The soil physical properties influence the way soils function; therefore, decisions
concerning suitability of soils for agriculture, hydrology, meteorology, construction
and environmental projects depend on the analysis of soil physical properties.
Figure 3.1: The various layers of soil that make up the horizons 2.
Soil is an extraordinarily complex medium made up of a heterogeneous mixture of
solid, liquid and gaseous materials and is composed of layers called horizons, see
Figure 3.1. The mineral fraction of soil contains particles of widely varying sizes,
shapes and chemical compositions. The solid phase of soil contains particles of vastly
different sizes, spanning the lower limits of the colloidal state to the coarsest fractions
of sand and gravel.
Now a day, the global worming is becoming a crucial issue for the whole world. The
salinization of soil is one of the major reasons causing desertification. Soil
degradation and salinization spoil large areas of farmlands, grasslands, and forests
especially in arid and semiarid regions 3. The rise in the sea level, which is sending
back the river water in coastal areas, is responsible for increase in salinity. This may
be due to decrease in water level in rivers because of tremendous increase in
consumption of easily available river water by very fast growing cities, factories,
industries, as well as the storage of water in dams to produce electricity reducing the
force and quantity of water reaching the sea. The use of improper irrigation
techniques is also one of the reasons for increasing salinization of arable land. Thus,
30
monitoring of salinity and mapping of the extent of the salinized soils are very much
important for the health o f the arable land.
The water and soil pollution by waste water o f large cities as well as the polluted
water liberated by industries in the rivers, lakes and underground water streams
decrease soil fertility, crop yield and growth by increasing soil contamination. The
smoke pollution o f cities and industries is also responsible for the increase in soil
pollution which may occur in terms of acid rain.
3.2.
Classification of S oils:
Soils are classified into groups o f various sizes based on the effective diameter of the
particles. The two prominent soil texture classifications that have evolved over the
years is the U.S Department o f Agriculture (USDA) system and the International
Society o f Soil Science (ISSS) system. A summary o f the two classification systems
are shown in Table 3.1.
Particle
USDA
ISSS
Clay
< 0.002 mm
< 0.002 mm
Fine Silt
0.002 - 0.006 mm
Medium Silt
0.006 - 0.02 mm
Coarse Silt
0.02 - 0.05 mm
Very Fine Sand
0.05 - 0.1 mm
Fine Sand
0.1 -0.25 mm
Medium Sand
0.25 - 0.5 mm
Coarse Sand
0.5 -1.0 mm
Very Coarse Sand
1 - 2 mm
Gravel/Stones
> 2 mm
0.002 - 0.05 mm
0.05 - 0.2 mm
0.2 - 2.0 mm
> 2 mm
Table 3.1: Particle size classification systems of the U.S. Department o f Agriculture
and the International Soil Science Society.
In addition to soil particles, organic materials (both decomposed and undecomposed),
numerous living organisms and chemical compounds, such as iron and aluminum
oxides are found in soils. Both water and air are also important components o f soil,
31
but being transient in nature they are usually not treated as constituents of soil
matrix4.
The clay fraction has a dominant influence on the electromagnetic properties of soil,
primarily because the small particles have such a large and reactive surface area and
therefore has the ability to retain water. In contrast, the sand and silt fractions
typically do not have as much effect due to the small surface area and the inability to
absorb or retain water to the extent that clays do. As a result, the sand and silt portions
of the soil matrix may be regarded as a passive entity whose influence on soil water is
manifested primarily by the geometric arrangement of the particles 5. In wet soil the
spaces between the solid particles are fully or partially filled with water and air, which
contains various chemicals that may have dissolved from the soil mineral phase or
have entered through the soil surface. Water in soil is pulled down by gravitational
forces but is also attracted to the surface o f the solid matrix. Soil has the ability to
retain water and the amount of water retention is based on the forces that act against
the water in the soil matrix such as gravitational force, ionic forces from solid
surfaces, etc. The most important characteristics o f the solid soil phase for our
purposes are the sizes and shapes o f the individual particles that comprise the soil
matrix and the amount of water contained within the mixture.
The physical properties o f the soil, including its ability to store water, are very much
related to the fraction o f the bulk soil volume that is filled with water and air. For
plant growth and development, a balance o f water and air in pore spaces must be
maintained. If water is limited, plant growth may be inhibited by water stress. On the
other hand if air is limited, by too much water, then growth may by limited by
insufficient aeration.
3.3.
T exture structure of Soils:
Soils are assigned unique names based on the concentrations o f sand, silt and clay and
are conveniently arranged in the soil textural triangle shown in Figure 3.2.
32
p e rc e n t SAND
Figure 3.2: Textural triangle showing the sand, silt and clay compositions for various
soil types. After Glen Rose Future Farmers o f America (FFA).
3.3.1
Porosity:
The voids or openings between the particles of soil are called as pore space or
porosity of the soil. The ideal soil should have proper assortment of large, medium
and small pores. A sufficient number of large or macro pores (with diameters > 0.06
mm), connected with each other, are required for rapid intake and distribution of
water in the soil and disposal of excess water by drainage into the substratum or into
artificial drains. In absence of water they serve as ducts. Cracks, old root cannels, and
animal burrows may serve as large pores. Soils with insufficient functional macro
porosity lose a great deal of rainfall and irrigation water as runoff. They drain slowly
and often remain poorly aerated after wetting. One of the first effects of compaction is
the reduction of the size and number of the larger pore spaces in the soil.
Small pores (diameter < 0.01 mm) are useful to hold water among soil particles. The
capillary movement of water takes place through medium-sized pores (0.06-0.01 mm
in diameter).
33
The porosity P of dry soil is defined as 6
P = 1 - ps / pr
where, ps = the density o f dry soil, and
pr = the density o f the associated solid rock.
The value o f ps for the soil samples considered in this thesis lies in the range from
1.06 to 1.7 g / cm3. The value o f pr varies between 2.6 and 2.75 g / cm3 and for
simplicity its value assumed as 2.65 g / cm3. The corresponding porosity value of the
soil samples varies between 0.35 and 0.6.
3.4.
Soils of G ujarat and their texture structure:
The geographical area o f the Gujarat state (India) is 19.60 million hectqre?’ 8 and the
reporting area is 18.82 million hectare,The forest area of the state is 1.88 million
hector. The barren and uncultivated land is 2.61 million hectore-The proportion of
cultivable waste in Gujarat is 2.32 million hectare,;
The agricultural production of land depends upon production capability of land, i.e.
soil and its moisture conservation. The inherent saline/ alkaline land o f about 1.20
million hector is low in productivity. The coastal strip from Bhavnagar to Una and
Malia to Lakhpat covering about 540 km of coastal length is facing grave situation of
salinity in ground water due to ingress of sea water at the rate o f 0.50 km per year.
Nearly 100,000 hector of fertile land have become unproductive.
This is partly due to poor recharge, accelerated withdrawal o f ground water, poor
drainage, and consequently, the increase of coastal saline areas. Nearly 270000 MCM
water is withdrawn from ground water and only 20% o f the total withdrawal is filled
by natural recharge.
The 2.32 million hector wasteland of the state is no man’s land.
The soils o f Gujarat are broadly classified into 9 major groups shown in figure 3.3.
34
There are black soils, mixed red and black soils, residual sandy soils, alluvial soils,
saline / alkali soils, lateritic soils, hilly soils, desert soils and forest soils 7’8.
ir
r
P A
tEOir
K
( s
i«
T A M
**
~ ~
-•»
A u .tjY ,/u
VAHUT
lA k n
|5^jS | A ..
LOAM
«tO AMO HIACV,
, AT t Mil 1C
5Hj (C A itA w
!” X | o c » i »
A l l u v iu m
t
m ii, y
[lf»jO l A u N t
-
-
Al.AA.,1
-------- - . . J ___________
Figure 3.3: The soils of Gujarat
7, 8
1. Black soils:
There are three types of black soils, namely (i) shallow, (ii) medium
and (iii) deep.
(i) The shallow black soils:
The shallow black soils are developed from basaltic trap in Saurashtra
region. These soils are mainly sandy clay loam in nature, and poor in
fertility. Deccan trap in extreme eastern part and the remaining strips in
Chhotaudepur and Saurashtra districts have developed from granite
35
and gneiss parent material. They are light gray in colour. In some
places they are gravelly but mainly they are sandy clay loam in texture.
(ii) The medium black soils:
They are residual soils having basaltic trap parent material. They are
found largely in Sabarkantha and Panchmahal districts, developed
from the granite and gneiss parent material. They are calcareous in
nature, and a layer of murrain (unconsolidated material o f decomposed
trap and limestone) is found below a depth o f about 40 cm, especially
in Saurashtra region. The colour of these soils varies from dark grey to
light grey, are silty loam to clay in texture, with neutral to alkaline in
reaction.
(iii) The deep black soils:
These soils have their origin in trap. They are dominating in the
districts o f Bhavnagar, S u rat, Valsad and South Vadodra. The depth
varies from 60 cm to as high as a few meters. The tract of ‘Bhal’
comprising of area between Dhandhuka and Bhavnagar has typical
deep black soils formed due to decomposition o f trap parent material
transported through flow of rivers radiating from the plateau o f Central
Saurashtra. The ‘Ghed’ tract of Junagadh district covering mainly
talukas of Porebander , Kutiyana , Manavadar and part o f Mangrol
have deep black soils formed due to deposition of basaltic trap material
transported by rivers. These soils are also impregnated with quite high
amount o f free line. The soils are dark brown to very dark grayish
brown in colour, containing 40-70% clay , are poor in drainage and
neutral to alkaline reaction. These soils are more fertile than other
black soils.
2. Mixed Red and Black soils:
These soils are shallow in depth with reddish brown colour at higher
elevations and grayish brown at lower elevations. These are clay loam
to clay and skeletal in nature, with up to 50% stony material in
36
subsurface layer , providing ideal drainage conditions to these soils.
These soils are highly calcareous in nature and alkaline in reaction.
3. Residual Sandy Soils:
These soils have developed in situ from the parent material originated
from red sandstone and shale. These residual soils are shallow in
depth, reddish brown in colour with fine weak granular structure,
sandy to loamy sand in texture dominated by coarse sand .They are
non-calcareous, neutral to alkaline in reaction with poor base
saturation. In Kutch district, they are affected due to salt accumulation.
These soils are also fertile.
4. Alluvial Soils:
These soils have been formed due to silting by the Indus river system.
These soils are very deep, coarse sandy, and are found in Banaskantha
and part o f Mehsana due to deposition o f coarse material from flowing
rivers. The alluvial sandy loam to sandy clay loam cover, the entire
northern
districts
of
Banaskantha,
Mehsana
and
boardering
Sabarkantha. These are non-calcareous, neutral to alkaline in reaction.
The alluvial sandy loam to sandy clay loam are found in Kheda and
Gandhinagar districts, eastern part o f Ahmedabad district, southern
part o f Mehasana and western part o f Vadodra district.
The coastal alluvial soils are sandy clay loam to clay in texture, neutral
to highly alkaline in reaction, and have medium fertility.
5. Laterites:
In the Dangs district, due to abundant forest vegetation and high annual
rain fall laterities are developed. They are yellowish red in upper
horizon, with thickness ranging from 20-40 cm. These are neutral to
slightly acidic in reaction.
37
6. Hill soils:
These soils occur in the hilly areas of Surendranagar, Amreli,
Jamnagar, Bhavnagar and Junagadh districts of Saurashtra, Kuttch and
eastern strip o f mainland Gujarat. They are shallow in depth, composed
of undecomposed rock fragments and are poor in fertility.
7. Desert Soils:
The two ranns (deserts) of Kuttch, viz. little runn and greater runn,
have these soils formed as a result of geological process of Pleistocene
age in Indogangetic depression. They are fairly deep, light gray in
colour, sandy to sandy loam, with silty clay loam texture in some
areas. The salt content in these soils is very high with NaCl as the
dominant salt.
8. Forest Soils:
The soils o f Junagadh forest contain more sand and are neutral. The
soils o f Dangs forest have higher silt fraction and are acidic. Organic
matter and lime contents are high in Junagadh soils compared to Dangs
soils.
9. Saline soils:
Salt affected soils occur in majority of soil groups identified in the
state. The major areas affected are due to the desert soils in Kuttch and
those affected along the sea coast due to ingress of sea water. The Bhal
tract is the area with flat topography formed due to the Alluvial
deposits brought by the rivers like Narmada, Mahi, Sabarmati, and
Bhogavo flowing in to the gulf o f Cambey. The rivers Bhadar, Ozat,
Minsar, Madhuvanti etc. have formed a delta which is known as Ghed.
Water loagging for a long time has created saline conditions.
To determine the dielectric properties of soils at radio and microwave frequencies soil
samples were collected from different regions of Gujarat state having different soil
texture. Referring to the soil map of Gujarat5’ 6 (Figure 3.3), the soil samples were
taken from the Sabarmati river bed at Ahmedabad, field at Amiapur (Gandhinagar
38
district), field at Kheralu (Palanpur district), field Jamnagar dist., field at Navsari
(Valsad district), Amreli district and field at Sayla (called Bhagat nu gaam)
(Surendranagar district) as well as from the sea bed near the Somnath temple
(Verawal).
The texture analysis was done by the KBM Engineering Company, Ahmedabad. The
texture analysis o f these soil samples is listed in tableS‘2. According to this table the
soils were mainly sand, sandy loam, silt loam and silty clay loam. The photograph of
the soil samples is shown in the figure 3.4.
Table 3.2:- The texture structure of soil samples.
Location
Soil texture (%) of
Soil
Density of Dry
(Region)
Sand
Silt
Clay
type
Soil g/cm3
Somnath
96
3.7
0.3
Sand
1.7
93
6.2
0.8
Sand
1.48
Palanpur dist.
82
16
1
Sand
1.59
Surendranagar
69
29
2
Sandy Loam
1.635
65
31
4
Sandy Loam
1.389
11
78
11
Silt Loam
1.179
Seabed
Sabarmati River
Bed, Ahmedabad
dist. (Sayla)
Gandhinagar
Dist.
Amreli Dist.
Valsad Dist.
7
62
31
Silty clay Loam
1.062
Jamnagar Dist.
12
50
38
Silty clay Loam
1.072
39
40
R eferences:
1. Environmental quality lab manual draft, Agriculture and Biological
Engineering Department, Prude University, Soil Properties, September 15
(2003).
2. United States Department of Agriculture (USDA), Natural Resources
Conservation Service (NRCS), Colorado Soil Survey Program, website:
http://www.co.nrcs.usda.gov/technical/soil/
3. Yun Shao,, Qingrong Hu, Huadong Guo, Yuan Lu, Qing Dong, and Chunming
Han,” Effect o f Dielectric Properties of Moist Salinized Soils on
Backseattering Coefficients Extracted From RADARSAT Image”, IEEE
Tram. Geosci and Remote Sens., 41/8 August (2003) 1879.
4. Behari J., Microwave Dielectric Characterization o f soil in relation to
Agriculture Applications, DST Sponsored project, 2001.
5. Jury W. A., and Horton R., Soil Physics Hoboken, NJ: John Wiley & Sons,
Inc., 2004.
6. Wang J. R., and Schmugge T.J., “An empirical model for the complex
dielectric permitivity o f soils as a function o f water content,” IEEE
Transactions on Geoscience and Remote seming, 18 /4, pp.288-295, October,
1980.
7. Joshi Sujan, Agriculture in Gujarat, Progress and Potential, Helios Enprint
Ltd.
8. Biswas B. C., Yadav D. S. and Maheshwari Stish, Soils o f India and Their
Management, Published by ‘The Fertilizer Association o f India’, New Delhi,
(1985).
41
Chapter IV
Experimental Methods for determination of complex
PERMITTIVITY
4.1.
INTRODUCTION:
The selection o f experimental method for estimation o f complex permittivity o f the
material depends on the following factors:
> Frequency range to be covered
> Material properties (i.e., homogeneous, isotropic)
> Form of material (i.e., liquid, powder, solid, sheet)
> Sample size and shape restrictions
> Destructive or nondestructive
The low frequency measurement of complex permittivity of the material can be
carried out by measuring the capacitance and resistance of capacitor with a sample
using an LCR meter. The microwave frequency measurement techniques are divided
it to two categories; Time Domain Reflectometry (TDR) and Frequency Domain
Technique. The TDR measures the time taken by an electromagnetic pulse to
propagate along a transmission line that is surrounded by the medium. In frequency
domain technique, e.g., VNA which measures the reflection coefficient of a simple
sample cell filled with the material under test (MUT).
4.2
FREQUENCY DOMAIN TECHNIQUE:
4.2.1
TECHNIQUES
FOR
THE
MEASUREMENT
OF
COMPLEX
PERMITTIVITY USING MICROWAVE BENCH SET UP AT SINGLE
FREQUENCY:
A variety of methods o f measurement of complex permittivity of materials at
microwave frequency exist. They are mainly the transmission line techniques and the
free space techniques.
42
The free space measurement techniques axe employed for the measurement of
complex permittivity o f materials in the sheet form. The free space transmission
technique is difficult to use at low frequencies because o f the need for a large
structure and a large amount of (soil) sample requirement *. Further in laboratory
conditions at microwave frequencies it is difficult to launch a plane wave in a limited
space, and diffraction from the edges of the sample may reduce the accuracy of
measurement 2. The transmission line or waveguide methods are easier to use in
which the sample is cut in small size of the dimensions o f the waveguide, even a very
small (or thin) sample is required in cavity or cavity perturbation techniques.
The transmission line methods of measurement o f complex permittivity are based on
the use of a slotted line and a variable short circuit. Slotted line techniques are well
suited to complex permittivity measurement (except for very low loss samples) at
microwave frequencies. The accuracy o f measurement depends to a large extent on
the accuracy with which the VSWR and the position o f the voltage minimum can be
found.
The most widely used and versatile microwave measurement technique is the twopoint method of measuring the complex dielectric constant involving the solution of a
transcendental equation 2. The input impedance o f a short circuited waveguide is
measured with and without the sample and a transcendental equation is solved. If the
approximate dielectric constant is not known, then two such measurements are carried
out with samples o f different lengths. In the present work the two-point method for
the measurement o f complex permittivity o f dry and wet soil samples at 5.56 GHz and
9.5 GHz microwave frequencies is used.
The “infinite” line method (also called infinite sample method) is useful when a
dielectric material has a very high loss tangent (tan 8). In this method a physically
reasonable length o f the dielectric sample dissipates a sufficiently large portion o f the
microwave energy entering the sample, so that no energy is reflected to the in-port,
and the sample may be considered to be o f infinite length. This method is very simple
but may lack accuracy depending on the exact sample length. Further for the dry and
wet soil samples, which is non homogeneous medium having soil particles, air voids
43
and water, it is difficult to insert the soil sample uniformly with same density profile
in the total length o f the sample holder waveguide.
4.2.1.1
THEORY OF THE TWO POINT METHOD FOR MEASURING
COMPLEX PERMITTIVITY INVOLVING THE SOLUTION OF A
TRANSCENDENTAL EQUATION:
The two-point method is best known and most widely used for the measurement of
complex permittivity. It is best suited to either “lossless” dielectrics or the dielectrics
with medium loss.
3li
l g +
h
1 ,
,
>
Z
q
,
k
Short Circuit
Figure :4.1
(a)
Sample
(b)
Figure (4.1-a) shows an empty short-circuited waveguide with a probe located at a
voltage minimum D r. Figure (4.1-b) shows the same waveguide, containing a sample
of length /e with the probe located at a new voltage minimum D. The sample is
adjacent to the short circuit. At the open boundary o f sample from figure (4.1-b), one
can write the impedance equation
Z0tan kl = - Z e tan k j s
-( 1)
Similarly in figure (4.1-a), looking toward the right, one can write
Z0tm k(lR + /e) = 0
...(2)
Now,
44
tan k(DR - D + le) = tm k[(lR + /e) - ( / + /6) + /e]
= tan&[(/fi + /e) - / ]
tan k(lR +le) ~ tm k l
1 - tan k(lR +le).im k l
Since tan(er + fi) =
tan a + tan ft
1- t a n a , tan ft
= - tan kl
Since, tan k(lR + /g) = 0 from eq (2).
.%tan k(DR - D + /g) = -
tan k j £
v
From equation (1)
/. tan k(DR- D + le) = — tan k j e
...(3)
But, h . - — , which gives
,K
1
:.im .k{pR - D+!e)= — tankJe
K
Dividing both sides o f this equation by kl€, we get
tm k(D R ~ D + le)
U.
tm k el6
=“
m
T
fAS
""(>
In equation (4) all the quantities on the left hand side are measurable, where as the
ton 2*
right hand side term is of the form ------ , with Z = k j e , so that once the
Z
measurements have been carried out, the complex number, Z = k j e , can be found by
45
solving the transcendental equation, and from it, ke . There exist infinite solutions of
the transcendental equation for e r . To obtain proper solution, the measurements are
carried out with a sample o f different sample length /'e . A solution common to the
two sets o f solutions will be the proper solution for e r . The solution is shown as an
“intersection point” in the graph as shown in the figure (4.2).
Figure (4.2): A solution common to the two sets of solutions.
It should be noted that the accuracy of the experimental results of complex
permittivity using this method depends to a large extent on the smoothness o f the
sample, the fitting o f the sample in the waveguide, the care which has been taken to
insure that its surfaces are properly “squared” with respect to each other, the accuracy
of measurement o f length le o f the sample, the position o f minima DR and D and the
accuracy in the measurement o f VSWR.
4.2.1.2 EXPERIMENTAL SET UP FOR ESTIMATION OF COMPLEX
PERMITTIVITY EMPLOYING TWO POINT METHOD:
The dielectric constant e' and dielectric loss e" of the soil samples were measured at
5.65 GHz (C-band) and 9.50 GHz (X-band) microwave frequencies using microwave
benches and employing the two-point method 2. The reflex Klystron and Gunn diode
were used to generate X and C-band microwave frequencies, respectively. The
experimental set up is shown in fig (4.3) and (4.4).
46
The sample holders for X-band and C-band measurements were fabricated from the
standard wave guides available. At the one end of the sample holder a metallic flange
was connected so that it can be connected to main line and other end was carefully
shorted as shown in figure (4.5).
47
------- r
-
Figure 4.3: The experimental set up for the two-point method at
X-band microwave frequency o f 9.5 GHz
\i\J
Klystron
2K 25
Isolator
Frequency
Meter
Microameter
Slotted
Section
Sample
Holder
Waveguide
Band
oo
Gunn Diode
Frequency
Slotted
Section
Figure 4.4 The experimental set up for the two-point method
at C-band microwave frequency o f 5.65 GHz
Isolator
Pin
Modulator
Sample
Holder
Waveguide
Band
49
3^
C or X-band
Wave guide
sample
holder
Short
Figure 4.5: The soil sample holder with metallic flange.
First, with no dielectric in the short-circuited line, the position of the first minimum
Dr in the slotted line was measured (Figure 4.6). Now the soil sample of certain
length (/e ) having certain moisture content was placed in the sample holder, such that
the sample touches the short-circuited end. Then the position of the first minimum D
on the slotted line and the corresponding VSWR,
r
were measured (Figure 4.7). The
VSWR was measured using either by a microameter or a VSWR meter. For the
measurement of VSWR using microameter no amplitude modulation was applied to
the microwave signal, but when the VSWR meter was connected, the amplitude
modulation using pin diode was applied to the microwave signal set up. This
procedure was repeated for another soil sample of same moisture content for another
soil sample length (/'6 ).
50
Dr
"
7
Slotted
Empty Wave
guide sample
holder
Short
"
Section
Flange
Figure: 4.6
Soil sample
Length /€
D and VSWR r
Short
~7r
Slotted
Section
Flange
Figure: 4.7
Sample
holder with
soil sample
The propagation constant (in the empty wave-guide) is calculated as
. 2n
k =—
..(5)
Where, Xg =2 x (distance between successive minima with empty short circuited
wave- guide sample holder)
The complex number C Z -VF is calculated using the equation
ip _ 1 * i -|r|*exP(/^)
jk ls
1 + |T| * expOV)
where, § = 2k*(D-DR-/e)
.(6)
(7)
and,
r +1
(8)
51
The solution o f the complex transcendental equation
tanhfFZr)
C Z - 'F
(9)
TZt
was obtained3 to get conductance Ge and susceptance Se The dielectric constant s'
and dielectric loss e" o f the soil sample were then calculated as
and,
Ge + (K'2a
.(10)
\ 2
1+
'2a
(n)
e"=-
(X„
1+ v2a
where, a = width of the wave-guide.
For more accurate results, the length of the sample should be kept near ( XgE / 4) one
quarter o f the wavelength in the dielectric filled wave guide. For estimation of
(Xge / 4) we assumed dielectric constant of dry soil as 2.5 and Xg€ was calculated
using the relation
2n
(12)
T
where, X = free space wavelength o f microwave signal,
Xc = 2a, for the dominant mode propagating in the rectangular waveguide.
ftr=
1.
Taking this value as a reference value measurements were carried out for many
samples whose lengths are nearly
/ 4, till the same values o f conductance Ge and
susceptance Se were obtained for the two soil samples. These values o f Ge and Se
were used for further calculations o f the dielectric constant e' and the dielectric loss
52
e " . As the moisture in the soil increased the sample length were reduced and similar
exercise was done for wet samples.
4.2.2
SIMULTANEOUS
MULTI
FREQUENCY
MEASUREMENT
TECHNIQUES:
There are many techniques for the simultaneous multi frequency measurements o f the
complex permittivity o f samples; they are either in time domain or in frequency
domain. These methods are either destructive methods, in which sample preparation is
needed for accurate evaluation, or nondestructive methods, which require very little or
no sample preparation. We used the frequency domain technique to measure complex
permittivity using a vector network analyzer 4,
4.2.2.1
MEASUREMENT OF COMPLEX PERM ITTIVITY USING
VNA:
A vector network analyzer consists of a signal source, a receiver and a display. The
source launches a signal at a single frequency to the material under test (MUT). At the
same time the receiver is timed to that frequency to detect the reflected and
transmitted signals from the material. The measured response produces the display of
the magnitude and phase of reflected and transmitted signal at that frequency. The
source is then stepped to the next frequency and the measurement is repeated to
display the reflection and transmission measurement response as a function of
frequency.
Various methods are developed by researchers for the estimation o f complex
permittivity o f liquid and solid samples using VNA1, 4* 6. Yan-Zhen Wei and S.
Sridhar 4’ 5 developed experimental technique for measurement o f complex
permittivity of liquids, which may be extended for colloids and solutions. The method
involves the determination o f complex impedance from the measurements o f the
reflection coefficient o f a simple sample cell consisting o f a coaxial semi-rigid cable
terminated by the liquid sample. The complex impedance measured at the network
analyzer plane is de-embedded to deduce the sample impedance, and from it, the
complex permittivity o f the liquid was estimated. The liquid-coax interface was
53
modeled as an impedance Z(co, g ) = [/©C/ + j(oCoe. ]'L. The de-embedding requires
calibrations with specific terminations like an open (air), a short circuit using liquid
mercury, and a standard liquid such as acetone. It eliminates the connector
impedances and any other line mismatches, and also the fringe-field capacitance Cj
within the coax, and the capacitance parameters Co, all of which are dependent on
frequency and hard to measure. The method was successfully applied for the
measurement of complex permittivity of variety of liquid samples in the frequency
range from 45 MHz to 20 GHz producing results with errors less than 2% for low
dielectric constant liquids (e ’ < 30). In the presented work the method suggested by
Yan-Zhen Wei and S. Sridhar is used for estimation o f complex permittivity of dry
and wet soil samples.
4.2.2.2
THEORY OF DE-EMBEDDING OF SAMPLE IMPEDANCE
AND MODELING OF COAX TERMINATION:
Various kinds o f calibration techniques for one-port measurements are available 6 that
can not be carried out at the reference plane of the network analyzer. For the
measurements using a semi rigid coaxial probe, the proper modeling of the frequency
a and sample dielectric permittivity e dependent impedance Z(co, e ) o f the probe-end
is crucial4’ 3. This calibration technique eliminates uncertain factors like fringe-field
(complex, frequency dependent) capacitance and the frequency dependence of the
probe parameters.
For a TEM mode propagating along a transmission line with characteristic resistance
Zo, the transmission line can be represented by equivalent T- network using Zi, Z2 and
Z3 impedances as well as Z(oa, e ) as the impedance o f the end of the probe (Figure
4.8-a). Zm (t»,e) is the measured output impedance which can be obtained from the
complex reflection coefficient
P „= rv'
.... (13)
measured by VNA Model- 8714 ES, Agilent made,, and
ZU =Z0
..... (14)
1~P m
54
z,
z3
(a): Z( e ,co) represents the impedance o f the coaxial-sample interface, and
the remaining spurious impedances are represented by the T network with
impedances Z\,Z2, Z3.
Z( e j to)
Cf(a>)
C (e,co)
(b) The equivalent circuit for Z( e , co) with complex fringe field
capacitances approximation for the coax-liquid interface.
Zi
z3
€ C0(co)
(c) The resultant equivalent circuit with part o f the fringe field capacitance
incorporated into the T-network.
Figure 4.8: Equivalent circuit o f the measurement configuration using VNA.
55
Also from figure (4.8-c),
2
= 2 ] Z2[Z3+ Z ( g)]
M
1 Z2 + Z 3+ Z (e)
.(15)
ZM[Z2 + Z3 + Z(e)] = Z,[Z2 + Z3 + Z(e)] + Z2[Z3 + Z(e)]
.‘.Z MZ 2 +ZMZ, + Z mZ( g) = Z,Z2 +Z ,Z 3 + Z jZ( g) + Z2Z3 + Z 2Z(e)
Z|Z2 +Z,Z3 + Z 2Z3 +(Z, + Z 2)Z (e )-(Z 2 + Z3)Z^ = Z ^ Z ( g)
A'+A’ 2 Z ( e ) -A '23 ZM(e) = ZA/(e)Z(e)
..... (16)
where
A'= ZtZ2 +ZjZ3 + Z2Z3
(17)
A'12 = Z1 + Z2
(18)
A'23 = Z2 + Z ,
(19)
By knowing the way in which Z depends on frequency co and the dielectric constant
e of the surrounding medium
Z(ro,e) = f(co,e)
....(20)
we can get Z(to, e ) from known e ( to) and vice versa.
For three calibration media of known e a, 6 b and e & by measuring the reflection
coefficients pA, Pb and pc, we can calculate Zma>Zmb and Zmc using equation (14),
and Za, Zb and Zc from equation (20). Equation (16) leads to three equations which
include three unknown parameters A', A'12 and A'23 as
A +A |2 ZB —A 23 Zm
= Z jmbZ jj
.(21)
,(22)
A +A |2 Zc ~ A
= ZUCZC
.(23)
A '+A
IS
Tts' n
7
- AA' 23 7
Z
=
56
For short, we use liquid mercury. Hence Zb = 0. Also Z mb is measured and known at
each frequency. Hence equation (22) gives
A'-A'jb Zm = 0
..... (24)
The coax terminated by a medium filling the other half space can be described by the
equivalent circuit given in figure (4.8-b), which represents e and © dependent
complex capacitance due to the fringing field 4. Hence we have
Z(©, e ) = [ jB(©, e ) + G(©, € )]'1.
Z(©, 6 ) = [ jfflCrfo) +j <dC(<d, e )] -1
..... (25)
« 1 , we can write 4
when
C(©,€)=Cc«(©) e
.... (26)
Cf represents the fringe field effects inside the probe, and Co the fringe field coupled
to the sample liquid. The effect o f Cf can be included into the three network
parameters as shown in figure (4.8-c). Hence from equation (25) and (26), we may
write
Z = 1/j©C = 1/ j©Co e
Substituting this value in equations (21) to (23), we get
A' 12
A'+j® c0 s A
‘ 23 ^ M A ~
ja)C0
:.jm C 0A*+^3- ■j®C0A’;23
6
,
57
A + - *12
(27)
^ 23%MA ~
where A = ja>C0A', A I2 = A'12, and A23 = ja)C0A'23
Similarly,
=hm _
..(28)
A
+^ll-A
Qa +
l* 7 3 Z7
jMC =—M
....(29)
a t+ ^ h - ^73^
a z MB
&
ec
€c
Also for sample liquid or solid dielectric
A +i
^12- Aa 23zZ,w -=
G
...(30)
G
By solving equations (27), (28) and (29) we get the values o f the three parameters
A, A12 and A23. Equation (30) can be simplified as
a-a a
-^-A
l
G
G
. - . A - A A = Z^
• c—
~ ^12
A —^23
.........(31)
Substituting the values o f A, A12 and A 23 in equation (31), and Zm from equation (14)
by measuring tm for the sample, we get
g
o f the sample.
The measurement does not depend on any information o f the fringing capacitance.
Also the frequency dependent parameter Co (©) which is complex, is determined at
every measurement frequency by this procedure. For “open” in which the probe is in
air, we use equation (27) with
g
’ = 1 and
g”
= 0. For standard, which is usually
acetone, we use equation (29) with a Debye model for
g
:
58
e = e » + ( e 0- 6 oo)/ (1+jcox)
(32)
with parameters 4 , e W= 1.9, eo =21.2 and t = 3.3*10'12s.
We used a vector network analyzer Model- 8714 ES, Agilent made, working in the
frequency range from 0.3 MHz to 3 GHz. The measurements were conducted at
Department o f Physics, Bhavnagar University, Bhavnagar, Gujarat, India. In the
present work the network analyzer was operated in frequency range from 30 MHz to
1.5 GHz with typical selections o f 51-201 points. A personal computer (PC) was
connected to receive the pairs o f data (Real and Imaginary values of impedance or
reflection coefficient) for the given frequency range.
4.2.2.3
DESIGN OF COAXIAL PROBE:
Several design configurations and methods are explained by Stuchly M. and Stuchly
S. 6. The configuration o f semi rigid coaxial probe used for the measurement of
complex permittivity o f dry and wet soils is shown in figure (4.9). We modified the
design o f semi rigid coaxial probe as prescribed by Yan-Zhen Wei and Sridhar S 4’5,
and attached a metal disc at the other end o f the probe keeping the central probe tip
outside 7. The diameter of coaxial cable was 0.141inch, with a metal disc (flange) of
diameter 3.2 cm connected at one end and the probe tip was kept 0.3 cm outside the
probe, while at the other end o f the probe an N-type female connector was attached.
