ΔΡΑΣΗ: «ΑΡΙΣΤΕΙΑ ΙΙ»

ΕΠΙΧΕΙΡΗΣΙΑΚΟ ΠΡΟΓΡΑΜΜΑ: «ΕΚΠΑΙΔΕΥΣΗ ΚΑΙ ΔΙΑ ΒΙΟΥ ΜΑΘΗΣΗ»

ΙΔΡΥΜΑ ΤΕΧΝΟΛΟΓΙΑΣ ΚΑΙ ΕΡΕΥΝΑΣ

Έργο: AncientCity: «Εφαρμογή Καινοτόμων Τεχνολογιών Γεωπληροφορικής για τη Μελέτη της Αστικοποίησης στην

Αρχαία Ελλάδα»

(Κωδικός ΠΔΕ: 2013ΣΕ01380048, Κωδικός ΣΑΕ: 013/3)

Διάρκεια Έργου: 31/1/2014 – 31/7/2015 (17 μήνες)

ΠΑΡΑΔΟΤΕΟ Π.4.1

Τίτλος: Τεχνική έκθεση επεξεργασίας και ερμηνείας δεδομένων γεωφυσικών διασκοπήσεων

GEOPHYSICAL INVESTIGATIONS AT ONCHESTOS, BOEOTIA

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Geophysical Investigations at Onchestos, Boeotia

NIKOS PAPADOPOULOS

GeoSat ReSeArch Lab, Institute for Mediterranean Studies, Foundation for Research and Technology Hellas

Abstract The Laboratory of Geophysical, Satellite Remote Sensing and Archaeoenvironment (GeoSat ReSeArch) of the

Institute for Mediterranean Studies (FORTH) conducted a geophysical survey at the ancient Onchestos during 1-10

June 2014. This reports describes the methodology and the results of the geophysical mapping in an effort to

reconstruct the buried archaeological relics and the build environment around the sanctuary of Poseidon.

Scope of the survey and field geophysical team The geophysical prospection survey at the

archaeological site of Onchestos (Fig. 1, 2) was carried

out in the period of July 1st – June 8th 2014 under the

collaboration of the Foundation for Research and

Technology, Hellas (F.O.R.T.H.) with the Columbia

University (Prof. Ioannis Mylonopoulos). The goals of

the geophysical prospection research campaign were

to try to investigate and map the possible architectural

structures and reconstruct the built environment

around the sanctuary of Poseidon.

Despite the challenging conditions originating

from the cultivated fields and the vegetation the

manifold methodologies employed in the geophysical

campaign of 2014 proved promising. In order to

maximize the results of the geophysical prospection

and test the quality of the collected signals, three

methods were applied in the site: magnetic

gradiometry, electrical resistance mapping, and

ground penetrating radar. The layout of the individual

geophysical grids was carried out by a differential

GPS survey that was supervised by the topographer

Mr. Goumas Panagiotis.

The geophysical mapping of the site was

conducted under the guidance of Dr. Nikos

Papadopoulos, Dr. Kleanthis Simirdanis and Stella

Kirkou from the Laboratory of Geophysical - Satellite

Remote Sensing and Archaeoenvironment (IMS-

FORTH) with the support of Prof. Ioannis

Mylonopoulos and the students that participated in the

excavation team. Details of the areas that were

approached with the geophysical prospection

techniques at the archaeological site of Onchestos are

shown in figures 3 and 4.

Archaeological background The rather limited references to the Sanctuary

of Poseidon in Onchestos and its overall significance

within the Boeotia territory are mainly found in the

work of Homer, Pindaros, Stavon and Pausanias

(Schachter, 1986). The ancient texts roughly describe

a specific ceremony that was used to choose the most

appropriate young horses to drag the chariots. Recent

evidences show that this ceremony comprises residue

of Mycenaean traditions, when the chariots were

considered extremely important in the battle fields

(Teffeller, 2001). If this interpretation is proved to be

correct, the sanctuary of Poseidon at Onchestos should

have a significant role in the living customs of the the

upper class of the Mycenaean Boeotia.

