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Geoelectric prospection of a city wall by multi-electrode resistivity image survey at the prehistoric site of Asea southern Greece.

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Archaeological Prospection
Archaeol. Prospect. 10, 241–248 (2003)
Published online 6 October 2003 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/arp.215
Geoelectric Prospection ofa City Wall
by Multi-electrode Resistivity Image
Surveyat the Prehistoric Site of Asea
(Southern Greece)
M. DOGAN* AND S. PAPAMARINOPOULOS
University of Patras, Department of Geology, Laboratory of Geophysics, 26110, Rio, Patras,
Greece
ABSTRACT
Electric resistivity imaging survey was carried out at the archaeological site of Asea as part of a programme ofcooperativeresearchbetweenthe Swedish Instituteat Athensandthe Universityof Patras.
This paper presents the geophysical results of the first field season 2001. It is intended to explore the
buried extension of the city wall in order to contribute to progressive research on the responses of the
resistivity method to intact subsurface archaeological features (wall, void, tunnel, etc.). The survey
was performed by means of a Geopulse imager system with 25 electrodes and an electrode spacing
of1m.TheWenner^Schlumberger array was selected for measurements.The city wall was imaged in
both north and south directions. The results indicate that the resistivity method can provide a clear
pseudo-image of medium to large size walls with high accuracy on archaeological sites.
Copyright 2003 JohnWiley & Sons,Ltd.
Key words: Asea; city wall; electric resistivity imaging; Geopulse; pseudo-image; archaeological
feature; Res2Dinv
Introduction
The application of geoarchaeological and geophysical methods changed the starting point
of the scientific approach to archaeological investigation the twentieth century. Traditional methods used in archaeology gave limited or
sometimes no information about unexcavated
subsurface archaeological features. However,
geophysical methods can aid archaeology at a
large scale (Weymouth, 1986).
Archaeological structures are material bodies
that have certain physical properties (Smekalova,
1993). Therefore, they can be easily investigated
* Correspondence to: M. Dogan, University of Patras, Department of Geology, Laboratory of Geophysics, 261 10, Rio,
Patras, Greece. E-mail: meliha_dogan@yahoo.com
Copyright # 2003 John Wiley & Sons, Ltd.
by using physical methods. The methods of
geophysical science allow one to locate them by
measuring their properties (i.e. magnetic intensity, resistance, conductivity).
One of the most commonly applied techniques
of geophysical survey is the resistivity method
(the measurement of the specific electrical resistance of soil.) because of its suitability in detecting walls, cavities and other structures at
differing depths. The use of resistivity in identification of walls has long been a common practice
(Sarris and Jones, 2000).
The method
Multi-electrode resistivity imaging techniques
using the Wenner–Schlumberger electrode array
have been applied for mapping an area of Asea
Received 2 June 2002
Accepted 10 March 2002
242
M. Dogan and S. Papamarinopoulos
Figure1. Location map of the research area.
in order to obtain a two-dimensional view of the
subsurface. This was found to be the most appropriate method for resolving the archaeological
problem under consideration at the site. The
vertical cross-sections or pseudo-sections were
measured by the tomography method. The
pseudo-section obtained forms the input for
inversion techniques which produce a twodimensional subsurface model (Loke and Barker,
1996). Inversion model sections of the wall were
obtained by using the Res2dinv software.
The site
The Asea valley is located between Tegea and
Megalopolis and measures 8 7 km in size. Geographically, It is a restricted area between the
larger alluvial areas in the Megalopolis basin and
Copyright # 2003 John Wiley & Sons, Ltd.
the plains of Tripolis (Figure 1). The valley bottom is the source area of two large Peloponnesian
Rivers, the Alpheios and the Eurotas. The most
typical bedrock at the site is Cretaceous and
Tertiary flysch and limestone (Forsen and
Forsen, 2002).
Early studies at the Asea prehistoric site
The Asea site was first excavated in the 1930s by
a Swedish archaeologist, Erik J. Holmberg. He
found a prehistoric settlement with abundant
remains from the later part of the Neolithic
period to the Middle Hellenistic period. He excavated the Paleokastro close to hill.
