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Exploration of the Hellenistic fortification complex at Asea using a multigeophysical prospection approach.

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Archaeological Prospection
Archaeol. Prospect. 13, 1–9 (2006)
Published online 6 June 2005 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/arp.262
Exploration ofthe Hellenistic
Fortification Complexat Asea Usinga
Multigeophysical Prospection Approach
M. DOGAN* AND S. PAPAMARINOPOULOS
Laboratory ofApplied Geophysics, Department of Geology, University of Patras, 26110, Rio,
Patras, Greece
ABSTRACT
Multi-electrode resistivity tomography, twin-probe and ground-penetrating radar (GPR) geophysical
techniques were applied at the multicultural archaeological site of Asea as a collaboration between
the Swedish Instituteat Athensandthe Universityof Patras.Thiswork presentsthe geophysicalresults
of Grid16 where a subsurface polygonalcity wallwas discoveredinthe field season 2002.It isintended
to explore especially thelocation, depthand trending direction ofthe city wallto contribute progressive
studies of methods applied to subsurface walls. The survey was performed using the Geopulse
imager, with 25 electrodes at 0.8 m electrode spacing, Geoscan RM4 and SIR-10 GPR systems. The
surveysused a 20 20 m grid mode and theWenner^Schlumberger hybrid array was selected for resistivity measurement. The Hellenistic period city wall was imaged in the south^north direction in a
well-preserved situation below subsurface. The results revealed that the resistivity method can
provideaclearpseudo-image ofamediumtolarge sizewallwithhighaccuracyonarchaeologicalsites.
Interpretation of GPR three-dimensional depth-slices identified the position, dimension and orientation ofthe subsurface city wall.Copyright 2005 JohnWiley & Sons,Ltd.
Key words: Asea; city wall; multi-electrode resistivity tomography; Campuse-Geopulse resistivity
meter;Wenner^Schlumbergerarray; pseudo-image; SIR-10
Introduction
The application of geophysical methods changed
the approach to the scientific involvement
in archaeology in the twentieth century
(Weymouth, 1986). Modern developments in
geophysical related digital electronic technology
have increased the ability of geophysical capabilities to provide an extremely rapid, three-dimensional reconnaissance of a site (Scollar et al.,
1990). In addition the use of multiple techniques
on a given site can vastly expand our understanding of its geophysical characteristics and
archaeological structure (Clay, 2001). In this
* Correspondence to: M. Dogan, Laboratory of Applied Geophysics, Department of Geology, University of Patras, 261 10,
Rio, Patras, Greece. E-mail: meliha_dogan@yahoo.com
Copyright # 2005 John Wiley & Sons, Ltd.
respect, a combination of various data sets
obtained using the different methods and their
three-dimensional image presentation from different processing techniques can help provide a
clear picture of the studied areas. Concordance
between the results of two or more different
techniques in signalling a specific target
increases the reliability and confidence of the
target detection itself.
The site
The Asea valley is located between Tegea and
Megalopolis and measures 8 7 km in size
(Forsen and Forsen, 2002). It has always been of
great strategic importance, as it provides the
only easy way of communication between the
Received 18 July 2004
Accepted 14 February 2005
M. Dogan and S. Papamarinopoulos
2
prosperous plain of Tripolis, with the strong
ancient cities of Tegea and Mantinea, and the
plain of Megalopolis. In the early 1990s, a first
reconnoitering tour of the valley was directed by
J. Forsen and the Asea valley again came into
focus of archaeological research as the Asea
Valley Survey 1994–1996 and the excavation of
the Doric temple on Agios Elias in 1997.
Geographically, it is a restricted region
between the larger alluvial areas in the Megalopolis basin and the plains of Tripolis (Figure 1).
The valley bottom is the source area of two
large Peloponnesian Rivers, the Alpheios and
the Eurotas. Asea Paleokastro is a large
(250 100 m) flat-topped hill with steep sides
that is detached from the lowermost southeast
slope of Agios Elias (Forsen, 1996). Immediately
to the southwest of the hill lies the railway station
of Asea, around which geophysical surveys were
carried out (Figure 1).
The geology at Paleokastro comprises limestone lenses in flysch (Jacobshagen, 1986). This
evidently separates this area from the alluvium
deposits of the Alpheios River downhill to the
southeast (Figure 1) where the grid of interest
was laid out.
