BRAȘOV COUNTY HISTORY MUSEUM
MUSEUM OF BRĂILA
The Thracians and their Neighbors in
the Bronze and Iron Ages
PROCEEDINGS OF THE 12TH
INTERNATIONAL
CONGRESS OF THRACOLOGY
TÂRGOVIȘTE
10TH
-14TH
SEPTEMBER 2013
“Necropolises, Cult places, Religion, Mythology” - Volume II -
Editorial Board
Valeriu Sîrbu and Radu Ștefănescu
MUZEUL BRĂILEI EDITURA ISTROS
BRAȘOV
2013
TWO AND THREE DIMENSIONAL ELECTRORESISTIVITY SURVEYS OF
THRACIAN BURIAL MOUNDS
Nikola Tonkov (Sofia - Bulgaria)
Key words: Thrace, barrow, resistivity prospection, 2D ERT, 3D ERT
Abstract: Burial mounds are undoubtedly amongst the most remarkable monuments remaining from the times
of the Thracians. That is why, reasonably, they have always been of special scholar interest since the dawn of
Bulgarian archaeology. Geophysical surveys of Thracian barrows have also long history in Bulgaria. For a long
time, however, they have been focused mainly on the discovery of large structures – monumental tombs.
Development of geophysical equipment as well as of computer programming in recent years have allowed the
technique of geophysical prospection to be improved and thus maximum useful information to be derived. This
enhanced technique includes the joint application of routine electroresistivity and geomagnetic mapping
supplemented now by the new opportunities provided by the continuous vertical electrical sounding (CVES)
and, respectively, by the two and three dimensional inverse modelling known also as two and three dimensional
resistivity tomography (2D and 3D ERT). First successful 2D ERT survey was performed on Svetitsa tumulus
of Shipka necropolis where not only a stone grave was detected but its dimensions and depth beneath the surface
were precisely determined. Inverse modelling indicated also that the grave had been probably built into an
already raised tumulus embankment. In the present communication the potential of 2D ERT is illustrated with
more examples of surveys of several Thracian barrows where wide range of structures has been detected and
their characteristics successfully predicted: a stone tomb, stone heaps, crepises, primary mounds, etc. The only
example of tumulus 3D ERT whose prognosis for the presence of a primary mound has been checked by
subsequent archaeological excavations is also reported.
Geophysical prospection of Thracian burial mounds in Bulgaria has a long history.
And it is quite reasonable. These are amongst the most numerous and attractive monuments
remained in the Bulgarian lands from antiquity. Tumulus survey is not a routine procedure
since it requires big depth of investigation so it lies on the boundary between archaeological
and geological prospection. In this respect, several techniques have been tested:
electroresistivity, geomagnetic, seismic, low and high frequency electromagnetic, etc.
Ultimately, at this stage, the most powerful technique has appeared electroresistivity (or
combination of resistivity and magnetic) survey. For a long time the resistivity prospection of
Thracian barrows has been focused on the detection of big structures – tombs. First
investigations have been performed by simple traversing or mapping with a single fixed
electrode configuration. Subsequently, the surveys have been expanded by mapping with
more electrode separations allowing respectively different depths of investigation. This
manner of measurements, however, allows only qualitative or at best semiquantitative
interpretation of the data. This means: detection of eventual disturbing body and
determination of its approximate dimensions in plan but not the depth and the vertical
dimensions. Nevertheless, several rich graves and tombs were detected, like those from the
region of Kazanlak (Tonkov 1996).
The rapid development of geophysical equipment as well as of computer programs in
the last two decades allowed the technique of resistivity prospection to be improved and thus
the whole tumulus embankment to be more completely and precisely examined. This
enhanced technique includes the joint application of common electroresistivity and
geomagnetic mapping, supplemented now by the new opportunities provided by the
continuous vertical electrical sounding (CVES) and, respectively, by the two and three
dimensional inverse modelling known also as two and three dimensional resistivity
tomography (2D and 3D ERT).
