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Magnetic signal prospecting using multiparameter measurementsthe case study of the Gallic Site of Levroux.

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Archaeological Prospection
Archaeol. Prospect. 17, 141–150 (2010)
Published online 5 August 2010 in Wiley Online Library
(wileyonlinelibrary.com) DOI: 10.1002/arp.384
Magnetic Signal Prospecting using
Multiparameter Measurements: the Case
Study of the Gallic Site of Levroux
M. PE¤TRONILLE1*, J.THIESSON1, F.-X. SIMON1,2 AND O. BUCHSENSCHUTZ3
1
De¤partement de ge¤ophysique applique¤e, UMR 7619 Sisyphe, Universite¤ Pierre et Marie Curie Paris 6, 4
Place Jussieu, 75252 Paris cedex 05, France
2
Po“le d’Arche¤ologie Interde¤partemental Rhe¤nan, Ope¤rations Arche¤ologiques, 2 Alle¤eThomas Edison,
ZA sud ^ CIRSUD, 67600 Se¤lestat, France
3
UMR 8546 CNRS-ENS Arche¤ologies d’Orient et d’Occident et textes anciens (AOROC), 45 rue d’Ulm,
75230 Paris cedex 05, France
ABSTRACT
The‘magnetic signal’that combines both the induced (Ji) and the remanent (Jr) magnetization is widely used in archaeological and pedological prospecting. Magnetic prospecting recording the lateral variations of the total magnetization
is the most frequently used measurement before in-phase magnetic susceptibility (Kph) and magnetic viscosity (Kqu)
mapping. The work presented here brings together three types of prospecting technique: magnetic field survey and
electromagnetic measurements with both frequency and time domain devices that measure magnetic susceptibility
andviscosityrespectively.The site studied, the Gallic town of Levroux (Indre,France), isparticularly interesting because
it includes features such as pits and ditches dug into the calcareous substratum partly filled with topsoil and with residues of different metallurgical and fire activities.The field results indicated anomalies with different types of characterization: (i) many compact features filled with magnetic, electrically conductive and minimally viscous materials; and
(ii) elongated anomalies characterized by lower magnetic properties and electric conductivity but relatively higher
magnetic viscosity than those of the compact features. In addition to the location of the features, the combination of
the information brought by the different types of measurements allows us to evaluate the possible erosion of their
upper parts by ploughing, to assess their depth (never deeper than1.30 m) and to precise the nature of the feature’s fill.
Copyright # 2010 John Wiley & Sons, Ltd.
Key words: magnetic susceptibility; magneticviscosity; electromagneticinduction; magneticsignalmapping; metallurgical activity; Levroux
Introduction
The study of the magnetic properties of soils began
60 years ago with the work of Le Borgne (1955),
who showed that soils have usually higher magnetic
susceptibilities than their parent rock. This fact is at the
origin of significant developments in archaeological
prospecting (Aitken et al., 1958; Scollar et al., 1990) and
different techniques using the magnetic properties of
soils therefore have been developed.
* Correspondence to: M. Pétronille, Département de géophysique
appliquée, UMR 7619 Sisyphe, Université Pierre et Marie Curie Paris
6, 4 Place Jussieu, 75252 Paris cedex 05, France.
E-mail: petronille@ipgp.jussieu.fr
Copyright # 2010 John Wiley & Sons, Ltd.
The magnetic method which records the lateral
variations of the total magnetization (J) was firstly
undertaken and showed its efficiency to locate buried
features that were either heated, such as ovens and
kilns, or unheated, such as pits and ditches (Aitken
et al., 1958; Tite and Mullins, 1971; Mullins, 1974;
Marmet, 2000; Benech et al., 2002). In parallel
frequency-domain electromagnetic (FDEM) instruments began to be developed in the 1960s; they allow
simultaneous measurement of both magnetic susceptibility (Kph) and electrical conductivity (s) (Colani,
1966; Howell, 1968; Tabbagh, 1974; Tabbagh, 1986;