The connector end was mated to the N type male connector o f a flexible coaxial cable
attached with the Vector Network Analyzer.
59
N-Ty
‘ ro
Fema
Conn
NA
Semi Rigid
Coaxial
Probe
—
Flange
3.2 cm
Probe Tip
3 mm
Figure 4.9: The configuration of semi rigid coaxial cable.
The flexible coaxial cable of VNA was calibrated at the end by using the standard
open, short and matched loads provided by the manufacturer. Now the flanged semi
rigid coaxial probe was connected with the flexible coaxial cable attached to VNA as
shown in figure (4.10).
N-type
Connector
^
A
To
VNA
Coaxial probe
______________
Soil sample
Figure (4.10): Measurement set up for VNA
60
4.2.3 LOW
FREQUENCY
MEASUREMENTS
OF
COMPLEX
PERMITTIVITY:
At low frequencies, when the wavelength is much larger than the sample dimensions,
the material can be approximated with lumped elements (resistance R, inductance L
and capacitance C) connected in a series or parallel circuit with wires which have
little effect on the circuit8.
Sengwa et al? ' 10 measured the dielectric constant and loss tangent of shale, sandy
sandstone, calcareous sandstone, and some minerals like clay, Gypsum, Calcite,
Magnesium rock, Lignite etc. in the frequency range from 100 Hz to 100 KHz. They
used automatic Keithley LCZ meter (model 3330) to determine the capacitance and
dissipation factor of parallel plate capacitor filled with the samples of thickness ~0.14
cm prepared from these materials.
4.2.3.1
DESIGN OF COAXIAL CAPACITOR:
Even though a parallel plat capacitor with disk electrodes is most commonly used as a
sample holder in the frequency range up to 100 MHz 8, the limitation o f the parallel
plate capacitor is that when we put the soil sample between parallel plates, the soil
sample gets disturbed. Further it is difficult to verify about how much pressure is to be
applied between the two plates of the parallel plate capacitor, since increasing the
pressure between plates means to increase the density o f the soil sample and the
complex permittivity of the soil is dependent on the density, too.
To avoid these difficulties a coaxial capacitor with vertical cuts on the outer cylinder
(Figure-4.11) was designed which gives almost in-situ dielectric properties o f the soil
sample. The inner conducting rod o f diameter 2 mm was surrounded by a conducting
outer cylinder with inner diameter 6.65 mm and outer diameter 7.93 mm. The central
conductor and surrounding outer cylinder were separated by Teflon ring o f thickness
4.66 mm. Four vertical cuts of length 10.26 mm and 1 mm wide spacing were
prepared on the outer cylinder reaching up to the Teflon ring. The cuts on the outer
cylinder help in removing the air inside the coaxial capacitor as it was dipped in the
MUT. The total length o f the capacitor was 14.92 mm.
61
(a)
—
J “ {b)
1 ( e )
Figure (4.1 1): Design of Coaxial Capacitor.
jr /
A
(a) Vertical Cuts, (b) Outer Cylinder, (c) Connector of Central Conductor,
(d) Connecting wire for outer cylinder, (e) Inner Cylinder.
While measuring the capacitance and resistance of the coaxial capacitor filled with the
liquid or soil sample, the capacitance and resistance, of the Teflon spacer used
between the two coaxial cylinders and the connecting terminals, was taken in to
account during calibration. Further for the coaxial capacitor the limiting wavelength
related with the complex permittivity e * of the soil samples was taken into
consideration according to Levistkya 8'and Brandt 11 as follows:
/ L >Xyfe'(b + a)
where a and b are the radii of the inner and outer cylinders of the coaxial capacitor,
respectively.
4.2J.2
THEORY:
For the coaxial sample holder capacitor (,a = 0.1 cm, b = 0.325 cm, fore ’ ~ 80), the
limiting wavelength was calculated 8 as A,|jm >11.94 cm. Hence the limiting frequency
is/um < 2.5 GHz, and the LCR meter is operating in the frequency range from 20 Hz
to 2 MHz which is the safest dimension of the coaxial capacitor. Conversely with this
capacitor dimensions and maximum frequency of 2 MHz (X = 1.5 x 1010 cm), the
maximum value of dielectric constant that can be measured is
yfe' < -----------= 1.1234 x I010
n(b + a)
(33)
62
The stray capacitance is the capacitance o f the measuring system excluding the
sample capacitance, which consist of edge capacitance Ce and the cell capacitance Cc.
Thus Cs-Ce + Cc
..... (34)
While measuring the fluids or soils using the coaxial capacitor the capacitance of
Teflon spacer (ring) is included in the stray capacitance value which must be removed
from the measured capacitance 1. The edge capacitance Ce is the capacitance arising at
the edges of the capacitor as a fringing effect due to bending o f the flux lines. The
value o f Ce depends on the electrode arrangement, and on the sample and electrode
thickness. The value o f Cc also contains the capacitance between the connecting leads.
The total stray capacitance can be eliminated by measuring the capacitance of the
capacitor with liquids o f known dielectric constant as
e ' = m (C p -C 0) + n
........ (35)
which is the equation o f straight line o f slope m and crossing the dielectric constant
axis at n for the measurement of capacitor with air, and some liquids of known
dielectric constant.
The dielectric loss o f the sample can be calculated using the equation8
e "= e 'ta n £
Where tan S = ———
o e 'e 0
...... (36)
....... (37)
For the coaxial capacitor8 the conductivity and dielectric constant are
, ln(b / a)G ^ In(b/a)
°
2nH
, In{b!a)C
2nHR ’ S
2nH ^
(38)
63
Substituting results (37) and (38) in (36), we get
e"=e'tan J
cr1 _ In(b/a)
co e 0 2 nHRco e 0
6'
CRco
(39)
In equation (39),
e' = measured value of dielectric constant from equation (34).
e 0= free space dielectric constant = 8.854 x 10‘12 F/m,
a = radius o f inner cylinder
b = radius o f outer cylinder
/ = frequency o f measurement,
H= length (height) of the capacitor
Rp = resistance measured using LCR meter at each frequency,
Cp = capacitance of coaxial capacitor with soil sample, at each frequency,
Co = capacitance of empty (air filled) coaxial capacitor, at each frequency.
The behavior of a linear, homogeneous, isotropic dielectric material in an electric
field is described by the Ohm’s law which relates the current density J t o the electric
field intensity E through the complex conductivity or, which includes the effects o f the
displacement current a s 8
J = cr*E = {a'+ja)e')E
........ (40)
An equivalent form o f (40) is
J = jm e* E = ja>(e'-j —)E
co
......... (41)
Where o ’ and e' are the real parts of complex conductivity and complex dielectric
permittivity, which are interrelated as
cr* = jcoe*
....... (42)
64
The complex parameters a* and e * include the imaginary components representing
energy loss occurring in a material as
cr* = cr'—ja " and € * = e '- j e"
Where ct’= m e’, and e"=
....... (43)
are real numbers.
Wet soil is a heterogeneous material consisting of soil particles, air and w ater11 which
behave as a conductor at low frequencies (below 100 kHz), while at high frequencies
(above 100 MHz) the dielectric behavior prevail. In the frequency range between 100
kHz to 100 MHz, the conductivity o f wet soil plays an important role in dielectric
phenomena. Thus the wet soil behaves as both conductor and dielectric, affecting the
measured real parts o f the conductivity o ’ and dielectric constant e '. The real part of
the conductivity a ’ consists o f two components (i) the ohmic or DC conductivity Ode,
which corresponds to the direct current field, and (ii) the displacement current
“dielectric conductivity” Odiei, which is frequency dependent. Thus the total loss factor
e"Mgl includes the «ohmic losses (arising due to conduction phenomena) and dielectric
losses (due to polarization processes), as
<="total
+ e "dc = &"d,el +
_ <?'+<?'dc
a>en
(44)
Where the frequency-dependent real part of the alternating current (ac) conductivity
a' is obtained from the relation 13
£r'=© €0e"
........ (45)
The dc conductivity tide can be obtained from the intercept o f straight line f it14 o f the
a ’ values at low frequencies against the frequency graph.
Now subtracting the value of
from the total dielectric loss shown in
equation (44), the actual dielectric loss o f the lossy material can be obtained.
65
REFERENCES:
1. Hallikainen M. T., Ulaby F. T., Dobson M. C., El-Rays M. A., and Lin-Kun
Wu, “Microwave Dielectric Behavior o f wet Soil-part 1: Empirical Models
and Experimental Observations,” IEEE Trans. Geosci. Remote Sensing, 23/1
(1985) 25.
2. Sucher M. and Fox J., Handbook o f microwave measurements, (1963) 504.
3. Arthur R.Von Hippel, Dielectric Materials and Applications, (1954).
4. Yan-Zhen Wei and Sridhar S., “Technique for measuring the frequencydependent complex dielectric constants of liquids up to 20 GHz”, Rev. Sci.
Instrum., 60/9 (1989) 3041.
5. Yan-Zhen Wei and Sridhar S., “Radiation-Corrected Open-Ended Coax Line
Technique for dielectric Measurements of Liquids up to 20 GHz”, IEEE
Trans. Microwave Theory and Techniques, 39/3 (1991) 526.
6. Stuchly M. A. and Stuchly S. S., “Coaxial Line Reflection Methods for
Measuring Dielectric Properties o f Biological Substances at Radio and
Microwave Frequencies-A Review”, IEEE Trans. Instrumentation and
Measurement, IM-29/ 3 (1990) 176.
7. Starr G. C., Lowery B., and Cooley E. T., “Soil Water Content Determination
using a Network Analyzer and Coaxial Probe”, Soil Sci. Soc. Am. J., 64 (2000)
867.
8. Levitskaya T. M. and Sternberg B. K., “Laboratory measurement o f material
electrical properties: Extending the application o f lumped-circuit equivalent
models to 1 GHz”, Radio Science, 35/ 2 March-April (2000) 371.
9. Sengwa R. J., Soni A., Ram B., “Dielectric behavior o f shale and calcareous
sandstone o f Jodhpur region”, Indian J. Radio and Space Physics, 33 October
(2004) 329.
10. Sengwa R. J., and Soni A., “Dielectric properties o f some minerals o f western
Rajasthan”, Indian J. Radio and Space Physics, 37 February (2008) 57.
11. Brandt A. A., “Studying the Dielectrics at Very High Frequencies (in
Russian)”, State Publisher ofPhys.-Math. Lit., Moscow, (1963).
12. Sternberg B. K., Levitskaya T. M., “Electrical parameters of soils in the
frequency range from 1 kHz to 1 GHz, using lumped-circuit methods”, Radio
Science, 36/4, July/August (2001) 709.
66
13. Sengwa R. J., Choudhary S., and Sankhla S,, “Dielectric spectroscopy of
hydrolic polymers-montmorillonite clay nanocomposite aqueous colloidal
suspension”, Colloids and Surfaces A: Physicochemical and Engineering
Aspects, 336 (2009) 79.
14. Sengwa R. J., Sankhla S., and Choudhary S., “Dielectric characterization of
solution intercalation and melt intercalation poly (vinyl a!cohol)-poly (vinyl
pyrrolidone) blend-montmorillonite clay nanocomposit films”, Indian Journal
o f Pure & Applied Physics, 48 March (2010) 196.
67
CHAPTER-V
DIELECTRIC PROPERTIES OF DIFFERENT TYPE OF SOILS
WITH MOISTURE CONTENT AT RADIO AND MICROWAVE
FREQUENCIES
5.1
INTRODUCTION:
Electromagnetically wet soil medium is a four component dielectric mixture
consisting o f air, bulk soil, bound water, and free water l. According to the soil
classification o f USDA, the soil mainly consists o f sand-particles o f diameter d >
0.005 cm, silt - 0.0002 < d < 0.005 cm, and clay - d < 0.0002 cm, weight content o f
which is expressed in percent by dry weight o f the soil 2. It has been shown that sand
consists primarily o f quartz and feldspar with complex dielectric constant
e i0« 4.5 + z'0.05; silt - from quartz and muscovite with
4.5 + zO.Ol clay - from
kaolinite and montmorillonite with e c/~ 4.5 + z‘0.25 , in the frequency range from 1-50
GHz 2. Densities o f these strata are similar and in the range from 2.5-2.1 g/cm3.
Further it has been shown that the quantity o f bound water in soil depends on the
volume o f clay fraction in the soil, which increases with increase in the volume o f
clay. This has been explained due to large specific surface area o f clay particles
compared to other soil particles ’’ 2. For the moisture content below transition
moisture in the s o i l 1-3 the water is present in the shape o f films around soil particles
and is bound. It is difficult to polarize the bound water molecules and the bulk o f
water in soil shows a smaller dielectric constant than that o f free water above
transition moisture. Further the complex dielectric constant
o f water varies
with temperature and frequency in the microwave region *’ 3. Hence in general the
complex dielectric constant o f the soil-water mixture is a function o f frequency,
temperature, moisture content, and texture structure o f (he soil.
Hallikainen et al} measured the amplitude and phase o f the TEio mode transmission
coefficient Tm for a dielectric sample o f length L; by comparing the amplitude and
phase readings o f the network-analyzer/ phase-gain indicator for the sample holder
68
when it was empty and when filled with a soil sample. The attenuation coefficient a
and the phase factor P, comprising the propagation constant y = a +j P, of the
dielectric filled waveguide were calculated from the transmission coefficient using the
iterative procedure. These values were used to calculate the dielectric constant e' and
dielectric loss e" o f the soil samples. It was observed that the dielectric constant of
soil-water mixture is a function of volumetric moisture content in the soil over the
frequency range from 1.4 GHz to 18 GHz. For a given volumetric moisture content in
the soil e' decreases with increase in clay content between 1.4 GHz to 18 GHz, e"
increases with increase in clay content for Wv> 0.2 /- :.
below 5 GHz due to
salinity effects, e" increases with increase in sand content in the soil above 5 GHz
due to the increased ratio of free water to bound water in sandy soils. Further it has
been shown that if density effects are controlled than the dielectric constant of dry soil
is essentially independent of soil texture and frequency.
Njoku 4 measured the complex dielectric constant of sand as a function o f moisture
contents in the range from 0 to 30% by volume at frequencies o f 0.679,1.0, 3.0, 8.52,
14.0 and 20.0 GHz. It was observed that the relaxation occurs due to the presence of
soil moisture at high frequency end, and shifts to a lower relaxation frequency as
moisture content decreases.
Hoekstra and Delaney 5 measured the complex dielectric constant of soils as a
function o f water content and temperature over the frequency range from 108 to 2.6 x
1010 Hz. It was observed that the dielectric constant o f wet soil decreases with
increase in frequency in this frequency range, and the dielectric loss achieves
maximum value in between. The relaxation observed was attributed to the presence of
water in soils, since interfacial and Maxwell-Wagner dispersions are unlikely to occur
at these high frequencies. The measured complex permittivity data o f four soils fall
within a relatively narrow band, indicating that the main parameter determining
g *(a>)
at constant temperature is the volumetric water content in the soil. Two
closely spaced relaxations appear to be associated with water in the soils. The
dominant relaxation with maximum loss occurred between frequencies ranging from
1 x 109H z to 4 x 109 Hz.
69
Robinson et al. 6 measured the complex permittivity of soils ranging from sand to clay
using a Surface Capacitance Insertion Probe (SCIP) and time domain reflectometer
(TDR) for various moisture contents in the soils. It was observed that the TDR
measurements were much more consistent, producing apparent relative permittivity
values below those of the Topp curve for the finer textured soils. They also measured
complex permittivity of a silty clay loam soil at two moisture contents in the soils
using a network analyzer, for the frequency range from 10 MHz to 1 GHz, to aid the
data interpretation. A reasonable agreement was found between the TDR
measurements and the network analyzer real permittivity for a bulk density around
0.75 g cm'3; above which the data diverge, with TDR permittivity rising above Topp’s
curve and the network analyzer data remaining below it.
Calla et al. 7 measured the complex dielectric constant o f loamy sand for various
moisture contents by weight, in the frequency range from 2 GHz to 20 GHz. The
measurements were carried out using HP network analyzer and an HP dielectric probe
employing coaxial probe method. They observed that the dielectric constant increases
slowly up to certain moisture contents after which it increases rapidly. Further it has
been observed that the change in loss factor is more at higher frequencies than that at
lower frequencies.
A. D. V yas8 measured the values of dielectric constant and dielectric loss of sand and
sandy loam for various moisture contents at X-band microwave frequency. He
observed that the permittivity o f sandy loam soil increases slowly up to 8% moisture
content, after which it increases linearly with moisture content. For sandy soil he
observed a small increase in permittivity up to 8% moisture content after which it
increased sharply. He also calculated the emissivity values for normal incidence for
the sand and sandy loam soil with moisture content from the dielectric data. It has
been observed that the emissivity of sandy loam change from 0.93 to 0.60 for the
moisture content variation from 0 to 20% by dry weight o f the soil. The emissivity of
sand decreased from 0.93 to 0.49 for the same variation in the moisture content. It has
been explained that the decrease in emissivity with increase in permittivity which
causes a total increase in reflected energy and thereby a total decrease in emitted
energy.
70
Ghosh et al. 9 measured the dielectric constant and dielectric loss o f dry and wet soils
at 14.89 GHz. The two point method 10 using a KU-band set up was used for the
measurement o f complex permittivity o f eleven soil samples with various texture
structures like sand, sandy loam, loam, silt loam and loamy sand. They observed that
under dry conditions, soil texture has little effect on the value of dielectric
permittivity, but this behavior changes with increase in the water content in the soil.
Further it have been observed that the dielectric constant e' increase gradually (for all
soil samples) with increase in the moisture content in the soil up to transition moisture
Wt. Beyond Wh the increase in e' is rapid. Further the transition moisture Wt was
observed to be dependent on the clay content of the soil sample. The value of e" was
observed to be higher for soil samples having higher sand content. At this microwave
frequency, the dielectric loss e" o f clayey soils had lower value at any given moisture
content as compared to high sand content soils.
Mishra and B ehari11 measured the complex dielectric constant o f sandy loam soil for
various moisture contents at S (3.0 and 3.5 GHz), C (5.5 and 6.5 GHz), and X-band
(9.0 and 9.4 GHz) microwave frequencies. A pseudo vector method using hom
antenna was used for the verification of efficiency of the method. It was observed that
the dielectric constant and loss increase with increase in moisture content in the soil.
The emissivity o f the soil calculated from dielectric data for normal incidence was
found to decrease with increase in moisture content in the soil for all frequencies of
measurement.
Sengwa et al. 12,13 measured the dielectric constant and loss tangent o f shale, sandy
sandstone, calcareous sandstone, and some minerals like clay, siliceous earth and
fuller’s earth at the microwave frequency of 10.1 GHz. They used two-point method 2
and microwave bench setup with the samples of thickness ~0.9 cm and 1.4 cm
prepared from these materials.
Pancholi and Khameshra 14 measured the complex permittivity o f some Rajasthan
soils (sand, sandy loam, sandy clay loam and clay) for various moisture contents at
7.114 GHz microwave frequency using the infinite sample method. It has been
observed that the dielectric constant and dielectric loss o f the soils increase slowly
with increase in moisture content up to transition moisture and then increased rapidly
71
with moisture content. Further it was observed that the transition moisture values are
higher for high clay content soils. The measured values were compared with the
values calculated using the Wang and Schmugge model. From the measured values of
dielectric constant and dielectric loss the emissivity, reflection coefficient and the
brightness temperature o f the soils for various moisture content were also calculated.
It has been observed that the power reflection coefficient increase with increase in
moisture content due to increase in permittivity. The emissivity and brightness
temperature were found to decrease with increase in moisture content in the soils.
Chaudhary and Shinde 15 measured the dielectric constant and dielectric loss o f soils
of various texture structures, for various moisture contents, at X-band microwave
frequency o f 9.65 GHz employing the infinite sample method. They also calculated
the ac conductivity and relaxation time for various moisture contents in the soils. It
has been observed that the moisture in soil significantly affect the dielectric properties
o f the soils. Further it is observed that the dielectric constant o f the moist soil is not a
simple function o f the dielectric constant o f individual components. The dielectric
loss of the soil is found to be dependent on the presence o f organic carbon, Na and
CaCOa percentage in the sandy loam soil.
Peplinski el al. 16 measured the dielectric constant €' and dielectric loss e" of four
soil types for various moisture contents in the frequency range between 0.3 GHz and
1.3 GHz. The measurements were made using a coaxial probe technique with the
material under test (soil sample). The magnitude and phase o f the reflection
coefficient were measured using a network analyzer to derive the values of e' and e"
for the material under test.
Alex and Behari17 used the lumped element approach of Stuchly et a l18 for estimation
o f emissivity by measuring the reflection coefficient which is related to the
characteristic impedance o f the transmission line and that of the sensor probe. It was
observed that the dielectric loss e" increases abruptly as the water content in the soil
increases above transition moisture.
72
5.2
EXPERIMENTAL:
5.2.1
SAMPLE COLLECTION AND PREPARATION:
The soil samples were collected from different regions o f Gujarat state referring to the
soil map o f G ujarat19, 20 shown in figure (5.3) chapter III. The soil samples were
collected from the Sabarmati river bed, fields o f Gandhinagar district, Palanpur,
Valsad, Amreli, Jamnagar, and Surendranagar district (Sayla) as well as from the sea
bed near Somnath temple (Verawal). Stones and gravels were removed from the soil
samples and then the soil samples were oven dried. The distilled water was added in
the soil under investigation and allowed to saturate for 24 hours. As the days went on,
the moisture content in the soil decreased and the measurement o f complex dielectric
constant o f the soil samples for various moisture content were carried out. The wilting
point (WP) and transition moisture of each soil (Wt) were calculated using the Wang
and Sehmugge model3 as
WP = 0.06774 - 0.00064xSand + 0.00478xClay.
Wt = 0.49xWP + 0.165.
where, Sand and Clay are the sand and clay contents in percent of dry weight of the
soil.
The gravimetric moisture content as weight percent o f the soil sample was found
using the relation
Gravimetric moisture content Wm = [(weight o f the wet soil-weight of the dry
soil)/(weight o f the dry soil)]
Hence the volumetric moisture content in the soil sample was calculated as
Wv = Wm x (bulk density o f the dry soil sample).
The corresponding calculated values for soil samples of different texture structure are
shown in table (5.1).
.73
38
Loam
Silty clay
0.242
0.283
1.072
0.595
50
12
Jamnagar Dist.
Loam
0.599
1.062
0.269
0.211
Silty clay
31
Amreli Dist.
62
0.476
Valsad Dist.
1.389
0.555
0.187
1.179
0.045
0.223
Loam
Sandy
0.118
4
Silt Loam
31
11
11
65
78
Dist.
Gandhinagar
loam
0.383
1.635
0.181
0.033
Sandy
2
29
69
Surendranagar
dist. (Sayla)
0.400
0.441
1.59
1.48
0.169
0.171
0.021
0.012
Sand
Sand
0.359
1
0.8
1.7
0.169
0.008
P
16
6.2
Soilg/cm3
Wt cm3/c m 3
WP cm3/ cm3
Porosity
82
93
0.3
Density of Dry
Transition Moisture
Wilting Point
Palanpur dist.
Bed, Ahmedabad
Sabarmati River
Sea bed
3.7
96
Somnath
Sand
type
Silt
Sand
(Region)
Clay
Soil
Soil texture (%) o f
Location
Table 5.1:- The texture structure of soil samples.
r-
5.2.2
EXPERIM ENTAL
TECHNIQUE
FO R
MEASUREMENT
OF
COMPLEX PERM ITTIVITY OF SOILS AT X-BAND AND C-BAND
MICROW AVE FREQUENCIES:
The measurements for the determination of complex dielectric constant o f dry and wet
soil at 9.5 GHz (X-band) and at 5.65 GHz (C-band) microwave frequencies were
carried out using microwave bench set up. The two-point method was employed. The
microwave bench set up and measurement technique are described in chapter IV.
In order to ascertain the validity o f measurements the complex permittivity of organic
solvents carbon tetra chloride and 1-propyl alcohol were measured at X-band and Cband microwave frequencies at 25° C. The observed values were compared with the
literature values of complex permittivity of these solvents and are fairly in agreement
with the literature values (Table-5.2). The measurements of complex permittivity of
dry and wet soil samples were carried out at room temperature o f -30° C. The
accuracy o f measurement o f dielectric constant was 5 % and in dielectric loss was
10%.
Table (5.2):- The comparison of measured values of complex permittivity of carbon
tetra chloride and 1-propyl alcohol measured at X-band and C-band microwave
frequencies at 25 °C, with the literature21'23 values.
M easurement
Liquid
frequency
M easured Values
Literature values
s'
6"
€'
e"
9.50 GHz
CC14
2.26
0.06
2.23
0.0
5.65 GHz
CC14
2.20
0.08
2.24
0.0
9.50 GHz
1-Propyl
3.71
0.86
3.53
1.16
4.03
1.26
3.87
1.48
Alcohol
5.65 GHz
1-Propyl
Alcohol
75
5.2.3
RESULTS AND DISCUSSION:
The estimated values of dielectric constant e' and dielectric loss e" at 9.5 GHz
microwave frequency at various moisture contents for all soil samples are shown in
table 5.3.
From table (5.3) and figure 5.1, it can be seen that at 9.5 GHz microwave frequency
the dielectric constant e' and dielectric loss e" of the soil samples increase with
increase in moisture content in the soils. We used volumetric moisture content rather
than gravimetric moisture content, since it represents the number o f dipole moments
per unit volume that determine the contribution o f water to dielectric polarization 5.
The dielectric constant e' of dry soils varies between 2 and 5 at this frequency of
measurement. ( e'Amreh dry soil = 2.73, and
^ Jamnagariiiysoii
= 4.37). The dielectric constant
of soil samples increase slowly with moisture content up to transition moisture after
which it increases rapidly with increase in moisture content. For sandy soils of
Somnath seabed, Sabarmati riverbed and Palanpur district sand the dielectric constant
increases slowly with increase in moisture content up to Wv - 0 .1 5 after which it
increases rapidly. For high clay content soil of Valsad district the dielectric constant
e' increases slowly with moisture content upto Wv - 0.25 after which it increases
rapidly. Thus, the transition moisture content is found to be dependant on the sand and
clay content in the soil. For the soils having higher sand content the transition
moisture has lower value, whereas for soils having high clay content the transition
moisture has large value.
For the moisture content o f Wv < 0.05 the dielectric constant of all the soil samples
having different texture structure has similar value. But at higher moisture content
(Wv -0.20) the dielectric constant of soil having higher sand content (or lower clay
content) is higher than that of low sand content (or high clay content) soils. Thus, for
lower moisture contents there is no much variation in the values o f dielectric constant
e' or for soils. But at higher moisture content the dielectric constant s ' o f sandy soils
have higher value than that of high clay content soil. This difference becomes clearer
as the moisture content in the soils increase further. Hence, we can say that at 9.5
76
GHz microwave frequency, and for high moisture contents above Wv -0.10 in the
soils
The dielectric loss e" values of all soil samples increase slowly and linearly with
increase in moisture content in the soil. The regression values o f the linear trend lines
for the soil samples are shown in table (5.4) along with their corresponding equations.
At lower moisture contents Wv < 0.10, there is no much visible difference between
the dielectric loss o f the soils having different texture structure. But at given higher
moisture content the dielectric loss o f the sandy soils is less than that for the high clay
content soils.
Figure (5.2) shows the variation of dielectric constant e'and dielectric loss e" as a
function o f sand content in the soils at X-band microwave frequency o f 9.5 GHz at
moisture content of ~ 0.27.
For moisture content below transition moisture in the soil, most water molecules are
tightly bound to the soil particles. It is difficult to polarize these bound water
molecules and the bulk of water shows a smaller dielectric constant s ' than that of
free water 3. The amount o f bound water contained in the first molecular layer
adjoining the soil particles is directly proportional to the soil particles contained in a
unit volume of the given soil sample '. The total surface area o f the particles is
proportional to the size o f soil particles and mineral contents in the soil. The clay
particles have large specific surface area in comparison with that o f sand or silt
particles *’ ’’9. This large specific surface area of clay particles enables the high clay
content soils to retain greater moisture in the form o f bound water. As the dielectric
constant o f bound water is very low compared to that o f the bulk water, at given
moisture content the dielectric constant o f high clay content soils is less than that of
high sand content soils (which have more free water at the same moisture content). As
the moisture content in the soils increase further, the number o f free water molecules
increases in the soil increasing the dielectric constant e '. To verify this conclusion,
we draw the graph of dielectric constant e' and the dielectric loss e" o f the soils
measured at 9.5 GHz microwave frequency for moisture content o f Wv -0.27 against
77
sand content in the soils (Figure 5.2). It is clear from the trend line, that at higher
moisture content the dielectric constant s' of the soil increases with increase in sand
content (conversely with decrease in clay content) in the soils. Further it can be seen
that the dielectric loss e" decrease with increase in sand content in the soil.
At higher frequencies >8.0 GHz, mainly the dielectric relaxation o f water contributes
to the dielectric loss e" of wet soils (1). At 9.5 GHz microwave frequency the
contribution o f conductivity to e" is not significant, so e" is proportional to the
volume fraction o f bulk water. At given moisture content, sandy soils having lower
specific area have more volume fraction o f bulk water than that for high clay content
soils, resulting in larger value o f €" for sandy soils. Similar behavior was observed by
Vyas 8 at 8.97 GHz (X-band) microwave frequency, Pancholi and Khameshra 14 at
7.114 GHz (J-band) microwave frequency, Ghosh et al. 9 at 14.89 GHz microwave
frequency, and Hallikainen et al. 1 at 10 GHz microwave frequency.
Table (5.5) and figure (5.3) show the variation o f dielectric constant e' and dielectric
loss e" o f the Gandhinagar district sandy loam soil and the Sabarmati river bed sand
for various moisture contents in the soils measured at C-band microwave frequency of
5.65 GHz. The dielectric constants'of both the soils increases slowly initially up to
transition moisture after which it increases rapidly with increase in moisture content
in the soil. The dielectric loss increases slowly and linearly with increase in moisture
content in the soils.
The comparison o f complex permittivity ( s ', s " ) of both the soils measured at Cband (5.65 GHz) and X-band (9.5 GHz) microwave frequencies is shown in figure
5.3.1. It is clear from the figure that at given moisture content (except dryer soils) the
dielectric constant e' of the soils is higher at 5.65 GHz as compared to that at 9.5
GHz. Further the dielectric loss e" of the soils at 5.65 GHz is lower than that at 9.5
GHz. This may be due to the fact that wet soil is a heterogeneous mixture of soil-airwater and the dielectric constant of heterogeneous mixture is dependent on the
dielectric constant o f individual components in some manner. The dielectric constant
of dry soil and air for the given soil sample is constant. But the dielectric constant e'
o f water decreases with increase in frequency from 5.65 GHz to 9.5 GHz, which plays
78
dominant role in the bulk dielectric constant o f wet soil medium. Similarly, the
dielectric loss e" o f water at 5.65 GHz is lower than that at 9.5 GHz microwave
frequency, which plays dominant role in reducing the bulk dielectric loss €" of wet
soil medium. The dielectric loss e" is nearly independent o f soil texture at all soil
moistures for the frequency variation from 4.0 to 6.0 GHz l. Hence the increase in
dielectric loss e" with increase in moisture content of the soils may be mainly due to
the increase in free water molecules in the soils.
5.2.4
EXPERIM ENTAL TECHNIQUE FO R TH E MEASUREMENT
OF COM PLEX PERM ITTIVITY OF SOILS AT RADIO AND
LO W ER MICROW AVE FREQUENCIES AND RESULTS:
The complex permittivity o f dry and wet soils having different texture and for
different moisture content were measured in the frequency range 30 MHz to 1.23 GHz
using a vector network analyzer the details of experimental technique has been
described in chapter IV (4.2.2). A semi rigid coaxial probe with metallic flange and
central probe tip was mated with VNA. The procedure o f calibration o f probe and
results of measurements are presented here.
The procedure fo r calibration o f the probe is described below step by step.
A. Open: - With the coax end terminated by free space (air e a - 1), the measurement
plane of the VNA was moved to the coax end using the electrical delay provided. The
delay, which corresponds to the length o f the coax (typically 6-12 inch), was adjusted
to give a cluster o f points near the Re(p) = 1 and Im(p) = 0 point, at the middle of the
right hand side of the display (Figure-5.4-a). Because of the connector mismatches,
the cluster is not a point but can occupy a region. The pair wise p a data were read
into the computer.
B. Short: - A short at the coax end was created by raising a vessel filled with
mercury, until the coax end was well within the liquid. This resulted in a cluster of
points around the position Re(p) = -1 and Im(p) =0, at the middle of the left hand side
of the display (Figure-5.4-b). The pair wise data ps were again read into the computer.
79
C. Standard Liquid: - The mercury cell was removed. At this point, the display was
checked to ensure that the data returned to the configuration for an open. A beaker of
100 ml with a standard liquid acetone (« 20 ml), was taken so that the coax end was
well immersed in the liquid. The data pc were again read into the computer (Figure5.4-c). We have preferred acetone as a third standard liquid because it is easily
available in pure form, has a reasonable dielectric constant and exhibits little
dispersion in the preferred frequency range.
D. Sample Liquid (Methanol): - The standard liquid was removed, and time was
allowed for the acetone to evaporate completely from the end o f the coax, until the
display returned to the open configuration. The procedure was repeated with methanol
(AR grade) and data p m were collected (Figure-5.4-d).
To'repeat the procedure D for soil samples of different moisture content, the coax was
first cleaned each time with acetone. After evaporation of acetone the display was
checked to ensure that the data returned to the configuration for an open. Then the
measurements of the pMfor the dry and wet soil samples were carried out.
The results using this procedure are shown in figure (5.5) for methanol as a sample
liquid.
