ELECTROMAGNETIC SOUNDING OF THRACIAN TUMULI
IN THE SBORYANOVO NATIONAL RESERVE, NEAR THE TOWN OF ISPERIH
, RAZGRAD DISTRICT, NORTH EAST BULGARIA
Yuriy Bogdanov1,2, Volodymyr Pavlovych1, Sergiy Prokopenko2,
Diana Gergova3, Boyko Vachev4
1Institute of Nuclear Research, National Academy of Science of Ukraine,
2Spetsaviaindustry LTD, Kharkiv , 3Nationa Instiyute of Archaeology with Museum, Bulgarian
Academy of Sciences 4Institute for Nuclear Research and Nuclear Energy, BAS
Complementing the existing data from various geophysical methods of investigating near-
surface structures (the electrical resistivity and the georadar method) of the Thracian tumuli
near the village of Sveshtari, in the Sboryanovo National Reserve(Gergova, Katevski 2008
374–379), detailed measurements of the natural electromagnetic radiation of the Earth were
conducted in July 2013 in the area (Fig. 1). The neighboring region was studied with respect to
seismicity using the same device and methodology (see presentation/report of Bogdanov, Pavlovych,
Prokopenko on the conclusion BlackSeaHazNet workshop). The paleoseismic investigations had been
carried out since the discovery of several tombs in the Eastern necropolis of Sboryanovo ,
including the Great Sveshtari tumulus, where evidence of ancient earthquakes was also found on the
stone tomb in the southern part of the tumulus (see the photo below )
The essence of the method applied consists in the following. The Earth constantly emits
electromagnetic waves over a sufficiently broad frequency range. According to present ideas, this
radiation is generated throughout the thickness of the Earth’s crust and mantle, and the intensity of
radiation depends to a substantial extent on the stressed state of rock varieties – the greater the stress,
the higher the radiation intensity. When passing through the near-surface layers, this radiation is
scattered by internal inhomogeneities (and is reemitted in the most stressed areas) in such a manner
that measuring the electromagnetic field in different points at the day surface provides actually a fixed
diffraction pattern of electromagnetic wave scattering by internal inhomogeneities. The next main task
is to decrypt the measurement results and to restore the internal structure of the near-surface layers
(for the purposes of engineering geology, including the task of tumuli structure investigation). This
method can be also implemented in the studies of the deeper strata of the Earth’s crust and mantle, the
depth being essentially determined by the measurement profile length at the surface in accordance
with the laws of electromagnetic wave scattering (Bogdanov et al., 2002, 2008).
In this way the intensity of electromagnetic radiation (EMR) on the day surface represents the initial
information for further analysis. To measure this value, a device has been developed that measures
and records the amount of EMR pulses with amplitude
exceeding a preset threshold per unit time. EMR is
measured by means of a highly sensitive broadband
antenna, which measures the magnetic component of the
electromagnetic field (for higher accuracy the three
components of the magnetic field vector – x,y,z,
component x changes along NS direction, component y
– along EW direction and component z – along the
vertical). The time interval of pulse number
measurement is set in advance. The time interval,
accepted for the data of walking speed measurements, is
equal to 0.2 s, which means that for an average
pedestrian velocity of about 5 km/h the EMR pulse
amount is recorded approximately at every 30 cm. The
GPS coordinates have been recorded for each measurement point.
Further on the EMR data were
transferred together with the
coordinates of each point from the
device to the computer and
processed using a special set of
The first stage of this processing is
shown in Fig. 1: binding to the
terrain according to the GPS
coordinates (top), binding to the
relief by plotting the route of
measurements (middle) and
constructing a 3D model of the site
on the base of GPS coordinates and
photos (bottom). The next stage of
data processing consists in mapping
the signal intensity and its gradients.
It must not be forgotten that the
intensity data are recorded in the
device for each component of the
magnetic field vector separately and
that all three components – x, y,z,
have to be analyzed in order to build
the complete picture of the structural
features of the near-surface layer. As
an example the map of the magnetic
field x-component distribution in the
area of Tumulus I is shown in Fig. 2.
The vortex pattern of the
electromagnetic energy distribution
around the tumulus is worth
noticing, which, as shown by
experience, is typical for EMR
distribution around pyramidal
Fig. 1. The first stage of EMR data processing – binding to the terrain.
Such a pattern is probably also typical for cone-shaped structures but this assertion needs additional
verification since the present study is a first attempt of exploring sepulchral tumulus and large cone-
like structures in general.
Fig. 2. Intensity distribution of the magnetic
field x-component in the vicinity of Tumulus I
(Great Sveshtari tumulus). Distribution of EMR
energy around the tumulus in a vortex-like
pattern. The tumulus outline is shown with a blue
line taking into account the lower excavation.
The maps with the distribution of intensity
gradients of the field x-component are shown in
Figs. 3 and 4. They are useful in detecting
underground structures and sometimes it is
expedient to present them in a different contrast.
So, the distribution of the intensity gradient in
the area of Tumulus I is shown in Fig. 3 with
indication of the survey routes.
y gradients across the survey area is presented in Fig. 4.
