Download We Verg 16 Geophysical Investigation in the Temple of Poseidon at

We Verg 16
Geophysical Investigation in the Temple of
Poseidon at Cape Sounio, Attica, Greece
G. Apostolopoulos* (National Technical University of Athens), K.
Leontarakis (National Technical University of Athens), C. Orfanos (National
Technical University of Athens), G. Amolochitis (National Technical
University of Athens) & F. Loizos (National Technical University of Athens)
SUMMARY
The famous Temple of Poseidon at Cape Sounio in Attica, whose columns still stand today, was probably
built in 440 BC. by Athenians after its first destruction by Persian troops during Xerxes I's invasion of
Greece in 480 BC. and his defeat in the naval Battle of Salamis. The later construction was during the
ascendancy of Athenian statesman Pericles, who also rebuilt the Parthenon in Athens. An integrated
geophysical investigation in the Temple with EM, GPR and ERT measurements and the appropriate field
design, processing and interpretation, has detected horizontal features and vertical ones under the Temple
with the later being either walls of the previous temple or constructions to hold loose material on which
the existing Temple of Poseidon is. Sudden conductivity changes in EM, diffractions in GPR and lateral
inhomogeneities in ERT sections through pole-dipole array and MOST technique indicate in the same
position the vertical features.
20th European Meeting of Environmental and Engineering Geophysics
Athens, Greece, 14-18 September 2014
Introduction
Cape Sounion is the spot where Aegeus, king of Athens, leapt to his death off the cliff, thus giving his
name to the Aegean Sea. The original, Archaic Period Temple of Poseidon on the site, which was
built of tufa, was destroyed in 480 BC by Persian troops during shahanshah Xerxes I's invasion of
Greece. After Athenians defeated Xerxes in the naval Battle of Salamis, the later Temple at Sounion,
whose columns still stand today, was probably built in ca. 440 BC. This was during the ascendancy of
Athenian statesman Pericles, who also rebuilt the Parthenon in Athens. The 2nd Ephorate of
Prehistoric and Classical Antiquities of Ministry of Culture has set a project for geophysical
investigation to detect whatever is under the temple, constructions (possible relics of the previous
temple or others), material etc. which was undertaken by the Applied Geophysics Laboratory of the
School of Mining and Metallurgical Engineering of National Technical University of Athens.
Cretaceous limestones exist in the area of the Temple. At its eastern and southern sides, the temple
seems touching the landscape with limestones, while at northern and western sides its level is higher
than the landscape with stony walls surrounding the material existing underneath.
Geophysical investigation (Fig. 1) consists a) electromagnetic measurements with the conductivity
meter of GF Instruments (CMD-2,4), b) GPR measurements with MALA System of three channels,
250ȂǾz and 500MHz shielded antennas working in parallel and c) ERT profiles with SYSCAL PRO
72 channels Iris Instruments. All data were positioned with LEICA Differential GPS System.
Figure 1 The ancient Temple of Poseidon at Sounio Cape. The geophysical profiles are shown a)
Electromagnetic in yellow, b) GPR in blue and c) ERT in red. Small picture shows Attica with Athens
and Cape Sounio position. Everything is in Google Earth background.
20th European Meeting of Environmental and Engineering Geophysics
Athens, Greece, 14-18 September 2014
Figure 2 Apparent conductivity maps in the investigation depths of 3m (a) and 6m (b). Shaded relief
of apparent conductivity map of 3m investigation depth (c). Map of the difference in apparent
conductivity between investigation depths 6m and 3m (d).
Geophysical Survey
Electromagnetic Method
Electromagnetic measurements with 1m step and profile separation gave the apparent conductivity
maps for 3m and 6m investigation depths (Fig. 2a and 2b respectively).
The apparent conductivity map for investigation depth 3m (Fig. 2a) shows:
x An area “A1” of very low apparent conductivities due to the nearby wall, has to its east side the
area “A2” of high conductivities with fine grain loose material and further to the east the area “A4”
with intermediate values of conductivity indicating a medium of loose fine and coarse material.
x The sudden changes of conductivities, northern to the area “A4” to lower values (area “A3” with
very coarse material) and southern to higher values (area “A5” with fine grain material) exist
because of a human structure (walls?). Sudden change of conductivity exists also between the
areas “A2” and “A4”. These changes are better shown in Fig. 2c with the shading approach of the
map of Fig. 2a. indicating the probable structures.