The uncertainty in the dielectric constant e ’ and dielectric loss e ” o f methanol using
VNA from the values calculated using the Debye model are within 5 % and 10 %
respectively. Measured values of dielectric constant and dielectric loss using VNA
(frequency range 30 MHz to 1.5 GHz) and microwave test benches operating at
frequencies of 5.65 GHz and 9.5 GHz were fitted in to the Debye equation. LEVMW
software 29 was used to carry out the fitting procedure. In this fitting process € s, e «,
and relaxation time (t) were taken as fitting parameters and they were found to be
20.92, 4.22 and 237.5 pS, respectively. The reported literature values of Gs, e » and
relaxation time (x) at 30 0 C are respectively, 19.96,3.2 and 266 p S 30; 20.33,5.41 and
253.5 pS 3I, and 20.4, 3.0 and 275 pS 32.
80
The calibrations in this method enable to eliminate the connector and line
mismatches, and the analysis developed to de-imbed the impedance of the liquidprobe interface from which by proper modeling the sample dielectric constant is
extracted. The de-imbedding procedure eliminates the unknown fringe field
impedance parameters Cf and Co at each measurement frequency.
The complex permittivity ( e ’, e ”) of Sabarmati river bed sandy soil, Gandhinagar
district sandy loam soil, and Valsad district silty clay loam soil for various moisture
(distilled water) contents in the soil was determined using VNA in the frequency
range from 30 MHz to 1.23 GHz and the results are presented in figure (5.7).
From figure (5.7) it can be observed that the dielectric constant e 5 and dielectric loss
e ” of the dry soils remain unaltered in the frequency range from 30 MHz to 1.23
GHz. The values o f the dielectric constant e ’ and the dielectric loss e ” o f the soils,
increase with increase in moisture content in the soil over this frequency range. It can
be seen from figure (5.7) that in the frequency range 30-300 MHz for the moist soil
the values o f dielectric loss e ” is high and decreases with increase in frequency for
all the three soils and for all moisture contents. This effect is mainly due to ionic
conductivity of the soil. The effective conductivity is due to presence of salt
composed primarily o f calcium. The concentration o f calcium increases with the clay
content o f the soil. Theoretical and experimental studies have shown that the
Maxwell-Wagner effect can be observed in the soil at the frequency lower than 100
MHz, thus apart from ionic conductivity contribution to dielectric loss there is
possibility o f contribution from Maxwell-Wagner effect in the frequency range < 100
MHz. Similar results were observed by Sternberg and Levitskya26, Alex and Behari17,
andH ipp27.
In the frequency range 300 MHz to 1.23 GHz at a given moisture content in the
Sabarmati sand and the Gandhinagar sandy loam soils the values o f the dielectric
constant e ’does not vary appreciably with frequency, but the dielectric loss e ”
decrease with increase in frequency. For the Valsad district silty clay loam soil the
values o f die dielectric constant e ’ and the dielectric loss e ” decrease with increase
in frequency from 30 MHz to 1.23 GHz. Further at given moisture content in the soil
the dielectric constant of higher sand content soil is higher than the dielectric constant
81
of high clay content soil, in this frequency range. On the contrary, at given moisture
content, the dielectric loss of the soil containing higher sand content is smaller than
that of the soil containing higher clay content. To verify this conclusion the graph of
complex permittivity variation with moisture content for the three soil samples at 1
GHz microwave frequency is shown in figure (5.8). J. R. Wang 3 also observed
similar results as reported at 0.3 GHz and 1.4 GHz. Hoekstra and D elaney5 observed
similar behavior at 0.8 GHz microwave frequency. Hallikainen et al. 1 measured the
dielectric constant e ’ and the dielectric loss e ” o f five different types o f soils at 1.4
GHz microwave frequency. The results o f measurement are analogous to our
measurements. It can be seen from the figure (5.8), that with increase in moisture
contents in the soil the dielectric constant e ’ increases rapidly in comparison to that
in the dielectric loss e ”. Further it can be verified from the figure that the dielectric
constant e 5 increases slowly initially with increase in moisture content up to
transition moisture o f the given soil, after which it increases rapidly. Similar behavior
was observed by Wang and Schmugge 3 at 1.4 GHz from the experimental
observations o f Newton and Lundien. It was pointed out that the transition moisture
Wt varies with the soil texture, being larger for the soils having higher clay contents
as compared to that for high sand content soils.
Peplinski et al. 16 measured the real and imaginary parts o f complex permittivity for
various moisture contents in four soil types, in the frequency range from 0.3 GHz to
1.3 GHz. They observed that in this frequency range the complex dielectric constant
( s ’, e ”) of wet soil depends on the textural composition in the soil along with the
moisture content in the soil. The wet soil comprise o f four components: air, solid soil
particles, free water, and bound water 24. In wet soil the total volume fraction o f water
Wv is summation of free water volume and bound water volume. The volume fraction
of bound water is highly dependent on the specific surface area As (in m2/g) o f the soil
particles because the bound water is defined as the adsorbed cations that are tightly
held by the negatively charged particle surfaces mainly composed o f clay. In
comparison with sand and silt particles, the clay particles have large specific surface
area that determines die portion o f bound water in the total volumetric moisture
content. The dielectric constant of free water at given frequency is determined by
Debye equation. As an example at 1 GHz, the complex dielectric constant o f pure free
82
water is e fvv = 79.3 - j 4.3 at room temperature 24 (excluding the conductivity term),
but the complex dielectric constant of bound water is e bw = 35 —j 15.
Hence e ’bW< e ’fW, and
€
bw ^ €
Considering these facts, we can arrive at conclusion that at given moisture content in
wet soil, the soil sample having higher clay content has more bound water fraction
than that of free water content (conversely, for the soil having higher sand content the
volume fraction of free water is more than that o f bound water). Hence at given
moisture content in the soils in this frequency range, the dielectric constant e ’ of
higher sand content soils is more than that of high clay content soils, where as the
dielectric loss of high sand content soils is less than that o f high clay content soils.
Hence, for wet soils at given moisture content and given frequency range between 0.3
GHz and 1.3 GHz
£ ’sandy wet soil> G ’clayey wet soil; ^ d
6 ” Sandy wet soil<~ G
5.3
Clayey wet soil-
CALCULATION OF EMISSIVITY:
In microwave remote sensing using radar or radiometer the backscattering coefficient
and the emissivity of soil, respectively, are measured at given microwave frequencies.
The microwave emissivity of soils is dependent on the water content and physical
properties of the soils 25. The timely information of soil moisture is very much useful
to agriculturists, hydrologists and meteorologists. The complex permittivity of soils at
microwave frequencies is related with the emissivity o f the soils. Hence from the
measured values of the dielectric constant e ’ and the dielectric loss s ” of the
Gandhinager district sandy loam soil at 9.5 GHz microwave frequency the emissivity
•
values of the soil for normal incidence were calculated using the relation
18
83
l - ( e ) 1/2
I -
1 + ( e ) ,/ 2
(1)
where, e = the dielectric constant o f the soil, and
e = emissivity of the soil.
The variation o f calculated values o f emissivity o f soils at 9.5 GHz microwave
frequency with moisture content is shown in figure (5.9).
5.4
CALCULATION OF TH E DIELECTRIC CONSTANT e ’ AND THE
DIELECTRIC LOSS e ” OF TH E SOILS USING TH E HALLIKAINEN
E T A L . MODEL, AND WANG AND SCHMUGGE MODEL:
(a)
Hallikainen et al. Model:
Hallikainen et al. 1 measured the complex dielectric constant o f five soil samples of
various texture structure in the frequency range from 1-to 18-GHz. A waveguide
transmission technique (for the 1-2 and 4-6 GHz microwave frequency range) and a
free space transmission technique (4 to 18 GHz microwave frequency range) were
used for the measurements. For various moisture contents in the soil samples, the
volumetric moisture content was calculated as Wv =
Wm, where pb is the bulk
density o f dry soil and Wm is the gravimetric moisture content in the soil.
It have been observed that at any given moisture content the dielectric constant e'
was roughly proportional to sand content (and inversely proportional to the clay
content) in the soil. Thus e' was found to be texture dependent at all frequencies from
1.4 to 18 GHz, although the magnitude o f the effect was found to decrease with
frequency. At 1.4 GHz the value of dielectric loss was found to increase with soil clay
content for Wv > 0.2 cm3cm'3. At 4.0-6.0 GHz, e" was found to be nearly
independent of soil texture at all soil moisture conditions. At frequency o f 8.0 GHz
and above the value o f e" was found to decrease with soil clay fraction; further, the
magnitude of this behavior increased with frequency.
84
Further the measurements of complex permittivity o f moist soils carried out at
temperatures between -11 °C and -24 °C showed that both s ' and e" decrease with
decreasing temperature below 0 °C.
Halliakinen et al. generated polynomial equations for e' and e"as a function of
volumetric moisture content Wv for each frequency and soil type. At each frequency,
the individual polynomials were combined into a single polynomial which represents
the value of e' and e" as a function of Wv, S, and C, where S and C are, respectively
the sand and clay contents o f a soil in percent by weight.
(b)
Wang and Schmugge Model:
Wang and Schmugge 3 proposed an empirical model to describe the dielectric
behavior o f soil-water mixtures. Using the transition moisture as an adjustable
parameter, the mixing of either the dielectric constants or the refraction indices of ice,
water, rock and air was carried out. The model is based mainly on two observations:
(i) For all soil samples the dielectric constant increases slowly initially with moisture
content up to transition moisture, after which it increases steeply with moisture
content, (ii) The transition moisture is found to vary with texture structure o f the soil,
being smaller for sandy soils than that for high clay content soils.
The wilting point (WP) o f a soil in percent o f dry weight o f the soil was represented
as
WP = 0.06774 - 0.00064 x SAND + 0.00478 x C LA Y .................(1)
Where, SAND and CLAY are the sand and clay contents in percent o f dry weight of a
soil.
The WP characterizes a stage o f Wc in the soil-water system (Figure. 5.10) at which
the soil tension is about 15 atm. Between WP and field capacity FC (soil tension o f1/3 bar), water is held in pores by capillary attraction. At Wc > FC, water flows with
gravity. Further at Wc < WP, it is difficult for plants to extract water from soil. Water
85
held in soils at temperatures up to 105 °C is called hygroscopic water (at soil tension
of 31 bars) and is virtually a part of the mineral structure o f the soil.
The transition moisture o f the soil is calculated as
Wt = 0.49 WP + 0.165.
......... (2)
For the moisture contents less than transition moisture Wt, most o f the water
molecules are tightly bound to the soil particles called bound water. At any rate it is
difficult to polarize these bound water molecules and the bulk o f water has dielectric
constant less than that for free water (W c> W t ). It has been observed that, the very
first layer o f water molecules around the soil particles have an activation energy of
~12 Kcal/mole, which is comparable to that of ice, rather than ~ 4 Kcal/mole for free
water molecules. Due to similarity in activation energy between tightly bound water
and ice molecules, the dielectric constant o f ice is used to describe the dielectric
behavior o f the soil water mixtures at W c<W t. We used the first approach of
empirical model which was the direct mixing of the dielectric constants of the
constituents to obtain the complex dielectric constant o f soil-water mixture as
e=W cex +{P - Wc) e a +(1 - P) e r , for Wc > Wt
........ (3)
Where
+ ( € „ - £ ,) — - r
w,
..... (4)
And
e=
Wte ,
+{Wc-Wt) e w +(P - Wc) ea +(1 - P ) e r
....... (5)
Where
+ (ew - e >
........ (6)
Here P = porosity o f the dry soil as explained in chapter II.
86
The dielectric constants o f ice are assumed as e,'~ 3 .2 an d e,"~ 0.1. For the air
e 0'~ 1 and e 0"~ 0. The dielectric constants of solid rock vary, but respective values
are considered to be e r'~ 5.5 and e r"~ 0.2. The dielectric constant of water varies
with frequency in the microwave region. Here e xrepresents the dielectric constant of
initially absorbed water. The parameter y can be chosen to best fit equations (3) and
(5) to the experimental data which was obtained from linear regression analysis as
y = -0.57 WP+ 0.481
.......(7)
At low frequencies the conductivity loss is required to be added in the imaginary part
of the dielectric constant as
€,"=€?'+€/
........(8)
=e"+60 A c
=e" +aWc2
....... (9)
Where e CT" is the conductivity loss which is assumed to be proportional to Wc2. a is
the ionic conductivity in mho/cm, and X is the wavelength in cm. e" is the imaginary
part of the mixed dielectric constant from pure water and dry soil, which is obtained
from equations (3) and (5). The parameter a is chosen to best fit the measured e ”.
It has been observed that Wt varies with soil types. The value o f y for sandy soils is
larger than that o f the high clay content soils. This means that, for a given moisture
content Wc < Wt, a higher fraction of water is in a tightly bound state for high clay
content soils than that for sandy soils. The larger value o f a represents higher ionic
conductivity, for high clay content soils than for the sandy soils, which means that
high clay content soils contain more Phosporous, Potessium and Calcium than that for
sandy soils.
87
Further they observed that for soils having higher sand content, the measured values
of e ," had a linear relation with Wv.
(c)
Comparison o f measured values o f the dielectric constant e ’ and the
dielectric loss e ” o f the soils with the values calculated using Wang and
Schmugge model, and the Hallikainen et al. model:
The comparison of measured values of the dielectric constant e ’ and the dielectric
loss e ” of the soils for various moisture contents, with the values calculated using
Wang and Schmugge model, and the Hallikainen et al. model at X-band microwave
frequency o f 9.5 GHz are shown in figure (5.11).
It is clear from figure (5.11) that the measured values of complex dielectric constant
( s ’, e ”) of the soils for various moisture contents in the soils agree very well with
the values calculated using the Wang & Schmugge model and the Hallikainen et al.
model. The measured values o f dielectric constant e ’ o f sandy soils are in very good
agreement with the values calculated using the models, where as the values calculated
using the models are lower than the measured values o f e ’ for clayey soils at higher
moisture contents. On the contrary the measured values o f dielectric loss e ” o f clayey
soils are in very good agreement with the values o f e ” calculated using the models,
where as the values of e ” calculated using the models are higher than the measured
values for sandy soils at higher moisture contents.
The comparison o f measured values o f the dielectric constant e ’ and the dielectric
loss e ” o f the soils with the values calculated using Wang and Schmugge model, and
the Hallikainen et al. model at C-band microwave frequency o f 5.65 GHz are shown
in figure (5.12).
From figure (5.12) it is seen that the measured values o f complex dielectric constant
(e ’, e ”) o f the soils for various moisture contents in the soils agree very well with
the values calculated using the Wang and Schmugge model, and the Hallikainen et al.
model at C-band microwave frequency of 5.65 GHz.
88
Table (5.6) shows the R2 fitting values of trend lines for all soil samples measured at
9.5 GHz (X-band) and 5.65 GHz (C-band) microwave frequencies compared with the
values calculated using the Wang and Schmugge model and the Hallikainen et ah
model.
89
Table 5.3.1
Frequency = 9.5 GHz
Sabarmati River bed Sand
Volumetric moisture
Dielectric
Dielectric loss
content
constant
e"
Wv cm3 cm'3
e'
0
2.829
0.090
0.014
2.894
0.156
0.038
3.889
0.448
0.068
4.899
0.644
0.094
6.306
0.984
0.109
6.534
1.056
0.148
8.532
2.224
0.190
11.1
2.111
0.221
12.445
2.17
0.229
13
2.25
0.230
14.7
2.28
0.248
14.7
2.19
0.279
16.84
2.59
0.285
17.74
2.93
0.31
17.8
4.123
Table 5.3.2
Frequency = 9.5 GHz
Gandhinagar dist. Sandy Loam
Volumetric
Dielectric constant
Dielectric loss
moisture content
g'
g"
0
2.827
0.188
0.059
4.946
0.924
0.097
5.288
1.468
0.147
7.867
2.688
0.154
6.335
2.426
0.2
9.159
3.021
0.234
13.467
2.677
0.24
14.422
2.81
0.262
15.79
2.837
Wv cm3 cm'3
Table 5.3.3
Frequency = 9.5 GHz
Amreli Dist. Silt Loam
Volumetric moisture
Dielectric
Dielectric loss
content
constant
g"
Wv cm3 cm'3
G'
0
2.73
0.11
0.069
3.92
0.47
0.083
5.07
0.77
0.097
3.3
0.68
0.140
8.9
1.75
0.179
9.2
1.98
0.221
11.14
2.5
0.234
12.1
2.8
0.252
12.1
2.9
0.283
14.2
3.2
0.3
14.4
3.4
91
Table 5.3.4
Frequency = 9.5 GHz
Somnath Sea bed Sand
Volumetric
Dielectric
Dielectric loss
moisture content
constant
e"
Wv cm3 cm'3
€'
0
3.839
0.228
0.014
3.72
0.141
0.097
7.46
0.93
0.109
8.3
1.4
0.147
7.74
0.583
0.179
7.81
1.08
0.207
13.14
1.55
0.262
17.57
2.25
Table 5.3.5
Frequency = 9.5 GHz
Jam nagar Dist. Silty Clay Loam
Volumetric
Dielectric
Dielectric
moisture content
constant
loss
Wv cm3 cm'3
e'
e"
0
4.37
0.517
0.03
4.7
0.592
0.221
8.96
1.23
0.27
11.14
2.15
0.283
12.56
2.3
0.3
11.47
2.29
0.31
12.2
2.19
0.355
12.26
1.86
0.366
16.17
4.32
0.373
15.4
2.7
92
Table 5.3.6
Frequency = 9.5 GHz
Palanpur Dist. Sand
Volumetric
Dielectric
Dielectric
moisture content
constant
loss
Wv cm3 cm'3
e'
e"
0
2.9
0.055
0.003
2.85
0.06
0.005
2.797
0.064
0.009
3.04
0.133
0.03
3.203
0.18
0.059
5.325
0.465
0.072
4.87
0.37
0.083
6.61
0.56
0.095
5.25
0.584
0.117
6.81
0.81
0.121
7.33
1.03
0.16
8.85
1.08
0.207
13.6
1.98
0.27
14.44
1.61
0.271
14.7
1.47
93
Table 5.3.7
Frequency = 9.5 GHz
Valsad Dist. Silty Clay Loam
Volumetric
Dielectric
Dielectric
moisture content
constant
loss
Wv cm3 cm'3
e’
e"
0
2.834
0.106
0.050
3.7
0.425
0.143
6.09
0.93
0.154
6.56
0.983
0.170
7.61
1.255
0.229
10.5
2.297
0.285
12.3
2.8
0.31
14.3
2.94
Table 5.4: The regression values and corresponding equations o f the linear trend lines
representing the variation o f dielectric loss with volumetric moisture content o f the
soil samples.
Soil Name
Linear Trend Equation
R2 value
Somnath Sea bed Sand
Y = 6.8931X + 0.1449
0.7771
Sabarmati River bed Sand
Y = 10.847 X - 0.0646
0.912
Palanpur District field Sand
Y = 6.3523X4-0.0603
0.8938
Gandhinagar District field Sandy loam
Y = 10.497X4-0.49
0.8622
Surendranagar District field Sandy loam Y = 13.431 X - 0.1697
0.8963
Amreli District field Silt loam
Y = 12.019 X - 0.1633
0.9784
Valsad District Silty Clay loam
Y = 9.7757 X - 0.1722
0.9448
94
Table 5.5: The measured values o f dielectric constant and dielectric loss for various
moisture contents in the soils at C-band microwave frequency of 5.65 GHz.
Table 5.5.1
Frequency = 5.65 GHz
Gandhinagar Dist. Sandy Loam
Volumetric moisture content Dielectric constant Dielectric loss
Wv cm3 cm'3
e’
e"
0
2.424
0.177
0.032
3.051
0.413
0.058
3.783
0.611
0.079
4.721
0.904
0.104
6.280
1.531
0.126
7.139
1.853
0.151
8.813
2.793
0.175
9.911
2.751
0.225
12.048
3.721
0.279
15.728
4.071
0.305
16.745
3.074
0.318
16.147
4.005
95
Table 5.5.2
Frequency = 5.65 GHz
Sabarmati River bed Sand
Volumetric moisture content Dielectric constant Dielectric loss
Wv cm3 cm'3
e'
e"
0
2.442
0.238
0.021
2.857
0.294
0.024
2.899
0.280
0.028
3.088
0.383
0.044
3.584
0.459
0.045
3.925
0.481
0.047
3.691
0.522
0.077
4.809
0.857
0.118
6.309
1.317
0.131
7.080
2.067
0.163
9.102
2.321
0.189
10.184
3.168
0.212
10.283
3.509
0.246
14.330
3.494
0.295
18.036
3.572
0.334
19.408
5.186
Table 5,6: The R2 fitting values of trend lines for all soil samples measured at 9.5
GHz (X-band) and 5.65 GHz (C-band) microwave frequencies compared with the
values calculated using the Wang and Schmugge model and the Hallikainen et al.
model.
Soil Sample
Frequency
Wang
M.T. Hallikainen et al.
and
Of
and Schmugge
Model
Type
Measurement Model
Sabarmati
9.5 GHz
River, Sand
X-band
Gandhinagar
9.5 GHz
District
X-band
R2 value
R2 value
R2 value
R2 value
for e ’
for e"
for € ’
fo re"
0.98
0,87
0.99
0.89
0.94
0.647
0.929
0.717
0.922
0.906
0.94
0.955
0.935
0.77
0.963
0.804
0.977
0.934
0.991
0.968
0.989
0.917
0.983
0.887
0.964
0.763
0.971
0.764
Sandy loam
Amreli
9.5 GHz
District
X-band
Silt loam
Palanpur
9.5 GHz
District
X-band
Sand
Valsad
9.5 GHz
District
X-band
Silti Clay
loam
Sabarmati
5.65 GHz
River Sand
C-band
Gandhinagar
5.65 GHz
District
C-band
Sandy loam
97
Figure (5.1): The simultaneous graphical presentation of the dielectric constant e' and
dielectric loss e" estimated at X-band microwave frequency o f 9.5 GHz, for various
soil samples plotted against moisture content.
20
♦
S o m n a th
a
S a b a rm a ti
x
P a la n p u r
+
S u re n d ra n a g a r
x
A m re li
o
G a n d h in a g a r
■
V a ls a d
-------- E x p o n (S o m n a th )
E x p o n (S a b a rm a ti)
- - -E x p o n (P a la n p u r)
E x p o n . (S u re n d ra n a g a r)----------- E x p o n . (G a n d h in a g a r)^ .'
--------
C o m p lex Perm ittivity
•E x p o n . (A m re li)
01
0.15
0.2
0 25
0.35
V o lu m e tric M oisture C o ntent
98
Figure 5.2: The variation of dielectric constant e' and dielectric loss e" as a function of
sand content in the soils at X-band microwave frequency of 9.5 GHz at moisture
content of ~ 0.27.
99
Figure 5.3.1: The comparison of dielectric constant and dielectric loss of
Sabarmati River bed sandy soil for various moisture contents, measured at C-band
microwave frequency o f 5.65 GHz and X-band microwave frequency o f 9.5 GHz.
, CM
o
S a b a rm a ti R e v e r b e d S a n d
K)
GO
C o m p lex P erm ittivity
CD
0 .0 5
01
0 .15
0 .2
0 .25
0 .3
0 .35
V o lu m e tr ic M o is t u r e C o n t e n t
100
Figure 5.3.2: The comparison of dielectric constant and dielectric loss of
Gandhinagar district sandy loam soil for various moisture contents, measured at
C-band microwave frequency o f 5.65 GHz and X-band microwave frequency of
Complex Permittivity
9.5 GHz.
101
(a) Open
(b) Short
(c) Acetone
(d) Methanol
Figure (5.4): Polar diagrams o f open, short, Acetone and Methanol obtained using
VNA during calibration.
102
Complex Permittivity
Frequency
Figure (5.5): Comparison of measured values of the dielectric constant and dielectric
loss of methanol using VNA in the frequency range from 30 MHz to 1.5 GHz with the
values calculated using the Debye model
103
-im ag
Epsilon, Cbrrplex-plane plot
104
Figure 5.7:
Measured values of dielectric constant and dielectric loss o f the soils for various
D ie le c tr ic C o n s ta n t
moisture contents using VNA in the frequency range from 30 MHz to 1.23 GHz
Figure 5.7.1
Wv = 0
Wv = 0.18
Wv = 0 024
-Wv = 0 24
&
Wv = 0 12
O
Wv = 0.34
Wv = 0 39
D ie le c tric L o s s
Sabarm ati River bed Sand
’
3.00E+07
J t.' v - %
3.30E+08
6 30E+08
9.30E+08
1 23E+09
F re q u e n c y (H z)
Figure 5.7.2
105
40
♦ Wv = 0
35
o W v * 0 097
0
A Wv = 0 22
x W v = 0 35
+ W v = 0 38
oW v = 041
G a n d h in a g a r D is tric t S a n d y L o a m S o il
°oo
■#
D ie le c tric C o n s ta n t
30
°
°
o
+ o fct^ > ° ° ° 0 °°0 00V
+
000000000000000000000000000°0000°00°000000°0 °0 °°0 0 0°0<>000
+++f+ V
j --+
+f ++
W
I14+-^4+44*h|--H-+f
25
20
X X
X xXx
X Xx
15 H
M
A M A ^ A ^ |AftAA^AA f l ^ ^ ^ a a ig y i^ liy!laA!iAaAAAAAAflAAAAflMAAAAftftAaA6AaAAAAflAAAA
10
mm
5
0
3 OOE+07
3.30E+08
6.30E+08
9.30E+08
1.23E+09
F re q u e n c y (H z)
D ie e c tric L o s s
Figure 5.7.3
F re q u e n c y (H z )
Figure 5.7.4
106
D ie le c t r ic L o s s
Figure 5.7.5
Figure 5.7.6
107
D ie le c tr ic c o n s ta n t
Complex Permittivity
Figure 5.8: Comparison o f measured values o f complex permittivity ( e ',e " ) of
Sabarmati River bad sand, Gandhinagar district sandy loam, and Valsad district silty
clay loam soils for various moisture contents (cm3 cm-3), at 1 GHz microwave
frequency, carried out using Vector Network Analyzer
108
Emissivity
Emissivity of Soils at 9.5 GHz
04
02
♦
S a b a rm a ti S and
a
G a n d h in a g a r S a n d y Loam
*
A m re li S ilt Loam
+
V a lsa d S ilty C la y Loam
-
P a la n p u r S and
*
S o m n a th S and
d
J a m n a g a r S ilty C la y Loam
------- E xpon (S a b a rm a ti Sand)
—
E xpon (G a n d h in a g a r S andy Loam )
---------E xpon (A m re li S ilt Loam )
—
E xpon (V alsad S ilty C lay Loam )
- - - E xpon (P a la n p u r S and)
—
E xpon (S o m n a th S and)
“
0 05
0.1
015
0.2
- E xpon (J a m n a g a r S ilty C la y Loam )
0 25
0.3
0 35
04
Volumetric Moisture Content
"igure 5.9: Comparison o f calculated values o f emissivity for various moisture
;ontents in the soils, at 9.5 GHz (X-band) microwave frequency
109
110
Figure 5.11: The comparison o f measured values of the dielectric constant e ’ and the
dielectric loss e ” o f the soils with the values calculated using Wang and Schmugge
Complex Permittivity
model, and the Hallikainen et al. model at X-band microwave frequency of 9.5 GHz
Complex Permittivity
Figure (5.11.1)
Figure (5.11.2)
111
Figure (5.11.3)
Figure (5.11.4)
Figure (5.11.5)
Figure (5.11.6)
113
CM
,O
Experimental
• Hallikainen et al model
to
Valsad Silty clay loam
00
Complex Permittivity
CO
-Wang & Schmugge model
-o
-o
0,05
01
0.15
0,2
0.25
03
0 35
Volumetric Moisture Content
Figure (5.11.7)
114
Figure 5.12: The comparison o f measured values o f the dielectric cbnstant e ’ and the
dielectric loss e ” o f the soils with the values calculated using W ang and Schmugge
Complex Permittivity
model, and the Hallikainen et al. model at C-band microwave frequency o f 5.65 GHz
"igure (5.12.1)
115
CM
o
Experimental
—
M.T.hallikainen et al model
o
ot
------ Wang & Schmugge model
cn
Complex Permittivity
o
lO
CM
C-band Gandhinagar
o
0.05
0.1
0.15
0.2
0.25
0.3
Volumetric Moisture Content
7igure (5.12.2)
116
REFERENCES:
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Wu, “Microwave Dielectric Behaviour o f wet Soil-part 1: Empirical Models
and Experimental Observations”, IEEE Trans. Geosci Remote Sensing, 23/1
(1985)25.
2
Boyarskii D. A., Tikhonov V. V., and Komarova N. Yu., “Model of Dielectric
Constant o f Bound Water in Soil for Applications o f Microwave Remote
Sensing”, Progress In Electromagnetics Research, PIER 35 (2002) 251.
3
Wang J. R., and Schmugge T. J., “An empirical model for the complex
dielectric permitivity o f soils as a function of water content”, IEEE
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4
Njoku Eni G., and Kong Jin-Au, “Theory for Passive Microwave Remote
Sensing of Near-Surface Soil Moisture”, Journal o f Geophysical Research,
82/2(1977)3108.
5
Hoekstra P. & Delaney A., “Dielectric Properties o f Soils at UHF and
Microwave Frequencies”, Journal o f Geophysical Res., 79/11 (1974) 1699.
6
Robinson D. A., Kelleners T. J., Cooper J. D., Gardner C. M. K,, Wilson P.,
Lebron I., and Logsdon S., “Evaluation o f a Capacitance Probe Frequency
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7
Calla O. P. N, Borah M. C., Vashishtha P., Mishra R., Bhattacharya A. &
Purohit S. P., “Study o f the Properties of dry and wet loamy sand soil at
microwave frequencies”, Indian J. o f Radio and Space Physics, 28 (1999)109.
8 Vyas A. D., “Complex Permittivity o f Sand & Sandy Loam Soils at
Microwave Frequency,” Indian J. Radio & Space Physics, 2 (1982) 169.
9
Ghosh A., Behari J. & Pyne S., “Dielectric parameters o f dry and wet soils at
14.89 GHz”, Indian J. o f Radio and Space Physics, 27 (1998) 130.
10 Sucher M. and Fox J., Handbook o f microwave measurements, (1963) 504.
11 Mishra U. S. & Behari J., “In-situ measurement of Dielectric Parameter of Soil
at Microwave Frequencies,” Journal o f the Indian Society o f Remote Sensing,
28/1 (2000) 1.
117
12 Sengwa R. J., Soni A., Ram B., “Dielectric behaviour o f shale and calcareous
sandstone o f Jodhpur region”, Indian J, Radio and Space Physics, 33 October
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13 Sengwa R. J., and Soni A., “Dielectric properties o f some minerals o f western
Rajasthan”, Indian J. Radio and Space Physics, 37 February (2008) 57.
14 Pancholi K.C., Khameshra S.M., “Complex dielectric permittivity o f some
Rajasthan soils at 7.114 GHz”, Indian J. o f Radio and Space Physics”, 23
(1994) 201.
15 Chaudhary H. C., and Shinde V. J., “Dielectric study o f moisture laden soils at
X-band microwave frequency”, International Journal o f Physical Sciences,
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16 Peplinski N. R., Ulaby F. T., Dobson M. C., “Dielectric Properties o f Soils in
the 0.3-1.3 GHz Range”, IEEE Trans. Geosci. & Remote Sens., 33/3, May
(1995) 803.
17 Alex Z. C. and Behari J. “Laboratory evaluation o f emissivity o f soils”, Int. J.
Remote Sensing, 19/7 (1998) 1335.
18 Stuchly S. S., Rzepecka M. A., and Iskander M. F., “Permittivity
Measurements at Microwave Frequencies Using Lumped Elements”, IEEE
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19 Joshi S., Agriculture in Gujarat, Progress and Potential, Helios Enprint Ltd.
20 Biswas B. C., Yadav D. S., and Maheshwari S., Soils o f India and Their
Management, Published by “The Fertilizer Association of India”, New Delhi
(1985).
21 Hill N. E., Vaughan W. E,, Price A. H. & Davies M., “Dielectric Properties
and molecular behavior”, (Van Nostrand-Reinhold, London) 1969.
22 Garg S. K. and Smyth C. P., “Microwave Absorption and Molecular Structure
in Liquid. LXII. the Three Dielectric Dispersion Region of the Normal
Primary Alcohols”, Journal o f Physical Chemistry, 69/4, April (1965) 1294.
23 Barthel J. and Buchner R., “High Frequency Permittivity and its use in the
Investigation of Solution Properties”, Pure and Applied Chemistry, 63/10,
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24 Dobson M. C., Ulaby F. T., Hallikainen M. T., and El-Rays M. A.,
“Microwave Dielectric Behaviour of wet Soil-part II: Dielectric Mixing
Models”, IEEE Trans. Geosci. Remote Sensing, GE-23/1 (1985) 35.
118
25 Ho W. and Hall H. F., J. Geophysical Research (USA), 78 (1973) 603.
26 Sternberg B. K., Levitskaya T. M., “Electrical parameters o f soils in the
frequency range from 1 kHz to 1 GHz, using lumped-circuit methods”, Radio
Science, 36/4, July/August (2001) 709.
27 Hipp J. E., “Soil Electromagnetic Parameters as a function o f frequency, soil
density and soil moisture”, Proceedings o f IEEE, 62/1, January (1974) 98.
28 Godio A., “Open ended-coaxial cable measurement of sandy soils”, American
Journal o f Environmental Sciences, 3 (3), (2007) 175.
29 Mackdonald R., “LEVM/LEVMW Mannual, Issue 8.0, August 2005.
30 Sato T., Chiba A., and Nozaki R., Journal o f Chemical Physics, 110 (1999)
2508.
31 Vyas A. D., Rana V. A., Bhatnagar S. P., Vashisth V. M., “Dielectric
dispersion and relaxation of mixtures of 1-propanol and phenol at lower
microwave frequencies”, Indian J. o f Pure and Applied Physics, 46, July
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32 Mashimo Satoru and Toshihiro Umehara, “Structures o f water & primary
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November (1991) 6257.
