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).

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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)

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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.

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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

Loke, M. H. 2010. Tutorial: 2-D and 3-D electrical imaging surveys. Available at:

http://www.geoelectrical.com/downloads.php.

Papadopoulos, N., Myeong-Jong, Y., Jung-Ho, K., Tsourlos, P., Tsokas, G. 2010. Geophysical investigation of tumuli by means of surface 3D Electrical Resistivity

Tomography. Journal of Applied Geophysics, 70, 3, p. 192-205.

Stummer, P., Maurer, H., Green, A. 2004. Experimental design: Electrical resistivity data

sets that provide optimum subsurface information. Geophysics, 69, p. 120-129.

Tonkov, N. 1996. Geophysical Survey of Tumuli in the Valley of the Kings, Central Bulgaria.

Prognosis and Archaeological Evidence. Archaeological Prospection, 3, p. 209-217.

Tonkov, N., Loke, M. H. 2006. A resistivity survey of a burial mound in the ‘Valley of the

Thracian Kings’. Archaeological Prospection, 13, p. 129-136.

Wilkinson, P., Meldrum, P., Chambers, J., Kuras, O., Ogilvy, R. 2006. Improved

strategies for the automatic selection of optimized sets of electrical resistivity tomography

measurement configurations. Geophysical Journal International, 167, p. 1119-1126.

Димитров, З. 2011. Археологически разкопки на Читашката могила, с. Братя

Даскалови, Старозагорска област, p. 54-59. In: Трако-римски династичен център в

района на Чирпанските възвишения (Ed. М. Тонкова). София.

Димитрова, Д., Сираков, Н., Марков, М. 2010. Спасителни разкопки на Китова

могила в землището на с. Крушаре, община Сливен, p. 255-258. In: Археологически

открития и разкопки през 2009 г. (Ed. Д. Гергова). София,.

Пищалов, С. 1976. Електрически методи на проучване. Държавно издателство

„Техника”, София.

Русев, Н., Стаменов, С. 2012. Спасителни археологически разкопки на надгробна

могила в м. Мералък в гр. Опака, Търговищка област през 2011 г., p. 205-207. In:

Археологически открития и разкопки през 2009 г. (Ed. М. Гюрова). София.

Тонкова, М., Димитров, З. 2011. Археологически проучвания на Каракочова могила, с.

Братя Даскалови, Старозагорска област, p. 36-43. In: Трако-римски династичен

център в района на Чирпанските възвишения (Ed. М. Тонкова). София.

Тонкова, М., Иванов, Я. 2011. Тракийска куполна гробница от края на ІV – началото

на ІІІ в. пр. Хр. в Момина могила, с. Братя Даскалови, Старозагорска област, р. 10-17.

In: Трако-римски династичен център в района на Чирпанските възвишения (Ed. М.

Тонкова). София.

Франтов, Г., Пинкевич, А. 1966. Геофизика в археологии. Ленинград.

Nikola Tonkov,

National Archaeological Institute with Museum,

Bulgarian Academy of Sciences,

E-mail: tonkov_n@hotmail.com

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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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3. Momina mound. 2D inverse resistivity models.

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4. The stone tomb in Momina mound.

5. Chitaskata mound. 2D inverse model.

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6. The stone krepis in Chitashkata mound.

7. Karakochova mound. 2D inverse model.

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8. Granit 1 mound. 2D inverse model.

9. The stone heap reached by the grave robber’s trench.

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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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12. Tumulus at the town of Opaka.

3D view of the results of the 3D inverse resistivity modelling.