402
The essence of this technique consists of performing measurements along the
traverses with several (ten or more) different separations of the selected electrode array and
thus obtaining data for gradually increasing depth (fig. 1). Thereafter the data is processed by
the so called inverse programs which as a result give the distribution of the true resistivity in
depth (Loke 2010).
As an example of the two dimensional ERT can be given the traverse above the
fortress wall of the Roman colony Deultum (fig. 2). At the top is the pseudosection of the
measured apparent electrical resistivity. At the bottom is the calculated inverse model. In the
middle is the pseudosection of apparent resistivity which would be measured at this inverse
model. The indicator to what extent the model is correct is the RMS (the root mean square)
error. In this particular case the value is about 1.4 percent which is pretty good. Collating
several parallel traverses gives three dimensional model of the resistivity distribution.
The principal and reasonable surveying grid is the rectangular one. But it may be used
only for lower mounds – with height up to 2-2.5 m. For the higher tumuli the grid distortion
becomes greater and difficult to be removed. That is why for the larger mounds the radial
grid, matching the characteristic axial symmetry of tumuli, is preferable. Unfortunately, in
this case the resistivity tomography can be performed only in two- (sooner in two and a half)
dimensional mode since the inverse programs available do not support such irregular
surveying grid. In any case, the precise levelling of each electrode position along the
traverses is required in order the programs to be able to correctly calculate the terrain
corrections.
Below, the results of certain successful two and three dimensional resistivity
tomography surveys on Thracian burial mounds, all verified by archaeological excavations,
will be presented1.
1. Momina mound at the village of Bratia Daskalovi.
It is about 7.5 m high with a diameter of some 50 m in diameter.
Resistivity mapping showed comparatively high and variable values. This spoke of an
inhomogeneous embankment with very high content of gravel and boulders. Inverse models
revealed a tomb south of the tumulus centre as well as two elongated stone heaps in front of
it. The tomb’s dimension in south-north direction was about 3.5 m. It was interesting that the
inverse model does not detect a roof, but only the front and rear walls (fig. 3). The
archaeological excavations unearthed both the stone heaps and the stone tomb and confirmed
that the roof had been destroyed (fig. 4) (Тонкова 2011).
2. Chitashkata mount at the village of Bratia Daskalovi.
It is about 7 m high with a diameter of some 40 m in diameter. Both the mapping and
inverse models spoke of a homogeneous tumulus embankment built of earth with high clay
and sandy content. The inverse models revealed also that the mound was encircled by a stone
krepis that was revealed later on by the excavations (figs. 5, 6). The excavations revealed also
an artificial stone heap with diameter 4-5 meters and height of about 1 m situated in the
centre at the base of the mound (Димитров 2011) that was not detected by the resistivity
prospection. Obviously, a feature with such dimensions at so great depth, about 7 meters, is
beyond the potential of geophysics.
3. Kaakochova mound at the village of Bratia Daskalovi.
It is about 2.5 m high with a diameter of some 25 m in diameter.
The resistivity mapping did not detect any anomalous feature except the shallow
occurring bedrock. In contrast, the inverse model trough the centre revealed a very low
resistivity zone at the base slightly south of the centre (fig. 7). The prediction was for the
1 The first successful application of 2D ERT performed on the Svetitsa mound, where a rich fifth century built
BC grave was detected, had been already reported (Tonkov, Loke 2006)
403
existence of a primary mound with very high clay content of the earth. The excavations
discovered such primary mound and beneath it a thick burial pyre with lots of funeral
offerings (Тонкова, Димитров 2011). In this case, it may be supposed that this very
intensive negative anomaly is produced not only by the primary mound but rather by the
burial pyre and, respectively, by the sharp rise of the medium conductivity due to the high
carbon content similarly to the possessing electronic conductivity charcoal (Франтов,
Пинкевич 1966, p. 84), graphite or coal schists (Пищалов 1976, p. 18), a phenomenon that
gives another unexpected practical result.