The city wall
The lower city fortification walls of Asea were
built using the rustic polygonal technique and
belong to the third century BC, namely the
Hellenistic period (Forsen and Forsen, 2002).
The only visible parts of the wall that ringed
the lower city are the two spurs which run down
slopes of the Acropolis. The width of the wall is
3.3 m. The wall blocks are very large and frequently measure 1–1.5 m in length and 0.5–1.2 m
in height. Substantial towers measuring 6.6 m in
height and 6.45 m in width are located on the
wall at intervals of 100 Greek feet (33 m). They
were built with very large blocks up to 1.5 and
even 2 m length, and about 0.7 m in height (Forsen and Forsen, 2002).
Survey details and methods
The results achieved in the 2001 survey season
(Dogan and Papamarinopoulos, 2003) encourCopyright # 2005 John Wiley & Sons, Ltd.
aged the shift of survey location to downhill
side of the Paleokastro region on the alluvial
deposits of the River Alpheios in order to search
for the southern continuance of the subsurface
lower city wall, and to investigate its turning and
conjunction points in the Asea. The grid of interest, Grid 16 is 20 m 20 m in dimension, was set
at about 2 m away from eastern side of the
Megalopolis–Tripolis highway (Figure 1). The
survey area was cleared of long dry bushes by
means of a bulldozer.
RM-4 survey
Data were acquired at 1 m intervals along the
traverse with 1 m separations between traverses
using a Geoscan RM-4 resistance meter. The
colour image map of the resistance distribution
of the grid exposed an exciting reflection of the
city wall, which is clearly outlined by hot colours
(reddish-purple) at left side of the map
(Figure 2a). The resistance values of Grid 16
range between 297 and 314 Ohms. The map of
resistance distribution of Grid 16 (Figure 2a)
provides a better understanding of the location
and trend of the city wall (white arrow) and a
scattered fallen block. The S–N trending city wall
measures 1 to 3 m in width.
Multi-electrode resistivity tomography
survey (MRT)
The same grid was also covered by the multielectrode resistivity tomography method and 20
sections were measured. The resistivity data
were recorded using a Campus-Geopulse resistivity meter with 25 electrodes. A 0.8 m electrode
interval and Wenner–Schlumberger array were
selected for the measurements. The 1:800 scale
horizontal depth slices map of the grid produced
is shown in Figure 3. The depth slice 0.84 m was
specifically magnified (Figure 2b).
The city wall and its tower complex can
be seen starting at a depth of 0.20 m (Figure 3).
The scattered stones that are approximately 1 m
in length and 0.30 m height are most probably
the construction blocks of later dwellings
located outside of the city wall (Figure 3). The
tower blocks give the impression of being more
Archaeol. Prospect. 13, 1–9 (2006)
Exploration of the Hellenistic Fortification Complex at Asea
3
Figure1. Location of the surveyed Grid16.
Copyright # 2005 John Wiley & Sons, Ltd.
Archaeol. Prospect. 13, 1–9 (2006)
4
M. Dogan and S. Papamarinopoulos
weathered and disintegrated compared with the
blocks of the city wall. The inner room of
the tower complex could be between depths of
0.68 and 1.34 m (Figures 2b and 3) below the
subsurface.
The inverse model resistivity pseudo-section
(Loke and Barker, 1995) of Line 06 provides an
idea about the dimensions of the city wall as well
as its burial depth (Figure 4). The blocks are
approximately 1 to 1.5 m in height and their
resistivity ranges from 7 to 148 Ohm-m. This
dimension coincides well with the dimensions
(Forsen and Forsen, 2002) for the southern segment of the lower city wall.
Ground-penetrating radar (GPR) survey
Also, 20 vertical radar sections were carried out
on the same grid. The SIR-10 monostatic GPR
system was used with the nominal frequency of
the antenna 500 MHz. The spatial sampling was
set to 2 m. The acquisition range was 80 ns and
the sampling rate was 512 samples per scan.
An attempt to convert the time slices to depth
has been investigated (Figure 5) in which a
hyperbola was fitted to faint hyperbolic reflections seen in the raw data. Reflections from a
point source buried in the ground produce
hyperbolic reflections. Hyperbola fitting is a
standard process available in GPR-Slice software
(Goodman, 2003).