119
CHAPTER VI
EFFECT OF SALINITY ON DIELECTRIC PROPERTIES OF
SOILS
6.1
INTRODUCTION:
The identification of effect of saline water on soils with their location is useful to both
the planner’s and farmer’s point o f view. The delineation o f salt affected soils is
possible using visible and NIR data, but the delineation o f salt affected areas in
coastal and desert areas as well as in the black clay regions is d i f f i c u l t I n the coastal
and desert areas containing sandy soils, the reflectance o f surface salt encrustation and
that o f the sand is almost the same. On the contrary in black clay soils, the formation
of salt encrustation on the surface is not sufficient to give a good contrast so that the
effect o f salinity on the soil can not be distinguished. The problem becomes more
complicated under wet soil conditions.
Sreenivas et al. 1 measured the complex dielectric constant o f sand, sandy clay loam
and clay using an L-band dielectric probe (1.25 GHz). Various concentrations of
sodium chloride (NaCl) and sodium acetate (CHaCOONa) solutions were prepared in
distilled water and mixed with the soils. It has been observed that the dielectric
constant e' is dependent on the soil texture and volumetric moisture content in the
soil, where as salinity has no much influence on the dielectric constant at this
frequency. Further it has been observed that the dielectric loss e" increase with
increase in salinity and increase in moisture content in the soil. Under the dry soil
condition, the soil has low conductivity and the lower value o f dielectric loss. It has
been explained due to the fact that under the dry conditions, more air is entrapped in
the soil which have dielectric loss e ”air = 0, thus reducing the bulk dielectric loss of
the dryer soil. For wet soils, the conductivity of soil increases with increase in the salt
content in turn increasing the dielectric loss e". It has been explained that with
increase in soil moisture, a greater amount of salt gets dissolved into the soil system
increasing the conductivity o f the soil. Further it has been explained that the dielectric
loss e" of sandy soils increase linearly with increase in salinity level where as for
120
clay soil the dielectric loss variation with increase in salt and moisture content is
curvilinear.
Now a day, desertification is a serious consequence of global warming, and is a most
crucial issue for the global change community 2. The salinization o f soil is one of the
major factors responsible for desertification. Soil degradation and salinization
devastate vast farmlands, grasslands and forests particularly in the arid and semi arid
regions.
Yun Shao et al. 2 measured the dielectric properties of artificially moistened and
salinized soils, and the saline soil samples taken from a salt lake. The measurements
were carried out using a microwave network analyzer operating in the frequency
range from 1-18 GHz. The dielectric measurements were also compared with the back
scattering coefficients extracted from a RADARSAT image. It has been observed that
e' gradually decreases with increase in frequency from 1 to 18 GHz. The dielectric
constant e ‘ increased rapidly with increase in moisture content in the soil. Salinity has
little impact on the dielectric constant. Further it have been observed that the
dielectric loss e" decreased rapidly with increase in frequency in the lower frequency
range < 2 GHz. In the higher frequency range, the soil salinity had little impact on the
dielectric loss e" which remained almost constant. In the 1-6 GHz range, at higher
moisture content > 20%, the value of e' decreases slowly with increase in salinity, but
the value o f e" increased rapidly with increase in salinity in the soil. It has also been
concluded that the dielectric loss e" is more sensitive to salinity at L-band than at Cband.
The soil salinity and surface soil moisture can be estimated by passive microwave
remote sensing methods 3. Since both o f these properties vary over an area, it is
important to know how to distinguish salinity in the soil when the moisture content in
the soil is varied, and vice versa. Water in the soil changes its microwave dielectric
constant, which in turn is responsible for the change in its emissivity. Further, the
concentration o f salts in water also affects its microwave dielectric property. When
saline water is added in the soil, the dielectric properties o f this mixture are different
from those of the pure water and soil mixture.
121
Jackson and O’N e ill3 measured the brightness temperature o f Elinsboro loamy sand
at various moisture contents and various salinity levels o f water such as 0, 5000,
15000, and 30000 ppm in water. They used radiometers operating at frequency o f 1.4
GHz, and 5 GHz, as well as a hand-held thermal infrared radiometer for the
measurements. The emissivity o f the soil for various moisture contents and salinity
levels were calculated from the measured brightness temperature. The dielectric
mixing models proposed by Wang and Schmugge 4, and Dobson et al. m odel5 were
used in conjunction with the emissivity model to predict the emissivity from a bare
smooth uniform profile. Nearly identical results were produced by the models near
zero salinity which were well reproduced by the observed data at L-band (1.4 GHz)
microwave frequency. Discrepancies occurred at C-band (5 GHz) due to depth of
sampling problems.
The electrical parameters like emissivity and scattering coefficient can be derived
from the dielectric constant of the material. Calla and Kalita 6 estimated the scattering
coefficient of saline soil for slightly rough surface and undulating surface from the
measured values o f dielectric constant at X-band (8.2-10 GHz) microwave
frequencies. The salinity in the soil varied from 4000 ppm to 40,000 ppm for different
moisture contents ranging from 0% to 30.43%. Using the perturbation model and
geometric optical model, the scattering coefficient has been calculated for different
look angles varying from 0° to 60° for both the vertical and horizontal polarizations. It
has been observed that for the slightly rough surface (i) the scattering coefficient
increases with increase in moisture content in the soil at given frequency (8.7 GHz),
(ii) the scattering coefficient decreases with increase in frequency (8.3 GHz to 9.7
GHz) at 11% moisture content at 28,000 ppm salinity in the soil, (iii) the scattering
coefficient decreases with increase in salinity at 8.7 GHz for 11% moisture content in
the soil, (iv) the scattering coefficient decreases with increase in look angle for both
the horizontal and vertical polarizations. The variation o f the scattering coefficient for
horizontal polarization is found to be more than that for vertical polarization at 11%
moisture content in the soil at 8.7 GHz microwave frequency, (v) For undulating
surface the value o f the scattering coefficient increases with increase in look angle for
horizontal polarization, where as for vertical polarization the scattering coefficient
decreases with increase in look angle at 11% moisture content at 8.7 GHz microwave
frequency.
122
Lasne et al, 7 measured the effect of salinity on the dielectric properties o f sand in the
frequency range from 1 to 7 GHz using Vector Network Analyzer coupled to an openended coaxial probe (SMA type). The dielectric constant e' and dielectric loss e" of
sand/saline water mixture were measured as a function o f frequency and moisture
content for two salinity values of S = 40 and 100 °/0 0. It has been observed that the
dielectric constant s ' decreases with increase in salinity as well as frequency. The
dielectric loss e" increases rapidly with increase in salinity. Between 1-2 GHz
dielectric loss e" shows steep variation with change in frequency. The measured
values were also compared with the values calculated using Wang and Schmugge
m odel4 including the complex dielectric constant of water calculated using Stogryn
equations8. It has been observed that the values o f dielectric constant e 1 are in good
agreement with the measured values in 1-3 GHz frequency range. The model
underestimates the measured values o f dielectric loss e " . They also computed the
radar backscattering coefficients from the measured values o f complex permittivity at
L-band (1.5 GHz). It has been observed that the sensitivity o f the backscattering
coefficient to the salinity depends on the moisture content in the soil. The strong
dependence of the backscattering coefficient on the salinity is observed at lower
moisture contents.
Thus considerable work has been done to determine the effect o f salinity on dielectric
properties of moist soil at microwave frequencies; however, few attempts have been
made to study the dielectric properties o f moist soil at radio and microwave
frequencies. Therefore, to gain more information in this area, controlled experiments
were conducted to study dielectric properties o f wet saline soil o f Gujarat state at
radio and microwave frequencies. The following sections describe sample
preparation, measurement technique and results obtained by this study.
6.2
MATERIALS AND METHODS:
The sandy loam soil o f Gandhinagar district was oven dried, and then wet soil
samples were prepared by adding distilled water, saline water o f 10,000 ppm and
30,000 ppm in the soil samples. Now time of 24 hours was allowed to saturate. As the
days went on, the moisture content in the soil decreased and the measurement of
complex dielectric constant o f the soil samples for various moisture contents o f wet
123
and artificially salinized wet soil were carried out using microwave bench set up
operating at 5.65 GHz and in the frequency range from 100 MHz to 1.6 GHz using
VNA employing the methods explained in chapter IV.
6.3
RESULT AND DISCUSSION:
The measured values o f the dielectric constant e ’ and dielectric loss e ” of the sandy
loam soil using VNA for various moisture contents and for various salinity levels of
10000 ppm and 30000 ppm are shown in the figure (6.1), for the frequency range
from 100 MHz to 1.6 GHz. It can be observed from the figure (6.1) that the dielectric
constant and dielectric loss o f the dryer soil does not change appreciably with the
variation of frequency. The dielectric constant and dielectric loss o f the soil increases
with increase in moisture content in the soil, irrespective o f the salinity o f water or the
frequency o f measurement. Further at any given moisture content irrespective of
salinity, the dielectric constant o f the soil remains almost constant above 300 MHz.
But in the frequency range from 100 MHz to 300 MHz the dielectric constant
increases with decrease in the frequency. The value of dielectric loss increases rapidly
with increase in salinity level in the soil particularly in this frequency band. The
dielectric loss decreases with increase in frequency from 100 MHz to 1.6 GHz,
irrespective o f salinity level for all moisture contents. The rate o f decrease in the
dielectric loss with increase in frequency reduces as the salinity in the soil increases
and moisture content in the soil decreases.
The dielectric constant is not showing distinguishable variation with salinity in the
soil. This may be due to the fact that the real part is influenced mainly by the soil
texture and soil moisture, and not correlated with the salinity in the water. Where as
the increase in the salinity increases the conductivity o f the soil-water mixture,
increasing the dielectric loss. Further at higher moisture contents the effect of salt
dominates the dielectric constant value rather than the moisture content in the soil. For
very low moisture contents, the variation in the dielectric loss can not be distinguished
with the salinity variation. This may be due to the lower conductivity o f the soil-saline
water mixture producing weak variation in the imaginary part.
124
To distinguish the effect o f salinity from that o f the distilled water in the soil, the
dielectric constant and dielectric loss o f wet salinized soil against moisture content at
spot frequencies of 0.21 GHz, 0.5 GHz, 1.01 GHz and at 1.4 GHz, is shown in figure
(6.2) (the results for the soil-distilled water mixture are taken from VNA results of
chapter V). It is clear from the figure (6.2) that there is no much visible variation in
the dielectric constant at any moisture content with and without salinity in the soil.
But at higher moisture contents above transition moisture, the dielectric loss e ”
increases with increase in the salinity. Further the increase in the dielectric loss with
salinity is more at lower end o f the microwave frequency range. Hence at higher
moisture contents,
€
sotl+distilled water
6 soiI+10,000 ppm saline water
€
soil+30,000 ppm saline water
The measured values o f the dielectric constant and dielectric loss o f the sandy loam
soil of Gandhinagar district at C-band microwave frequency o f 5.65 GHz are shown
in figure (6.3) for various volumetric moisture contents (in cm3/cm3) of distilled
water, as well as for the water solutions o f 10,000 ppm and 30,000 ppm salinity. It is
seen from the trend lines of figure (6.3) that the dielectric constant and dielectric loss
of the soil increases with increase in moisture content in the soil. Further it is seen that
the salinity has no much effect on the dielectric constant o f the soil, but the dielectric
loss increases with increase in salinity. The results agree well with the available data
in the literature '~3.
From the measured values of the dielectric constant e ’ and the dielectric loss e ” of
the Gandhinager sandy loam soil, at 0.21 GHz, 0.5 GHz, 1.01 GHz, 1.4 GHz and at
5.65 GHz microwave frequencies, the emissivity values o f the soil for normal
incidence were calculated using the relation9
g - l - .1T.(e>1/2.a
l + (e)1/2
where, e = the complex dielectric constant of the soil.
125
The plot of emissivity versus moisture content for normal incidence, in case of
Gandhinagar district soil is shown in figure (6.4) at given fixed microwave
frequencies, for various salinity levels in the water.
It can be seen that for all frequencies emissivity decreases with increase in moisture
content in the soil. Further at given moisture content the emissivity o f the soil
decreases with increase in the salinity o f the water. For dryer soil the effect of salinity
can not be distinguished from the emissivity of the soil. The rate o f decrease in
emissivity with increase in moisture content in the soil is almost the same for all
frequencies considered here for the moist soil with distilled water. For the salinity of
10,000 ppm in the water, the emissivity decreases rapidly with decrease in frequency
at given moisture content in the soil. Further, it is observed that this decrease is more
rapid at salinity of 30,000 ppm in the water. Also, the decrease in emissivity with
increase in salinity o f water at given higher moisture content is more profound at
lower frequency. Thus, at given moisture content and given frequency of
measurement, the emissivity varies as
Csoil+30,000 ppm saline water
^soil+l0,000 ppm saline water <' ©sosl+distilied water
The comparison o f variation o f the emissivity o f the soil at particular frequencies o f
0.21 GHz, 0.5 GHz, 1.01 GHz, 1.4 GHz, and at 5.65 GHz for various moisture
contents may be very much useful for the detection o f salinity and moisture content in
the soil, as well as in remote sensing applications.
Figure (6.5) shows the comparison o f measured values o f dielectric constant e ’ and
the dielectric loss s ” o f moist salinized soil with the values calculated using the Wang
& Schmugge 4 model, in which the values o f dielectric constant e ’ and the dielectric
loss e ” o f water are calculated using Stogryn 8 equations for various salinity levels
and various frequencies. It is observed that for 10,000 ppm saline water/soil mixture,
at spot frequencies o f 0.5 GHz, 1.01 GHz, and 1.5 GHz, the measured values of
dielectric constant e ’ and the dielectric loss e ” agree very well up to 15 % moisture
content after which the calculated values using the model are lower than the measured
values. Further, for 30,000 ppm saline water/soil mixture, at 0.5 GHz, 1.01 GHz, and
126
1.4 GHz microwave frequencies, the measured values agree very well up to 20 %
moisture content in the soil after which the calculated values using the model are
lower than the measured values. At 5.65 GHz microwave frequency and 10,000 ppm
saline water/soil mixture the measured values o f dielectric constant e ’ and the
dielectric loss e ” agree very well with the values calculated using the Wang &
Schmugge - Stogryn equations up to 12 % moisture content in the soil after which the
calculated values using the model are lower than the measured values. Further, at 5.65
GHz microwave frequency and 30,000 ppm saline water/soil mixture the measured
values of dielectric constant e ’ and the dielectric loss e ” agree very well with the
values calculated using the Wang & Schmugge - Stogryn equations up to 10 %
moisture content in the soil after which the calculated values using the model are
lower than the measured values.
Figure (6.6) shows the comparison of measured values o f dielectric constant e ’ and
the dielectric loss e ” of water with the values calculated using the Stogryn equations,
for salinity levels o f 10,000 ppm and 30,000 ppm in the frequency range from 100
MHz to 1.5 GHz. It can be observed that the measured values o f dielectric constant s ’
and the dielectric loss e ” o f water agree very well with the values calculated using the
Stogryn equations. At 10,000 ppm salinity in the water the values o f e ” calculated
using the Stogryn equations are lower than the measured values after 1.1 GHz. At
30,000 ppm salinity in the water the values o f e ” calculated using the Stogryn
equations are higher than the measured values below 500 MHz. Further it is observed
that as the salinity in the water increases from 10,000 ppm to 30,000 ppm the
dielectric constant € ’ does not vary appreciably but the value o f the dielectric loss e ”
increases considerably. The increase in the value o f the dielectric loss e ” is more at
lower end of the frequency.
127
Figure 6.1: The measured values o f the dielectric constant e and dielectric loss e of
the sandy loam soil using VNA for various moisture contents and salinity levels of
10000 ppm and 30000 ppm in water for the frequency range from 100 MHz to 1.6
GHz.
40
35
A
O W v = 0 049666
□ W v = 0 081877
A W v = 0 125362
X W v = 0 158273
X W v = 0 197381
+ W v = 0 241314
G a n d h in a g a r 1 0 ,0 0 0 P P M
30 >
p tk
25
1
i
I
+ * H i m i n H i n i m i i i i n n i i H t m m n w i m i n n i w t t + | | | | m *H
20
15
Q
10
5
1 0E +08
iiiiiiiiiiiim m iiiiinm iiiiiniim iiiiiiiium iini
4 .0 E + 0 8
7 0E +08
1 0E +09
F req u e n c y
CHj^)
1 3E+09
1.6E +09
________
. —
—
D ie le c tr ic L o s s
*=•
Figure (6.1.1)
F re q u e n c y
( H i)
Figure (6.1.2)
128
35
30
(p
O
Wv = 0 049134
X
Wv = 0 163062
qf^
c
-
+
- Wv = 0 084249
A
Wv = 0 122464
Wv = 0 190067
D
Wv = 0 240314
Gandhinagar soil + 30000 PPM saline water
25
CO
ifi
o 20
o
u
■F^
t>
<o 15
«
Q
T IT
"W*W*f*HfH*t*tHwHi i iiiiiiiiii.|iiinntHHfHHt4<t||||H ||lH|||i|il<|t<.l|nl>WW4.
&
10
*■ ^ QteWl^ flaa^flflflfl0aafl^ ^ aftv>wvwvvywnvv^^
1.0E+08
4.0E+08
7.0E+08
1.0E+09
1.3E+09
1.6E+09
Frequency f t f e )
Figure (6.1.3)
Gandhinagar soil + 30000 PPM Saline water
0 0485
0 085542
0.113077
Dielectric Loss £
0178943
0 20986
0 240314
1.0E+08
4.0E+08
7.0E+08
1 0E+09
1.3E+09
1.6E+09
Frequency
Figure (6.1.4)
129
Figure 6.2: Measured values o f the dielectric constant e ’ and dielectric loss e ” o f the
sandy loam soil using VNA at spot frequencies o f 0.21 GHz, 0.5 GHz, 1.01 GHz and
at 1.4 GHz, for various moisture contents and salinity levels
1
o
io
o
co
—
♦—
—
e—
e p s D is t ille d W a t e r
eps"
D is t ille d W a t e r
e p s ’ 1 0 0 0 0 p p m S a lin e W a te r
o
N-
—
~
1 0 0 0 0 p p m S a lin e W a t e r
/
f
e p s ' 3 0 0 0 0 p p m S a lin e W a t e r
I- — e p s "
3 0 0 0 0 p p m S a lin e W a t e r
/
/
o
O
o
tO
Complex Permittivity
o
CO
- eps"
— x
0.21 GHz
/
/
/
/
005
01
0 15
02
0 25
03
0 35
Volumetric Moisture Content (cmA3/cmA3)
s
§
Figure (6.2.1)
—
* • — e p s ' D is tille d W a t e r
—
e—
e p s " D is tille d W a t e r
Complex Permittivity
s a n a
e p s * 1 0 , 0 0 0 p p m s a lin ity
- -
/*"
/
- - e p s ” 1 0 , 0 0 0 p p m s a lin ity
— - X — e p s ' 3 0 , 0 0 0 p p m s a lin ity
—
I- — e p s " 3 0 , 0 0 0 p p m s a lin ity
^
/
^
o
005
01
0 15
02
0 25
03
035
Volumetric Moisture Content (cmA3/cmA3)
Figure (6.2.2)
130
Figure (6.2.3)
-
- eps' dist water
—
— eps" dist water
- • A- - - e p s ' 10000 PPM
Complex Permittivity
- - * ■ --eps" 10000 PPM
— I— eps' 30000 PPM
— X— eps" 30000 PPM
1.4 GHz
o
0 05
0.1
0.15
0.2
0.25
0.3
Volumetric Moisture Content (cmA3/cmA3)
Figure (6.2.4)
0.3E
Figure 6.3: Measured values of the dielectric constant e ’ and dielectric loss e ” o f the
sandy loam soil using microwave bench set up at 5.65 GHz, for various moisture
contents and salinity levels
132
Figure 6.4: The emissivity values of the soil for normal incidence for the Gandhinagar
district sandy loam soil, at 0.21 GHz, 0.5 GHz, 1.01 GHz, 1.4 GHz and at 5.65 GHz
microwave frequencies
0.21 GHz
o
b>
o
Emissivity
o
co
CM
d
+ 30,000 ppm saline water in soil
0
l
!
i
i
i
0.05
0.1
0.15
0.2
0.25
0.3
0.35
Volumetric Moisture Content (cmA3/cmA3)
Figure (6.4.1)
bo
CD
4*
o Double Dist, water in soil
k)
o
Emissivity
o
o
o
0.51 GHz
a
10,000 ppm saline water in soil
O
+ 30,000 ppm saline water in soil
0.05
0.1
0.15
0.2
0.25
0.3
0.35
Volumetric Moisture Content (cmA3/cmA3)
Figure (6.4.2)
133
1.01 G H z
b>
Emissivity
o
o
00
'f
o
o D ouble Dist. W a te r in soil
CM
O
a
1 0 ,0 0 0 ppm saline w ater in soil
+ 3 0 ,0 0 0 ppm saline w ater in soil
0.05
0.1
0.15
0.2
0.25
0.3
Volumetric Moisture Content (cmA3/cmA3)
0.35
Figure (6.4.3)
is
o
o
o
ji
Emissivity
oo
1.4 G H z
a
io
o
o Double Dist. W ater in soil
10,000 ppm saline water in soil
+ 30,000 ppm saline water in soil
0
0 .0 5
0.1
0 .1 5
0 .2
0 .2 5
0 .3
0 .3 5
Volum etric Moisture content (cm A3/c m A3)
Figure (6.4.4)
134
••X-.
xr
o
Emissivity
5.65 GHz
♦ Double Dist. Water in soil
O
CM
a
10,000 ppm saline water in soil
x 30,000 ppm saline water in soil
o
0.05
0.1
0.15
0.2
0 25
0.3
Volumetric moisture content (emA3/emA3)
135
Figure 6.5: The comparison o f measured values of dielectric constant e ’ and the
dielectric loss e ” with the values calculated using the Wang & Schmugge-Stogryn
C o m p le x P erm ittivity
equations for various salinity levels and various frequencies.
Com plex Perm ittivity
Figure (6.5.1)
Figure (6.5.2)
136
Complex Permittivity
Figure (6.5,3)
Figure (6.5.4)
C o m p le x P e rm ittiv ity
Complex Permittivity
Figure (6.5.5)
Figure (6.5.6)
Complex Permittivity
C o m p le x P e rm ittiv ity
Figure (6.5,7)
Figure (6.5.8)
C o m p le x P e rm ittiv ity
Figure 6.6: The comparison of measured values of dielectric constant e ’ and the
dielectric loss e ” of water with the values calculated using the Stogryn equations, for
salinity levels of 10,000 ppm and 30,000 ppm in the frequency range from 100 MHz
C o m p le x P e rm ittiv ity
to 1.5 GHz
F re q u e n c y
Complex Permittivity
Figure (6.6.1)
Frequency
Figure (6.6.2)
140
REFERENCES:
1
Sreenivas K., Venkatratnam L., Narasimha Rao P. V., Dielectric Properties
of salt-affected soils, Int. J. Remote Sens., 16/4 (1995) 641.
2
Shao Yun, Hu Qingrong, Guo Huadong, Lu Yuan, Dong Qing, and Han
Chunming, Effect o f Dielectric Properties of Moist Salinized Soils on
Backscattering Coefficients Extracted From RADARSAT Image, IEEE
Trans. Geoscience and Remote Sensing, 41/8 (2003) 1879.
3
Jackson T. J., O’Neill P. E., Salinity Effects on the Microwave Emission of
Soils, IEEE Trans. Geo. Sc. Remote Sens, 25/2 (1987) 214.
4
Wang J. R., and Schmugge T. J., An empirical model for the complex
dielectric permitivity o f soils as a function o f water content, IEEE
Transactions on Geoscience and Remote sensing, 18/4 (1980) 288.
5
Dobson M. C., Ulaby F. T., Hallikainen M. T., and El-Rays M. A.,
Microwave Dielectric Behaviour of wet Soil-part II: Dielectric Mixing
Models, IEEE Trans. Geosci. Remote Sensing, GE-23/1 (1985) 35.
6
Calla O. P. N., and Kalita H. S., Estimation of scattering coefficient of saline
soil for slightly rough surface and undulating surface at microwave
frequencies, Indian Journal o f Radio & Space Physics, 33 December (2004)
405.
7
Lasne Y,, Paillou P, Rurrie G., Serradilla F., Freeman A., Farr T., McDonald
K., Chapman B., Effect of salinity on the dielectric properties o f geological
materials: Implication for soil moisture detection by means o f remote
sensing, IEEE Transactions on Geoscience and Remote sensing, 46 /6
(2008) 1674.
8
Stogryn A., Equations for calculating the dielectric constant o f Saline Water,
IEEE Trans. On Microwave Theory and Techniques, (1971) 733.
9
Ho W. and Hall H. F., J. Geophysical Research (USA), 78/27 (1973) 6301.
141
CHAPTER VU
DIELECTRIC PROPERTIES OF WET AND FERTILIZED SOILS
AT RADIO AND MICROWAVE FREQUENCIES
7.1
INTRODUCTION:
The complex dielectric spectrum e * = e ’ - j'e ” o f geological materials is very much
useful in planning ground penetrating radar (GPR) surveys *, applications to
microwave remote sensing, to understand the behavior o f induced-polarization and
their use in time domain reflectometry (TDR) measurements. The dielectric constant
e ’ is indicative o f the material’s capability for storing energy in the electric field
(electrical polarization) whereas dielectric loss e ” is indicative o f the material’s
capability for absorbing energy from the alternating electric field. In these materials,
energy loss (dissipation) results from conversion o f electrical energy to thermal
energy (Joule heating) through momentum transfer during collisions as the charge
move. The measurement o f dielectric properties of soils with moisture content and
fertilizer content is also very much useful in agriculture2.
Porosity o f soil greatly helps to judge the moisture content and moisture movement in
the soil. Any operation that reduces aggregation and decreases the amount of organic
matter in the soil decreases pore space. Vivek Yadav et al. 2 measured the dielectric
properties o f fertilized sand for various concentrations of Urea, Shree Ram-33, Shree
Ram-50P, D.A.P., and Mosaic at X-band microwave frequency using two-point
method. It 1ms been observed that the dielectric constant and dielectric loss of the soil
increases with increase in fertilizer content in the soil. Further it has been observed
that the dielectric constant increases slowly with increase in D.A.P. content in the soil,
where as it increases rapidly with increase in Shree Ram-33 content in the soil. It has
been explained that the fertilizer increases the pore space in the soil which is
responsible for increase in fertility of the soil.
Shaikh and Navar Khele 3 measured the dielectric properties o f black soil with
organic and inorganic matters at microwave frequency. It has been observed that the
142
dielectric constant decreases with increase in frequency, which may be due to
molecules having harder time to rotate with increase in frequency. Further it has been
observed that the dielectric constant increases linearly with increase in fertilizer
content in the soil. It has been explained that according to the theory o f electrolyte, in
the limit of low concentration, the dependence o f dielectric constant is approximately
linear. Further it has been explained that with the addition o f organic matter in the
soil, the water holding capacity o f the soil increases. The dielectric loss is found to
increase with increase in inorganic and organic matter content in the soil equally. It
has been explained that the dielectric loss is associated with ionic conduction.
A capacitive soil moisture sensor having fork like geometry have been designed and
used by Eller and Denoth 4 for the field measurements o f water content of natural
soils in frequency range up to 35 MHz, It estimates complex permittivity of soil for
various moisture contents by measuring the impedance with a twin T-bridge. A
symmetrical plate condenser o f 150 cm3 effective measuring volume was filled with a
soil sample and was connected to the feed unit by a coaxial cable with a two-port
vector analyzer, in combination with a programmable synthesizer. The amplitude and
reflection coefficient o f the material under test were measured using the simple Tjunetion method, to calculate the complex permittivity o f MUT.
At frequencies below 5 MHz, even a relatively large sample has dimensions less than
the wavelength, and the time of wave propagation remains short in comparison with
the wave period5. Methods of measuring lumped parameters are known to be used at
frequencies up to 100 MHz, provided the dimensions o f the sample can still be made
substantially less than the wavelength. A parallel plat capacitor with disk electrodes is
most commonly used as a sample holder in this frequency range. Levitskaya and
Sternberg 5 determined the complex permittivity of the material by measuring the
magnitude Z and phase 4> o f the sample impedance by using an impedance analyzer.
The measurements were carried out in the frequency range from 1 KHz to 100 MHz
using disk electrodes, and in the frequency range from 0.1 MHz to 1 GHz using
coaxial sample holder.
In reference 5, the lumped parameter approach was used, and the data processing
procedure included corrections for stray parameters such as: inductance L, resistance R
and capacitance C of the measuring system.
143
Kandiah and Mitchell 6 measured the dielectric properties o f kaolinite clay-waterelectrolyte systems in the frequency range from 30 to 105 Hz using a comparator
which is essentially a Wheatstone bridge. A cylindrical sample was held between two
flat circular platinum electrodes which were platinum black coated. It has been
observed that the dielectric increment ( e 0- e m) increases with increase in temperature;
whereas the characteristic frequency is independent o f temperature. Further it has
been observed that the larger the hydrated ion size, the larger is the increment and
lower characteristic frequency; and that higher the electrolyte concentration the larger
is the dielectric increment and the characteristic frequency. Also it is observed that the
dispersion o f the conductivity and dielectric constant and the characteristic frequency
are significantly affected by the ion type and ion concentration.
Sengwa et al. 1 measured the dielectric permittivity o f dry and water saturated shale,
sandy sandstone and calcareous sandy stone o f Jodhpur region at room temperature in
the frequency range from 100 Hz to 100 kHz, and also at 10.1 GHz microwave
frequency. It has been observed that the dielectric constant of these samples decrease
with increase in frequency. Further if has been observed that there is a large
enhancement in the e ’ values of water saturated samples in comparison to the e ’
values of the dry samples. The low frequency limiting dielectric constant e 0, high
frequency limiting dielectric constant €ro, the dielectric relaxation time x and
distribution parameter a of these samples have been determined by drawing the ColeCole plots of these samples. Further ac conductivity o f dry and water saturated
samples has also been evaluated and reported.
Sengwa and S o n i7 measured the dielectric constant e ’ and dielectric loss e ” of dry
samples o f clay, siliceous earth, fuller’s earth, gypsum, lignite, calcite, tourmaline and
magnesium rock o f opencast mines of western Rajasthan, India, in the frequency
range 100 Hz to 100 kHz and also at X-band microwave frequencies. It has been
reported that the dielectric constant e ’ decrease with increase in frequency in the
lower frequency region. The low frequency limiting dielectric constant e 0, high
frequency limiting dielectric constant €„, the dielectric relaxation time x and
distribution parameter a of these samples were evaluated by drawing the Cole-Cole
144
plots of these samples. It has been observed that all these minerals have large value of
a and their t values lie in the range from 0.1 to 11 ms. Calculated values o f frequency
dependent ac conductivity have shown a linear behavior between log a and log /
The dielectric properties of moist soil and fertilized soil have been measured in the
frequency range 10 kHz to 2 MHz. The following sections describe sample
preparation, experimental set up and measurement technique. The result of the study
has also been presented here.
7.2
SAMPLE PREPARATION:
The wet soil samples o f Gandhinagar district sandy loam soil for various moisture
contents were prepared in the laboratory by adding various proportions of double
distilled water in the dry soil, and the measurements for the estimation o f dielectric
properties of wet soil samples were carried out using the precision LCR meter.
The addition of fertilizer in the soil increases the water holding capacity o f the so il3.
To observe the effect o f fertilizer on the soil we selected two fertilizers (i) Sulphet of
Potash (SOP) also called Potassium Sulphet, and (ii) Zinc Chelate. The SOP contains
potash (as K 20) percent by weight minimum- 50.0, Sulpher (as S) percent by weight
minimum- 17.5. SOP is imported in India from Finland. It is prescribed to prepare
0.2% to 0.5% solution of SOP (by desolving 200 to 500 gm /1 0 0 liters o f water). It is
100% water soluble fertilizer for folier spray. It is recommended for use on Cotton
and Vegetables. The microgranual formulation of Zinc Chelate contains water soluble
Zinc (Zn) minimum = 12.0 %, Zinc (Zn) Chelated by EDTA minimum = 12.0 %, and
pH stability range was 4.9 in aqueous solution. It is recommended for use on Paddy,
Cotton, Chillies, Vegetables, Sugarcane, Groundnut, and Horticultular crops. It has
been prescribed to dissolve 100-150 gm o f Librel Zinc Chelate in 150-200 Ltrs. of
water and to be sprayed over one acre o f standing crop or if required the dose can be
increased. Zinc Chelate fertilizer is manufactured by Ciba UK Pic, Bradford, West
Yorkshire, UK.
145
Further the dry soil samples o f Palanpur district sand and Gandhinagar district sandy
loam soil were taken. Adding some fixed proportion o f fertilizers Sulphet o f Potash
(SOP) and Zinc Chelate in the double distilled water, various solutions in % of
fertilizer in water were prepared. 6 ml of these solutions were added in the soil
samples of 100 gm weight and mixed well. The % solutions o f fertilizers by weight in
the soil samples are shown in the table (7.1). The volumetric moisture content in the
soil samples for the fertilized water solutions was calculated as explained in chapter V
(5.2.1).
The dielectric constant e ’ and dielectric loss e ” o f these fertilized soil samples were
also measured at spot frequencies o f 0.5 GHz, 1.0 GHz and 1.5 GHz, using Vector
Network Analyzer employing the method as described in chapter IV (4.2.2).