Despite the importance of the sanctuary, the

archaeological excavation works were occasional and

focussed on two areas at the sides of the highway

connecting Thebes-Livadia. The first excavation

campaign in 1964 along the 91st kilometer of the road

Thebes-Livadia (Area A) verified the site as part of the

sanctuary of Poseidon and also revealed some other

artefacts (Τουλούπα, 1964). The continuation of the

excavation activities in 1971 brought to light the

temple of the sanctuary and a large rectangular

building that was correlated with an “early”

bouleutirion. The identification of three different

inscriptions verified the 1961 excavation results and

correlated Area A, without any doubt, as the central

part of the sanctuary of Poseidon, due to the existence

of the temple.

The first systematic excavation in Area B (92nd

kilometer Thebes-Livadia) that initiated in 1973

revealed a large building with length at least 48 meters.

The ceramics analysis indicated that this specific

building was in constant use from 4th century B.C.

until the Roman Times (Δακορώνια, 1973/74). The

1991 excavation season verified the existence of this

large building, which is probably a stoa, with 18 or 19

houses. The excavation also showed a construction

phase older than the 4th century B.C. Furthermore the

identification of two copper judicial votes linked the

specific area with the common sanctuary of Poseidon

(Χριστοπούλου, 1995).

Geophysical exploration and instrumentation

Geophysical methods can detect various types

of subsurface soil features such as pits, foundations,

ditches, middens, fire hearths, kilns and concentrations

of pottery. These methods are non-destructive and

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involve measuring the physical properties of soils

(such as magnetic susceptibility or electrical

resistance) on or below the surface of a site. The soil

resistance techniques are best suited for features that

contrast with the surrounding soils in porosity, density

and water content such as walls and ditches. The

magnetic methods are best suited for features that

contrast with the surrounding soils in the

concentrations of magnetic minerals they contain such

as pits or ditches filled with topsoils imbedded in sub-

soils. In particular, burned soils, habitation units and

ditches filled with organic material enhance the

magnetic susceptibility of the soil and thus are good

targets for magnetic methods. The electromagnetic

methods are ideal for obtaining information for both

soil conductivity and magnetic susceptibility of soils

when large penetration is needed.

Magnetic measurements deal with anomalies of

the geomagnetic field, which are caused by contrasts

of the rock magnetization or by soils rich in magnetic

oxides. The magnetization of rocks contains shares of

inductive and remnant magnetization. The inductive

magnetization originates from the magnetic earth field

and depends on its actual strength and direction and on

the susceptibility χ of rocks or soils. In contrast, the

remnant magnetization is constant and is not changed

by alterations of the recent magnetic field.

Ground Penetrating Radar is to the seismic

reflection method. A high frequency, small duration

electromagnetic pulse is transmitted into the ground.

This pulse (signal) is diffused in the subsurface

materials and its direction depends on its properties.

Part of the pulse energy is reflected on the surface that

separates materials with different properties and is

recorded at a receiver on the surface. The remaining

pulse energy is diffused at deeper levels. The time

between the transmitting and the receiving pulse

depends on the velocity along the trace the pulse

followed. This time can be measured and if the

electromagnetic wave propagation velocity is known

then the depth of the reflector can be determined.

Resistivity surveying, namely the measurement

of the specific resistance of soil, is the most commonly

applied technique of geophysical survey in the

Mediterranean. This is because of its suitability in

detecting walls, cavities, layers and other localized

structures of differing electrical, permittivity and

electrochemical properties (Mares, 1984:263).

Resistivity methods make use of DC or AC fields to

measure the electrical potential or potential gradient of

the corresponding current. The resistivity of the

underlying medium is calculated by Ohm's Law.