Marmet, 2000; Benech et al., 2002). At the same time,
time-domain electromagnetic (TDEM) instruments
that measure decayed impulse response of the ground
were proven to be sensitive to the loss or gain of
Received 19 March 2010
Accepted 30 June 2010
M. Pétronille et al.
142
induced magnetization with time (Ji): the so-called
magnetic viscosity (Kqu) (Colani and Aitken, 1966;
Dabas and Skinner, 1993; Thiesson et al., 2007). This
property depends on the presence of ferrimagnetic
grains characterized by very small size ( 10–20 nm)
(Dearing et al., 1996). Despite of some limitations of
these methods, many studies have shown their complementarity in terms of type of magnetization, depth
and geometry of the magnetic sources (Tabbagh, 1984;
Desvignes and Tabbagh, 1995; Benech et al., 2002).
The purpose of the present paper is to improve the
characterization of the features on the Gallic site of
Levroux (France) by non-destructive multiparameter
measurements and to test the reliability of the magnetic and EM (FDEM, TDEM) devices used in that
archaeological context. On this site significant contrasts were expected between the magnetic properties
of the surrounding soil and those of the archaeological
features dug into the calcareous substratum and
partly filled with topsoil and with residues of different
metallurgical and fire activities (Buchsenschutz et al.,
1993). The simultaneous use of FDEM and magnetic
devices allows the location of the anomaly sources
beneath 0.40 m, this estimate being achieved after
subtraction of the induced magnetization (Ji) of the
topsoil (Benech et al., 2002). The TDEM device allows
the strength of the viscous part of remanent magnetization (Jr) to be evaluated.
After a presentation of the geological and archaeological context of the site studied, we detail the characteristics of the three devices used for the geophysical
survey. The results obtained with the different methods
are then presented, followed by a general discussion
comparing geophysical results and archaeological
data.
cylindrical. They are usually comprised between 0.90 m
and 2 m wide and long, and less than 0.50 m deep,
although about a 100 of them reach 0.50 to 1.30 m in the
calcareous substratum, not to mention shafts that are
between 2 m and 9 m deep. The analysis of the pit
fillings revealed in most of cases earth mixed with
dumps of material characterized by charcoals, shards of
pottery, glass, coins, animal bones and numerous
metallic remains and slag resulting from an intense
metallurgical activity such as a smithy. Post holes
identified in the ‘Quartier des Arènes’ show a
diameter varying between 0.20 m and 1 m, and are
usually deeply carved in the calcareous substratum.
Like pits, they show different shapes (cylindrical,
rectangular, square) but are filled essentially with earth
and limestone. The ditches identified in this sector were
similarly filled with earth and limestone and most of
them are usually between 0.5 and 1 m wide and deep.
The archaeological and geological context of
Levroux therefore makes this site particularly interesting for both magnetic and electromagnetic surveys
to locate the different features and to assess the nature
of their fill. This work reports on the results of the
magnetic and EM surveys made in the south of field
304 located in the ‘Quartier des Arènes’ (fields L1 and
L1’, Figure 1) that has revealed in its northern part a
very large craft activity: 593 post holes, 153 pits filled
with lots of material including slag, as well as some
ditches including a large one 0.90 m wide and 0.60 m
deep, named R 271, filled with limestone-rich sediment
but poor in craft remains (Figure 1). This ditch, which
cuts all the features it encounters, corresponds to a
former Gallo-Roman fence.
Field devices and methods
Geological and archaeological context
Magnetic field and frequency-domain
electromagnetism devices
The city of Levroux (Indre, France) began to develop in
the second century BC with the settlement of an area that
was characterized by houses elevated by stilts at
least partly used for craft activities in the ‘Quartier
des Arènes’, south of the present-day village
(Buchsenschutz et al., 1993, 2000). The numerous
excavations made in this sector in the 1980s showed
a large number of features carved in the Oxfordian
(Jurassic) calcareous substratum located just below the
topsoil and these are aligned along east–west and
north–south dominant directions: 311 pits and 1419 post
holes as well as some ditches corresponding to possible
property divisions. Five types of pit have been
identified: elongated, square, shaft, hemispherical and
During a first survey we performed vertical gradiometer measurements using a G858 magnetometer
(Geometrics Ltd) with two caesium vapour sensors
located at 0.40 m and 1 m above the ground surface,
which allows measurements of the total magnetic field
pseudogradient regardless of time variations. This first
survey allowed us to make sure of the extent of the
human activity on the field studied and to quickly
locate the different magnetic anomalies.