146
(Librel)
o
o
Sandy Loam
r— <
i
0.08 %
40 mg
oo
Sand
district
o*o—■< ooH
(Librel)
Chelate
I*— <
oo
90 mg
60 mg
50 mg
0.62 %
310 mg
Sand
Zinc
6 ml
0.50 %
250 mg
(SOP)
district
Palanpur
6 ml
0.34 %
170 mg
of Potash
T— 4
0.18%
0.12 %
0.10 %
6 ml
6 ml
6 ml
6 ml
6 ml
6 ml
f—
0.24 %
120 mg
oo •< oo
6 ml
oo
0.20 %
Sulphate
0.083
6 ml
0.16 %
o
Palanpur
t““4
100 mg
80 mg
0.083
6 ml
0.12 %
60 mg
Chelate
<
district
0.0954
0.0954
0.0954
0.0954
0.0954
0.0954
0.0954
0.0954
0.083
0.083
6 ml
0.08 %
40 mg
Zinc
r “H
Gandhinagar
0.083
6 ml
0.50 %
25 mg
0.083
6 ml
0.40 %
20 mg
(SOP)
Sandy Loam
0.083
moisture content
0.083
6 ml
fertilized water
of Volumetric
6 ml
0.32 %
16 mg
of Potash
district
0.22 %
fertilizer
% solution of Volume
Sulphate
11 mg
In 50 ml double distilled water
soil (gm)
Type
Weight of Fertilizer(mg)
Weight of Fertilizer
Z5 C
o'C
o3} oCwH o
r“ <
Gandhinagar
Soil Type
Table (7.1): % solution content of fertilizer in the soil samples:
o
oo
147
7.3
EXPERIMENTAL SET UP:
A precision LCR meter Agilent make E-4980A operating in the frequency range from
20 Hz to 2 MHz was used for the measurements o f capacitance and resistance offered
by the coaxial capacitor. The LCR meter can take simultaneous measurements during
one trigger in the frequency range from 20 Hz to 2 MHz in 201 linear steps (or
logarithmic steps if required).
A standard four point probe Agilent 16089A with Kelvin clip leads was connected to
the LCR meter. The coaxial capacitor was connected at the end o f the probe and fixed
in a stand pointing downward.
The compensation o f the LCR meter and coaxial capacitor was done in following
steps:
(i)
Open: The LCR meter was compensated for open circuit coaxial capacitor
with air as dielectric medium.
(ii)
Short: A vessel containing mercury was raised from the lower side o f the
capacitor till it fills the capacitor completely and then LCR meter was
compensated for short.
The LCR meter is said to be compensated up to the end o f the coaxial capacitor.
7.4
STANDARDIZATION OF CAPACITOR:
The coaxial capacitor was standardized using the liquids o f known dielectric constant,
as follows:
(1) The capacitance Co and resistance Ro o f the coaxial capacitor were measured
using LCR meter for open circuit condition with air as dielectric for the
frequency ranging from 20 Hz to 2 MHz.
(2) N ow a small vessel containing CCI4 (AR grade) was kept below the coaxial
capacitor. Then raising the vessel until the CCI4 level completely fills the
capacitor, the capacitance Cp and resistance Rp were measured for the
frequency ranging from 20 Hz to 2 MHz.
(3) CCI4 was removed from the coaxial capacitor. Then a vessel containing
Acetone was raised below the capacitor so that it fills the capacitor
148
completely. Now Acetone was removed and time was allowed for the Acetone
to evaporate completely. Again the capacitance and resistance o f the empty
capacitor were measured to verify the initial values of the empty capacitor as
explained in step (1).
(4) Now the steps (2) and (3) were repeated for other standard liquids of known
dielectric constant like Benzene and Chloro-benzene.
For each capacitance value of standard liquid, the difference capacitance Cp-Co was
calculated. A graph was drawn for the dielectric constant against Cp-Co for the known
Dielectric Constant
standard liquids, as shown in figure (7.1).
i ---------------------- r
0
0 .5
1
1 .5
2
i
2 .5
Cp - Co in pF
Figure (7.1). A graph o f the dielectric constant e ’plotted against Cp-Co in pF.
The equation for the straight trend line connecting all points was obtained from the
graph as
y = 2.0139 x + 0.98
........ (1)
where, x = Cp - Co in pF,
y = Dielectric constant € ’,
2.0139 = slope o f the straight line from the graph of figure (7.1), and
0.98 = intercept o f the straight line from the graph o f figure (7.1).
149
The correlation coefficient of difference capacitance values and the dielectric constant
for the linear trend line was observed to be 0.9999.
The dielectric constant for each sample was calculated using the equation
e ’meas = 2.0139 (Cp - CQ) + 0.98
..... (2)
The dielectric loss of the sample (liquid or soil) for each frequency was calculated
using equation
e"=e'tan<J
(3)
Where tan <5 = --------
(4)
G>e'e0
For the coaxial capacitor5 the conductivity and dielectric constant are
In(b/a)G _ In(b/d)
2nH
, Injbla)C
2nHR ’ 6 2nH e0
( 5)
Substituting results (4) and (5) in (3), we get
€"=e’tand> = ------
M b! a) <='meas
2nHR& e 0 CRm
.(6)
In equation (6),
e ’ = measured value o f dielectric constant from equation (2).
a = radius o f inner cylinder
b = radius o f outer cylinder
f - frequency o f measurement,
H = length (height) of the capacitor
Rp = resistance measured using LCR meter at each frequency,
Cp = capacitance of coaxial capacitor with soil sample, at each frequency,
150
Co —capacitance of empty (air filled) coaxial capacitor, at each frequency.
In the actual calculation we considered C = C/> -Co and R = Rp as explained in
equation (2).
The conductivity & o f the soil samples for all frequencies o f measurements was
calculated using the equation
<j'= co e"e0
......... (7)
A graph of cr' against frequency was drawn for each soil sample. Extending the linear
fitting line towards lower frequency end, the value o f intercept of & for zero
frequency was obtained, called dc conductivity 0 ’dc- The value o f dielectric loss e ”
due to dc conductivity was calculated at each frequency using the equation
Hence subtracting the dc conductivity loss e ”dCfrom the measured value o f dielectric
loss e ” from the equation (44), we get the actual dielectric loss € ”actuai a s 8’9
actual
=e
tx’
coen
...... (9)
The measurements were carried out for the other liquids like 1-Propanol and Acetone
(AR Grade) as explained in steps (2) and (3) in measurement procedure. Again
subtracting the capacitance of air filled capacitor from the measured values of the
capacitor filled with 1-Propanol and Acetone. Substituting the respective values in
equation (2), we get the dielectric constant o f 1-Propanol and Acetone. The calculated
values are compared with the known standard values given in the literature as shown
in the table (3), and are in good agreement with literature values with error less than
3%.
Table (7.2) shows the comparison o f the measured values with the values obtained
from literature. The results are in very good agreement with the literature values.
151
Table (7.2): Comparison o f measured values and literature values 11 o f dielectric
constant
e o known a t given
6 *m easured
A e ’ / e ’ in
tem perature
at 2 MHz
percent
1
0.98
2%
CC14
2.23811 (20° C)
2.225
0 581 %
Benzene
2.2836 11(20° C)
2.294
-0.455 %
Chloro-benzene
5.708 11(20° C)
5.491
0 396 %
1-Propanol
19.5 11(20° C)
19.253
1 267 %
Acetone
20.7 11 (25° C)
20.630
0 338 %
M aterial
Air
A vessel containing wet soil sample o f Gandhinagar district sandy loam soil was
raised up slowly taking care that the position of the capacitor does not change, and
slowly knocking the vessel it was raised till the coaxial capacitor was completely
filled with the wet soil (the air in empty part came out through vertical groves). Now
the capacitance Cp and resistance Rp o f the soil filled capacitor were measured using
the LCR meter for the whole range o f the frequency. Subtracting the capacitance of
empty air filled capacitor from the capacitance o f the soil filled capacitor; the
dielectric constant e ’ and dielectric loss e ” of the Gandhinagar district soil were
calculated using equations (40) and (47).
The measurements were carried out for the estimation of dielectric constant, dielectric
loss, and loss tangent o f Gandhinagar district sandy loam soil, for various moisture
contents and frequency variation from 10 kHz to 2 MHz using the LCR meter. The dc
conductivity o f the soil for all moisture contents was also calculated using the method
explained by Sengwa et al. 9. The value o f dielectric loss is obtained by subtracting
the dc conductivity from the measured value of dielectric loss.
The measurements for fertilized wet soil samples o f Gandhinagar district sandy loam
soil and Palanpur district sand were also carried out.
152
7.5
RESULTS AND DISCUSSION:
It can be observed from figure (7.2) that the dielectric constant e ’ and dielectric loss
g”
o f the (Gandhinagar district sandy loam) wet soil decreases with increase in
frequency from 10 kHz to 2 MHz. Similar results have been reported in literature ’’4’
8. The decrease in e ’ with increase in frequency range from 100 Hz to 100 kHz is the
common characteristic o f the geological materials l. Further it is observed that in this
frequency range the dielectric constant e ’ and dielectric loss
g”
o f the soil increases
vary rapidly with increase in moisture content in the soil. A very large enhancement
in the values o f c ’ and
g
” is observed at lower frequency with increase in moisture
content in the soil. This very large enhancement in the permittivity value of wet soil
may be due to electro chemical polarization 1 which arises due to increase in surface
charge carrier density in presence o f water molecules in the pore spaces o f the soil.
The variation of the measured dielectric constant
and corrected dielectric loss
g”
g
’ (eps’), dielectric loss
g”
(eps”),
(actual eps”) after subtracting the contribution of
ohmic (dc) conductivity contribution is shown in figure (7.3) for Wv : 0.219
cm /cm . It can be observed that the ohmic (dc) conductivity contribution to the
dielectric loss in wet soil is more at lower frequency end.
The variation of loss tan 8 o f the Gandhinagar district sandy loam soil for various
moisture contents in the frequency range from 10 kHz to 2 MHz is shown in figure
(7.4). It is observed that at very low moisture content of Wv = 0.005 cm3/cm3, a very
small peak is observed at 1.262 MHz (tan 8 = 0.37). As moisture content in the soil
increases the value of tan 8 increases (tan 8 = 7.88 at 238 kHz for Wv = 0.097
cm3/cm3; tan 8 = 12.95 at 160 kHz for Wv = 0.178 cm3/cm3; and tan 8 = 19.99 at 150
kHz for Wv = 0.219 cm3/cm3), which also show shift o f tan 8 towards lower
frequency side as moisture content in the soil increases. Analogous behavior has been
reported in literature I0. The shifting o f loss peak towards lower frequency with
increase in moisture content in the soil suggests the change in size o f the orienting
ions in the presence o f pore water in the samples 10.
153
Figure (7.5) shows the variation of real and imaginary values of conductivity with
variation in frequency for various moisture contents in the Gandhinagar district sandy
loam soil. It can be observed from figure (7.5-a) that for dryer soil (Wv = 0.005
A
<J
cm /cm ) the frequency dependent real part o f conductivity o ’ o f the soil is very small
at lower frequency (cr’~ 1.74 xlO'6 at 10 kHz) and increases with increase in
frequency (o ’~ 1.6 xlO"4 at 2 MHz). At higher moisture contents in the soil a ’
increases slowly with increase in frequency. Analogous behaviour was observed by
Sternberg and Levitskaya8, Sangwa et al. \ and Sangwa and S o n i10. The increase in
the value o f conductivity cr’ of wet soil samples shows that the conductivity o f soilwater matrix increases with increase in moisture content in the soil 1‘ 10. The
imaginary conductivity
a”o f the dryer soil
(Wv = 0.005 cm3/cm3) is very small at
lower frequency (cr”~ 5 xlO'6 at 10 kHz) and increases with increase in frequency
(o”~ 0.00045 at 2 MHz). At given higher moisture content in the soil the conductivity
or” decreases with increase in frequency up to certain minimum value after which it
increases with increase in frequency. It has been observed that the minimum value of
0 ” shifts towards lower frequency end as moisture content in the soil increases (cr”~
0.0014 at 178.25 kHz for Wv = 0.097 cm3/cm3; a ”~ 0.00187 at 158.865 kHz for Wv
= 0.178 cm3/cm3; tr”~ 0.0021 at 149.987 kHz for Wv = 0.219 cm3/cm3 ). Further it
has been observed that the value o f a ”mimmum increases with increase in moisture
content in the soil.
Figure (7.6) shows the variation o f dc (ohmic) conductivity with moisture content in
the sandy loam soil o f Gandhinagar district. It has been observed that for dryer soil
(Wv = 0.005 cm3/cm3) the dc conductivity is o f the order o f 4 x 10'7, where as for wet
soil it increases with increase in moisture content in the soil approaching saturation
value o f 0.0214 at (Wv = 0.219 em3/cm3) for die given soil sample.
Figure (7.7) shows the variation o f dielectric constant e ’ and dielectric loss e ” o f the
wet fertilized soils (Sandy loam soil of Gandhinagar district, and Sandy soil of
Palanpur district) for various concentrations o f different fertilizers [Sulphet of Potash
(SOP), and Zinc Chelate] in the frequency range from 10 kHz to 2 MHz. It has been
observed that the dielectric constant e ’ and dielectric loss e ” of the wet fertilized
soils decrease with increase in frequency from 10 kHz to 2 MHz. The dielectric
154
constant e ’and dielectric loss e ” increases with increase in % concentration of
fertilizer content in the wet soil. This behavior is mainly dependent on the moisture
content in the soil. There is approximately linear increasing the e ’ with percentage
volume of organic and inorganic metal. This may be due to the fact that the added
organic and inorganic matter forms a chemical composition o f low concentration
along with the chemicals present in the soil. According to the theory o f Electrolyte, in
the limit of low concentration the dependence of € ’ is approximately linear (3). By
adding fertilizer the water holding capacity o f soil improves. The dielectric
permittivity o f soil directly depends on the amount of moisture content present in the
soil. The higher moisture content increases the dielectric constant o f the soil. The
dielectric loss € ” o f soil increases with increase in % volume o f fertilizer. The reason
may be that e ” is a parameter which describes the motion o f electric charge i.e. is a
conduction phenomenon 3. Certain dielectrics display conduction which arises from
the actual charge transport (ionic conduction in electrolytes) rather than due to the
displacement current. Such conduction is described by volume conductivity which
adds an additional term to the dielectric loss e ”. Due to this additional term the
dielectric loss increases with increase in fertilizer content in the soil.
Figure (7.8) shows the variation of dielectric constant e ’ and dielectric loss e ”of the
soils (Sandy loam soil o f Gandhinagar district, and Sandy soil o f Palanpur district) for
various % concentrations o f fertilizers [Sulphet of Potash (SOP), and Zinc Chelate] at
spot frequency o f 2 MHz. It can be observed that the dielectric constant of the soils
increases slowly with increase in % concentration of fertilizers SOP and Zinc Chelate.
The dielectric loss increases rapidly with increase in % concentration o f fertilizers in
the soils.
Figure (7.8-a) shows the variation of dielectric constant and dielectric loss of
Gandhinagar district sandy loam soil for various % concentrations of Sulphet of
Potash (SOP) in the soil at 2 MHz. It can be observed that for the variation of
fertilizer SOP from 0.22% to 0.50% in the soil the dielectric constant e ’ increases
from 6.12 to 8.99 and dielectric loss e ” increases from 60.4 to 90.4. Figure (7.8-b)
represents the variation o f dielectric constant and dielectric loss of Gandhinagar
district sandy loam soil for various % concentrations o f Zinc Chelate in the soil at 2
MHz. It can be observed that for the variation of fertilizer Zinc Chelate from 0.08% to
155
0. 20% in the soil the dielectric constant e ’ increases from 9.99 to 12.64 and dielectric
loss e ” increases from 82 to 134.5. The variation of dielectric constant and dielectric
loss of Palanpur district sandy soil for various % concentrations o f Sulphet of Potash
(SOP) in the soil at 2 MHz is shown in figure (7.8-c). It can be observed that for the
variation of fertilizer SOP from 0.24% to 0.62% in the soil the dielectric constant e ’
increases from 8.12 to 11.25 and dielectric loss e ” increases from 68.9 to 115.4.
Further the variation o f dielectric constant and dielectric loss of Palanpur district
sandy soil for various % concentrations o f Zinc Chelate in the soil at 2 MHz is
represented in figure (7.8-d). It can be observed that for the variation o f fertilizer Zinc
Chelate from 0.08% to 0.18% in the soil the dielectric constant e ’ increases from
8.10 to 9.94 and dielectric loss e ” increases from 63.2 to 98.65. The reason for large
increase in e ” may be due to the fact that e ” describes the motion o f electric charge
1. e. is a conduction phenomenon 3, which arises from the actual charge transport (ionic
conduction in electrolytes), described by volume conductivity that adds an additional
term to the dielectric loss e ”.
Figure (7.9) shows the variation o f loss tan 8 o f the wet fertilized soils for various
concentrations of different fertilizers in the frequency range from 10 kHz to 2 MHz. It
has been observed that a loss peak appears near 105 Hz for the wet soils for all the %
concentrations of fertilizers in the soils. The loss peak is observed to shift towards
lower frequency end with increase in fertilizer content in the soils except that of SOP
in Palanpur district sandy soil in which the loss peak is observed to shift towards
higher frequency end, the reason for this behavior is yet to be found by taking more
observations on different types of soils.
The variation o f real and imaginary conductivity o f the wet fertilized soils (Sandy
loam soil of Gandhinagar district, and Sandy soil of Palanpur district) for various
concentrations of different fertilizers [Sulphet o f Potash (SOP), and Zinc Chelate] in
the frequency range from 10 kHz to 2 MHz is shown in figure (7.10). It has been
observed that the real conductivity a ’ o f the soils increase slowly with increase in
frequency. The real conductivity & of the soils also increase with increase in %
concentration o f fertilizer in the soil. The imaginary conductivity a ” o f the soils
decrease with increase in frequency up to certain value o f a ”mmimum after which it
156
increases with increase in frequency. The value o f <r”mmimum is observed to shift
towards higher frequency end with increase in % concentration o f fertilizer in the
soils except that for SOP in Gandhinagar district sandy loam soil in which the value
of
is observed to shift towards lower frequency end with increase in %
concentration o f SOP in the soil.
Figure (7.11) shows the variation of dc conductivity o f the wet fertilized soils for
various concentrations of fertilizers. It has been observed from figure (7.11-a) that the
dc conductivity o f Gandhinagar district sandy loam soil increases from aac = 0.0035
to 0.0057 for % solution variation o f SOP from 0.22% to 0.50%. Figure (7.11-b)
shows that the dc conductivity o f Gandhinagar district sandy loam soil increases from
adc - 0.0052 to 0.009 for % solution variation of Zinc Chelate from 0.08% to 0.20%.
Further it can be observed from figure (7.11-c) that the dc conductivity of Palanpur
district sandy soil increases from tide = 0.0045 to 0.0099 for % solution variation of
SOP from 0.24% to 0.62%. Figure (7,11-d) shows the variation o f dc conductivity of
Palanpur district sandy soil from a<jC= 0.0034 to 0.0067 for % solution variation of
Zinc Chelate from 0.08% to 0.18%. Thus it can be concluded that Zinc Chelate
increases the dc conductivity of the soil more rapidly in comparison with SOP.
The variation of dielectric constant and dielectric loss of the wet fertilized soils for
various concentrations o f different fertilizers at spot frequencies o f 0.5 GHz, 1.0 GHz
and 1.5 GHz measured using Vector Network Analyzer (method o f measurement
described in chapter IV) is shown in figure (7.12). It has been observed that at given
fertilizer content in the soil the dielectric constant e ’ and dielectric losse ” o f the soils
decrease with increase in frequency from 0.5 GHz to 1.5 GHz. Further it is observed
that at given frequency o f measurement the dielectric constant e ’ and dielectric loss
e ” o f the soils increase with increase in fertilizer content in the soil. Further it is
observed that the values o f the dielectric constant e ’ and dielectric loss e ” o f the
soils for the frequency band from 0.5 GHz to 1.5 GHz are very small in comparison
with those obtained for the lower (radio) frequency band from 10 kHz to 2 MHz. At 2
MHz for the variation o f fertilizer SOP from 0.24% to 0.62% in the soil the dielectric
constante5 increases from 8.12 to 11.25 and dielectric lo s se ” increases from 68.9 to
115.4; where as at 1.0 GHz for the variation of fertilizer SOP from 0.24% to 0.62% in
157
the soil the dielectric constant e ’ increases from 4.47 to 4.94 and dielectric loss € ”
increases from 0.58 to 0.72. This shows that for given variation o f fertilizer in the soil
the variation o f dielectric constant and dielectric loss values is more for lower (radio)
frequencies (e.g. at 2 MHz), rather than that for the microwave frequencies form 0.5
GHz to 1.5 GHz.
158
Figure 7.2: The measured values o f dielectric constant and dielectric loss of
Gandhinagar district wet sandy loam soil for various moisture contents in the
Dielectric Constant
frequency range from 10 kHz to 2 MHz.
Dielectric Loss
Figure 7.2-a
Figure 7.2-b
159
Figure 7.3: The measured values o f dielectric constant, dielectric loss, and actual
dielectric loss after subtracting dc conductivity dependent dielectric loss of
Gandhinagar district wet sandy loam soil at Wv = 0.219 in the frequency range from
10 kHz to 2 MHz.
Figure 7.4: The calculated values of loss tan 5 o f Gandhinagar district wet sandy loam
soil for various moisture contents in the frequency range from 10 kHz to 2 MHz.
160
Figure 7.5: The real and imaginary values of conductivity for variation o f frequency
Real Conductivity, ex
from 10 kHz to 2 MHz, for various moisture contents in the soil
Frequency (Hz)
x W v = 0.219
G an dhinagar district Sandy Loam Soil
1E-07
1E+04
1E+05
1E+06
1E+07
Frequency (Hz)
Figure 7.5-b
161
Figure 7.6: DC conductivity for various moisture contents (cm3/cm3) in the soil.
1.0E-01
G a n d h in ag ar S andy Loam
DC C o n d u ctivity
a DC
1.0E-02
1.0E-03
1.0E-04
1.0E-05
y = -3.4514X3 + 1.0526X2 - 0.0084x + 2E -05
1.0E-06
1.0E-07
R2 = 1
i
1.0E-08
1.0E-09
0.05
0.1
0.15
0.2
0.25
V o lum etric M oisture C o n ten t
162
Figure 7.7: The variation of dielectric constant and dielectric loss o f the wet fertilized
Dielectric Constant
soils for various concentrations of different fertilizers.
Dielectric Loss
Figure 7.7-a
Figure 7.7-b
163
D ie e c t r ic L o s s
Figure 7.7 -c
Figure 7.7-d
164
D ie le c tric C o n s ta n t
Dielectric Constant
Frequency (Hz)
Dielectric Loss
Figure 7.7-e
Frequency (Hz)
Figure 7.7-f
165
D ie le c tric L o s s
Figure 7.7-g
F re q u e n c y (H z )
Figure 7.7-h
166
D ie le c tric C o n s ta n t
Figure 7.8: The variation o f dielectric constant and dielectric loss o f the wet fertilized
soils for various concentrations o f different fertilizers at spot frequency o f 2 MHz.
too
G a n d h in a g a r s a n d y lo a m s o il + S O P S o lu tio n
C o m p le x P erm ittivity
80
60
- Dielectric Constant
40
• Dielectnc Loss
20 A
0
0.1
0.2
0.3
0.4
0.5
0.6
% S o lu tio n o f S O P
Figure 7.8-a
160
G an d h in ag ar Sandy Loam + Z in c C helate Solution
2 M Hz
C o m p lex P erm ittivity
140
120
100
80
60
• Dielectric Loss
40
20
0
0 05
0.1
0.15
0.2
0 25
% Solution o f Zinc C hilate
Figure 7.8-b
167
i
O
Palanpur district Sand + Solution of SOP
2 MHz
CO
o
O
o
Complex Permittivity
O
CM
o
CD
O
- • - D ie le c tr ic Loss
o
CM
♦ — ------------------------------- ♦ — ---------- ♦
— I----------------------- 1-------------------------- 1------------------------ i------------------------ 1-------------------------1------------------------ j
01
02
0.3
0.4
05
0.6
0.7
% Solution of SOP
?
00
o
CM
Palanpur district Sand + Zinc Chelate
Solution
o
00
o
o
Complex Permittivity
O
o
CO
—
Dielectric Constant
—a — Dielectric Loss
O
CM
«— _«------•----------------------- ♦
------!-------------------------------------- 1-------------------------------------- j-------------------------------------- 1
O
0.05
0.1
015
0.2
00
V
8
Tj
% Solution of Zinc Chilet
168
Figure 7.9: The variation o f loss tan 8 of the wet fertilized soils for various
concentrations o f different fertilizers in the frequency range from 10 kHz to 2 MHz.
10
++++^++^ ++++++++++++
++++++
"*++
a4 4
4+ •"
,T
4>
to
-
.+ -'
e
43
■
+ + !--
+*.•
• “
. O
A
o o 0 o o o o o o o ° » ° o o o < ,o 0 o
aoO®
**A
°00
°0 •- V.
v-%
0v
V . +4
a*
44 0°'
O - + A
O - + A
A °°
°S
*4
a%
a a O °
a4°
G an d h in ag ar san dy lo am + SO P
1.0E+04
* Soil + 0 22 % SO P Solution
<= Soil + 0 32 % S O P Solution
+ Soil + 0 4 0 % S O P Solution
■Soil+ 0 5 0 % S O P Solution
1 0E +05
1 0E +06
Frequency (H z)
Figure 7.9-a
Figure 7.9-b
169
170
Figure 7.10: The variation o f real and imaginary conductivity o f the wet fertilized
soils for various concentrations o f different fertilizers in the frequency range from 10
kHz to 2 MHz.
0 01
!
G a n d h in a g a r s a n d y loam + SO P
0 008
,0 0 0 0 0 4 0 0 0 0
H I | I I I I I »-*•»
R eal C o nd u ctivity a*
.- h - h -h -h - h
OOOOO'
OOOOO'
OOOOOO'
++++-H-H+*
ooooooo
,0 0 0 0 0 0 0 0 '
OOOOOOOOOOO'
,0 0 0 0 0 0 0 0 0 0 0 0 0 0 '
1) 0 0 0 0 0 0 0 0 0 0 '
,4 4 4 4 4
,4 4 4 4 4 4 4 4 4 4 4 '
0 006
4 4 4 4 4 4 4 4 4 4 4 4 4
,44444444444444444444444,
,1 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 ,
0.004
4
Soil + 0 22 % S O P Solution
o Soil + 0 32 % S O P Solution
0 002
-
+ Soil + 0 4 0 % S O P Solution
- Soil + 0 50 % S O P Solution
1 OE+04
1 0E +05
1.0E+06
F requency (H z)
Figure 7.10-a
0.0019
+ Soil + 0 4 0 % SOP Solution
• Soil + 0 50 % SO P Solution
b
° Soil + 0.32 % SO P Solution
o
o
o
Oo°o!+++++J.
.****
°°°o 0o> .
"A,44A
b
o
o
‘*4,
o
Im ag in ary C o nd u ctivity a"
CO
T—
o
o
o
4 Soil + 0.22 % SOP Solution
'**4*44,*444,
441444,
4444,
4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 1
444444“ ^
0.0004
G a n d h in a g ar san d y loam + SO P
0.0001
1 0E+04
1 0E+05
1 OE+06
Frequency (Hz)
Figure 7.10-b
171
0.003
a Soil
+ 0.08 % Solution
+ Soil + 0.12 % Solution
o Soil + 0.16 % Solution
x Soil + 0.20 % Solution
0
1E+04
1E+05
1E+06
1E+07
Frequency (Hz)
Im aginary C onductivity,
cr
Figure 7 .10-c
Frequen cy (H z)
Figure 7.10-d
172
Real Conductivity o'
Frequency (Hz)
Imaginary Conductivity o"
Figure 7.10-e
Frequency (Hz)
Figure 7 .10-f
173
Figure 7.10-g
Figure 7.10-h
174
Figure 7.11: The variation o f dc conductivity of the wet fertilized soils for various
concentrations o f different fertilizers.
O
o
G a n d h in a g a r s a n d y lo am s o il + S O P S o lu tio n
O
G
CO
o
o
eg
o
o
O
O
o
o
DC Conductivity o
GO
O
O
o
0.1
02
0.3
04
0.5
06
% Solution of SOP
0
\............ .............................. “T------------------------------ “
0
0 05
I-----------------------------------1-------------------------------- “ I-------------------------------- “ I
0.1
0.15
0.2
0.25
% Solution of Zinc Chilate
Figure 7.11-b
175
DC Conductivity a
0.012 i
0.01
-
0.008
-
Palanpur district Sand + Solution of SOP
0.006
0.004
0.002
0
0.2
0.4
0.6
0.8
% Solution of SOP
n
«o
o
o
©
CM
-M
O
O
CM
o
o
d
DC Conductivity o'
00
o
o
Palanpur district Sand + Zinc Chelate Solution
o
0.05
0.1
015
0.2
o>
»Tj
t
% Solution of Zinc Chilet
176
Figure 7.12: The variation of dielectric constant and dielectric loss o f the wet
fertilized soils for various concentrations of different fertilizers at spot frequencies of
Dielectric Constant
0.5 GHz, 1.0 GHz and 1.5 GHz measured using VNA.
Frequency (Hz)
Figure 7.12-a
Frequency (Hz)
Figure 7.12-b
177
D electric Constant
Frequency (Hz)
Dielectric loss
Figure 7.12-c
Frequency (Hz)
Figure 7.12-d
178
Palanpur district Sand + SOP
Dielectric constant
55
35
5.00E+08
■e — Soil + 0.24 % Solution
— ■— Soil + 0 34 % Solution
• A- - • Soil + 0 50 % Solution
— K— Soil + 0.62 % Solution
1 00E+09
1 50E+09
Frequency (Hz)
Dielectric loss
Figure 7.12-e
Frequency (Hz)
Figure 7.12-f
179
Dielectric constant
Dielectric loss
Figure 7.12-g
Frequency (Hz)
Figure 7.12-h
180
REFERENCES:
1. Sengwa R. J., Soni A., Ram B., “Dielectric behaviour o f shale and calcareous
sandstone o f Jodhpur region”, Indian J. Radio and Space Physics, 33 October
(2004) 329.
2. Yadav V., Anil Kumar, Sharan S, & Sinha A. K., Yadav M., Gupta V. K. &
Jangid R. A., “Measurement o f Dielectric Behavior o f Fertilized Soil at
Microwave Frequency”, Journal o f Agricultural Science, 1/2 December
(2009) 42.
3. Shaikh A. A., Nawarkhele V. V., “Dielectric Study o f Black soil with Organic
and Inorganic Matter at Microwave Frequency”, Frontiers o f Microwaves and
Optoelectronics Proceedings o f International Conference on Microwaves and
Optoelectronics, Anamaya Publishers, New Delhi, India, (200S) 879.
4. Eller H., Denoth A., “A capacitive soil moisture sensor”, Journal o f
Hydrology, 185 (1996) 137.
5. Levitskaya T. M., Sternberg B. K., “Laboratory measurement o f material
electrical properties: Extending the application o f lumped-circuit equivalent
models to 1 GHz”, Radio Science, 35/2 March-April (2000) 371.
6. Arulanandan Kandiah and Mitchell J. K., “Low frequency dielectric dispersion
o f clay-water-electrolyte systems”, Clays and Clay Minerals, 16 (1968) 337.
7. Sengwa R. J., Soni A., “Dielectric properties o f some minerals o f western
Rajasthan”, Indian Journal o f Radio % Space Physics, 37 February (2008) 57.
8. Sternberg B. K., Levitskaya T. M., “Electrical parameters o f soils in the
frequency range from 1 kHz to 1 GHz, using lumped-circuit methods”, Radio
Science, 36/4 July-August (2001) 709.
9. Sengwa R. J., Sankhla S. & Choudhary S., “Dielectric characterization of
solution intercalation and melt intercalation poly (vinyl alcohol) - poly (vinyl
pyrrolidone) blend-montmorillonite clay nanocomposite films”, Indian
Journal o f Pure & Applied Physics, 48 March (2010) 196.
10. Sengwa R. J., and Soni A., “Low-frequency dielectric dispersion and
microwave dielectric properties o f dry and water-saturated limestones of
Jodhpur region”, Geophysics, 71/5, September - October (2006) G269.
11. Hill N. E., Vaughan W. E., Price A. H. & Davies M., “Dielectric Properties
and molecular behavior”, (Van Nostrand-Reinhold, London) (1969).
181
CHAPTER VIII
SUMMARY
Systematic experimental study o f dielectric properties o f soils, as a function of texture
structure, moisture content, salinity, and fertilizer content at various frequencies have
been carried out. The texture analysis has shown that the soils o f Gujarat undertaken
for observation were sandy, sandy loam, silt loam, and silty clay loam soils. The bulk
density o f the dry soils is found to vary from 1.062 for Valsad district silty clay loam
to 1.7 for Somnath sea bed sand.
Various methods are available for the estimation of complex dielectric constant o f the
soil. Depending on the possible range o f variation o f the dielectric constant e ' and the
dielectric loss e ” o f the dry and wet soils, and availability o f the instrument, the twopoint method o f measurement involving the solution o f complex transcendental
equation (Sucher & Fox) have been used at 9.5 GHz and 5.65 GHz microwave
frequencies. The soil samples were oven dried and then distilled water was added in
the soil. Time period o f 24 hrs was allowed for the water to distribute evenly in the
soil, saturate the soil and was allowed to facilitate internal drainage. As the days went
on, the moisture content in the soil decreased and the measurement o f complex
dielectric constant o f the soil samples for various moisture content were carried out.
The gravimetric moisture content as weight percent of the soil sample was found and
then the volumetric moisture content in the soil sample was calculated by multiplying
it with the bulk density o f the dry soil sample.
It has been observed that the dielectric constant o f dry soil remains constant at
frequencies o f 5.65 GHz and 9.5 GHz. The dielectric constant e ' and the dielectric
loss e ” o f the soils is found to increase with increase in moisture content in the soil.
The dielectric constant o f soil is found to increase slowly initially up to transition
moisture, after which it increases rapidly. The dielectric loss increases slowly and
linearly with increase in moisture content in the soil. Further it has been observed that
the variation o f dielectric constant and dielectric loss o f wet soils is dependent on the
182
texture structure of the soils and frequency of measurement. The dielectric constant e '
of sandy soils is found to be higher than that o f high clay content soils at higher
moisture contents.