Normally employed in mapping mode (namely a fixed

electrode configuration moving along a profile or grid,

giving the horizontal distribution of resistance). It can

also be adapted to measure the vertical distribution of

resistance and thus the depth of a feature by increasing

electrode separation while the center of the

configuration remains stationary (resistivity

soundings). It can be also applied in the tomographic

mode where both the vertical and horizontal

distributions of resistivity are measured along

transects. Multi-probe, wheeled and tractor-based

systems have also been able to speed up resistivity

surveying (mostly in smooth, conductive soils), and

vertical profiling (or vertical electrical profiling, VES)

and tomographic techniques have been employed to

provide stratigraphic information as well as horizontal

mapping at different depths.

During the geophysical prospection project at

the archaeological site of Onchestos in 2014,

magnetic, soil resistance and ground penetrating radar

(GPR) were employed to record the subsurface

information at specific areas of the archaeological site

(Fig. 5). These techniques were chosen as the most

appropriate for meeting the goals of the project,

according to the needs of the research, the

geomorphological characteristics of the site and the

expected subsurface archaeological targets - with

respect to the detection and mapping of them.

Emphasis was given to the detailed (high resolution)

coverage of the specific areas.Table 1 summarizes the

area coverage by each technique and the technical

details concerning the sampling intervals of the

different geophysical methods that were applied in the

area, Figure 6 outlines the area that was covered with

the geophysical techniques.

Table 1: Technical details of the geophysical survey parameters and the area that was covered with the different geophysical

methods.

Geophysical data processing

The geophysical techniques were used in a

systematic way. Magnetic measurements were carried

out with sampling interval of Δx=0.5m & Δy=0.25m

in magnetic surveys East and North directions

respectively. The coverage of the areas of interest was

carried out by moving along transects in an S-N

direction. The raw geophysical data were entered in a

portable PC right after fieldwork. Magnetic data were

dumped into a portable PC through an RS232 serial

cable. Each data set was coded after a grid number.

Data sets were given the appropriate coordinates

according to the position of the adjacent grids and an

area code was given for each cluster of grids. A

specific map coordinate system was chosen for each

geophysical mosaic of grids, which was registered to

the appropriate geodetic system of coordinates (local

coordinate system), based on the Geodetic GPS

mapping data. Thus, after the rectification of the

satellite image, it was possible to overlay the

geophysical maps at their corresponding location.

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All data were characterized by a constant shift

of the average value within each surveyed grid due to

differences in balancing the instrument and the

shifting of the base/reference stations. For this reason,

pre-processing of the data was needed in order to

create a common base level (0-level base line) for all

grids. Statistical analysis of both the common rows

and the calculation of the average level of adjacent

grids was carried out in order to provide a correction

factor for each grid. Both, the change of coordinates

and the correction factors were able to create the

mosaic of the grids in each area. In this way,

processing of the adjacent grids was conducted

simultaneously.

Most data sets were processed with a specific

methodology. Kriging interpolation was used for

gridding the data. In some cases, selective despiking

techniques were used to isolate the extreme values that

masked the anomalies of interest. Selective

compression of the dynamic range of values was also

employed to isolate anomalies close to the background

level. A mask file was created to isolate the areas that

were not surveyed due to the existence of thick

vegetation, fences, modern structural remains, and

other surface features.

The GPR sections were at first given the

relative X, Y coordinates according to a local

reference system that was used for each one of the

sites. Initially, the first peak was determined in order

to define the initial useful signal from each line. This

determination was based on the intensity percentage of

the first reflected wave (5-30%). The line equalization

based on the selected first peak was followed trying to

bring the first reflections of each line in a common

starting time. Then the application of AGC, Dewow

and DCshift filters enhanced the reflected signal,

while the rejection of the background noise and the

data smoothing was accomplished by a trace-to-trace

averaging filter. Finally, horizontal depth slices at

different depth levels were created by the original

vertical sections assuming a velocity for the

electromagnetic waves equal to 0.1m/nsec. The

synthesis of the processed sections was accomplished

with the Sensors&Software software (EKKO

MAPPER & EKKO 3D).