Moreover, the distribution above Tumulus I (Great Sveshtari
tumulus) is the same as in Fig. 3, but it is shown in black-
The careful analysis of the figures proves that:
1. In the central, southern and southwestern part of
Tumulus I(Great Sveshtari tumulus) anomalies in the EMR
field are observed, which may be associated with
underground structures, and the anomaly in the southern part
is obviously related to the excavated destroyed sepulcher.
2. The field anomalies in Tumulus II(Tumulus 27)
clearly exhibit geometric structures that can be associated
with underground facilities.
3. The field above Tumulus III (Tumulus 31) is
sufficiently homogeneous with the exception of its central
part, which requires a more detailed study.
4. Typical strips are traced from Tumulus III (Tumulus
31) to Tumulus I(Great Sveshtari tumulus), which, as proved
by experience in seismic structure investigation, may be
associated with crustal faults due to paleoearthquakes.
Fig. 3. Anomalous field distribution in the vicinity of
Tumulus I (Great Sveshtari tumulus). Measurement routes
are shown in the top.
Fig. 4. Field distribution of intensity gradients on the surveyed site. The tumulus are indicated by blue
outlines and numbers.
The next step in data processing is the composition of in-depth sections along the measurement
profiles. The procedure of constructing the sections is sufficiently complicated mathematically and it
will not be considered in detail here. It has to be noted only that it is based on the laws of
electromagnetic wave scattering and on investigating the correlation functions.
Several sections of the surveyed Tumuli I(Great Sveshtari tumulus), II( Tumulus 27) and III(Tumulus
31) are presented below. It has to be reminded that the sections are constructed on the base of analysis
only of the magnetic field x-component. The analysis of the other two components may contribute to
introducing additional detail in the obtained patterns.
Figure 5 shows two sections of Tumulus I (Great Sveshtari tumulus) and their comparison with the
geophysical section obtained by the electrical resistivity method. The location of the sections is
clearly seen in the figure (profiles A-A’ and B-B’). The places with suggested heterogeneous
inclusions are shaded. An interesting feature of this tumulus is distinctly seen in section B-B’: it
seems as if this tumulus consists of three tumuli, formed on top of one another, with the destroyed
sepulcher (left, bottom of profile B-B’) being under the external tumulus.
The data from the exploration of Tumulus II (Tumulus 27) are shown in Fig. 6 – this is the most
intriguing information from the viewpoint of underground structures that have not been discovered
earlier. The scheme of survey routes and the distribution of field intensity at the surface are shown in
the center of the top part of the figure. Field distribution anomalies are clearly visible here.
Unfortunately, the data for this tumulus are not in sufficient detail because the manifestation of
“surprises” has not been expected here at all. Further on, based on the analysis of six in-depth profiles
(bottom of the figure) the supposed situation of underground structures has been localized (right, top
part of the figure).
Fig. 5. Geophysical
sections of Tumulus I
tumulus) based on
EMR sounding data
and comparison with
data from the
method. The places
inclusions are shaded.
Fig. 6. Scheme of Tumulus II( Tumlus 27), location of survey routes, scheme of sections with
supposed location of underground structures and 3D distribution of field intensity (top part of the
figure); geophysical sections according to EMR data (bottom part of the figure)
Fig.7. Distribution of survey routes, field intensity and geophysical sections of Tumulus III(Tumulus
Fig. 8. Geophysical sections
of Tumulus III Tumulus 31).
The existence of geometric
structures may be supposed
in the center of the bottom
part of the tumulus.
Tumulus III (Tumulus 31)
(Figs. 7, 8) seems empty at
first sight, but the detailed
analysis of the sections
suggests the existence of
certain structures in its
To compose the conceptual
models of information
processing for the tumulus,
an EMR survey was
conducted along two profiles
of the well-studied sepulcher under Tumulus 13. These data provide information concerning the
specific features of the geophysical sections that have to be taken into account when processing the
results for unexplored tumulus.
Finally, the field intensity distribution was obtained from airborne survey.
In conclusion, it has to be pointed out that the above results have been obtained by processing only the
EMR magnetic field x-component for a limited number of profiles, passing above the most interesting
places of the tumulus. The processing of all profiles in all three coordinates, taking under
consideration the additional information recorded (dispersion and automatic changes in the detection
threshold in the course of measurement), may provide the possibility of obtaining three-dimensional
distribution patterns of underground cavities and structures.
The excavations in Tumulus I (Great Sveshtari tumulus) in 2013 did not confirm the initial
assumptions based on the existing anomalies.
Subsequently, at the end of the year burials and remains of structures were found in Tumuli
II(Tumulus 27) and III( Tumulus 31)(Gergova 2013; 2014 in print). The burial of horses with a 2-
wheel chariot, encountered in Tumulus II(Tumulus 27) is the first of this type in Bulgaria and is of
special interest(see photos below and Figs. 6 and 7).
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electromagnetic emission analysis. Geophysical journal (Ukraine), V30, 2, pp 32 – 41.
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