The measurement o f complex permittivity were carried out in the frequency range 30
MHz to 3 GHz using Vector Network Analyzer, the de-embedded technique
developed by Yan-Zhen Wei and S. Sridhar were used. A semi-rigid coaxial probe
with a central tip and surrounding flange has been designed and connected to Vector
Network Analyzer. The measured values o f complex dielectric constant o f dry and
wet soils in the frequency range from 30 MHz and 1.23 GHz has shown that the
dielectric constant and dielectric loss of given dry soil remains constant in the given
frequency range. The dielectric constant o f wet soil increases with increase in
moisture content in the soil but at given moisture content the dielectric constant of
given soil remains unaltered with variation in frequency. Further it has been observed
that at given moisture content in the soil the dielectric loss of the soil decreases with
increase in frequency from 30 MHz to 1.23 GHz. The dielectric constant s ' o f sandy
soils at spot frequency o f 1 GHz is found to be higher than that o f high clay content
soils at higher moisture contents. The dielectric loss e ” o f sandy soils at spot
frequency o f 1 GHz is found to be lower than that of high clay content soils at higher
moisture contents.
The emissivity o f the soils calculated at 9.5 GHz for various moisture contents show
that at given moisture content in the soil the emissivity o f sandy soils is lower than
that of high clay content soils.
The measured values o f complex dielectric constant ( e
e ”) o f the soils for various
moisture contents in the soils are compared with the values calculated using the Wang
and Schmugge model, and the M. T. Hallikainen et al. model at X- and C-band
microwave frequencies of 9.5 GHz and 5.65 GHz, respectively. The measured values
of complex dielectric constant (e
e ”) of the soils for various moisture contents in
the soils agree well with the values calculated using the Wang and Schmugge model,
and the M. T. Hallikainen et al. model.
183
The measurements o f the dielectric constant e ’ and dielectric loss e ” o f the sandy
loam soil for various moisture contents and for salinity levels of 10000 ppm and
30000 ppm for the frequency range from 100 MHz to 1.6 GHz using VNA have
shown that the dielectric constant and dielectric loss of the dryer soil does not change
appreciably with the variation in frequency. The dielectric constant and dielectric loss
of the soil increases with increase in moisture content in the soil, irrespective of the
salinity o f water or the frequency of measurement. Further at given moisture content
in the soil irrespective of salinity, the dielectric constant o f the soil remains almost
constant above 300 MHz. But in the frequency range from 100 MHz to 300 MHz the
dielectric constant increases with decrease in the frequency. The value o f dielectric
loss increases rapidly with increase in salinity level in the soil particularly in this
frequency band. The dielectric loss decreases with increase in frequency from 100
MHz to 1.6 GHz, irrespective o f salinity level for all moisture contents. The rate of
decrease in the dielectric loss with increase in frequency reduces as the salinity in the
soil increases and moisture content in the soil decreases. The measured values o f the
dielectric constant and dielectric loss of the sandy loam soil o f Gandhinagar district at
C-band microwave frequency o f 5.65 GHz for various volumetric moisture contents
(in cm3/cm3) of distilled water, as well as for the water solutions o f 10,000 ppm and
30,000 ppm salinity show that the dielectric constant and dielectric loss o f the soil
increases with increase in moisture content in the soil. Further it is seen that the
salinity has not much effect on the dielectric constant o f the soil, but the dielectric loss
increases with increase in salinity.
The emissivity values o f the Gandhinager sandy loam soil for normal incidence
calculated from the measured values of the dielectric constant e ’ and the dielectric
loss e ” o f the soil at 0.21 GHz, 0.5 GHz, 1.01 GHz, 1.4 GHz and at 5.65 GHz show
that for all frequencies emissivity decreases with increase in moisture content in the
soil. Further at given moisture content the emissivity o f the soil decreases with
increase in the salinity of the water. For dryer soil the effect o f salinity can not be
distinguished from the emissivity o f the soil. Also, the decrease in emissivity with
increase in salinity o f water at given higher moisture content is more pronounced at
lower frequency band.
184
The measured values of dielectric constant e ’ and dielectric loss e ” o f sandy laom
soil of Gandhinagar district for various moisture contents, in the frequency range from
10 kHz to 2 MHz have shown that there is a large enhancement in the values o f e ’
and e ” with moisture content, at the lower frequency end. This large enhancement in
the values o f e ’ and e ” may be associated with the electro chemical polarization
which arises due to increase in surface charge carrier density in presence o f water
molecules in the pore spaces of the soil. The increase in the value of tan 8 with
increase in moisture content, and shift o f tan 8 towards lower frequency side with
increase in moisture content in the soil suggests the change in size o f the orienting
ions in the presence o f pore wafer in the samples. The increase in the value of
conductivity or’ of wet soil has shown that the conductivity o f soil-water matrix
increases with increase in moisture content in the soil.
The dielectric constants ’ and dielectric losse ” of the wet fertilized soils(Sandy loam
soil o f Gandhinagar district, and Sandy soil o f Palanpur district) is found to decrease
with increase in frequency from 10 kHz to 2 MHz. The values of e 5 a n d e ” for the
wet fertilized soil increases with increase in % concentration o f fertilizer [Sulphet of
Potash (SOP), and Zinc Chelate] content in the soil. This may be associated with the
added organic and inorganic matter which forms a chemical composition of low
concentration along with the chemicals present in the soil. According to the theory of
Electrolyte, in the limit of low concentration the dependence o fe ’ is approximately
linear. Fertilizer increases the water holding capacity of soil which in turn increases
the dielectric constant o f the soil. Conduction arising due to the actual charge
transport (ionic conduction in electrolytes) adds an additional term to the dielectric
lo sse ” which increases the dielectric loss with increase in fertilizer content in the soil.
The real conductivity o ’ o f the soils is found to increase with increase in %
concentration o f fertilizer in the soil, as well as with increase in frequency of
measurement from 10 kHz to 2 MHz. The imaginary conductivity cr” o f the soils is
found to decrease with increase in frequency up to certain value o f cr”mimmum after
which, it increases with increase in frequency. Further it has been observed that Zinc
Chelate increases the dc conductivity of the soil more rapidly in comparison with
SOP.
185
From the measurements o f dielectric permittivity in the frequency range 0.5 to 1.5
GHz using VNA it has been observed that, for a given fertilizer content in the soil the
dielectric constant e ’ and dielectric loss e ” of the soils decrease with increase in
frequency. Further, at given frequency o f measurement the dielectric constant e ’ and
dielectric loss e ” o f the soils is found to increase with increase in fertilizer content in
the soil.
FUTURE PLAN:
In the thesis the attempt is made to study the effect of moisture content, salinity and
fertilizer content in the soil at various frequencies. The measurements for the
dielectric properties o f some of the high clay content soils for various moisture
contents o f distilled water and saline water will be carried out at radio and microwave
frequencies, the data will be useful for microwave remote sensing technique
applications. Microwave dielectric constant of fertilized soil which has been carried
out up to 1.5 GHz, will be extended to higher microwave frequencies.
An empirical model for the estimation of complex permittivity o f wet soil, moist
saline soil at different microwave frequencies using soil characteristics will be
developed. The developed empirical model will be tested with the help o f present data
and the data which will be collected in near future.
186
APPENDIX
Publications:
1. Vyas A.D. & Gadani D.H., “Study of Complex Permittivity of Soils
of Gujarat at Microwave Frequencies”, Physical Methods o f Soil
Characterization, Edited by Prof. Behari J., Narosa Publishing
House,New Delhi (2001) 129. Oral presentation.
2 . Vyas A.D. & Gadani D.H., “Study of Emissivity of Soil of Gujarat at
Microwave Frequencies”, Proceedings National Conference on
Applications o f Radio Techniques for Remote Sensing, ICRS, Jodhpur
(2002) 1-2. Oral presentation.
3 . Vyas A.D. & Gadani D.H., “Dielectric Properties of Dry and Wet
Soils
at
Microwave
Frequencies”,
Microwave
Measurement
Techniques and Applications, Edited by Prof. J.Behari, Anamaya
Publishers, New Delhi, India (2003) 80. Oral presentation.
4 . Gadani D.H. and Vyas A.D., “Measurement of Permittivity of Soils at
X-band Microwave Frequency”, Proceedings National Conference on
Microwaves, Antenna, Propagation and Remote Sensing, ICRS,
Jodhpur (2002) VIH-5. Oral presentation.
5 . Gadani D.H. and Vyas A.D., “Complex Permittivity of Soil at C- Mid
X-band Microwave Frequencies”, Microwaves and Optoelectronics,
Edited by M.D. Shirsat, Nawarkhele, Raju G.S. & Khirade, Anamaya
Publishers, New Delhi, India (2004) 266. Oral presentation.
6. Vyas A.D. & Gadani D.H., “Microwave Dielectric Constant of
Soils”, Topics in Electromagnetic Waves, Devices, Effects and
Applications, Edited by Prof. J.Behari, Anamaya Publishers, New
Delhi, India (2005) 120.
7. Gadani D. H., Rana V. A., Bhatnagar S. P. and Vyas A. D.,
“Measurement of complex dielectric constant of dry and wet soil
using
VNA”
Proceedings
National
Conference
on
Recent
Advancements in Microwave Technique & Applications, Jaipur.
October-(2006) 320. Oral presentation.
8. Gadani D. H. and Vyas A. D., Measurement of Complex Dielectric
constant of Soils of Gujarat at X and C-band Microwave Frequencies,
Indian J. Radio & Space Physics, 37 (2008) 221.
Seminars and Conferences attended:1. National Seminar on “Soil Studies and Characterization by Physical
Methods”,
PHYSOIL-2000,
3-4,
February,
2000,
School
of
Environmental Sciences, Jawaharlal Nehru University, New Delhi.
(Oral Presentation).
2. National conference on “Applications of Radio Techniques for
Remote Sensing”, 23-24, November-2001, International Center for
Radio Science, Jodhpur, Rajasthan. (Oral Presentation).
3. National Workshop cum Symposium on “Microwave Measurement,
Techniques and Applications”, 4-6, February -2002, School of
Environmental Sciences, Jawaharlal Nehru University, New Delhi.
(Oral Presentation).
4. National Conference on “Microwaves, Antenna, Propagation and
Remote Sensing”, 19-20, December-2002, International Center for
Radio Science, Jodhpur, Rajasthan. (Oral Presentation.).
5. National Conference on “Microwaves and Optoelectronics (NCMO2004)”, 29-30, June-2004, Department of Physics, Dr. Babasaheb
Ambedkar Marathwada University, Aurangabad ,Maharastra. (Oral
Presentation.).
6. National Conference on “Recent Advancements in Microwave
Technique & Applications (Microwave-2006)”, 6-8 October, 2006,
Department of Physics, University of Rajasthan, Jaipur. (Oral
Presentation.).
7. State level One day seminar on “Microwave and its Applications”,
Microwave-2007,21st January, 2007. (Oral Presentation.).
Workshop Attended:
UGC
Sponsored
Regional
Workshop
on
SATELLITE
DATA
PROCESSING (Remote Sensing Applications), 14-15, March, 2005,
Organized by Post Graduate Department of Physics, J. E. S. College,
Jalna and Supported by Education and Training Group of National
Remote Sensing Agency, Department of Space, Hydrabad.
Physical Methods o f Soil Characterization
Edited by J. Behari
Copyright © 2001, Narosa Publishing House, N ew Delhi, India
13. Study of Complex Permittivity of Soils of
Gujarat at Microwave Frequencies
A.D. Vyas and D.H. Gadani
Department o f Physics, University School o f Sciences, Gujarat University
Ahmedabad-380009, India
Abstract
Dielectric constant and dielectric loss o f sand with various moisture contents has
been measured at microwave frequency. The observed values have been compared
with the values calculated by empirical m odels. It is found that measured values o f
real part of dielectric constant (s') are in good agreement with those calculated by
empirical models.
Introduction
R em ote sensing of soils at microwave frequencies is carried out either by active
technique (radar) which measures the backscatter coefficient or passive technique
(radiometer) which measures the emissivity. T he backscatter coefficient (Ulaby
et al., 1986) and the emissivity (Wang, 1987; Wang et al., 1982; Wang and
Choudhary, 1981) both depend on the complex permittivity which in turn depends
upon moisture content o f soil, soil texture and frequency of measurement. Thus
measurement of complex permittivity of soils at microwave frequency has gained
considerable interest. A comprehensive study o f dielectric properties o f different
soils o f Gujarat state for various moisture contents at microwave frequencies has
been undertaken. To start with, the complex permittivity of sand with varying
m oisture contents has been measured at X -band microwave frequency and the
results are reported in this paper.
Materials and Methods
T he sample of soil was collected from the bed of the Sabarmati river (neap
Usmanpura, Ahmedabad City). The soil was passed through a sieve o f mesh No.
50 and then collected in a metallic tray. T he soil was analysed, for its texture,
structure and its constituents are shown in Table 1.
T h e wilting point and transition moisture o f soil in terms of volumetric water
content (cm3/cm3) are calculated by using die Wang and Schmugge model (1980), as
WP = 0.06774 - 0.00064 x Sand + 0.00478 x Clay
w here sand and clay are the sand and clay contents in percent o f dry weight of
the soil.
130
V yas and G adani
Table 1 Composition and physical parameters of soil
Sand
Silt
Clay
Density
Wilting point (WP)
Transition moisture (Wt)
93%
6 .2 %
0.8 %
1.52 gm/cc
0.012 cnrVcm3'
0.171 cm3/cm3
ps
The transition mositure is calculated as
Wt = 0.49 WP + 0.165
Two point method is used for the measurement of complex permittivity because
it is suitable for the low and medium loss dielectrics (Sucher apd J. Fox, 1963)
and used by several workers (Vyas, 1982, Ghosh et al., 1998). The experimental
set up for the measurement is shown in Fig. 1.
Fig. 1 Schematic diagram of the X-band set up used for measuring e' and e"
With no sample, dielectric in the short circuited line, the position of the first
minimum DR in the slotted line was measured. At the same time the distance
between successive minima in the slotted line was also measured to calculate the
guide wavelength, Ag. The soil sample of length lEwith certain moisture content
was introduced in the waveguide, such that one end of the sample touched the
short circuit. With the sample in the wave guide the position o f the minimum D
in the slotted line and the VSWR r were measured. Similar measurements were
made for another sample of length l's having the same moisture content. The
complex trancedental equation
tanh (J Z l)
TZi
was solved for the two samples to obtain the conductance Ge and susceptanee
SE. The permittivity e' and dielectric loss e" were then calculated using the
relations:
c >_ GE + (Ag/2 a )2
£ “
1 + (Ag/2 a )2
__________________ ■ S ' e
1 + (A g/2 a )2
where a is the width of the wave guide.
Study o f Complex Permittivity of Soils o f Gujarat at MF
131
The measured values of the permittivity e' and the dielectric loss £ for sand
have been plotted against volumetric moisture content Wv as shown m Fig. 2.
20
01
0.2
03
Volumetric moisture (Wv)
Fig. 2
Plot o f e' and € versus volumetric soil moisture content Wv.
Series 1 = J.R, Wang’s Model
Senes 2 = M.T. Hallikainen’s Model
Series 3 = Experimental
Results and Discussion
The solid lines in the graph (Fig. 2) represent the curves o f experimental values
of s' and € ’ against Wv.The values of € and £" both increase with increase in the
moisture content. The increase in € is rapid compared to that in £" with moisture
content. The permittivity C increases slowly up to 10% moisture content, and
thereafter it increases rapidly. The initial slow increase in the dielectric constant
up to 10% moisture content may be due to less number of free water molecules.
At higher moisture contents the number of free water molecules in the soil water
mixture increases. The free water molecules have higher dielectric constant
compared to bound water molecules. Hence the dielectric constant increases
rapidly. The sand has a small surface area per unit volume. Hence it is a capillary
system with less volume of capillary pores and with a large volume of noncapillary pore spaces which ensures good drainage and aeration and so has low
water holding capacity.
132
V yas
and
G adani
The measured values of the com plex permittivity are compared (Fig. 2) with
the values obtained using the tw o empirical m odels based on soil texture
(Hallikainen et al, 1985, and Wang et al., 1980). The graph shows that the real
part of the dielectric constant (t) is in good agreement with the values calculated
by empirical models. The loss factor (£") is also in good agreement with the
values calculated by empirical m odels up to about 15% moisture content, after
which the measured values are lower than the calculated values. Triis discrepancy
may be due to several reasons, i.e., chemical composition o f soil, soil temperature
and the experimental method used for measurement o f complex permittivity of
soils.
Acknowledgement
Authors are thankful to Prof. V.B. Gohel, Head, Department of Physics, for
providing constant encouragement and laboratory facilities.
References
1. Ghosh Anirbid, Behari Jitendra and Pyne Suprio, 1998, Dielectric parameters of
dry and wet soils at 14.89 GHz, Ind. J. Radio and Space Physics, Vol. 27. 130—
134.
2. Hallikainen M.T., Ulaby F.T., Dobson M.C., EL Rayes M.A., and Lin-Kun WU
1985, Mircowave Dielectric behaviour of wet soil Part-I: Empirical models and
Experimental Observation, IEEE Transactions on Geoscience and Remote Sensing,
Vol. GE - 23, 1. 25-35.
3. Sucher M. and Fox, J. 1963, Handbook of microwave measurements.
4. Ulaby F.T., Moore R.K. and Fung A.K., 1986. Microwave remote sensing active
and passive, Vol. 3 (Addison Wesley).
5. Vyas A.D., Complex Permittivity of Sand and Sandy Loam Soils at Microwave
Freqnency, Ind. J. Radio and Space Physics, II, 169-170.
6. Wang, J.R., 1987, Microwave emission from smooth bare Fields and soil moisture
sampling depth. IEEE., Transactions on Geoscience and Remote Sensing, 25,
616-622.
7. Wang. J.R., McMurtrey, J.E., Engman, E.T., Jackson, T.J., Schmugge, T.J., Goule,
W.I., Fuch, J.E., and Glazier, W., 1982, Readiometric measurements over bare
and vegetated Fields at 1.4 GHz and 5 GHz frequencies. Remote Sensing o f
Environment, 12, 295-311.
8. Wang. J.R., and Choudhary, B.J., 1981, Remote sensing of soil moisture over
bare Fields at 1.4 GHz frequency. J. Geophysical Research, 86, 5277-5282.
9. Wang J.R., and Schmugge T.J., 1980, An empirical model for the complelx
dielectric permidvity of soils as a function of water content, IEEE Transactions
on Geoscience and Remote sensing, Vol. GE- 18, No. 4, 288-295.
NATIONAL CONFERENCE
ON
Applications o f
Radio Techniques for R em ote Sensing
2 3 - 2 4 November 2001.
Organized
By
International Centre for
Radio Science
PROCEEDINGS
•OM -NIW AS" A -23, Shastri Nagar,
Jodhpur 342003
Study Of Emissivity Of Soil Of Gujarat At Microwave Frequencies
A. D. Vyas and D.H. Gadani
Department of Physics, University School of Sciences
G ujarat University, Ahmedabad-380 009
Abstract
Emissivity o f wet and dry soils have been estimated using measured values of dielectric
constant e ’ and dielectric loss s ” at an X-band microwave frequency .The emissivity o f
soils (dry and wet) was also calculated using dielectric parameters obtained by M.T
Hallikainen et al model [3] It is found that emissivity o f soils depend upon the moisture
content in the soil and soil texture. The emissivity o f the soil (dry and wet) calculated by
both the methods are in agreement
Introduction
Emissivity o f soil depends upon its
dielectric properties, which in
turn
depends upon its wetness, surface
roughness, soil textural composition,
physical
temperature
and
sensor
parameter (frequency, polarization and
incidence angle). The measured values of
emissivity therefore can be used to
estimate moisture content Observations,
usually done at satellite platform provide
soil moisture estimates over large areas.
For the proper selection o f sensor
parameters and to interpret remotely
sensed data, it is essential to study
interaction o f microwaves with soils (dry
and wet)
Tablel.
Composition and physical
parameter-of soils
Location
(Region)
Sabarmati
River (1)
(Ahmedabad)
Gandhinagar
Dist. (11)
Soil texture {%)
Sand
Slit
93
6.2
0.8
65
31
4
To provide database for dielectric
properties of different kinds of soils, we
undertook a programme to study
dielectric properties o f soils o f Gujrat
state. In this paper we present result o f
dielectric measurement on two types o f
soils at X-band microwave frequency
The dielectric constants o f these soils
were also calculated by empirical model
and compared with experimental values.
The dielectric constants o f soils are used
to calculate their emissivities.
Material and Method
Soil samples collected from different
regions o f Gujarat state were analyzed
for their textural composition. The
wilting point and transition moisture
Soil type
Wilting point
(WP)
cm3/ cm3
Transition
moisture (Wt) ,
cm3 /cm3
Sand
0.012
0.1708
Sandy
loam
0.045
0.1872
Clay
1-2
where, GE is the conductance and
is
the susceptance; lg is the guide wave
length and 'a' is the wider dimension of the
wave guide.
of these soils were also calculated
using Wang and Schmugge model [ 2 \
According to this model, WP and
transition moisture are given by the
relations
WP = 0.06774 - 0.00064 * Sand +
0.00478*Clay.
... (1)
Em pirical model
Several empirical models have been
developed to estimate soil dielectric
constants These models include Wang
and Schmugge model [2], and Hallikainen
Wt = 0.49*WP + 0.165
...(2 )
The textjral composition along with WP,
and transition moisture of soils are given
in table(l).
The dielectric constant e ’ and dielectric
loss r.” of soil (i) sand and soil (I I )
sandy loam were measured at an X-band
microwave frequency using the two point
method (Sucher and J.Fox [7]). The
experimental set up used for the
measurement is shown in figure (1).
The dielectric constant e ’ and dielectric
loss e” were calculated with the help o f
following expressions,
et al model [3],.The dielectric constant e ’
and dielectric loss s ” calculated by these
models
match
fairly
well
with
experimental values. In the present
investigation dielectric constant and
dielectric loss o f soils were calculated
with Hallikanen et al. model [3],
According to this model the dielectric
constant and dielectric
loss can be
calculated by the polynomial expression.
e = (ao+ai*S+a2*C)+(b0+bi*S+b 2*
C)nVv+(co+c,*S+c2*C)*Wv2
...(5)
Where S and C are respectively sand and
clay contents in percent of dry weight of
soil. For the given frequency
the
constants a;, b, and c, (i=0, 1,2) are taken
from the table given by Hallikainen et al.
model [3] The estimated values o f e ’ and
e” for,
2
I
Gunn
Power
VSW R
2a J
Meter
Supply
Gunn
Oscillator
PIN
Modulator
Attenuator
Frequency
Meter
Slotted
Selection
Wave- Dielectric
Guide Sample
Band Holder
Fig (1). Schematic diagram of tire X-band set up used for measurement of e 7and e ’ ’.
1-3
moisture contents and for
different types of soils are presented in
the figure(2)
d if f e r e n t
'
F ig .(3 ).E x p erim en tal v a u es o f c ' and e "
versus volum etric m o istu re c o n ten t Wv
Fif (2Mj>virical v'a llJCS °tdielectric constant
c and dielectric loss e " plated against
volurrclric moisture content Wv
G a n d h in a g a r dtsL soil
workers
(Lundien,
Wang,
HaJlikainen et al. J.Behari, Caila etc
[l]-[6]) The dielectric constant of
---------- S a b a rm a ti l i v e soil
♦ Sabarmati river sand
a Gandhinagar dist. sandy loam
Results and Discussion
The measured values of the dielectric
constant e’ and dielectric lqss e ” , for
sand and sandy loam soil were plotted
against volumetric soil moisture content.
The plots are shown in figure (3).It is
evident from figure (3) that for both the
soils there is slow increase in dielectric
constant e ’ with moisture content up to
10% after which it increases rapidly with
moisture content. This is due to the fact
that for small moisture content there is '
more bound water in comparison to free
water and the dielectric constant of free
water is higher in comparison to that for
bound water After this concentration the
dielectric constant increases rapidly
because now there are more free water
molecules in comparison to bound water.
The results are in agreement with our
earlier results [8] and several other
1-4
sand is higher than that o f sandy
loam at all moisture contents . The
sandy loam, which contains less than
8% o f silt and clay, is a simple
capillary system with a large volume
o f non-capillary pore spaces, which
ensure good drainage and aeration.
Loam soils on the other hand contain
35% silt and clay, and thus have
smaller non-capillary pore spaces
and do not have good drainage and
aeration and have higher water
holding capacity than sand. This
behavior has been reflected in their
dielectric properties. The calculated
values of dielectric constant of soil
from different places and having
different textural composition have
shown similar results (figure 2)
A
comparison
of experimentally
measured values of e’ and e ” and those
calculated from empirical model [3] for
sand and sandy loam is given in figure
(4). It is clear from the figure that the
calculated values of dielectric constant
matches fairly well with the e' for sand
Fig (5).Comparision o f cmpcrical and
experimental values o f errrissivity of
bolh soils
1
t
0 .8
0.6
0.4
F ig .( 4 ) . C o m p a r is o n o f e m p ir ic a l a n d
m e a s u r e d v a lu e s o f c a n d c " v e r s u s V W
0 .2
-
0
0.1
0
0.3
0.2
Wv — ►-------- Sabarm ali river empirical
■ - - G andhinagar dist. empirical
a
Sabarmati river experim ental
X
G andhinagar dist. experim ental
--------- Sabarmali river sand model
A
Sabarmali river sand experimental values
------ Gandhinagar disl. Sandy loam model
o
Gandhinagar disc Sandy loam
experimental values
and sandy loam soils experimental
values. The dielectric loss e" is in
agreement, with experimental values for
both the soils for low moisture contents
up to 15%,after which calculated values
are higher than the observed values. This
needs more attention, in microwave
measurements, where wave guide is used
as a sample holder, there is a possibility
of water leakage from wave guide at
higher moisture content. Further at low
water concentration the system can be
treated as medium lossy material and at
higher water concentration it is
considered as a high loss material where
the
accuracy
of
measurement
detoriate.
The
discrepancy
in
experimental and empirical values of
s ’and e” may also be due to some
assumptions in the model [3],
The errrissivity of sand and sandy loam
are calculated from the measured
values of e’and e” using the equation
1-Ve
2
... ( 6 )
e =11+Ve
The calculated values o f emissivity
from the estimated values of (e’,e” )
using empirical model are shown in
figure (5) along with values of
emissivity
calculated
from
the
observed values of ( £ \ e ” ). The values
o f emissivity calculated by both
procedures are in good agreement.
The figure shows that emissivity of
sand and sandy loam decrease with
increase in moisture content. The
4 Anirbid Ghosh, Jitendra Bihari and
Suprio Pyne, “Dielectric
parameter
' of dry and wet soil at 14 89 GFIz,”
Indian J Radio & Space Physics,
Vol 27,pp 130-134, June 1998.
5 0 P N Calla, M.C Borah , RMishra,
P Vashishtha, A. Bhattarcharya &
S P Purohit “Study of the properties
of dry and wet loamy sand soil at
microwave frequencies”, Indian J.
Radio & Space Phys , Vol.28, pp.109112, June 1999
6 Vyas A D , “Complex Permittivity o f
Sand & Sandy
Loam
Soils at
Microwave Frequency,” Indian J.
Radio & Space Physics, Vol. II. pp.
169-170 August 1982,
7 Sucher M.and J. Fox, 1963, Handbook
of microwave measurements.
8 A-D.Vyas and D.H. Gadani, Study o f
Complex Permittivity of soils of
Gujrat at Microwave Frequencies,
’’Physical
Methods
of
Soil
Characterization, Edited by P ro f J.
Behari. pp 129-132,2001.
decrease in ermssivity with moisture
content is due to increase in permittivity
which causes a total increase in reflected
energy and there by decrease in emitted
energy
Acknowledgement
The
authors are thankful to Prof
V.B Gohel,
Head
Department of
Physics, Gujrat University, Ahmedabad,
for providing laboratory facilities and
constant encouragement
References
1. JR . Lundin “Terrain analysis by
electromagnetic means,” U.S Army
Engineer
Waterways
Station,
Vicksburg, MS, Tech. R ep , pp.3-727,
Feb. 1971
2. J R Wang., and T.J Schmugge "An
empirical model for the complex
dielectric permittivity of soil as a
function of water content,” IEEE
Transaction on Geosciences and
Remote Sensing, Vol. GE-I8, No.4,
' pp. 288-295.0ctober 1980.
3. M.T. Hallikainen, F.T. Ulaby, M.C.
Dobson, M.A. El-Rays, and LinKun
Wu,
“Microwave
Dielectric
Behaviour of wet Soil-part T
Empirical Models and Experimental
Observations,” EEEE Trans. Geosci.
Remote
Sensing,
vol.
GE23, N o.pp.25-33, January 1985.
1-6
Reprint
Microwave Measurement
Techniques and
Applications
Editor
Jitendra Behari
Anamaya Publishers
New Delhi
Microwave Measurement Techniques and Applications
Edited by J Behan
Copyright © 2003, Anamaya Publishers. New Delhi, India
12. Dielectric Properties of Dry and Wet
Soils at Microwave Frequencies
A.D. Vyas and D.H. Gadani
Department o f Physics, Gujarat Univetsity, Ahmedabad-380 009, India
Abstract The complex permittivity s' and e" with moisture content have been
measured for different types of soils at an X-band microwave frequency The
measured values o f s' and e" are com pared with the empirical values calculated
using Hallikainen et al model [2] The com plex permittivity data has been used
to estimate ermssivity of these soils with the moisture content.
1. Introduction
The soil has physical, chemical and electrical properties. The physical properties
o f the soil are its colour, texture, grain size etc The chemical properties are
the salinity and the alkaline or acidic nature o f the soil. The electrical properties
include the dielectric constant and conductivity. It has been observed by
several workers [ 1-5] that the variation o f dielectric constant o f the soil with
moisture content, at a given m icrowave frequency, is directly related to the
texture structure o f the soil. We studied [6] dielectric properties of sand
(Sabaramati river) at microwave frequency and found that its dielectric
constant increases with the moisture content in the soil. In continuation o f
this study we measured complex permittivity o f different types o f soil (collected
from different regions o f Gujarat state) having different moisture content at
an X-band m icrowave frequency and results are presented in this article. The
dielectric permittivity has been used to compute em issivity o f dry and wet
soils. These studies are useful in interpretation o f microwave remote sensing
data.
• .
2. Materials and Methods
The soil samples were collected from different regions o f Gujarat state. The
textural analysis o f the soils, wilting coefficient and transition moisture are
given in Table 1.
The wilting coefficient and transition moisture are calculated using the
Wang and Schm ugge model [7] as
WP = 0.06774 - 0.00064 x Sand + 0.00478 x Clay
(1)
Dielectric Properties o f Dry and Wet Sods ♦
81
Table 1. Textural composition and physical parameters of soils
Location
(Region)
Sand
Sabarmati River 93
(Ahmedabad)
Wilting
point (WP)
(cm 3/cm3)
Sod
type
Sod texture (%)
Transition
moisture (Wt)
(cra3/cm 3)
Silt
Clay
62
0.8
Sand
0.012
0 1708
Sandy
loam
0.045
0 1872
Gandhinagar
Dist I
65
31
4
Amreli Dist
11
78
11
Silt loam
0 118
0 2228
Valsad Dist.
7
62
31
Silty clay
loam
0.211
0 2686
Wt = 0.49 x WP + 0.165
(2)
The dielectric constant of the soil sample at the given moisture content
was calculated using the two-point method [8] at an X-band microwave
frequency. The block diagram for the experimental set up is shown in Fig. 1.
VSWR
meter
Gunn
oscillator
PIN
modulator
Attenuator
Frequency
meter
Slotted
section
Waveguide
band
Dielectric
sample
holder
Fig. 1. Schematic diagram of the X-band set up used for measurement of e' and e"
The complex transcendental equation
was S0}Ved for the
two soil samples of different length (at a given moisture content) to obtain the
conductance GE and susceptance SE.
The dielectric constant e' and dielectric loss e " were then calculated using
the relations [8]
G, +
e
2a
(3 )
-
1+
£
=
( \
1+
^2
(4 )
82
♦ V yas and G adani
where a is the width of the w ave guide
The empirical values of e' and e" for the soils having different texture are
calculated for various moisture contents at the X-band micro wave frequency
using the Halltkainen et al model [2].
£
—
(fig + iq
+
(C q 4-
X
C|
S 4" &2 X C) 4~
X
S
4-
Cj
X
O
X
X
S
4*
b 2 X C3) X \Yv
Wl)^
(5)
where S and C are, respectively, the sand and clay contents of a soil in percent
by weight, Wv is the volumetric moisture content in the soil sample and the
values o f a„ b„ ct (i - 1, 2, 3) are taken from [2].
The measured values o f s' and e" for the sand, sandy loam, silty loam and
silty clay loam soils at various moisture contents are plotted in Fig 2.
Fig. 2.
□
Sabarmati
A
Gandhinagar
o
Am reli
4-
Valsad
Variation of ef and €' of the soils with moisture content.
It is seen from Fig. 2 that the dielectric constant e' and dielectric loss e"
increase with increase in moisture content for all types o f the soil. The increase
in e' with moisture content is more compared to that in e". At the given
moisture content
^ Sand ^ ^ Sandy loam
^ Silty loam ^ ^ Silty clay loam
The dielectric loss for all the soils increases slow ly and linearly with the
increase in moisture content.
3. Results and Discussion
Figure 3 show s the comparison o f the measured values and empirical values
Dielectric Properties o f Dry and Wet Soils
♦
83
of s ' and e" at various moisture contents for the different type of soils It is
evident from Fig. 3 that the measured and em pirical values of real part of
complex permittivity s ' is in good agreement with empirical values for different
moisture content, but, the imaginary part s" is in good agreement with empirical
values up to 15% moisture content (up to transition moisture), after which
empirical values are higher than the measured values. This feature has been
found in all types of soils.
O Sabarmati
A Gandhinagar
X Amreli
--------- Sabarmati model
--------- Gandhinagar model
--------- Amreh model
+ Valsad
--------- Valsad model
Fig. 3. Comparison of experimentally measured values and empirically calculated
values of s' and e" with volumetric moisture content of the soils.
The emissivtty of the soils from the measured values of complex permittivity
for normal incidence can be calculated from the relation [9]
2
e -
\
-
-
T
(6)
b
1 + Ve
Figure 4 shows the variation of emissivity o f the soils calculated from
measured values of complex permittivity using equation (6), with the moisture
content. It is seen that the emissivity of dry soils is less than 1 and decreases
with increase in moisture content for all the soils.
At lower moisture content there are more bound water molecules in the
soils compared to free w ater molecules. Hence the dielectric constant is low.