In the early 1990s, a first reconnaissance tour of
the valley was made by the Forsens and archaeological fieldwork started in 1994. Concurrently
Archaeol. Prospect. 10, 241–248 (2003)
Geoelectric Prospection at Asea, Southern Greece
243
Figure 2. Topographic map of the Asea site illustrating locations of surveyed sections (after Holmberg,1944).
Copyright # 2003 John Wiley & Sons, Ltd.
Archaeol. Prospect. 10, 241–248 (2003)
244
with the archaeological survey, a geological team
led by Mika Lavento studied the geomorphology
of the valley.
The city wall
The lower city fortification walls of Asea were
built using the rustic polygonal technique in 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 that run down slopes of the Acropolis in
the north and south. The northern wall, which is
the subject of the resistivity survey in Asea, runs
downhill from the northeastern corner of the
M. Dogan and S. Papamarinopoulos
Acropolis straight down to the valley in a northeasterly direction (Figure 2). The distance from
the Acropolis walls to the first tower is 40 m. It is
visible in all its length. The width of the wall is
3.3 m. The two wall faces are built with large
blocks in the polygonal technique of the same
kind as the southern lower city wall. The blocks
have straight sides and were placed on the bedrock without a footing course. The blocks of the
two wall faces, seemingly stepping up the hillside like a staircase, have been visible through
the centuries. A substantial tower is located 40 m
from the Acropolis on the wall. It measures 6.6 m
in height and 6.45 m in width. It was built with
very large blocks up to 1.5 and even 2 m length,
and about 0.7 m in height (Forsen and Forsen,
Figure 3. Resistivity pseudo-sections of L1and L2 acquired by means of the Res2DInv inverse modelling software.High resistivity
regions showing the position of the subsurface extension of the Southern wall.
Copyright # 2003 John Wiley & Sons, Ltd.
Archaeol. Prospect. 10, 241–248 (2003)
Geoelectric Prospection at Asea, Southern Greece
245
Figure 4. Inverse model resistivity sections of L4, L5, L6, L7 and L8 from the survey conducted at the prehistoric site of Asea.
Copyright # 2003 John Wiley & Sons, Ltd.
Archaeol. Prospect. 10, 241–248 (2003)
246
M. Dogan and S. Papamarinopoulos
Figure 5. Pseudo-sections of L9, L10, L11and L12 at AseaValley illustrating the location of buried city wall and terraces.
Copyright # 2003 John Wiley & Sons, Ltd.
Archaeol. Prospect. 10, 241–248 (2003)
Geoelectric Prospection at Asea, Southern Greece
2002). All the towers located along the city wall
have the same dimensions and are placed at
intervals of 33 m.
Measurements and interpretation
Line 1 is on the downhill side of the continuation
of the southern city wall, keeping a water filled
ditch as reference point (N23 E). Line 2 (N23 E)
and line 3(N25 E) have been purposely placed to
ascertain if any subsurface extension of the wall
exists. The distance between L1 and L2 is 35 m;
L3 is 7.5 m away from L2 (Figure 2). The high
resistance regions in line 1 (13–15 m), line 2 (12.5–
15 m) and line 3 (17.5–20 m) are predicted to be
the mentioned extension of the southern wall
(Figure 3). Those high values are continued in
all three sections.
The other measurements have been carried out
keeping just below the tower of the northern
fortification wall as a reference point (Figure 2).
Line 5 (N116 E) is 2 m away from line 4 (N100 E)
and 10 m from line 6 (N156 E). The measured
sections giving the best image of the wall determined lay out of subsequent sections. All sections have been shifted and measured by taking
the position of the image of the wall in previous
section as reference. The measured distance
between line 7 (N156 E) and line 8 (N154 E) is
20 m and line 9 is (N140 E) is 29 m. Line 10
(N54 E) is 11 m from line 9 and 9 m from line
11 (N50 E), which is 10 m distant from line 12.