In a second survey we performed simultaneously
magnetic field and FDEM (CS60) measurements in
order to evaluate the part of the total magnetic field
anomaly due to induced or remanent magnetization of
features lying beneath 0.40 m. We used a G858
Copyright # 2010 John Wiley & Sons, Ltd.
Archaeol. Prospect. 17, 141–150 (2010)
DOI: 10.1002/arp
Magnetic signal prospecting using multiparameter measurements
143
Figure 1. Location of the excavated zone in the northern part of field 304 (‘Quartier des Are'nes’, Levroux) and presentation of the main features discovered in the1980s (types of pit and ditch R 271).The fields L1and L1’prospected in 2008 and located in the south of field 304 are also indicated.
magnetometer with two sensors located at 0.40 m above
the ground and horizontally separated by 0.60 m, which
allows two parallel profiles to be simultaneously
recorded (Tabbagh, 2003) (Figure 2i). It was coupled
with a CS60 Slingram apparatus (0.60 m intercoil
spacing and 27.96 kHz frequency) in vertical coplanar
(VCP) configuration (Figure 2i), which allows two
measurements under the hypothesis of low induction
number: in-phase magnetic susceptibility, Kph, down to
a depth of 0.40 m, proportional to the in-phase part of
the secondary to primary field ratio on the one hand,
and electrical resistivity, r, down to a depth of around
0.80 m, proportional to the quadrature part of this ratio
on the other hand (Thiesson et al., 2009).
Data sets obtained during these two surveys were
recorded continuously along each profile over a
24 m 50 m rectangular field named L1 (Figure 1).
The profiles were 1 m apart, which gives a 0.50 m
meshed grid after extraction of the data.
Time-domain electromagnetism device
We performed Kqu measurements with the VC100
TDEM Slingram device (1.10 m intercoil spacing) in
Copyright # 2010 John Wiley & Sons, Ltd.
perpendicular configuration (Figure 2ii) (Thiesson
et al., 2007). The transmitter coil of the VC100 creates
a primary field that magnetizes the magnetic minerals
of the soil. At different times after shut-off of this field,
the receiver coil gets the signal corresponding to the
decayed loss of induced magnetization (Ji) of the
soil, the so-called magnetic viscosity (Kqu), that
depends on the presence of ferrimagnetic grains
characterized by very small size ( 10–20 nm; Dearing
et al., 1996). In the VC100, five sampling measurements
are considered after shut-off (12.2, 22.9, 44.2, 86.9 and
172 ms) to obtain the responses for both short and long
time-delays.
We prospected a 20 20 m square in the middle
of field L1, named L1’ (Figure 1), with two different
heights of the VC100 in order to get information
about the depth of possible Kqu anomaly source(s).
The device was first positioned on the ground
(lower position) and then at 0.19 m above the
ground (higher position), which corresponds to
respective investigation depths of 0.80 m and 0.60 m,
these latter depending on the geometry of the
transmitter and of the receiver coils (Thiesson et al.,
2007). The data were acquired over a 1 1 m mesh
grid.
Archaeol. Prospect. 17, 141–150 (2010)
DOI: 10.1002/arp
M. Pétronille et al.
144
Field results
Magnetic field and frequency-domain
electromagnetism surveys
Figure 2. (i) The G858 magnetometer (Geometrics Ltd) coupled with
the CS60 frequency-domain electromagnetism device. (ii) TheVC100
time-domain electromagnetism device.
Two types of features can be distinguished on the
magnetic maps in Figure 3: the compact ones with a
very strong anomaly magnitude and the elongated
ones of lower magnitude. The strongest compact
anomalies have amplitudes varying between 60 and
140 nT. The largest anomaly is rectangular in shape
(5 10 m) and 90–140 nT in amplitude (anomaly a,
Figure 3). Additionally one can distinguish smaller
circular features characterized by 2–5 m diameters and
60–100 nT amplitudes (anomalies b to h, Figure 3). The
polarity of these anomalies (negative part in the north,
positive in the south) is characteristic of features more
magnetic than the surrounding soil.