As moisture content increases there are more free water molecules available
in the soils, which increases the dielectric constant s'. At a particular moisture
content
^ Sand ^ ^ Sandy loam ^ & Silty loam
& Silty clay loam
84
♦ V yas and G adavi
| — Aroreli
Fig. 4.
o Gandhinagar
+ Sabarmati
x Valsad
Variation of emissivity of the soils with volumetric moisture content.
T h is shows that the real p art of the dielectric constant e' depends on the sat
co n ten t of the soil. S in ce the sand has a low water holding capacity,
m oisture content increases m ore free water m olecules are available m the sc
having higher sand con tent. Hence the e ' for the soil having h ig h er sai
content is higher.
Acknowledgement
A uthors are thankful to Prof. V.B. Gohel, H ead, Department o f P hysics, |
constant encouragem ent and for providing laboratory facilities.
References
1. Lundien, J R., "Terrain analysis by electromagnetic means,” US. A rmy Engirt
Waterways Station, Vicksburg, MS, Tech.Rep., pp 3-727, February 1971.
2. Halhkainen, M.T., Ulaby, F.T., Dobson, M.C., El-Rays, M.A., and Lin-K
Wu, “Microwave Dielectric Behaviour o f wet Soil-part P Empirical Mod
and Experimental Observations,” IEEE Trans Geosci. Remote Sensing >
GE-23, No. 1, pp. 25-33, January 1985.
3. Ghosh, Amrbid, Behari, Jitendra and Pyne, Supno, “Dielectric parameter:
dry and wet soils at 14.89 GHz,” Indian J. Radio and Space Physics, Vol.
pp. 130-134,June 1998.
4. Calla, O.PN., Borah, M.C., Vashishtha, E, Mishra, R., Bhattacharya, A .;
Purohit, S.P., “Study of the properties o f dry and wet loamy sand soi
microwave frequencies," Indian J R a d i o and Space Physics, Vol.
pp. 109-112, June 1999-
Dielectric P>operties o f D iy a n d Wet Soils ♦
85
5 Vyas, A D , “Complex Permittivity of Sand and Sandy Loam Soils at Microwave
Frequency,” Indian J Radio and Space Physics, Vol II. pp. 169-170, August
1982,
6 Vyas. A D and Gadam, D H , “Study of Complex Permittivity of soils of
Gujarat at Microwave Frequencies,” Physical Methods of Sod Characterization,
Edited by Prof J. Behan, pp 129-132, 2001
7 Wang, J R and Schmugge, T J , “An empirical model for the complex dielectric
permitivity of soils as a function of water contents,” IEEE Transactions on
Geoscience and Remote Sensing, Vol. GE- 18, No 4, pp 288-295 October
1980
8 Sucher. M and Fox J , 1963, Handbook of mictowave measurements
9 Ho, W and Hall, H F, J. Geophys Res (USA), 78, pp 603, 1973.
NATIONAL CONFERENCE
ON
Microwaves, Antenna, Propagation &Remote Sensing
19 - 21 December 2002
Organized
By
International Centre for Radio Science
PROCEEDINGS
- P -
H
-
"OM - NIWAS” A - 23, Shastri Nagar, Jodhpur 342003
M easurement o f Permittivity o f Soils at X-band M icro w a v e F r e q u e n c y
D.H. G a d a n i and A.D. Vyas*
Departm ent of Physics, G ujrat U niversity,
•
Ahrnedabad
permittivity o f the soils with difterent
moisture content collected from Palanpur
Dist., and the results are presented in this
paper. The data of our earlier studies have
been included for comparison. The complex
permittivity o f soils with different moisture
content has
been calculated
using
M.T.Hallikainen model10. These evaluated
values of complex permittivity have been
compared w ith the observed values for
different types o f soils and for different
moisture contents.
ABSTRACT
The complex permittivity of the soils,
collected from different regions of Gujrat
state, have been measured for various
moisture contents at an X-band microwave
frequency. The soils were analyzed for their
texture structure. These textural data were
used to calculate e’ and e” for different
moisture contents using M.T.Hallikair.en
model. The calculated values of complex
permittivity of soils were compared with
observed values for different moisture
contents.
INTRODUCTION
MATERIALS AND METHODS
The complex permittivity of dry and v/et
soils plays an important role in determining
emissivity and scattering coefficient o f the
agricultural fields. These parameters are
essential in interpreting satellite data
obtained by radiometer or radar. Several
attempts have been made to measure
dielectric properties o f Indian soils with
various moisture content in the laboratory1'3,
as well as in the field condition4, at the
microwave
frequencies.
These
measurements have shown that the
dielectric property o f soils depend on
moisture content and hence provides a tool
to determine moisture content in the soil
which is an important parameter for
agricultural scientists and meteorologists. A
programme of comprehensive study o f
complex permittivity o f soils with moisture
content from different regions o f Gujarat
state has been undertaken. Some results o f
our measurements have been reported in our
earlier papers5'7 which included the soils o f
Sabarmati river, Gandhinagar District,
Amreli District, and Valsad District. In
continuation of this we measured complex
Table (1) shows the textural analysis of the
soil samples collected from different regions
of Gujarat state. The wilting coefficient and
transition moisture are calculated using the
Wang and Schmugge model9 as
WP
=
0.06774
0.00064xSand
+0.00478xClay.
..............(1)
Wt = 0.49xWP + 0.165..............(2)
The experimental set up
for the
measurement o f permittivity' of tire soils at
various moisture contents is shown in figure
(1). The two-point method (Sucher and
J.Fox) is used for the measurement of
permittivity
at
X-band
microwave
frequency.
The measured values o f e ’ and e”
for the soil samples of various moisture
contents are plotted in figure (2). It is seen
from figure (2) that the value o f e’ increases
with increase in moisture content for all the
soils. The value o f e’ increase slowly up to
about 15% moisture content, after which it
increases rapidly with moisture content for
all ty-pes of the soils.
VIII-5
Table (l): - Textural composition and physical parameters ofsoils.
Location
(Region)
Sabarmati
River
(Ahmcdabad)
Soil type
Soil texture(%)
Sand Silt Clay
6.2 0.8
93
Sand
Wilting
Point (WP)
cm3/ cm3
0012
Transition
MoisturefWt)
cm3 1 cm
0 1708
Gandhinagar
Dist.
65
31
4
Sandy
Loam
0.045
0.1872
Amreli Dist.
11
78
11
Silt loam
0.118
0 2228
Valsad Dist.
7
62
31
Silty clay
Loam
0.211
0.2686
Palanpur Dist
82
Sand
0.02114
0.1698
16
I
RESULTS AND DISCUSSIONS
The measured values of permittivity o f soils
at various moisture contents are compared
with the empirically calculated values using
the M.T.Hallikainen model10. The procedure
to calculate e’ and e” for the soils from the
model, are described in our earlier paper5.
Figure (3) shows the comparison o f the
measured and empirical values of
permittivity of the soils for the various
moisture contents.
It is visible from figure (3) that the
measured values o f e’ for all types of the
soils are in good agreement with the values
calculated from the empirical model. The
values o f s ” calculated from the model for
all the types o f soils are also in agreement
with the measured values for the soils up to
about 15% moisture content, after which the
empirical values are higher than the
measured values.
Figure (1): Schematic diagram of the X-band Set up used for measurement o f e’and e ”
VIII-6
encouragement and for providing laboratory
facilities.
• SABARMATI
» GANDHINAGAR
X AMRELI
+ VALSAD
.
SABARMAT1 EXPT.
*
GANDHINAGAR EXPT.
o
AMR ELI EXPT.
+
VALSAD EXPT.
X
PALAN PUR EXPT.
----------- SABARMATI MODEL
------------GANDHINAGAR MODEL
------------ AMRELI MODEL
............... VALSAD MODEL
------------ PALANPUR MODEL
o PALANPUR
V o lu m e tric m o is tu re c o n te n t W v
V o lu m e tric m o is tu re c o n te n t W v
Figure (2):-The measured values of s'
and e” for the soil samples of various
moisture contents.
It is evident from ftg.(2) and (3) that tire
permittivity of dry soils is almost same for
all the soils, which shows that permittivity is
independent of texture structure for tire dry
soils. As the moisture content increases the
increase in e’ and e” shows dependence on
the texture structure of soils. At higher
moisture contents the value of s’ . is in
general higher for the higher sand content.
This can be explained by tire fact that the
sand has lower specific area as compared to
clay; the water holding capacity of sandy
soil is less. Therefore as the moisture
content increases, more free water
molecules are available in the soil having
higher sand content. Thus, the value of s ’
for the soil having higher sand content is
higher at a given moisture content.
ACKNOWLEDGEMENT
Authors are thankful to Prof. V.B. Gohel,
Head Department of Physics, for constant
Figure (3):- The comparison of the
measured and empirically calculated
values of permittivity of the soils for the
various moisture contents.
REFERENCES
[1] Ghosh, Anirbid., Bihari, Jitendra and
Pyne, Suprio., “Dielectric parameters of
dry and wet soils at 14.89 GHz,” Indian
J.
Radio
&
Space
Physics,
vol.27,pp. 130-134, June 1998.
[2] Vyas, A.D., “Complex Permittivity of
Sand & Sandy Loam Soils at
Microwave Frequency,” Indian J. Radio
& Space Physics, Vol. II, pp. 169-170
August 1982.
[3] Calla O.P.N., Borah M.C., Vashishtha
P., Mishra R., Bhaltacharya A. and
Purohit S.P., “Study of the properties of
dry and wet loamy sand soil at
microwave frequencies,” Indian J. Radio
& Space Phys., vol.28, pp. 109-112, June
1999.
VIII-7
[4] Mislira, Lima Shankar and Bihari
Jitendra, “In Situ Measurement of
Dielectric Parameter of Soil at
Microwave Frequencies,” Journal of
the Indian Society of Remote Sensing,
Vol.28, Numberl, pp 1-7, 2000.
[5] Vyas A.D. and Gadani D.U., “Study of
Complex Permittivity of soils of Gujarat
at Microwave Frequencies,” Physical
Methods of Soil Characterization,
Edited by Prof. J.Behari, pp. 129-132,
2001.
[6] Vyas A.D. and Gadani D.H., “Study of
Emissivity of Soils of Gujarat at
microwave frequencies”, Presented in
the
National
Conference
on
Applications of Radio Techniques for
Remote Sensing at the International
Centre for Radio Science. 23-24
November, 2001.
[7] Vyas A.D. and Gadani D.H., “Dielectric
Properties of Dry and Wet Soils at
microwave frequencies”, Presented in
the National Workshop cum Symposium
Microwave Measurement Techniques
and Applications at JNU, New Delhi, 46 Feb’2002.
[8] Sucher M. and Fox J., 1963, Handbook
of microwave measurements.
[9] Wang J.R. and Schmugge T.J., “An
empirical model for the complex
dielectric permittivity of soils as a
function of water content,” IEEE
Transactions on Geosciences • and
Remote sensing, Vol. GE-18, No.4,
pp.288-295, October 1980.
[10] Hallikainen M.T., Ulaby F.T., Dobson
M.C., El-Rays M.A. and Lin-Kun Wu,
“Microwave Dielectric Behaviour of
wet Soil-part 1: Empirical Models and
Experimental
Observations,”
IEEE
Trans. Geosc. Remote Sensing, Vol.GE23, No. 1, pp.25-33, January 1985.
VIII-8
Microwaves and Optoelectronics
M D Shirsat, V V Nawarkhele, G S Raju and P W Khirade (Eds.)
Copyright © 2004, Anamaya Pubhsheis, New Delhi, India
Complex Permittivity of Soil at C- and
X-band Microwave Frequencies
D.H. Gadani* and A.D. Vyas
Department o f Physics, School o f Sciences, Gujaiat University, Ahmedabad, Gujarat
*E-mail dhgadani@yahoo.com
Abstract
The complex permittivity o f soil is measured in the laboratory conditions for various
moisture contents at 5 65 and 9 5 GHz microwave frequencies. The comparison o f
results show that in comparison to frequency, the moisture content plays an important
role m determining the complex permittivity o f the soil The measured values o f
permittivity o f dry and wet soils were compated with the values estimated using the
Wang and Schmugge, and the M T Hallikainen et al model
Introduction
Earlier studies [1,2, 5-8] on the measurement of complex permittivity of soil at
various microwave frequencies have shown the dependence of permittivity on
the moisture content of the soil The estimation of soil moisture is very much
useful in hydrology, metrology and in agriculture crop yield Recently,
microwave remote sensing techniques have been used to estimate soil moisture.
In passive microwave remote sensing, the emissivity o f soil is measured by the
radiometer and in active remote sensing, the back scattering coefficient o f the
soil is measured by radar to estimate the moisture content in the soil. The
measured value of the complex permittivity of the soil at various moisture
content can be used to calculate the emissivity and the back scatter coefficient
of the soil at a given frequency and moisture content. Thus, laboratory
measurement o f a complex permittivity of the soil for various moisture contents
and at various frequencies provides useful information for the passive and active
remote sensing applications. In the present work, the measured values o f
complex permittivity of sand of the Sabarmati river is presented for various
moisture contents at C- and X-band microwave frequencies.
Materials and Method
At microwave frequencies, the slotted line technique is generally used because
it is convenient and readily available. The accuracy o f measurements o f
dielectric constant depends on the accuracy with which the VSWR and the
position of the voltage minimum can be found.
We used the two-point method for measurement of dielectric constant
involving the solution of a complex transcendental equation, which is suitable
for lossless and medium loss dielectric [3].
Complex Permittivity of Soil at C- and X-band M icrow ave Frequencies
267
The experim ental setup for the two-point m ethod at C-band m icrowave
frequency is shown in Fig 1 The soil sample was collected from the Sabarm ati
riverbed and oven dried Distilled water was added in the soil and allow ed to
saturate for 24 h First, with no dielectric in the short-circuited line, the position
o f the first m inim um DR in the slotted line was m easured. Then, the soil sam ple
o f certain length /Ehaving certain moisture content was placed m the sam ple
holder, such that the sample touches the short-circuited end. Now the position o f
the first m inim um D on the slotted line and the corresponding VSWR, r were
measured This procedure was repeated for another soil sample o f-s a m e
moisture content for another soil sample length /'r Now the propagation
constant (in the empty wave-guide) is calculated as
( 1)
k=
where
Aj = 2 x (distance between successive minima with empty short circuited wave-guide).
VSWR
Meter
Gunn
Oscillator
PIN
Modulator
Attenuator
Frequency
Meter
Slotted
Section
Wave­
guide
Bend
Dielectric
Sample
Holder
Fig. 1. Experimental set up for the two-point method at C-band microwave frequency.
The complex num ber C Z - VF is calculated using the equation
1
H r |x e *
C Z -w = ------x — H ---- r
jk lz l + |r ] x e J*
where,
<{>= 2k x (D - D r - 4)
(2)
(3)
and
|r | = —
1 1 r+1
(4)
268
Gadani and V yas
The solution of the complex transcendental equation
CZ -v|/
tanh(rZT)
( 5)
TZx
was obtained [4] to get conductance GE and susceptance SE • The dielectric
constant e' and the dielectric loss e" of the soil sample is then calculated as
( 6)
and
( 7)
where, a = width of the wave-guide.
The moisture content as weight percent of the soil sample is given by
..
(weight of the wet soil - weight of the dry soil)
% moisture content Wm=-— St------------------------§------------ 1---- - x 100%
(weight of the dry soil)
ro-v
W
Hence the volumetric moisture content in the soil sample is calculated as
Wv - Wmx (dry density of the soil sample)
(9)
The measured values of the dielectric constant s' and the dielectric loss s"
at C- and X-band microwave frequencies for the soil have been plotted against
volumetric moisture content as shown in Fig. 2. The texture structure of
Sabarmati river sand is shown in Table 1.
Table 1. Texture structure of Sabarmati river soil
Sand
Silt
Clay
Wilting point WP, cm3/cm3
Transition moisture Wt, cm3/cm3
93%
6.2%
0.8%
0.012
0.1708
The wilting point and transition moisture o f soil in terms o f volumetric water
content ( c r a W ) are calculated by using the Wang and Schmugge model as
WP = 0.06774 - 0.00064 x Sand + 0.00478 x Clay
(10)
where Sand and Clay are the sand and clay contents (%) of dry weight of the soil.
The transition moisture is calculated as
Wt = 0.49 WP + 0.165
( 11)
Complex Permittivity o f Soil at C- and X-band Microwave Frequencies
O C-Band EXPT
269
X X-Band Expt
Fig. 2. Comparison o f Sabarm ati C- and X-band experimental results.
A
C-bandExpt.
O
X-band Expt.
t
C-Hallikainen Model
— *
C-Wang Model j
X-Hallikainen Model
- a
X-Wang Model |
Fig. 3. Com parison o f C- and X-band results with Hallikaiuen and W ang model.
270
Gadani
and
V
yas
Depending on the texture structure o f the soil, the complex permittivity o f
the soil, for various moisture contents, at C- and X-band microwave frequencies
have been calculated using the M T. Hallikamen et al. model [6] and the Wang
and Schmugge model [5], Figure 3 shows the plots o f the complex permittivity
of the soil for various moisture contents at C- and X-band microwave frequencies
giving comparison o f experimental and empirically calculated values of the
complex permittivity
Results and Discussion
Figure 2 shows the comparison o f the C- and X-band results o f the complex
permittivity o f the soil for various moisture contents. It is seen from the graph
that at any moisture content in the soil, the real part o f the dielectric constant at
C-band microwave frequency is more than that at X-band microwave frequency.
It is also seen that at lower moisture content up to the transition moisture (Wt)
the dielectric constant o f the soil increases slowly with the increase in moisture
content for both the frequencies At higher moisture contents, above (Wt), the
dielectric constant o f the soil increases rapidly with increase in moisture content.
On the contrary, the dielectric loss s" for the soil at C-band microwave
frequency is less than that at X-band microwave frequency. The dielectric loss
increases slowly and linearly with the increase in moisture contents in the soil
for both the frequencies.
Figure 3 shows the comparison o f C- and X-band results o f the complex
permittivity o f the soil for various moisture contents with the empirically
calculated values using the M.T. Hallikainen et al. model [6J and the Wang and
Schmugge model [6], It is seen that the experimentally measured values o f the
dielectric constant e' o f the soil are in good agreement with the empirically
calculated values for various moisture contents for both the frequencies
The experimentally measured values o f the dielectric loss s" of the soil are
lower than those calculated using the two empirical models at higher moisture
contents. This may be due to the typical composition o f the Indian soil
Acknowledgement
The authors are thankful to Prof. B.P. Agrawal, Head, Department of Physics,
Gujarat University, Ahmedabad for providing laboratory facilities.
References
1
2.
3.
4.
5
Vyas, A.D., ‘Complex permittivity of sand and sandy loam soils at microwave
frequency’, Indian J. Radio and Space Physics, VoL II, August 1982, pp. 169-170.
Vyas, A.D. and Gadani, D.H., ‘Study of complex permittivity o f soils of Gujarat at
microwave frequencies’, m Physical Methods of Soil Characterization, J. Behari
(Ed.), Narosa Publishing House, New Delhi, 2001, pp. 129-132.
Sucher, M. and Fox J., ‘Handbook o f microwave measurements', 1963.
Von Hippel, A.R. (Ed.), ‘Dielectric materials and applications', 1954.
Wang, J.R. and Schmugge, T.J., ‘An empirical model for the complex dielectric
permittivity of soils as a function of water content’, IEEE Transactions on
Geoscience and Remote Sensing, Vol. GE-I8, No. 4, October 1980, pp. 288-295.
Com plex Permittivity o f Soil at C- and X-band Microwave Frequencies
6
7
8
9
271
Halhkamen, M T , Ulaby, F .T , Dobson, M C„ El-Rays, M A and Wu, L -K ,
‘Microwave dielectric behaviour of wet soil- Part 1. Empirical models and
experimental observations’, IEEE Trans Geosci. Remote Sensing, Vol. GE-23, No. 1,
January 1985, pp 25-33
Mishra, U S and Behart, J., ‘In-situ measurement of dielectric parameter of soil at
microwave frequencies’, Journal o f the Indian Society o f Remote Sensing, Vol. 28,
No. 1, 2000
Hoekstra, P and Delaney, A , ‘Dielectric properties of soils at UHF and microwave
' frequencies’, Journal o f Geophysical Res , Vol. 79, No. 11, April 1974, pp 16991708.
'Vyas, A.D and Gadani, D H., ‘Dielectric properties of dry and wet soils at
microwave frequencies’, in Microwave Measurement Techniques and Applications,
J Behari (Ed.), Anamaya Publishers, 2003, pp. 79-85
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Topics in electromagnetic Waves: Devices. Effects and Applications
Edited b> J Behan
Copyright 0 2005, Anamaya Publishers, New Delhi. India
14. Microwave Dielectric Constant of Soils
A.D. Vyas and D.H. Gadani
School o f Sciences, Gujarat University, Ahmedabad, India
Abstract: Dielectric properties o f six soil samples collected from vanous parts o f
Gujarat are measured at an X-band microwave frequency. The permittivity and
dielectric loss of soils increase with moisture content in the soil. For sandy soil, an
anomalous behaviour is observed, particularly for more than 80% sand content. The
measured values o f complex permittivity with moisture content were compared
with those calculated by Hallikainen et al. model and Wang and Schmugge model.
It is found that estimated values o f e' for different moisture content are in good
agreement with the observed values of £'. The estimated value of emissivity calculated
from the measured values o f complex permittivity decreases with the increase in
moisture content in the soil.
Introduction
Microwave remote sensing technique to estimate the moisture content in the
soil has gained considerable attention, because microwave sensors can operate
in all weather conditions [ I ]. Further, microwave can penetrate deep in the soil
in comparison to visible and infrared radiations. The technique involves either
measurement of emissivity o f soils using radiometers, or back scattering
coefficient using an active sensor. Both the techniques depend on the dielectric
constant of the soils, which is considerably affected by the moisture content in
the soil. In order to interpret the data obtained by remote sensors, the study of
variation of complex permittivity o f the soils with moisture content is required.
Considerable work has been done in this area in different countries [1]. India,
being a large country, an attempt has been made to study the dielectric properties
o f soils of different regions o f India (w ith moisture content) by some
workers [2-7].
A comprehensive study o f complex permittivity of soils of Gujarat State has
been undertaken. Soil samples were collected from different regions o f Gujarat
and measurements o f complex permittivity (laboratory conditions) were done
for dry arid wet soils at an X-band microwave frequency, as reported in our
earlier papers. In continuation of this we report here complex permittivity of
soil o f Jamnagar district with moisture content. The earlier reported data on
dielectric properties o f other regions have also been included for comparison.
Empirical models [8-9] and mixing formula [10-11] have been developed for
estimation of complex permittivity of soil water mixture. The determined values
o f e* o f soils were also compared with the calculated values from the empirical
models [8-9], and these results are also presented in this article.
Microwave Dielectric Constant o f Soils
121
Sample Collection
The samples of soils were collected from di fferent regions of Gujarat (Saurashtra,
South Gujarat, Sabarmati River, North Gujarat and Central Gujarat) in polythene
bags and brought to the laboratory for measurement.
Experimental Procedure
Texture size was deteremined by mechanical fractionation and sedimentation
technique and is listed in Table I. Samples were dried in an oven at 110 °C for
24 h. Different levels o f moisture were determined by adding distilled water to
dry samples and allowing it to stand for 24 h to aid setting.
Table I.
Textural composition and physical parameters o f soils
Soil texture{%)
Location
Sand
(Region)
Silt
Soil type
Clay
Wilting
Transition
point (WP)
moisture (iV,)
cmVcm3
cmVcm1
Sabarmali
River
(Ahmedabad)
Gandhinagar
District
Amreh Dist
Valsad Dist.
Palanpur
District
Jamnagar
District
93
62
0.8 Sand
0.012
0 1798
65
31
4
Sandy loam
0 045
0.1872
11
7
82
78
62
16
II
31
I
Silt loam
Silty clay loam
Sand
0 118
0211
002114
0.2228
0.2686
0.1698
12
50
38
Silty clay loam
0 2417
0.2834
t
The gravimetric moisture content was found by the relation
Percent moisture content
(W eight o f the wet soil - W eight o f the dry soil)
rr
, AAO/
— ----------------------------------------------------------------------- X lUU /0
(Weight o f the dry soil)
Hence the volumetric moisture content in the soil sample is calculated as
Wx = WmX (Bulk density o f the dry soil sample)
The wilting point and transition moisture o f soil in terms o f volumetric water .
content (cm3/cm 3) were calculated using the Wang and Schmugge model as
WP = 0.06774 - 0.00064 x S a n d * 0.00478 x Clay
where Sand and Clay are the sand and clay contents in percent o f dry weight of
the soil
The transition moisture calculated as
Wt = 0 .4 9 WP + 0.165
are listed in Table 1.
122
A.D V yasand D.H. Gadani-
At microwave frequencies the slotted line technique is generally used because
i t is convenient and readily available. The accuracy o f measurements o f dielectric
constantdepends on the accuracy with which the VS WR and the position of the
voltage minimum can be found.
We used the two-point method for measurement ofdielectnc constant involving
the solution of a complex transcendental equation, which is suitable for lossless
and medium loss dielectric [12].
The experimental set up for the two-point method at X-band microwave
frequency is shown in Fig. I. First, with no dielectric in the short-circuited line,
the position o f the first minimum DR in the slotted line was m easured. Now the
soil sample of certain length lE having certain moisture content was placed in
the sample holder, such that the sample touches the short-circuited end. Now
the position o f the first minimum D on the slotted line and the corresponding
VS WR, r were m easured. This procedure was repeated for another soil sample
o f same moisture content for another soil sample length
Now the propagation
constant (in the em pty waveguide) is calculated as
( 1)
k =
where = 2 x (Distance between successive mimima with empty short circuited
waveguide).
Gurm
power
supply
—
i
Attenuator
PIN
modulator
Gunn
oscillator
VSWR
meter
____ 1______
Frequency
Slotted
meter
section
Wave­
guide
bend
Dielectric
sample
holder
Fig. 1 Experimental set up forthe two-point method at X-band microwave frequency.
The complex num ber C Z ~'F is calculated using the equation
i J - Ir lV
jk le
. ..where
and
$ = 2£*(D - Dr - IJ
r|
r —1
r +1
(2)
l +jrj*e*
(3)
(4)
Microwave Diclesliic Coosianujit Sods . 123
The solution o f theeomplex transcendental equation
lanhfF Z r)
‘— FZT-
( i)
was obtained {14] to get conductance GE and susceptance SE. The dielectric
Constance'and (Ire dielectric loss e" o f the sod sampleare then calculated as
Ge +
^2 a
(6 )
and
V)
where a is the width a f the waveguide.
Results and Discussion
A plot o f f:' and e" against volumetric soil moisture content for the soil o f
Jamnagar district is shown, in Fig. 2 The real pant o f com plex permittivity €
increases gradually with the moisture content upto the transition moisture, after
which it increases sharply with the moisture content in the soil. The imaginary
part o f the complex dielectric constant e" increases linearly with the moisture
content in the soil and does not show a sharp increase after transition moisture.
Similar behaviour was observed in our earlier studies for soils o f different regions
o f G ujarat This is due to the fact that in above transition moisture there are
more free water molecules in comparison to bound water molecules. The bound
water has low dielectric constant in comparison to free water. A comparison o f
E o f Jamnagar district soilw ith the soils collected from other regions ofGujarat
is shown in Fig. 3. The value o f z' for dry soil is higher than those collected
from other regions ofGujarat. This m aybe due to the presence o f higher mineral
content in this soil in comparison to other soils. It is evident from Fig. 3 that
below transition moisture the data points (e', e" } for all six soils fall within a
relatively narrow band, indicating that upto this moisture content the main
parameter determining £ is the volumetric water content. After the transition
moisture iVjy the variation o f z and s" with moisture content is soil texture
dependent. It is noticed that above transition m oisture e ' value o f soil is low er
for sam ples having higher clay contents. Further, the ratio Ae7AITvis higher for
124
A D V yasandD .H Gadam
♦
Fig. 2
Fig. 3
J a m n a g a r Dist soil
Graph o f complex permittivity of Jamnagar District soil for various
moisture contents.
♦ Sabarm ati
a
G andhinagar D ist
+ V alsad Dtst.
o Pafanpur Dist.
x Amreii Dist.
x
Jam nagar Dist
Comparison of e* o f Jamnagar District soil with the soils collected from
other regions o f Gujarat.
Microwave Dielectric Constant o f S oils
125
soil samples having higher sand content; this has been observed by previous
investigators (Haihkainen et al, Wang and Schmugge, and Alex and Behari). It
is due to a large specific surface area o f clay particles in comparison to other
basic components of sod, silt and sand. The large specific surface area o f clay
particles enables soil to retain greater moisture content in the form o f bound
water.
w v = 0%
W„ = 0%
— o— wv = 15%
■e—
Wv =
— *— Wv = 27%
15%
Wv = 27%
Sand %
Fig. 4
Plot of e' and e" against sand percentage.
A plot of e' and z" against sand percent has been shown in Fig. 4, for dry,
medium and high moisture contents. The value o f z ' and e " increases with
m oisture content in the soil. Above 80% o f sand content there is a sharp peak/
hump present, which cannot be explained with the available data. This anomalous
behaviour o f high sand content soil needs further attention, because, it m ay be
exploited for soil characterization. Behari [13] observed similar behaviour in
his study o f frequency dependent dielectric behaviour o f wet soil.
The complex permittivity o f wet soils can be predicted, if the soil physical
parameters and textures are known, by two prom inent models given by Wang
126
A.D Vyas and D.H. Gadam
and Schmugge and Hallikainen et al. The complex permittivity of six sods of
Gujarat with moisture content, calculated using both the models, are shown m
Fig. 5 for sand, sandy loam and silty clay loam from Sabarmati, Gandhinagar
and Valsad.
o S ab arm ati E xp e n m en tal
.............
Hallikainen M o d e l
-----------W an g M o d e l
20
16
12
OJ
8
4
0
(a)
0
0 .0 5
0.1
0 .1 5
0 .2
V o lu m e tric m o istu re co n ten t
(b )
0 .2 5
0 .3
Micr'ovwnc Diclcctric’Conslant o f Soils
^ ">
V a ls a ti E x p t
..............
H a llik a m e n M o d e l
----------
127
W ang M o d el
Fig. 5 (a) Sabarmati river sand, (b) sandy loam and (c) Valsad silty clay loam.
The experimentally observed values of e have also been included in the Figure
for comparison The e ' values with moisture content for sand and sandy loam
soils are m agreement with those calculated from both the models. The e' for
various moisture content for silty clay loam soils are predicted well with
Hallikainen et al. model, but Wang and Schmugge model predicts slightly less
values of e' with moisture content for these type o f soils. The values o f e"
calculated from empirical models for all types o f soils are in good agreement
with the experimentally observed values upto 15% of moisture content in (he
soil, after which predicted e" values with moisture content are higher than the
measured values o f e"
sandy loam
x
Jam nagar
silty d a y loam
E m is s iv ity ( e )
♦ G a n d h in a g a r
Volumetnc Moisture content
Fig. 6
Variation o f emissivity with moisture content for sandy and silty clay loam soils
128
A D . Vyasand D.H Gadam
The emissivity o f soils from the measured values o f complex permittivity for
normal incidance can be calculated from the relation
e
=
l
( 8)
Fig. 6 shows the variation o f emissivity calculated from the measured values
o f e* with moisture content for sandy loam and silty clay loam soils. It is evident
from the figure that, for dry soils emissivity is less than one and decreases with
increase in moisture content in the soil.
R eferen ces
t. Microwave Remote Sensing (Active and Passive), F.T. Ulaby, R.K. Moore and
A K.Fung, vol. I, II, III Artech House Inc. 1981,1982,1986.
2. Vyas, A.D., “Complex Permittivity of Sand and Sandy Loam Soils at Microwave
Frequency,” Indian J Radio & Space Physics, II, pp. 169-170. August 1982
3. Z.C AlexandBehari J. “Laboratory evaluation ofemissivity ofsoils”,/n£ J. Remote
Sensing, 19, No.7, pp 1335-1340, 1998.
4 Mishra U.S. and Behari J. “In-situ measurement o f Dielectric Parameter o f Soil at
Microwave Frequencies," Journal o f the Indian Society o f Remote Sensing, 28,
No 1, 2000.
5. Panchoh K.C and Khameshra, S.M “Complex dielectric permittivity o f some
Rajasthan soils at 7 114 GHz”, Indian J o f Radio and Space Physics”, 23, June
1994, pp.201-204
6 Shrivastava, S.K. “Transmission Line Model for Predicting the microwave emission
from soil1’, Proceedings, National conference on Application of Remote Sensing,
November 20!, pp. I -7
7 Calla, O.P.N, Borah, M.C., Vashishtha, P., Mishra, R., Bhaftacharya, A. and Purohit,
S.P. “Study o f the Properties of dry and wet loamy sand soil at microwave
frequencies", Indian J. o f Radio and Space Physics, 28, June 1999, pp. 109-112.
8 Hallikainen, M .T., Ulaby, F.T., D obson, M .C., El-Rays, M A. and
Lin-Kun Wu, “Microwave Dielectric Behaviour o f wet Soil-part 1: Empirical Models
and Experimental Observations,” IEEE Trans. Geosci. Remote Sensing, GE-23,
No.I, pp.25-33, January 1985.
9. Wang, J.R. and Schmugge, TJ. “An empirical model for the complex dielectric
permitivity of soils as a function ofwater content," IEEE Transactions on Geoscience
and Remote Sensing, GE-18, No.4, pp. 288-295, October 1980.