Line 13 (N52 E) was surveyed separately at a
location (10 m away from Megalopolis road)
supposed to be on the line of the extension of
the southern wall as a 50 m section using a 2-m
electrode interval. The trace of the dislocated
wall can be seen in this section.
The pseudo-sections along lines 4, 5 and 6
indicate a very clear picture of the northern
wall (Figure 4). The high resistivity areas near
the surface of the pseudo-sections correspond
with the part of the buried northern fortification
wall. On those lines, the wall produces very high
resistivity values (342, 284 and 98 ohm-m) and
the resistivity distribution changes depending on
the thickness and concentration of stones in the
burial field. This becomes clear when the resistivity section of lines 4, 5 and 6 are compared
Copyright # 2003 John Wiley & Sons, Ltd.
247
with lines 7 and 8. Line 7 (Figure 4) most probably indicates an area of a collapsed tower,
because, it is almost 100 Greek feet (33 m) away
from the visible tower on the wall (Forsen and
Forsen, 2001) and it also shows rock pieces
belonging to a modern well and a buried building at the site (Figure 2). Line 9 is located so as to
change the direction of survey (Figure 5). Here, it
is seen that the anomaly belonging to the wall has
shifted through an E–W direction (Figures 2 and
5). Line 10 illustrates the existence of high resistivity anomalies (Figure 5) from the wall as well
as stone blocks, which probably belong to the
settlement inside the city wall. At line 11, the
anomaly of a rock pile that follows the whole
ravine is clearly seen, but the wall itself does not
appear as a distinct geophysical marker. It is
thought that at this point, the wall is parallel to
the ravine under the modern rock pile (Figure 5).
The high resistivity area close to the surface
might be a terrace built with stone and soil to
prevent downslope erosion. They might be used
for agricultural purposes and also to protect the
houses in the past from high floods from the
Alpheios River. The modern rock pile, the wall
and another terrace-like feature can be recognized again in line 12 (Figure 5).
Conclusion
The Asea archaeological site has been explored
geophysically in order to check the response of
the buried lower city fortification wall. By means
of small-scale electric tomography imaging survey. The data obtained have been inverted into
resistivity pseudo-sections by using the Res2Dinv
software package. Subsurface continuation of the
northern side of the Hellenistic city wall has been
clearly imaged. It is intended to measure the
surveyed portion of the site intensively using
ground-penetrating radar in the 2002 archaeological season.
Acknowledgements
The authors are grateful to Drs Jeannette Forsen
and Bjorn Forsen for inviting them to work in
Asea. A full account of the results of the collaboration 2001–2002, taking into account both the
Archaeol. Prospect. 10, 241–248 (2003)
248
archaeological and geophysical results, is in preparation by Jeannette Forsen, Bjorn Forsen,
Meliha Dogan and Stavros Papamarinopoulos.
References
Forsen J, Forsen B. 2002. The Asea Valley Survey. An
Arcadian Mountain Valley from the Paleolithic
Period until Modern Times. ActaAth-4: 51,
Stockholm.
Holmberg EJ. 1944. The Swedish Excavations at Asea
in Arcadia. ActaRom-4: 11, Lund and Leipzig.
Copyright # 2003 John Wiley & Sons, Ltd.
M. Dogan and S. Papamarinopoulos
Loke MH, Barker RD. 1996. Rapid least-squares
inversion of apparent resistivity pseudosections
by a quasi-Newton method. Geophysical Prospecting 44: 131–152.
Sarris A, Jones RE. 2000. Geophysical and related
techniques applied to archaeological survey in
the Mediterranean: a review. Journal of Mediterranean Archaeology 13: 3–75.
Smekalova TN. 1993. The Use of Magnetic Prospecting
in Archaeology. Unpublished, Physics Institute,
St Petersburg State University: St Petersburg.
Weymouth JW. 1986. Archaeological site surveying
program at the University of Nebraska. Geophysics 51(3): 538–552.
Archaeol. Prospect. 10, 241–248 (2003)
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