παραδοτεο π.4.1 - ancient city

ΔΡΑΣΗ: «ΑΡΙΣΤΕΙΑ ΙΙ»
ΕΠΙΧΕΙΡΗΣΙΑΚΟ ΠΡΟΓΡΑΜΜΑ: «ΕΚΠΑΙΔΕΥΣΗ ΚΑΙ ΔΙΑ ΒΙΟΥ
ΜΑΘΗΣΗ»
h
ΙΔΡΥΜΑ ΤΕΧΝΟΛΟΓΙΑΣ ΚΑΙ ΕΡΕΥΝΑΣ
eo
S
IM at
S- R
FO eS
e
R A
TH rc
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
G
GEOPHYSICAL INVESTIGATIONS AT ONCHESTOS, BOEOTIA
Geophysical Investigations at Onchestos, Boeotia
NIKOS PAPADOPOULOS
GeoSat ReSeArch Lab, Institute for Mediterranean Studies, Foundation for Research and Technology Hellas
eo
S
IM at
S- R
FO eS
e
R A
TH rc
h
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.
G
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 (IMSFORTH) 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
2
h
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.
G
eo
S
IM at
S- R
FO eS
e
R A
TH rc
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 subsoils. 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
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.
3
Integration of geophysical data
h
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-
G
eo
S
IM at
S- R
FO eS
e
R A
TH rc
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.
4
h
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.
eo
S
IM at
S- R
FO eS
e
R A
TH rc
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.
G
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 southnorth 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
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
5
h
eo
S
IM at
S- R
FO eS
e
R A
TH rc
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.
G
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.
6
h
eo
S
IM at
S- R
FO eS
e
R A
TH rc
G
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.
7
h
eo
S
IM at
S- R
FO eS
e
R A
TH rc
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
G
View of Area A-Section 1 from the SW
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.
8
h
eo
S
IM at
S- R
FO eS
e
R A
TH rc
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
G
Figure 4. Details from the Area B that was surveyed the archaeological site of Onchestos. Geophysical campaign 2014.
9
Method
Magnetic
Survey
Depth
of
Investigation
2m
eo
S
IM at
S- R
FO eS
e
R A
TH rc
h
1.
Instrument
s
Bartington
Grad 601
2.
GPR
Sensors &
Software
Noggin Plus
Smart Cart
with
250
MHz
antennas
2-3m
3.
Electrical
Resistance
Geoscan
RM85
1-2m
G
Figure 5: Details of the geophysical instrumentation that was used in the prospection of Onchestos - 2014 geophysical
campaign season.
10
h
eo
S
IM at
S- R
FO eS
e
R A
TH rc
G
Figure 6: Overlay of the geophysical grids on the satellite image for Areas A and B that were surveyed in Onchestos.
11
h
eo
S
IM at
S- R
FO eS
e
R A
TH rc
G
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.
12
h
eo
S
IM at
S- R
FO eS
e
R A
TH rc
G
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.
13
h
eo
S
IM at
S- R
FO eS
e
R A
TH rc
G
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.
14
h
eo
S
IM at
S- R
FO eS
e
R A
TH rc
G
Figure 12. Magnetic gradiometry map and diagrammatic interpretation of the magnetic anomalies from Area B at
Onchestos.
15
h
eo
S
IM at
S- R
FO eS
e
R A
TH rc
G
Figure 13. GPR depth slices (0.5-0.6 m) and diagrammatic interpretation of the GPR anomalies from Area B at
Onchestos.
16
h
eo
S
IM at
S- R
FO eS
e
R A
TH rc
G
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.
17
h
eo
S
IM at
S- R
FO eS
e
R A
TH rc
G
Figure 15. Integrated diagrammatic interpretation of the geophysical anomalies that registered by all the geophysical
anomalies at Onchestos from Areas A and B.
18