4. Tumulus 1 at the village of Granit near Bratia Daskalovi.
It is about 3 m high with a diameter of some 35 m.
Resistivity mapping detected a low magnitude positive anomaly slightly southwest
from the centre. The inverse models outlined an isometric high resistivity feature in the base,
about 3 m in diameter, 1 meter height, the top side at 1 meter from the surface (fig. 8). Most
likely it is a stone heap. The mound has not been a subject of archaeological excavations. But
last year grave robbers made a trench and obviously reached the stone heap (fig. 9).
Thankfully, they were stopped in time and the trench was immediately refilled.
5. Tumulus 1 at the village of Krushare.
It is about 6 m high with a diameter of some 50 m.
The mapping did not register any local anomaly except a broad low resistivity zone in
the centre of the mound. The inverse models outlined by its very low resistance a big feature,
most likely a primary mound with diameter about 15 meters and height of about 2.5 m (fig.
10), which might indirectly indicate the existence of a primary grave. The excavation carried
out later on confirmed this prediction and discovered both the mound and a primary grave
(Димитрова et alii 2010).
6. Thracian burial mound at the town of Opaka.
It was about 2.5 m high with a diameter of some 30 m in diameter.
Its dimensions were not very clear because it situated on a slope and it had been
subjected to modern interventions.
The survey of the tumulus at the village of Opaka for the first time made use of 3D
ERT on burial mound applying dipole-dipole array.
In fact, there is only one example of 3D ERT of one small tumulus from North Greece
performed by a team from the Korean Institute of Geoscience and the University of
Thessaloniki a couple of years ago (Papadopoulos et alii 2010). They have used the so called
three- (or half-Schlumberger) electrode array. But I am not aware if in this case excavations
have been carried out there and the geophysical results have been checked up.
Our surveying grid was rectangular with density 1x1 m. The measurements were
carried out with ten different lengths of the dipole-dipole array. The inverse modelling was
performed both in two-dimensional mode along each single traverse and in three-dimensional
mode of the whole data available. The results of the 2D modelling showed that the thickness
of tumulus embankment did not exceed 2 m (fig. 11). The result of the 3D inverse modelling
outlined a primary tumulus just east of the centre of the mound (fig. 12). Such tumulus was
really unveiled by the subsequent archaeological excavations. It was erected over two graves
with cremations in pits dug into the ground (Русев, Стаменов 2012). The elongated anomaly
at the south periphery of the grid may be caused by the border line between the surrounding
terrain and the tumulus embankment but in any case it must not be taken into account because
the measurement points there are not enough so the three dimensional inverse modelling is
not completely correct. The performed comparisons showed that in this case 3D ERT gave
much better results than 2D (along every single profile) and therefore it has to be always
applied in appropriate conditions, i. e. on tumuli 2-3 meters high.
404
The achieved results at this stage of research allow some directions to be drawn
regarding the development of resistivity prospection of Thracian burial mounds. These are
primarily in the application of 3D ERT on all funeral tumuli including big ones. In this
respect, both the acceleration of measurement process and the overall prospection quality
forces the use of multichannel resistivimeters, like those already applied in geological survey,
as well as of the so called optimized electrode arrays (Stummer et alii 2004; Wilkinson et alii
2006) and, respectively, more advanced inverse programs for data processing.
BIBLIOGRAPHY
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на ІІІ в. пр. Хр. в Момина могила, с. Братя Даскалови, Старозагорска област, р. 10-17.
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Nikola Tonkov,
National Archaeological Institute with Museum,
Bulgarian Academy of Sciences,
E-mail: [email protected]
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1. Scheme of the measurement points in two dimensional continuous vertical
electrical sounding (2D CVES)
2. Roman colony Deultum. Results of the two dimensional inverse modelling.
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10. Tumulus 1 at the village of Krushare. 2D inverse model.
11. Tumulus at the town of Opaka.
2D inverse model along a traverse through the centre of the mound.
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