Some elongated anomalies also can be observed on
the magnetic maps in Figure 3. They are characterized
by a relatively weak total magnetization (anomalies
between 5 and 15 nT) compared with the compact
ones. Two of them (anomalies j and k, Figure 3) are 1 m
wide and seem to be at right angles to one another.
Anomaly l shows a similar shape and strength as
anomalies j and k, but with an oval form surrounding
the compact anomaly a.
We tried to estimate the depth of the different
magnetic sources using Euler’s deconvolution
(Thompson, 1982; Desvignes et al., 1999). We chose a
structural index equal to 2.5 for the compact
anomalies, which is a likely value for compact features,
and a structural index equal to 2 for the elongated ones.
We found a depth below the ground surface estimated
between 0.50 and 0.90 m for all the compact anomalies
Figure 3. (i) Vertical magnetic pseudogradient map and (ii) total magnetic field map obtained with the G858 (Geometrics Ltd) on the field L1.
Letters a to iindicate the principal compact anomalies, and letters j to lindicate the elongated ones.
Copyright # 2010 John Wiley & Sons, Ltd.
Archaeol. Prospect. 17, 141–150 (2010)
DOI: 10.1002/arp
Magnetic signal prospecting using multiparameter measurements
described above, and between 0.60 and 0.70 m for the
three elongated anomalies identified in this work.
The measurements made with the CS60 (Figure 4)
show that the compact anomalies observed on the
magnetic maps have both high Kph in their upper parts
and relatively low electrical resistivity. The elongated
features are less visible on the maps obtained with the
CS60 than the compact ones.
The coupled measurements of the G858 and the
CS60 allow a more precise assessment of the depth of
the anomaly sources and of the nature of the
magnetizations. The best way to compare the EM
Kph map with the magnetic survey results is to
transform the Kph map derived from the CS60 into a
magnetic anomaly map generated by the topsoil (first
0.40 m) induced magnetization variations. The EM
data were filtered with a function calculated for two
145
dipoles located at 0.10 m and 0.25 m depth (Benech
et al., 2002). The magnetic anomaly map thus obtained
(Figure 5i) shows weak anomalies corresponding to
5% of the total magnetic anomalies amplitude. Consequently the differences that exist between this and
the original magnetic map may have two possible
explanations: on the one hand, the magnetic features
may bear an important remanent magnetization Jr; on
the other hand, 95% of their volume may be located
beyond the investigation depth of the CS60 (which
only slightly exceeds the topsoil thickness). However,
considering that the superficial layer has been
ploughed and cannot present any macroscopic Jr,
the subtraction of the CS60 induced magnetic anomaly
map from the total magnetic field anomaly measurements delivers the magnetic anomaly map corresponding to features lying beneath 0.40 m (Figure 5ii).
Figure 4. (i) Apparent in-phase magnetic susceptibility (Kph) map and (ii) apparent electric resistivity (r) map obtained with the CS60 for field L1.
Letters a to iindicate the principal compact anomalies, and letters j to lindicate the elongated ones.
Figure 5. (i) Induced magnetic anomaly map computed from the Kph map obtained with the CS60 for field L1 (Figure 4i). (ii) Magnetic anomaly
map of field L1 corresponding to the features lying beneath 0.40 m. Letters a to i indicate the principal compact anomalies, and letters j to l
indicate elongated ones.
Copyright # 2010 John Wiley & Sons, Ltd.
Archaeol. Prospect. 17, 141–150 (2010)
DOI: 10.1002/arp
146
M. Pétronille et al.
According to the values obtained for this resulting
map, CS60 measurements confirm that the sources of
the principal magnetic anomalies observed on the
magnetic maps in Figure 3 correspond to features lying
beneath 0.40 m, the distinction between induced and
remanent magnetization is not possible given that
these sources lie out of the volume investigated by the
CS60.