Colour and grey scale geophysical maps were

produced: Hot colours (reddish colours) in colour

maps and light (white) colours in grey scale maps

represent high intensity values. Cold colours (bluish

colours) in colour maps and dark (black) colours in

grey scale maps represent low intensity anomalies.

GIS software (ArcGIS v.10) was used to rectify

the geophysical maps and overlay them on the

topographic plan of the site.

Integration of geophysical data

Area A

The magnetic gradiometry method scanned

more than 11,500 square meters separated in

(regularly) 20 by 20 meters grids oriented along the

southeast-northwest direction. The raw magnetic data

exhibited large values of the vertical magnetic gradient

ranging between +/- 80 nT/m. After despiking these

extreme values the range of the measurements reduced

to +/- 35 nT/m. The grid and line equalization filters

smoothed the values between different grids and along

individual lines (Fig. 7). The complete diagrammatic

interpretation of the magnetic anomalies of this section

is shown in Figure 8.

The final image resulted by the processing of

the magnetic data shows to be quite noisy, with some

magnetic dipoles scattered mainly towards the north,

east and west of the surveyed area. These dipoles are

caused by buried or visible metal fragments, masking

the signal that could be created by the potential buried

archaeological structures in a radius of at least 1-2

meters around the metal object. The southern part of

section 1 in Area A exhibits high magnetic gradients

occupying areas that are oriented along the SE-NW

direction. These areas are correlated with the bedrock

outcrops that are visible on the surface to the west, the

different terraces to the central part of the area and the

backfill soil material due to the excavation activities at

the temple.

The most prominent magnetic anomalies are

registered towards the flat part of the area at the north

west of the excavated temple. The magnetic data

outline three long linear anomalies that form a

rectangular structure probably related architectural

relics, buried no more than 1.5-2 meters in depth. The

SE-NW linear anomalies appear with negative

magnetic gradients. On the other hand the SW-NE

linear structure has a positive signature. The specific

structure is oriented along the SE-NW direction and its

dimensions is 52m by 34m. This structure seems to

enclose three other architectural parts signifying a

potential different construction phase. The linear

magnetic anomaly at the north of the excavation is

probably related to structural remains of the temple

towards this direction. The linear features to the west

of the surveyed area should be treated with caution due

to the existence of surface metallic objects that have

definitely influenced the magnetic signal.

As we move further to the north and enter the

field with the olive trees the magnetic map shows two

linear magnetic anomalies with perpendicular

orientation towards the western and eastern grids of

the surveyed area respectively. The most important

geophysical feature that is probable related to an

architectural structure is outlined towards the northern

most part of the area. The magnetic data clearly show

as positive magnetic gradients the southern part of this

structure running for about 34 meters along the SW-

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NE direction. The eastern and western parts of this

feature seem to continue further to the north entering

the northern field with olive trees. This structure also

seems to enclose two other rectangular structures.

Overall the specific feature has the same SE-NW

orientation with the corresponding feature that was

registered on the magnetic map about 50 metes to the

south.

More than 8,500 square meters in the section 1

at Area A were covered with the RM85, overlapping

most of the area that was covered with the magnetic

method (Fig. 9 & 10). The resistivity map was less

informative in terms of possible buried archaeological

features. The bedrock outcrops, the terraces and the

backfill excavation material have been also registered

as high resistivity values. The most promising

resistivity linear anomaly that is related with the same

architectural complex seen in the magnetic map, is

shown at the central part of the surveyed area, having

a SE-NW orientation and about 13 meters length.

The GPR survey in the section 1 of Area A

extended within 8 different grids covering an area of

3,200 square meters at the central part of the area (Fig.

11). At the central part of the investigated area the

GRP maps managed to outline as strong reflectors the

shape of a rectangular building with dimensions 8m by

12m. The building has SE-NW orientation showing at

the same time good preservation. The western wall of

the building is further extended to the north correlating

quite well with the corresponding linear magnetic

anomaly at the specific location.