The apparent conductivity map for investigation depth 6m (Fig. 2b) shows the limestone with
conductivities increasing southwards because of the existing fine conductive material above. Area
“A2” with high conductivities and fine material confirms the presence of a human structure since the
general trend of limestone appearance does not excuse such a sudden change in conductivities.
Another processing technique, as it is the subtraction of conductivities of the maps for the two
investigation depths (Fig. 2d), confirms the previous comments.
GPR Method
The depth of interest for the geophysical survey leads to use the GPR sections with the 250MHz
antenna (Fig. 3b) and since the combination of 250MHz and 500 Mhz antennas shows finer detection
(Fig. 3a) we will ultimately use these records to see what results this method can provide.
20th European Meeting of Environmental and Engineering Geophysics
Athens, Greece, 14-18 September 2014
Figure 3 GPR section “Sv5” taken either with combination of 250Mhz and 500MHz shielded
antennas (a) or with only 250MHz antenna (b). Parallel GPR sections of N-S direction with the
combination of antennas presented in a 3D mode with the perimeter of the Temple and some columns
also shown (c).
The processing sequence for the GPR sections with REFLEXW (Sandmeier, 2012) was: a) subtract
mean (dewow), b) manual gain, c) background removal, d) band-pass frequency. Diffractions and
multiple reflections show areas of localities which can be walls or piles of coarse material. The
shallow ones are outlined with yellow line and the deeper ones with red in Fig. 3. The GPR profiles of
N-S direction show in the respective sections (Fig. 3c) localities in line that correspond to the position
we observed the sudden changes in conductivities in electromagnetics separating areas “A3”, “A4”,
“A5”. The GPR sections of W-E direction presented in a 3D mode have also shown vertical localities
in line comparable with what we observed in electromagnetics.
ERT Method.
ERT profiles of N-S and W-E directions are expected to show the previous structures along with
various layers of different texture under the Temple. In the case we couldn’t use conventional
electrodes, we used lead plates with medicine gel to have good electrode contact (Fig. 4a) with the
marbles. Mainly Pole-Dipole array (forward and reverse) and Dipole-Dipole array were used to detect
lateral and horizontal discontinuities. Resistivity models with RES2DINV (Loke and Barker, 1996)
for each array and combined arrays and models with stacking technique “MOST” (Leontarakis and
Apostolopoulos, 2012) were evaluated and finally the Pole-Dipole with “MOST” technique models
were chosen to conclude for what the method could detect under the Temple. Vertical resistive
features, separating the loose material were detected in the N-S profiles. These features are in the
same position we observed the sudden changes in conductivities in electromagnetic and the diffraction
20th European Meeting of Environmental and Engineering Geophysics
Athens, Greece, 14-18 September 2014
in GPR sections. Vertical resistive features “A” and “B” are also detected in the W-E profiles (Fig.
4c). These features seem to hold between them the loose material on which the Temple is. Horizontal
resistivity slices also show the regions of different texture.
Figure 4 GPR a) Photo with the Temple and an ERT profile, b) photo with the lead plate used as an
electrode, c) W-E ERT profiles shown in a 3D mode with perimeter and some columns.
Conclusions
The geophysical survey with electromagnetics, GPR and ERT profiles with the appropriate field
design and interpretation has detected horizontal features and vertical ones under the Temple with the
later being either walls of the previous temple or constructions to hold loose material on which the
existing Temple of Poseidon is.
Acknowledgements
The authors wish to thank the 2nd Ephorate of Prehistoric and Classical Antiquities of Ministry of
Culture for the permission to present this geophysical survey and personally Dr. E. Andrikou, director
archaeologist, and Dr. F. Karasavva, architect, for their constructive collaboration.
References
Leontarakis, K. and Apostolopoulos, G.V. [2012] Laboratory study of the cross-hole resistivity
tomography: the Model Stacking (MOST) Technique. Journal of Applied Geophysics 80, 67–82.
Loke, M.H., and Barker R.D. [1996] Rapid least-squares inversion of apparent resistivity
pseudosections by a quasi-Newton method. Geophysical Prospecting, 44, 131-152.
Sandmeier KJ [2012]. REFLEXW – software for the processing of seismic, acoustic or
electromagnetic reflection, refraction and transmission data. http://www.sandmeier-geo.de/.
20th European Meeting of Environmental and Engineering Geophysics
Athens, Greece, 14-18 September 2014