Time-domain electromagnetism surveys
Figure 6. Location of field L1’ (black square) on the vertical magnetic
pseudogradient map obtained with the magnetometer G858 for field L1.
The TDEM measurements using the VC100 were
performed on a 20 m 20 m square (field L1’) in the
middle of the field L1 (Figure 6) where the main
compact and elongated anomalies appear on the
Figure 7. VC100 Kqu mapsoffield L1’obtainedwhenthe deviceisinthelowerposition (onthe ground).Letters a, band cindicatetheprincipalcompact
anomalies.
Copyright # 2010 John Wiley & Sons, Ltd.
Archaeol. Prospect. 17, 141–150 (2010)
DOI: 10.1002/arp
Magnetic signal prospecting using multiparameter measurements
maps obtained with both the G858 and the CS60.
Figures 7 and 8 show the Kqu maps obtained for the
four last channels of the VC100 prototype when
the device is placed both in lower (on the ground
surface) and higher (0.19 m above the ground
surface) positions. The first channel of the VC100,
corresponding to 12.2 ms, is not presented here
because the response due to soil conductivity
(decreasing as t 3/2) remains important in that
channel. By comparison the magnetic viscosity
response (decreasing as t 1) dominates the four last
channels.
147
Some anomalies are observed in the northwest part
of the maps obtained with the device in the lower
position (Figure 7). The shape and location of one
suggest that it corresponds to anomaly a, discussed
above. It has maximum amplitudes at 22.9, 44.2 and
86.9 ms but decreases at 172 ms. Another compact
anomaly can be seen on the four maps in Figure 7. Its
shape and location (southwest part of the maps)
suggest that it corresponds to the group of anomalies b
and c. Its amplitude and size lie in the same range as
the anomaly a but with a more marked amplitude
decrease as the time after shut-off increases.
Figure 8. VC100 Kqu maps of field L1’ obtained when the device is in the higher position (0.19 m above the ground). A particular scale has been
adapted to the smaller amplitude of the anomalies detected on the three maps obtained between 44.2 and 172 ms in order not to lose information,
which would be the case in taking the scale of the map obtained at 22.9 ms. Letters a, b and c indicate the principal compact anomalies.
Copyright # 2010 John Wiley & Sons, Ltd.
Archaeol. Prospect. 17, 141–150 (2010)
DOI: 10.1002/arp
148
The maps in Figure 8, obtained with the device in the
higher position, show the same patterning as those in
Figure 7. Compact anomalies a, b and c can be seen, but
their amplitude is significantly reduced according to
the maxima observed on the maps obtained with the
device in the lower position (Figure 7). These results
confirm that the sources of these viscous materials is
around 0.80 m.
In order to analyse the previous results, we assessed
the decrease of the signal with time, which can be
expressed in the form A/tn and we drew the map of the
parameter n when the VC100 is in the lower position
(Figure 9i). The resulting map, obtained by taking into
account the times between 22.9 and 172 ms, shows that
n lies between 0.98 and 1.21. This range corresponds to
the behaviour of randomly dispersed single domain
grains (Néel, 1949; Le Borgne, 1960; Dabas et al, 1993).
Consequently the different anomalies have similar
viscous grain-size distributions, even if it must be
noted that the compact anomalies a, b and c have
higher values, about 1.15.
It would be generally interesting to map the Kqu /
Kph ratio to describe the relative abundance variations
of the grains responsible for Kqu, but we did not have
Kph measurements between a depth of 0.60 m and
0.80 m. Nevertheless one can attempt to extrapolate the
effect of the viscous magnetization cumulated over
M. Pétronille et al.
2200 years in using the values of magnetic viscosity at
22.9 ms (the device being in the lower position) and to
calculate the equivalent vertically magnetized layer at
0.60 m depth corresponding to these anomalies
(Desvignes et al., 1999). We can compare it to the
equivalent vertically magnetized layer at 0.60 m depth
deduced from the magnetic anomaly created by the
features beneath 0.40 m (Figure 9ii). The contribution
of the magnetic viscosity to total magnetization
recorded by the G858 is very limited: this implies that
the magnetization responsible for the magnetic sources
can be either induced or remanent, but of limited
viscosity. In contrast when considering the results
obtained for elongated features ( j to l), it is clear that
they are relatively more viscous than the compact
features (Figure 9ii). This difference can be explained
by a higher content in small ferrimagnetic grains, thus
by a difference in the fills composing the magnetic
sources (Dearing et al., 1996).