Area B

Area B is attached to the east side of the

highway that connects Thebes and Livadia along the

92nd kilometer of the highway. After the extensive

cleaning of the area from the thick vegetation, it was

made possible to cover totally 6,000 square meters

with both the magnetic (Fig. 12) and GPR (Fig. 13)

methods.

The magnetic map is extremely revealing and

informative regarding the detection of buried

architectural relics, especially at the eastern part of the

investigated area. The data clearly outline the

foundations of four almost square rooms, with

negative magnetic gradients, with dimensions 5.3m by

5 m that are placed next to each other along the south-

north direction. To the south of the last room, a larger

rectangular building with dimension 22m by 23m is

visible. The northern and the western part of this large

building seem to appear with double parallel walls.

The circular negative magnetic anomaly that

crosses the interior part of this large building is

attributed to recent construction activities and is

probably related to the foundations of an old round

mill. Towards the northeastern corner of the area we

can also see the continuation of the road that is visible

on the satellite image and leads to the site from the

highway. The three concentric circular faint anomalies

are related to modern activities and should not be

considered as archaeological features. The large

dipolar anomaly at the south west corner of the large

building is caused by the electricity pole that is located

in the specific spot.

As we move to the western part of the area, the

older excavation trenches have masked the data by

disturbing the magnetic gradiometry readings around

them. The scattered dipoles all over this part of the

area are caused by visible or hidden in the top surface

layers metal fragments. The south edge of the area that

is attached next to the highway is extremely disturbed

due to thrown garbage. Besides the above negative

findings, the magnetic data shows two small sections

at the central west and at the north west corner of the

area that probably host architectural features.

The GPR data in Area B were much less

informative regarding the magnetic measurements.

Besides some strong reflections that are scattered

around the whole area GPR was completely unable to

reconstruct the four rooms and the large building at the

eastern part of the area. The specific section is shown

as fuzzy area of strong reflections without forming a

specific geometric feature. This is attributed to the

physical properties of subsurface relics that exhibit

substantial magnetization that was finally registered in

the magnetic data. On the other hand their resistivity

contrast with respect to the background soil matrix

was limited thus rendering the GPR unable to detect

them.

The plans outlining the archaeological features

that have been excavated in previous campaigns have

been rectified and overlaid on the satellite image of

Area B (Fig. 14). They are also presented together with

the architectural relics that have identified with the

geophysical methods during the 2014 campaign. It is

shown that the walls at the central part of the area are

correlated well with the visible excavation trenches. It

is also noted that the large rectangular building with

the rooms are not presented in any of these old

excavation plans.

Concluding remarks

The synthesis of the geophysical results clearly

demonstrates the importance of the manifold

geophysical strategy to survey the area of Onchestos.

Each one of the methods applied has been able to

suggest specific targets in terms of the physical

quantity measured and the properties of the

subsurface. The employment of different methods for

the scanning of the site was valuable, since they

provided complementary information and thus helped

the delineation of the most significant features that

were suggested by the various approaches.

It is shown that the magnetic gradiometry method

proved to be the most suitable for reconstructing the

architectural relics of Onchestos. Resistivity and GPR

data were severely affected by the geological and local

environmental setting of the site. Figure 15 provides

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an integrated image of the most prominent geophysical

anomalies, resulting by all the applied techniques for

both areas. The confidence level of the particular

anomalies (potential targets) is given either by the

intensity of their signal (taking always in account their

correlation to modern features) or the complementary

character of the signal produced by the various

methods. The future planning for continuing the

geophysical survey at Onchestos should mainly focus

on the employment of the magnetic gradiometry

method in an effort to cover specific sections between

Areas A and B and complete the picture of the

structured environment.

Bibliogaphy

Mares, S., Introduction to Applied Geophysics, D.