10 de Loor G.P., “Dielectric Properties o f Heterogenious Mixtures containing Water”,
J. Microwave Power, 3, pp. 67-73,1968
11
Behari, J., Sahu, S.K and Mishra, U.S. “Microwave Dielectric Constants o f Soil”,
Physical Methods o f Soil Characterization, Narosa, New Delhi, 2001, p. 25
12. Sucher, M. and Fox, J. 1963, Handbook o f Microwave Measurements.
Microwave Dielectric Constant of Soils
129
13 Behan, J . “ Frequency Dependent Variation of Dielectric Parameters of Wet Soil”,
Microwave Measurement Technique and Applications, Anamaya Publishers, New
Delhi, 2003, p. 71
14. Dielectric Matenals and Applications, Edited by Arthur R. Von Hippel, 1954
320
Proceedings o f National Conference: Microwave 2006
Measurement of complex dielectric constant of dry and
wet soil using VNA
D.H.Gadani*, V.A.Rana*, S.P.Bhatnagar** and A.D.Vyas*
♦Department o f Physics, University School o f Sciences,Gujarat University, Ahmedabad,
♦♦Department o f Physics, Bhavnagar University, Bhavnagar.
E-mail:- dhgadani@vahoo.com
Abstract
A technique for the measurement of
the
frequency dependent
dielectric
constant o f soil using the vector network
analyzer (VNA) operating in range from
30 MHz to 3 GHz has been described. A
coaxial probe attached with the VNA is
used. W ith the help o f three standard
calibrations the (unknown) fringe-field
impedance is removed. The technique is
used to measure the complex dielectric
constant o f dry and wet soil of
Gandhinagar district (Gujarat).
Index Terras --- Complex dielectric
constant, Coaxial probe, Soil, Moisture,
Vector Network Analyzer (VNA).
L INTRODUCTION
The complex dielectric constant o f dry
and wet soils is a useful parameter to
interpret microwave remote sensing data
produced for hydrology, agriculture and
meteorology. There are many methods (3'9)
to measure the dielectric constant o f soils for
various moisture contents in the laboratory
conditions, e.g., microwave bench set- up
using the transmission and reflection
method, the time doma'iir reflectometry and
the frequency domain technique using
vector network analyzer (VNA). Microwave
benches used for such measurements
provide complex dielectric constant at spot
frequencies in a particular microwave band.
Time domain reflectometry (TDR) is
frequently used for measurement o f
dielectric constant o f dry and wet soils. TDR
is wide band method and measurements are
done rapidly. The advantage with VNA
technique over TDR technique is that
Fourier transform is not required. In this
paper we present our results o f complex
permittivity measurements o f dry and wet
soils collected from Gandhinagar district o f
Gujarat using vector network analyzer in
frequency range 30 MHz to 3 GHz. Our
earlier data(10) for dielectric constant o f soils
(dry and wet) obtained at ‘X ’ band
microwave frequency have also been
included for comparison.
II. EXPERIMENTAL
The de-embedded technique developed
by Yan-zen Wei and S. Sridhar is used for
the measurement o f frequency dependent
dielectric constants o f dry and wet soils. The
technique involves the determination o f the
complex impedance o f a simple sample cell,
consisting o f a coaxial semi rigid flanged
cable terminated by liquid or soil sample.
The liquid-coax interface is modeled as
Z(0,0) □ [jd C f+ j D C o e ]'1.
The de-embedding requires calibrations
with specific terminations to eliminate
connector impedances and any other line
mismatches, and also the fringe field
capacitance Cf within the coax as well as the
capacitance
parameters Co that are
frequency dependent and difficult to
measure. The three terminations used for
calibration are an open, a short using liquid
mercury and acetone as standard liquid.
A semi rigid coaxial cable is designed which
is shown in figure (1). The diameter o f
coaxial cable is 0.141inch, with a meted disc
(flange) o f diameter 3.2 cm connected at one
end and the probe tip is kept 3 mm outside
the probe, .while at the other end o f the probe
an N-type female connector is connected.
The connector end is mated to the N type
Proceedings of National Conference: Microwave 2006
32!
male connector of the coaxial probe attached around the position Re (Z) = -1 and Im (Z)
=0, at the middle of the left hand side of the
with the HP-8417-ES Network Analyzer.
The network analyzer is operated in display. The pair wise data Zs were again
frequency range from 30 MHz to 1.5 GHz read into the computer.
with typical selections of 51-201 points. A C. Standard Liquid: - The mercury cell was
personal computer (PC) was connected to removed. At this point, the display was
receive the pairs of data (Real and checked to ensure that the data returned to
Imaginary values of impedance) for the the configuration for an open. A beaker of
100 ml with a standard liquid (Q20 ml),
given frequency range.
The flexible coaxial probe of VNA was usually acetone, was taken so that the coax
calibrated at other end by using the standard end was well immersed in the Liquid. The
open, short and matched loads provided by data Zc were again read into the computer.
D. Sample Liquid (Methanol): - The
the manufacturer. Now the flanged semi
rigid coax probe was connected at the other standard liquid was removed, and time was
end of the flexible coaxial probe attached to allowed for the acetone to evaporate
VNA.
completely from the end of the coax, until
the display returned to the open
configuration. The procedure was repeated
with methanol (AR grade) and data Zm were
collected.
To repeat the procedure D for soil samples
of different moisture content, the coax was
first cleaned each time with acetone. After
evaporation of acetone the display was
checked to ensure that the data returned to
the configuration for an open. Then the
measurement for Zm of the soil samples for
various moisture contents was carried out.
Fig.l:
For the measurement the sandy loam soil o f
The configuration of semi rigid coaxial
Gandhinagar district was taken whose
cable.
texture structure is given in the table (1),
. The procedure for experimental data where the wilting point and transition
collection is described below step by step.
moisture are calculated using Wang and
A. Open: - With the coax terminated by free Schmugge model(t).__________________
space, the measurement plane of the VNA
Table 1
was moved to the coax end using the
Composition and Phy:deal parameters
electrical delay provided. The delay, which
o f Gandhinagar c istrict soil.
corresponds to the length of the coax
Sand
65%
(typically 6-12 inch), was adjusted to give a
Silt
31%
cluster of points near the
Re (Z) = 1 and
4%
Clay
Im (Z) = 0 point, at the middle o f the right
1.389 gm/cc
Density Ds
hand side o f the display. Because of the
0.045
Wilting point (WP)
connector mismatches, the cluster is not a
Transition moisture cm^/ern^
point but can occupy a region. The pair wise
0.1872
(Wt)
ZAdata were read into the computer.
c m ^ /c rrP
B. Short: - A short at the coax end was
Now the complex dielectric constantD O
created by raising a vessel filled with
Z
i*
jO ” of the soil samples (and methanol)
mercury, until the coax end was well within
is
calculated
using the equation(41
the liquid. This resulted in a cluster o f points
322
Proceedings of National Conference: M icrowave 2006
£ _ 'Z'u
A + Z MA23
III. RESULTS
The measured value o f dielectric
constant □ ’ and dielectric loss □ ” for
methanol at different frequencies is shown
in figure (2). The complex permittivity o f
this liquid is also estimated using Debye
model and shown in the same figure. The
'measured values of □ for methanol are
fairly in agreement with the values obtained
from the model up to the frequency o f I
GHz, after which there is a deviation
between observed values and estimated
values.
Figure (3-a) and (3-b) show the plots o f
dielectric constant □ ’ and dielectric loss □ ”
o f Gandhinagar district soil at different
frequencies, for various moisture contents in
the soil. It is visible that the dielectric
constant □ ’ and dielectric loss □ ” o f the soil
increases with increase in moisture content
in the soil at all frequencies.
.
--------- eps'vna
'
-
eps' debye
Figure (4) shows the plot o f dielectric
constant O ’ and dielectric loss 0 ” o f the soil
plotted for various moisture contents at 0.9
GHz frequency. It is seen that □ ’ and □ ”
increases with increase in moisture content
in the soil. With increase in moisture content
the value o f O ’ increases slowly up to the
transition moisture, after which it increases
rapidly. The value o f □ ” increases slowly
with moisture content as compared to that of
□ This can be explained by the fact that
wet soil is a mixture o f soil particles, air
voids and liquid water. The high dielectric
constant o f water ((0 W~80) depends on the
molecules ability to align its dipole moment
along an applied field. Anything that hinders
the molecule’s rotation
~W v~ = 0
-
VW *= 002 43 1 21
-------------VW = 0 0 4 4 0 9 6
---------- - V W = 0 0 4 7 8 2 2 1
-------------V W * 0 1 0 1 1 0 5
------------- V W * Q 111831 (
j -------------V W * 0 176171
------------- VW = 0 255471 j
,3
*
w*
i
----------eps"vna
600000000
........... eps’ debye
1E+09
1 5E-KJ9
!
F re q u e n c y
Fig.3-b: Measured values of dielectric loss
0 ” o f Gandhinagar dist. Soil against
various frequencies, for different
moisture contents Wv.
35 j
0
5E+08
I
1E+09
15E +09
I
Frequency___________ l
Fig.2: Comparison of the measured values
of Methanol with those calculated using
Debye model.
---------- VW «0
30 i
25 i
Wfeng &
Schmugge model
20 1
o
o
10 ■
------ VW >0 0243121
*
«°-4'
0
*-----------VW = 0 0 4 4 0 9 6 ............. V W * 0047822
o ExpL 0 9 GHz
---------- VW *0101105 -----------VW *01116316’
0
----------- V W * 0 1 7 6 1 ? 1 ----------- VW * 0 255471
. 335
"
0 f —
I
'tc
10
;
-
— -----------
’
I
*
____________ __
5
1 500000000
1E+09
i
F re q u e n c y
1*
1 .5 E + 0 9
'
j
Fig.3-a Measured values of dielectric
constant O’ of Gandlihiagardist. Soil
against various frequencies, for different
moisture contents Wv.
0.1
Wv
0.2
Fig.4: The comparison of the measured
values of □ ’ and 0 ” of the soil for various
moisture contents at 0.9 GHz with those
calculated using the empirical relations of
Wang and Schmugge model.
(e.g. freezing, tight binding to a soil
particle, etc.) therefore reduces the dielectric
constant o f water. The water molecules
contained in the first molecular layers
surrounding the soil particles are tightly held
Proceedings of National Conference: Microwave 2006
by the soil particles, due to the influence of
matric and osmotic forces, called bound
water (1, 2). Hence the dielectric constant of
bound water is low. The matric forces acting
on a water molecule decrease rapidly with
the distance away from the soil particle
surface. Hence the water molecules located
several molecular layers away from the soil
particles are able to move freely within the
soil medium, called free water Thus the
dielectric constant of free water is high. At
moisture contents below the transition
moisture in the soil there are more bound
water molecules as compared to free water
molecules. Hence the dielectric constant of
soil at lower moisture contents is low. As
the moisture content in the soil increases
above transition moisture in the soil, the free
water molecules increase rapidly in the soil
increasing the dielectric constant □ ’ rapidly.
The comparison o f the measured values of
O’ and □ ” for the soil for various moisture
contents at 0.9 GHz with those calculated
using the empirical relations using the Wang
and Schmugge model is also shown in figure
(4). The measured values are in good
agreement with the values calculated using
the model.
;
♦
X-8and
X
0 9 GHz
:
--------- Export (0 9 GHz)........... Bcpon (X-Band)
The comparison of the measured values of
0 ’ and 0 ” at 9.5 GHz with those
measured by VNA at 0.9 GHz frequency.
Figure (5) shows the measured values o f
O’ and O” at 9.5 GHz frequency using the
microwave bench set up and the measured
values at 0.9 GHz using VNA. It can be seen
that for given moisture content, the value of
3 ' decreases with increase in frequency
323
indicating that the dielectric dispersion
occurs in this region o f frequency.
A CKN O W LED G EM EN T
Authors
are
thankful
to
Prof.
R.V.Upadhyay, Head, Department o f
Physics, Bhavnagar University, Bhavnagar,
for providing laboratory facilities to use the
Network Analyzer.
REFERENCES
1.
J.R.Wang, and T.J.Schmugge,“An
empirical model for the complex
dielectric permitivity of soils as a
function o f water content,” IEEE
Transactions on Geoscience and
Remote sensing, vol.18, no.4 pp. 288.
1980.
2.
M.T.Hallikainen,
F.T.Ulaby,
M.C.Dobson, M.A.El-Rays, and LinKun Wu, “Microwave Dielectric
Behaviour o f wet Soil-part 1:
Empirical M odels and Experimental
Observations”, IEEE Tram. Geosci.
Remote Sensing, vol GE-23, no.l,
pp,25-33, January-1885.
3.
Devendra Mishra, Mohinder Chabra,
Benjamin R. Epstein, Mark Mirotznik,
Kenneth R. Foster, “Noninvasive
Electrical
Characterization
of
Materials at Microwave Frequencies
Using an Open-Ended Coaxial line”,
IEEE Trans. Geos.'Remote Sens., vol38, no.l, pp.8-13, January-1990.
4.
Y.Z.Wei and S.Sridhar, “Technique
for measuring complex- dielectric
constants o f liquids upto 20 GHz”,
Rev. Sci. Instrum., vol-60, no.9,
pp.3041-3046, May-1998.
5.
David V.Blackham, Roger D.Pollard,
“An
Improved ' Technique
for
Permittivity Measurements Using a
IEEE Tran.
Coaxial probe”,
Instrumentation and Measurement,
vol-46, no.5, pp.1093-1099, October1997.
6.
G. C. Starr, B. Lowery, and E. T.
Cooley,
“Soil
Water
Content
Determination using., a Network
Analyzer and Coaxial Probe”, Soil Sci.
324
7.
8.
Proceedings o f National Conference: Microwave 2006
SocA m J. , vol-64, pp.867-872, MayJune, 2000.
W. Skierucha, R Walczak, and A.
Wilczek, “Comparison o f Open-Ended
Coax and TDR sensors for the
measurement of
soil
dielectric
permittivity
in
microwave
frequencies”, Int Agrophysics, vol-18,
pp.355-362, Ocotober-2004.
Z.C.Alex and J.Behari, “Laboratory
evaluation of emissivity o f Soils”, Int
J
Remote Sensing, vol.19, no.7,
pp. 1335-1340, 1998.
9.
10.
O. P. N. Calla, M. C. Borah, P.
Vashishtha, R. Mishr?., A. Bhattachrya
and S. P. Purohit, “Study of the
properties of dry' and wet loamy sand
soil at microwave frequencies”, Indian
J. Radio and Space Physics, vol-28,
pp. 109-112, June-1999.
A. D. Vyas & D. H. Gadani,
“Microwave Dielectric Constant o f
Soils”, Topics in Electromagnetic
Waves,
Devices,
Effects
and
Applications^Edited by Prof. J.Behari,
Anamaya Publishers, New Delhi, India
pp.120,2005.
Indian Journal of Radio & Space Physics
Vol. 37, June 2008, pp. 221-229
Measurement of complex dielectric constant of soils of Gujarat at X- and
C-band microwave frequencies
D H Gadani1& A D Vyas2
'Physics Department, C U Shall Science College, Ashram Road, Ahmedabad 380 014, Gujarat. India
■Department of Physics. University School of Sciences, Gujarat University. Ahmedabad 380 009, India
E- mai 1: dhgadan i @yahoo.com
Received 16 February 2006; revised 6 Jane 2007; accepted 05 May 2008
Dielectric constant and dielectric loss of soils collected from different districts of Gujarat state for various moisture
contents have been measured at X- and C-band microwave frequencies. It has been observed that the dielectric constant of
soils depend on the moisture content in the soils and frequency of measurement. Dielectric constant of soils increases slowly
with increase in the moisture content in the soil up to the transition moisture, after which it increases rapidly with moisture
content. The measured values of complex permittivity of dry and wet soils are compared with the values calculated from the
empirical models and arc found to be in agreement. The observed complex permittivity is used to calculate emissivity of
soils for various moisture contents. It has been found that emissivity of soils decreases with increase in moisture content in
the soil.
Keywords: Dielectric constant. Microwave frequency. Soil moisture content. Emissivity
PACS No.: 77.22.Gm
1 Introduction
The dielectric constant of a soil is a measure of the
response of the soil to an electromagnetic wave. This
response is composed of two parts (real and
imaginary), which determine the wave velocity and
energy losses, respectively. In a non-homogeneous
medium such as soil, the bulk dielectric constant (eb)
is a combination of the individual dielectric constants
of its components (i.e. air, water, dry soil, etc.), but is
not a weighted average. The large contrast between
the dielectric constant of air (e;1~ 1), dry soil1 (es ~ 2
to 4) and water (ew- 80) in microwave region, result
in a range of £b from 2 to 40 for a soil-water interface.
From the measured value of dielectric constant, the
emissivity and back-scattering coefficient of soil at a
given frequency can be calculated. In the passive
microwave remote sensing the radiometer measures
the emissivity of soil, whereas in active remote
sensing the radar measures the back-scattering
coefficient of the soil. Thus the knowledge of
variation of dielectric constant with moisture content
of a soil is useful for the interpretation of data
obtained by various sensors for microwave remote
sensing applications, e.g. agriculture, hydrology and
meteorology. It has been observed by several
workers2' 16 that the dielectric constant of dry soil lies
between 2 and 4 and increases with increase in
moisture content in the soil and hence provides a tool
to determine moisture content in the soil. The authors
studied17'22 moisture dependence of complex
permittivity for soils collected from Sabarmati river
bed, fields of Gandhinagar, Palanpur, Valsad and
Amreli district at X-band microwave frequency and
found that dielectric constant of these soils increases
slowly with moisture content up to transition
moisture, after which it increases rapidly with the
increase in moisture content in the soil. To gain more
information, complex permittivity of dry and wet soils
collected from Surendranagar district (Sayla) and
Gandhinagar district was measured. The complex
permittivity of Surendranagar district soil was
measured at X-band, the complex permittivity of
Gandhinagar district soil was measured at C-band
microwave frequency and the results are presented
here. The results of a previous study17'22 of complex
permittivity of soils have also been included in this
paper for comparison.
Various empirical3'415 models have been reported
in the literature to calculate dielectric constants of
moist soil from its texture and frequency of
measurement. The determined-dielectric constants of
soils with various moisture contents at microwave
frequencies were also compared with the calculated
values obtained from different models.
222
INDIAN J RADIO & SPACE PHYS, JUNE 2008
2 Materials and method
The soil samples were collected from different
regions of Gujarat24 state (Fig 1) The soil samples
were collected from the Sabarmati river bed, fields of
Gandhinagar district, Palanpur, Valsad and Amreli
districts and from the field of Surendranagar district
(Sayla) as well as from the sea bed near Somnath
temple Stones and gravels were removed from the
soil samples and then the soil samples were oven
dried. Distilled water was added to the soil and
allowed to saturate for 24 h. As the days went on, the
moisture content in the soil decreased and the
measurement of dielectric constant of the soil samples
for various moisture contents were carried out. The
texture structure of various soil samples was obtained
from the KBM Engineering Company, Ahmedabad
(shown in Table 1)
The wilting point (WP) and transition moisture
«»
(Wt) are calculated using the Wang and Schmugge3
model as follows.
WP = 0 06774 - 0 00064 x Sand + 0.00478 x Clay
Wt = 0 4 9 x W P + 0.165
where, Sand and Clay stand for the sand and clay
contents in percent of dry weight of the soil
The dielectric constant s' and dielectric loss e" of
the soil samples were measured at 5 65 GHz (C-band)
and 9 50 GHz (X-band) microwave frequencies, using
the two-point method25 The reflex Klystron and Gunn
diode were used to generate X- and C-band
microwave
frequencies,
respectively
The
experimental set up is shown in Fig 2
The sample holders for X-band and C-band
measurements were fabricated from the standard
waveguides available. At the one end of he sample
TO
T-
•>)
Tl
T
“T“
K*W J
P A
K
43
O
»l
S HALLO W
w
IC K C
M E D IU M B L A C K
S
D EEP BLACK
B
U II
E 23
1S3
iSZ
i
r n
ELS
GHJ
£3
m
S»J
*
4$
R E S ID U A L S A N D Y
40
tfo
I $ T A M*
\
JF*
A L L U V IA L S A N D Y
A L L U V IA L S A N D Y L O A M
M IX E D R E D A N D B L A C K
L A T E R JT 1 C
C O A S T A L A L L U V IU M
DESERT
H IL L Y
FOREST
S A L IN E A L K A L I
n
& »’
Fig. 1— Soil map of Gujarat
0
**
*>
GADANI &VYAS: DIELECTRIC CONSTANT OF GUJARAT SOILS AT X- & C-BAND MW FREQUENCIES
223
Tabic 1— The texture structure of soil samples
Location (Region)
Soil texture (%) of
Clay
Sand
Sill
Sea bed near Somnath
Sabarmati river bed (Ahmedabad)
Palanpur Dist.
Surendranagar Dist. (Sayla)
Gandhinagar Dist
Amreli Dist.
Valsad Dist.
0.3
0.8
I
2
4
11
31
3.7
6.2
16
29
31
78
62
11
7
Wilting point
WP, cm3/ citT
Transition moisture Wt,
cnrV cm'’
0.007734
0.012
0.021
0.03314
0.045
0.118
0.211
0.16879
0.1708
0.1698
0.18124
0.1872
0.2228
0.2686
Sand
Sand
Sand
Sandy loam
Sandy loam
Silt loam
Silty clay loam
X-band Wave guide
Gunn
Power
Supply
Microwave
Source
96
93
82
69
65
Soil
type
VSWR
Meter
PIN
Modulator
Attenuator
Frequency
Meter
Slotted
Section
Waveguide
Bend
S h o rt
Flange
Dielectric
Sample
Holder
Fig. 2—The experimental set up for the two-point method at Xband microwave frequency
holder a metallic flange was connected, so that it can
be connected to the main line and the other end was
carefully shorted as shown in Fig. 3. Lengths of the
X-band and C-band sample holders are 3 cm and 5
cm, respectively.
First, with no dielectric in the short-circuited line,
the position of the first minimum DR in the slotted line
was measured. Now the soil sample of certain length
(/e ) having certain moisture content was placed in the
sample holder, such that the sample touches the shortcircuited end. Now the position of the first minimum
D on the slotted line and the corresponding VSWR, r
were measured. This procedure was repeated for
another soil sample of same moisture content for
another soil sample length (/f' ).
Now the propagation constant (in the empty wave­
guide) is calculated as
Wave
Fig. 3—Construction of the sample holder for X-band and C-band
microwave bench
where
(J) = 2k*(D - D r - /E)
and
|r| = —
where, A.g = 2 x (distance between successive minima
with empty short circuited wave-guide sample holder)
The complex number CZ - H' is calculated using
the equation
r=
1 &l-|r|*expO'(|»
jkl? 1+ r|*expO'<t))
...
( 2)
... (3)
... (4)
The solution of the complex transcendental equation
c z _ T = .anh(rZT)
TZ t
was obtained1^ to get conductance Ge and susceptance
S e. The dielectric constant e' and the dielectric loss £”
of the soil sample were then calculated as
224
INDIAN I RADIO & SPACE PHYS, JUNE 2008
■ ( 6)
Measurement
frequency,
GHz
and
£
—
• - (7)
-
1+
V
/la
where, a - width of the waveguide.
For more accurate results, the length of the sample
should be kept near 2.^/4, one-quarter of the
wavelength in the dielectric field waveguide For
estimation of X&e/ 4, dielectric constant of dry soil as
2.5 was assumed and XgE was calculated using the
relation25
2%
T
Table 2—The comparison of measured values of complex
permittivity of carbon tetra chloride and 1-propyl alcohol
measured at X-band and C-band microwave frequencies at 25°C,
with the literature27”29 values
G rib-
where, X = free space wavelength of microwave
signal, Xc = la , for the dominant mode propagating in
the rectangular waveguide and
1.
Taking this value as a reference value,
measurements were carried out for many samples
whose lengths are nearly Xmf 4, till the same values of
conductance GBand susceptance -S'E were obtained for
the two samples. These values of Ge and SE were used
for further calculations of the dielectric constant e'
and the dielectric loss £" As the moisture in the soil
increased the sample length were reduced and similar
exercise was done for other wet samples
In order to ascertain the validity of our
measurements the complex permittivity of carbon
tetrachloride and 1-propyl alcohol were measured at
X-band and C-band microwave frequencies at 25°
and were compared with the literature values of
complex permittivity of these solvents (Table 2)
They are fairly m agreement with the literature values
The measurement of complex permittivity of dry and
wet soils was done at room temperature (which was
around 30°C).
The gravimetric moisture content as weight percent
of the soil sample was found by the relation
Percent moisture content Wm
= [(weight of the wet soil - weight of the dry
soil)/(weight of the dry soil)] x 100%
9 50
5 65
9 50
(X-band)
5 65
(C-band)
Liquid
CC14
CC14
1-Propyl
Alcohol
1-Propyl
Alcohol
Measured
values
Literature
values
E'
2 26
2 20
3 71
£"
0 06
0 08
0 86
2 23
2 24
3 53
00
00
1 16
4 03
1 26
3 87
1 48
t"
Hence the volumetric moisture content m the soil
sample is calculated as
Wv = Wm x (bulk density of the dry soil sample)
From the measured values of the dielectric constant
e' and the dielectric loss e" of the Sabarmati sand and
Gandhinager sandy loam soils, at X- and C-band
microwave frequencies, the emissivity of the soils for
normal incidence were calculated using the relation10
where, 8 = the dielectric constant of the soil.
3 Results
The measured values of the dielectric constant e'
and dielectric loss e" at X-band microwave frequency
for different soil samples were plotted against various
values of moisture content. The plots are shown in
Fig. 4.
The dielectric constant £' and the dielectric loss £"
of soils increase with the increase m moisture content
for all the soil samples. Further, dielectric constant £'
increases slowly up to the transition moisture for all
soil samples, after which it increases rapidly, but the
dielectric loss e" increases linearly with the moisture
content Wv The variation of £' with moisture content
is almost similar for all the soil samples up to the
transition moisture For the moisture content above
transition moisture the increase in the dielectric
constant e' of sandy soils is more as compared to that
of the high clay content soils
Figures 5(a) and 5(b) show the variation of
dielectric constant e' and dielectric loss £"
respectively, as a function of sand content in the soils
. GADANI &VYAS: DIELECTRIC CONSTANT OF GUJARAT SOILS ATX- & C-BAND MW FREQUENCIES
a Saaarmaii
* Palanpur
+ Sa^a
0 Gandhinagar
x A n re li
soil, moisture content in the soil and the frequency of
measurement''4'15. Some typical results of comparison
of measured values of the dielectric constant e’ and
the dielectric loss e" of the soil samples with the
values calculated using the empirical models are
shown in Fig. 6(a) and Fig. 6(b) for sand)' loam and
silty clay loam soils. It is evident from Fig. 6 that the
measured values of s' are in good agreement with the
values calculated using the three empirical models,
for various moisture contents. The values of e" are
t*
O
CD
o
to
A . O >
• Valsad
to
DIELECTRIC CONSTANT {£') / LOSS <e")
CO
o
* Somnath
TP 5
0
0.05
0,1
0.15
0.2
0.25
0.3
0.35
VOLUMETRIC MOISTURE CONTENT
Fig. 4—The measured values of ihe dielectric constant e' and
dielectric loss C at X-band microwave frequency for different soil
samples plotted against moisture content
O)
20
16
12
■
8
4
0
0
20
40
SAND. %
60
80
100
Fig. 5—The variation of (a) dielectric constant e' and (b)
dielectric loss e" as a function of sand content in the soils at Xband microwave frequency
for Wv 0.27 at X-band microwave frequency. It is
evident from the figure that, the value of e' increases
with increase in sand content in the soils. Thus at Xband microwave frequency, at higher moisture
contents above transition moisture
~
F Sand ^
F Sandy loam
F S ill to.'.in ^
F s illy clnv loam
From Fig. 5(b) it is evident that for wet soil (Wv ~
0.27) the value of s" decreases with increase in sand
content at X-band microwave frequency.
The dielectric constant e' and the dielectric loss e”
of various soil samples were calculated using the
empirical models based on the texture structure of the
VOLUMETRIC MOISTURE CONTENT
Fig. 6—Comparison of measured values of die dielectric constant
c' and the dielectric loss e " of the soil samples of
(a) Surendranagar sandy loam and (b) Valsad silty clay loam with
the values calculated using the empirical models
INDIAN J RADIO & SPACE PHYS, JUNE 2008
o
OJ
O
l
»o
to
O
O
l
increases with increase m frequency from C- to Xband range for Sabarmati sand, but for Gandhinagar
sandy loam e" slightly decreases with increase m
frequency Similar results were obtained by other
workers5'8
Figures 9(a) and 9(b) show comparison of
experimentally measured values of s' and s", at Cband microwave frequency, for Sabarmati sand and
Gandhmager sandy loam soils, with the calculated
values using the empirical models3,4,15 It is seen that
the measured values of e' for both soils are m good
agreement with the calculated values using the
empirical models The measured values of e" for both
(b ) G a n d h in a g a r S a n d y L o a m
O
8
4
O
!
D IE LE C TR IC C O N S T A N T (s') / L O S S (s")
also m good agreement with the empirical models up
to the transition moisture, after which the calculated
values using the empirical models are higher than the
measured values.
Figure 7 shows the graph of experimentally
measured values of the dielectric constant e' and the
dielectric loss e" for the Sabarmati river sand and
Gandhinagar district sandy loam soils for various
moisture contents at C-band microwave frequency.
Again it is seen that the dielectric constant increases
with increase m moisture content in the soils For the
moisture contents up to the transition moisture the
value of e' increases slowly initially, and after
transition moisture it increases rapidly.
The value of dielectric loss e" increases linearly
with increase m moisture content for the sand and
sandy loam soils Above transition moisture the value
of e" for sand is slightly higher than that of the sandy
loam
For a comparison, C- and X-band measured values
of the dielectric constant e' and the dielectric loss e"
for Sabarmati sand and Gandhmager sandy loam soils
plotted against various moisture contents are shown m
Figs 8(a) and 8(b). It is seen that the value of 8'
decreases with increase in frequency from 5 65 GHz
to 9.5 GHz for both the soils The value of e" slightly
DIELECTRIC CONSTANT (s') / LOSS (s 'l
226
0
0
0.1
0 .2
0.3
0.4
V O L U M E T R IC M O IS T U R E C O N T E N T
Fig 7—Measured values of the dielectric constant e' and the
dielectric loss e" for the Sabarmati nver sand and Gandhinagar
district sandy loam soils for various moisture contents at C-band
microwave frequency
0
01
02
0 3l
VOLUMETRIC MOISTURE CONTENT
Fig 8—Comparison of C- and X-band measured values of the
dielectric constant e’ and the dielectric loss e" foi (a) Sabarmati
sand and (b) Gandhmager sandy loam soils plotted against various
moisture contents
227
o
o
E M IS S IV IT Y
o
. GADANI &VYAS DIELECTRIC CONSTANT OF GUJARAT SOILS AT X- & C-BAND MW FREQUENCIES
x X-Band
0.2
o C-Band
0
0.1
0.2
0.3
V O L U M E T R IC M O IS T U R E C O N T E N T
Fig 10—-The variation of emissivity of Gandhinagar sandy loam
soil at C-band microwave frequency as compared to that at Xband microwave frequency
4 Discussion
Fig 9—Comparison of experimentally measured values of e' and
e", at C-band microwave frequency, for (a) Sabarmati sand and
(b) Gandhmager sand> loam soils, with the calculated values
using the empirical models
the soils are higher than the values calculated using
the empirical models.
A typical plot of emissivity versus moisture content
at normal incidence for the soil collected from
Gandhinagar district is shown in Fig. 10 It can be
seen that at both the frequencies emissivity decreases
with increase ir„ moisture content in the soil Similar
results were observed for the soils collected from
other regions of Gujarat
Wet soil is a mixture of soil particles', air voids and
liquid water. The high dielectric constant of water (ew
~ 80) depends on the molecule's’ ability to align its
dipole moment along an applied field Anything that
hinders the molecule’s rotation (e g freezing, tight
binding to a soil particle, etc.) therefore reduces the
dielectric constant of water The water molecules
contained m the first molecular layers surrounding the
soil particles are tightly held by the soil particles, due
to the influence of matric and osmotic forces, called
bound water3,4 Hence the dielectric constant of bound
water is low. The matric forces acting on a water
molecule decrease rapidly with the distance away
from the soil particle surface. Hence the water
molecules located several molecular layers away from
the soil particles are able to move freely within the
soil medium, called free water Thus the dielectric
constant of free water is high
At moisture contents below the transition moisture
in the soil there are more bound water molecules as
compared to free water molecules Hence the
dielectric constant of soil at lower moisture contents
is low As the moisture content in the soil increases
above transition moisture in the soil, the free water
molecules increase rapidly m the soil increasing the
dielectric constant e' rapidly
The sand has a small surface area per unit volume
as compared to that of clay particles Hence at given
228
INDIAN J RADIO & SPACE PHYS. JUNE 2008
moisture content the clay particles are capable of
holding more bound water molecules as compared to
sand. i.e. there are more free water molecules in sand
as compared to that in clay at given moisture content.
Hence the increase in the dielectric constant of sandy
soils is more as compared to high clay content soils,
above the transition moisture.
This is the reason why in Figs 4 and 7 the dielectric
constant e' of the soils increase slowly initially up to
the transition moisture and rapidly after the transition
moisture, and why the e' of sandy soils is more as
compared to high clay content soils at moisture
content above transition moisture.
In Fig. 5(a) for Wv ~ 0.27 the dielectric constant e'
increases as sand content increases, due to the lower
specific surface area of sand per unit volume.
At given moisture content above W/, the dielectric
constant e' decreases with increase in frequency for
Sabarmati river sand and Gandhinagar sandy loam
soil in Fig. 8. But the dielectric loss 8" for Sabarmati
river sand increases with increase in frequency from
5.65 GHz to 9.50 GHz. where as e" for Gandhinagar
sandy loam soil decreases with increase in frequency.
This can be explained by the fact that at any given
moisture content and at all frequencies, e' is found to
be roughly proportional to sand content (and inversely
proportional to clay content). Thus e' is soil texture
dependent in the same fashion at 5.65 GHz and 9.50
GHz. But between 4 and 6 GHz 8" is nearly
independent of the soil texture3,4 at all soil moisture
conditions At frequencies above 8 GHz e' decreases
with soil clay fraction4 (or increase with increase in
soil sand fraction).
5
6
7
8
9
10
11
12
13
14
15
16
17
Acknowledgement
Authors are thankful to the Head, Department of
Physics, University School of Sciences, Gujarat
University. Ahmedabad, for providing constant
encouragement and laboratory facilities.
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