Figure 5 shows the hyperbola obtained with
GSSI 500 MHz antenna at the Asea site. The
horizontal axis represents metres. The range setting is 80 ns two-way travel time (TWTT). The
ground surface reflection is approximately at
14 ns, the peak of the hyperbolic reflection is at
approximately 33.3 ns, and a good choice for the
bottom of the hyperbola is at about 40 ns TWTT.
The distance (2x) of the hyperbola is approximately 1.07 m
Figure 2. The results of Rm-4, MRTand GPR measurements.
(a) Geoscan RM4 twin probe resistance survey of Grid16.Colour plot of acquired resistance data, white arrow points to the
city wall. (b) Tomography survey, magnified horizontal depth
slice 0.84 m. (c) Magnified GPR depth slice 52.08^75.36 cm,
white arrow shows the city wall and its tower complex.
Copyright # 2005 John Wiley & Sons, Ltd.
2x ¼ (12.15 13.22) ¼ 1.07 m; x ¼ 0.535 m
t1 ¼ (40 14)/2 ¼ 13 ns (divided by 2 to obtain
one-way travel time)
t2 ¼ (33.3 14)/2 ¼ 9.65 ns
Soil velocity
p
ð1Þ
V ¼ x ðt21 t22 Þ
p
V ¼ 0:535= ð9:652 132 Þ ¼ 0:06 m ns1
Archaeol. Prospect. 13, 1–9 (2006)
Exploration of the Hellenistic Fortification Complex at Asea
Figure 3. Demonstration of electrical tomography for Grid16: horizontal depth slices in metres.
5
Copyright # 2005 John Wiley & Sons, Ltd.
Archaeol. Prospect. 13, 1–9 (2006)
6
M. Dogan and S. Papamarinopoulos
Figure 4. Inverse model resistivity section of Line06 from the survey conducted at Grid16. Sampling interval is 0.8 m.
Figure 5. Process of hyperbola fitting in an attempt to convert the time slices to depth.
Thus, a velocity of 6 cm ns1 is found for the
saturated soil (Annan and Cosway, 1992)
matrix material of Grid 16. Given the velocity,
Copyright # 2005 John Wiley & Sons, Ltd.
the depth can be assigned to the radar profiles.
With the peak of the hyperbola at t2 (ns) below
the surface, the depth to the anomaly may be
Archaeol. Prospect. 13, 1–9 (2006)
Exploration of the Hellenistic Fortification Complex at Asea
estimated by equation (2)
d¼
Vt2
2
ð2Þ
d ¼ 0:06 m ns1 9:65 ns ¼ 0:6 m
With an estimate of soil velocity the dielectric
constant (K) can be computed by equation (3)
p
K ¼ C=V
ð3Þ
p
K ¼ 0:2998 m ns1 =0:06 m ns1
K ¼ 25
where C is the speed of light in a vacuum
(0.2998 m ns1) and V is the velocity of radar
energy (in m ns1).
With this information, 12 time-slices were generated with a 5.7 ns interval (or approximately
17.28 cm depth) and converted into depth slices
using the calculated velocity 6 cm ns1 (Figure 6).
The data were subjected to background filtering
in order to cut out low-frequency anomalies
created by subsurface geology. The depth level
52.8–75.36 cm was enlarged (Figure 6), so that the
pattern of the city wall can be well seen.
Despite the relatively high water table causing
attenuation of the radar signal, (which proved to
be more problematic than was thought originally) useful results were obtained in Grid 16. An
advanced technique (Goodman, 2003) of threedimensional visualization of GPR depth-slices
allowed fine details of the archaeological remains
to be seen.
A number of points may be made from these
images. Interpretation of the GPR data revealed
two sets of structures located at the west and east
sides of the grid, within the top 2 m of stratigraphy, and separated by a natural depression running between them (Figure 6). The main feature
standing out on the western side is obviously the
city wall in respect of the size of blocks. It may be
also inferred that the northern part of the wall
constitutes the tower complex when comparing
the stone block magnitude with the rest of it. The
eastern feature is presumed to fit in with later
dwellings on the site owing to its shallower
depth. The GPR was better resolved for this
feature than the resistivity tomography. The
Copyright # 2005 John Wiley & Sons, Ltd.
