We PA1 25
Reconnaissance Geophysical Survey for the
Detection of Salinization and Stratigraphy in
Thorikos Valley, Attica, Greece
G. Apostolopoulos* (National Technical University of Athens), A. Kallioras
(National Technical University of Athens), K. Pavlopoulos (Paris-Sorbonne
University Abu Dhabi), K. Stathopoulou (National Technical University of
Athens) & A. Vlassopoulou (National Technical University of Athens)
SUMMARY
A reconnaissance geophysical survey is a starting task of a multitasking collaboration to detect all
hydrogeological (extent of salinization), stratigraphical, geological features of Thoriko valley in order to
reconstruct the environment in ancient times as well as to interpret the position of the various
archaeological monuments existing in the area. Electromagnetic measurements with conductivity meter,
made in a quick mode with a cart dragged by car, have given, after data evaluation and filtering, valuable
information regarding near surface salinization phenomenon. In addition ERT and IP profiles give deeper
detection of this phenomenon discriminating media of various clayey content.
20th European Meeting of Environmental and Engineering Geophysics
Athens, Greece, 14-18 September 2014
Introduction
Thoricus (or Thorikos) was an ancient Greek city in southern Attica (Fig. 1d), the mining centre of the
Laureotica, where lead and silver was mined. Mycenaean tholos tombs (15th c. BC) and a Late
Mycenaean installation (12th c. BC), probably connected with the mines in the area, have been
uncovered. The town's harbour was to the south of the acropolis; the island of Makronisi provides
natural protection. The ancient city's centre, its acropolis and the theatre (ca. 525-480 BC in a
significant survival) are on Velatouri hill. The large Doric Temple (late 5th century BC), may have
been a Telesterion for the cult of Demeter and Persephone
A scientific team with hydrologists, geologists, geophysicists and of course archaeologists has started
a collaboration to detect all hydrogeological (extent of salinization), stratigraphical, geological
features of the area of Thoriko in order to reconstruct the environment in ancient times as well as to
interpret the position of the various archaeological monuments existing in the area.
The need of geophysical investigation in urban environment or in the country side along roads for
near surface studies (environmental, geotechnical, archaeological) in a quick mode, led the Applied
Geophysics Lab of NTUA to build a wooden-plastic cart (Fig.1a) where either EM conductivity meter
or GPR shielded antennas could be mounted. Differential GPS Leica is used for positioning.
The Applied Geophysics Lab of NTUA in order to contribute in the previous collaboration has started
a reconnaissance survey for the first 6 m depth of investigation with EM measurements and
conductivity meters CMD2,4 of GF Instruments mounted to the cart (Fig. 1b,c) and dragged by car
covering in a quick mode the valley of Thorikos (Fig.1e) along the existing roads.
Figure 1 a) A wooden-plactic cart made for urban measurements with EM conductivity meter or GPR
shielded antennas. b,c) The cart in the country side of Thoriko with CMD 4, CMD-2 EM conductivity
meters of GF Instruments for 6m and 3m investigation depth respectively, d) Attica map with Athens
and Thoriko position, e) Thoriko survey area with the routes of EM measurements (blue line CMD4,
red line CMD2.
20th European Meeting of Environmental and Engineering Geophysics
Athens, Greece, 14-18 September 2014
Figure 2 Electromagnetic maps in Thorikos valley.
20th European Meeting of Environmental and Engineering Geophysics
Athens, Greece, 14-18 September 2014
ERT and IP profiles in various places that EM results suggest as interesting give additional
information deeper and more accurate.
The valley of Thorikos is covered by torrential deposits and is surrounded by hills of marbles
(Kamariza upper marble) and schists.
Geophysical Survey
Electromagnetic Measurements
The electromagnetic measurements with the conductivity meter and the cart for the two investigation
depths 3m and 6m, after despiking filtering procedure in order to extract local picks gave maps of
apparent conductivity and in phase component (top part of Fig. 2). This combination of the two
parameters helps us to discriminate in high conductivity areas salinization from anthropogenic or
other metallic origin structures. In order to filter out the later influence we extract the values with in
phase component greater than 5ppt and the final maps are shown in the bottom part of Fig.2.
In addition map of the subtraction of conductivities in the two investigation depths is shown with all
measurements and filtered measurements in order to have a better picture under the depth of 3m. The
darker blue color the greater conductivity and salinization.
In all maps but most clearly in the maps with filtered values the conductive region influenced by
salinization lies within the south part of the survey area. This region can be better shown with Google
Earth as basemap (Fig.3 a,b,c) and we observe that in its edge the Temple of Demeter and Persephone
(Fig. 3.e) exists. Probably the nearby ships to the Temple shown in gravure of 1758 (Fig. 3d) is not in
the fantasy of the painter... In any case the conductive region is spread to the valley with obvious
problems to the agricultural activity in the area. It is worth to mention that deposition of torrents must
be great as we see in the photo of Fig. 2e the basement of the Temple and the surface now.
Figure 3 a,b) Apparent conductivity maps in detection depths 3m and 6m and c) Map of the
subtraction of the previous two conductivities. All maps in Google Earth background. d) Gravure of
the Temple of Demeter and Persephone by J.D.Le Roy 1758 Paris with foreigners stealing ancient
monuments and ships nearby. e) Photo of the Temple today.
20th European Meeting of Environmental and Engineering Geophysics
Athens, Greece, 14-18 September 2014
Figure 4 Resistivity and chargeability sections presented in a 3D mode of ERT and IP profiles eith
the routes of electromagnetic measurements in the south part of Thorikos valley.
ERT and IP survey.
ERT and IP profiles with 175m length (50m investigation depth) and pole-dipole electrode array were
positioned in such a way to follow somehow the salinization front entering to the valley from the sea
shore.Also, 5m electrode distance was used with the current and potential electrodes separated in
parallel lines (in order to avoid polarization effects, Dahlin and Leroux, 2012)
All resistivity sections (Fig. 4) show that in even in depths of 40m we have conductive media
implying salinization as a dominant phenomenon. The chargeability sections give an additional
information regarding the conductive media with the higher values implying higher clayey character.
This clayey presence near the sea is in depths of 30m going to 20m as we enter the valley but they
vanish in northern areas.
Conclusions
The multitasking collaboration for Thorikos area is in its starting stage and geophysical survey has
given the first valuable information either for horizons near the surface with electromagnetic
measurements but for deeper as well through ERT and IP sections. This collaboration is to be
continued as well as the geophysical survey going to northern areas of the valley.
Acknowledgements
The authors wish to thank firstly Mr. G. Amolochitis, geophysicist of NTUA Applied Geophysics Lab
as well as the students F. Loizos and K. Michalopoulos for their valuable help in field acquisition.
Thanks also go to Mr. A. Kapetanios, archaeologist, for his first information and comments with
whom more interactive collaboration will be continued in the multitasking approach
References
Dahlin T. and Leroux V. [2012] Improvement in time-domain induced polarization data quality with
multi-electrode systems by separating current and potential cables, Near Surface Geophysics 10,
545-565.
20th European Meeting of Environmental and Engineering Geophysics
Athens, Greece, 14-18 September 2014