Reidel Publishing Company, Prague, 1984.

Schachter, A. 1986. Cults of Boiotia 2. Herakles to

Poseidon. BICS Suppl. 38.2. London.

Teffeller, A. 2001. “The Chariot Rite at Onchestos:

Homeric Hymn to Apollo 229-38,” Journal of

Hellenic Studies, 121, 159-166.

Δακορώνια, Φ. 1973/74. “Σεϊντή Μαυροματίου,”

Αρχαιολογικό Δελτίο, 29, Β 2, 442.

Τουλούπα, Ε. 1964. “Στενή,” Αρχαιολογικό Δελτίο,

19, Β 2, 200-201.

Χριστοπούλου, Α.Χ. 1995. “Ειδήσεις από τη Στενή

Μαυροματίου,” στο: Β΄ Διεθνές Συνέδριο

Βοιωτικών Μελετών, Λιβαδειά, 6-10 Σεπτεμβρίου

1992. Επετηρίς της Εταιρείας Βοιωτικών

Μελετών, Β 1. Αθήνα, 429-445.

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Figure 1: World View 2 satellite image (Acquisition Date: August 2nd, 2010) of the wider region of Onchestos where

the city of Aliartos is shown to the west.

Figure 2: Details of the Areas A and B that were surveyed at Onchestos. The polygons outline the regions that were

covered with the geophysical prospection methods.

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View of Area A-Section 1 from the SW

View of the Sanctuary of Poseidon from the south

View of the Area A-Section 1 from the east

Northern “Terrace” Wall at the north of Poseidon temple

Surveyed section inside the olive trees (Area A- Section 1)

as it is seen from south.

View of the Area 1 – Section 2 from the south.

Figure 3. Details from the Area A that was surveyed the archaeological site of Onchestos. Geophysical campaign 2014.

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View of Area B from North West

Older excavation trenches in Area B

Visible architectural relics at the west of Area B

Older excavation trenches in Area B

Figure 4. Details from the Area B that was surveyed the archaeological site of Onchestos. Geophysical campaign 2014.

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Method Instrument

s

Depth of

Investigation

1. Magnetic

Survey

Bartington

Grad 601

2m

2. GPR Sensors &

Software

Noggin Plus

Smart Cart

with 250

MHz

antennas

2-3m

3. Electrical

Resistance

Geoscan

RM85

1-2m

Figure 5: Details of the geophysical instrumentation that was used in the prospection of Onchestos - 2014 geophysical

campaign season.

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Figure 6: Overlay of the geophysical grids on the satellite image for Areas A and B that were surveyed in Onchestos.

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Figure 7. Overlay of magnetic gradiometry map from Area A-Section 1 on the satellite image of the site

Figure 8. Diagrammatic interpretation of the magnetic anomalies in Area A-Section 1 at Onchestos.

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Figure 9: Resistivity map of Area A-Section 1 at Onchestos overlaid on the satellite image of the wider area.

Figure 10: Diagrammatic interpretation of the most prominent resistivity anomalies of Area A-Section 1 at Onchestos.

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Figure 11. GPR slices from 0.1m to 0.8m below the ground surface at the Area A-Section 1 at Onchestos overlaid on

the satellite image of the wider area and diagrammatic interpretation of the most prominent GPR reflectors.

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Figure 12. Magnetic gradiometry map and diagrammatic interpretation of the magnetic anomalies from Area B at

Onchestos.

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Figure 13. GPR depth slices (0.5-0.6 m) and diagrammatic interpretation of the GPR anomalies from Area B at

Onchestos.

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Figure 14. Integrated diagrammatic interpretation of magnetic and GPR anomalies from Area B at Onchestos that

have been overlain on the satellite image. The plans of the older excavation plans have been also superimposed on the

satellite image.

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Figure 15. Integrated diagrammatic interpretation of the geophysical anomalies that registered by all the geophysical

anomalies at Onchestos from Areas A and B.

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