7
block in the construction size (about 1 m) and
the location of this later period wall can be
well seen between depths of 20 cm and 75 cm
(Figure 6, first four depth slices).
The results from this survey provided subsurface evidence of the city wall preserved within
the topsoil between 20 cm and 2.10 m (5.76 ns and
69.3 ns). Especially, the S–N trending wall and its
well-preserved blocks (Figures 2c and 6) in rectangular shape can be well discerned at depths of
52.08 and 75.36 cm. The wall blocks are approximately 1 and 3 m in size.
A careful visualization and interpretation of
the data allowed a detailed description of the
structures and the different and changing depths
with which each wall was constructed to be
made. Additionally, from the height of a single
stone layer and the total height of the wall, the
changing number of stone layers preserved over
the length of the wall can be determined. Taking
into account some archaeological constraints,
this might show if more than the base of a wall
is conserved and therefore if there are still some
archaeological layers buried in the surrounding
subsurface. In the example of the east wall the
mean layer thickness is less than 75 cm, showing
that it was constructed of rather small stones
than the rest of the structure.
Discussion and conclusions
One of the conclusions from the above experiment was that the information concerning location, dimensions and depth of the buried city wall
has been improved. From an archaeological point
of view, critical problems regarding the shape of
the city wall have been resolved. A previously
unknown direction of the wall was discovered. In
this case, the results of the geophysical survey
may suggest that there is no need for excavation
to provide a clear answer to the southern section
fortification wall. However, it is strongly suggested that the grid should be extended for
further geophysical investigations.
The results of prospection techniques have
also provided a prediction of the other subsurface possible archaeological features in the Grid
16 area. Furthermore, from this experience, it has
emerged that a combination of the information
Archaeol. Prospect. 13, 1–9 (2006)
M. Dogan and S. Papamarinopoulos
Figure 6. Ground-penetrating radar depth slices for Grid16.
8
Copyright # 2005 John Wiley & Sons, Ltd.
Archaeol. Prospect. 13, 1–9 (2006)
Exploration of the Hellenistic Fortification Complex at Asea
obtained from the different method and their
three-dimensional image presentation using different processing techniques can help provide a
clear picture of the area studied. Concordance
between the results of two or more different
techniques for a specific target should increase
the reliability and confidence of the target detection itself.
Generally, the city wall was recognized with
all methods used. The electrical resistance RM4
twin-probe method was successful in tracing the
extent and distribution of the city wall and
nearby archaeological features. Its results can
be considered similar to those of multi-electrode
resistivity tomography techniques. However, the
tomography technique particularly has proved
to be a powerful technique in the study of subsurface walls. The city wall, characterized by
high resistivity, was better resolved with the
MRT than any other method. The inverse model
pseudosections and the resulting depth slices of
resistivity data delineated the dimensions and
burial depths of the fortification wall in the site
with remarkable accuracy. Thus, it can be stated
that the tomography technique provides a more
detailed view of the subsurface structure than
can be obtained using other geophysical techniques and may therefore lead to better understanding of depth. On the other hand, it is
important to remember that electrical imaging
is not a reconnaissance tool (Barker et al., 2001). It
is too slow and more suitable for the detailed
study of problem areas that have been selected
using other techniques such as GPR.
Similarly, the use of SIR-10 GPR has shown the
advantages and disadvantages of the system.
Applying the correction of its malfunctions and
keeping the sampling interval appropriate to the
buried archaeological feature of interest to record
much denser data, the instrument can be prove
valuable in buried wall surveys. The GPR survey
confirmed the existence of the city wall and gave
information regarding its dimension and depth
using time-slices technique at the Asea.
The GPR and MRT techniques provide better
results than the RM-4 twin-probe method for the
subsurface city wall at the Asea archaeological
site. The ‘depth’ using RM-4 becomes only an
Copyright # 2005 John Wiley & Sons, Ltd.
9
approximation in complex archaeological deposits. There is a strong correlation between GPR
and MRT data. Thus, it can be concluded that
together with MRT, the GPR can be used as an
alternative method in the geophysical prospection of subsurface walls in archaeological sites.
Acknowledgements
The authors are grateful to Drs Jeannette Forsen
and Bjorn Forsen for inviting us to work at the
Asea archaeological site.
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