General discussion and conclusions
Considering their geometrical shape, the magnetic
anomalies identified on the Gallic site of Levroux can
be separated into two types: compact and elongated, as
in the case of archaeological features carved in the
Figure 9. (i) Map of the parameter n calculated for field L1’when theVC100 is in the lower position and for times between 22.9 and172 ms; nis given
by the decrease of the signal of the VC100 with time expressed in the form A/tn. (ii) Map of field L1’ representing the evaluation of the viscous
part of the total magnetic field anomaly. Letters a, b and c indicate the principal compact anomalies, and letters j to lindicate the elongated ones.
Copyright # 2010 John Wiley & Sons, Ltd.
Archaeol. Prospect. 17, 141–150 (2010)
DOI: 10.1002/arp
Magnetic signal prospecting using multiparameter measurements
Oxfordian limestone found during the excavations of
the 1980s: pits and post holes on the one hand, and
ditches on the other hand. Their spatial distribution
confirms what their shapes and their sizes suggest:
small compact features can be interpreted as pits or
post holes, and elongated ones as ditches. For example
anomalies j and k, shown in Figure 10, are in continuity
with the ditch R 271 discovered in the 1980s in the
north part of field 304 (Buchsenschutz et al., 1993,
2000) – j is in direct continuity with this feature – and
correspond to a southward extension of this ditch
partitioning the area.
Moreover two major facts revealed by the simultaneous interpretation of magnetic and EM results
must be emphasized. First, the depths of anomaly
sources have been assessed, which is proved by the
small contribution of the induced magnetization of the
first 40 cm to the total anomalies delivered by magnetic
149
measurements and confirmed by Euler’s deconvolution. The range of depth thus obtained is in agreement
with the previous excavations at Levroux. Second,
original characteristics can be identified for the filling
of strong compact anomalies. Their good electrical
conductivity suggests that they do not correspond to
kilns (or other type of features built with bricks) but to
pits having a significant content of fine soil particles
and increased moisture. Their high magnetic signal
associated with a low ratio of viscous magnetization to
total magnetization suggests the presence of metallurgical wastes: ore pieces, slag or even iron particles
(Mullins, 1974; Dabas, 1989; Pétronille, 2009), as
already observed in most of the pits excavated on
this site. More precisely, according to the amplitude of
the main compact anomalies and their size, we can
propose that these features (a to i on Figure 10)
correspond to pits and probably not to post holes, the
Figure 10. Interpretationofthemagneticanomaliesidentifiedontheverticalmagneticpseudogradient mapobtainedfor field L1.Compact anomalies
a to i are interpreted as pits. Elongated anomalies are interpreted as ditches, j and k being in continuity with the ditch R 271discovered during excavations in the northern part of field 304.The different types of pit found in the excavated part of this parcel are indicated too.
Copyright # 2010 John Wiley & Sons, Ltd.
Archaeol. Prospect. 17, 141–150 (2010)
DOI: 10.1002/arp
150
fill of the latter being essentially made with earth and
limestone that are weakly magnetic and electrically
conductive materials. Contrary to the response from
the pits, the elongated anomalies are characterized by
weak total magnetization but relatively strong magnetic viscosity, as is common for earthen archaeological features (Graham and Scollar, 1976; Dabas et al.,
1993; Pétronille, 2009).
The geophysical surveys presented in this paper
confirm the advantage of the simultaneous use of
magnetic and EM properties in investigations to better
characterize the magnetic signal of soils on an
archaeological site. Combining three properties (total
magnetic field anomaly, near-surface magnetic
susceptibility and magnetic viscosity) we are able to
discriminate features by their shape and their depth
but also by their infills. This categorization significantly adds to the interpretation of this site.
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DOI: 10.1002/arp
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