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CHARACTERISTICS OF THE SURFACE HEAT FLOW DENSITY ON THE
TERRITORY OF BULGARIA AND THE NEIGHBOURING BLACK SEA SHELF
T.Dobrev , S.Dimovski1, S.Kostyanev2, G.Aleksiev 3, V.Stoyanov 4
1, 2, 4 – University of Mining and Geology “St. Ivan Rilski”, Sofia, Bulgaria
3 – Geographical Institute, Bulgarian Academy of Science, Sofia, Bulgaria
1 – [email protected], 2 – [email protected], 4 – [email protected]
Abstract
Schematic maps of surface heat flow density are developed and analyzed. Averaging
windows of 400 km2 and 1600 km2 are applied respectively. The density of the regional heat
flow is in the range of 40-80 mW/m2. Five geothermal zones are well distinguished on both
composed maps.
The first geothermal zone (I) is detached along the line Lom – Veliko Turnovo – Aitos. This
zone has a sub-parallel orientation and is characterized by medium activity. The maximum
heat flow values for the country are forming the well-distinguished South-Western
geothermal zone (II). This zone has a sub-meridian orientation and a well-expressed crustmantle geoenergy origin. The second most important geoenergy zone is the Central Rhodope
one (III). This zone has an endogenetic character. The Berkovitsa – Etropole – Elhovo belt,
representing the fourth geothermal zone (IV), is mapped by minimal values of the heat flow
density. The territory of North East Bulgaria (geothermal zone V), and especially the NorthBulgarian uplift, are characterized by low fluctuations in the heat flow density values.
Some preliminary notes and a review of published data concerning the heat flow in
Bulgaria.
Main objective of the presented study is regional study and zoning of the surface heat flow
density on the territory of Bulgaria and the neighbouring Black Sea shelf. The investigation is
based on a review of published geothermal data (Velinov, 1986; Dachev, 1986; Bojadgieva
and Gasharov, 2001, etc.). The applied methodic is presented by Dobrev et al., 2004.
The surface heat flow density (heat flow ) is a very important physical parameter that gives
the most detailed information about the Earth’s natural heat. It can be used for estimating the
resources of geothermal energy, for studying the geothermal regime, and for calculating the
thermal field on different depths in the lithosphere. The heat flow value through the surface of
the Earth as a planet is equal to about 61.5 mW/m2. The scientists who have performed the
first studies of the geothermal field in Bulgaria have pointed their efforts towards estimating
the thermal conductivity coefficient of the rocks and the geothermal gradient of the geological
section. If data of thermal conductivity coefficient for different rocks are missing or not
sufficient, than the general practice is to accept a constant value for the thermal conductivity
coefficient in the entire area under study. The first schematic map of heat flow in Bulgaria
(T.Velinov, K.Boyadjieva, I.Petkov, 1979) was developed on the base of the geothermal
gradient in depth interval between 100 and 500 m taking the averaged value of 2.1 W/(m.K)
as a constant one characterizing the thermal conductivity coefficient in the country. According
to the cited scheme, the average value for surface heat flow density on the territory of
Bulgaria is about 65 mW/m2. On the quoted map, the heat flow has anomalous low values in
North East Bulgaria (the North-Bulgarian uplift) and high values in parts of West Sredna
Gora and Rhodope Central Massif.
The heat flow map of Europe published in 1979 is revealing the global trends in the behaviour
of this geothermal indicator on the territory of Bulgaria. The map was developed after taking
into account data from the above-mentioned scheme (T.Velinov, I.Petkov, 1976; T.Velinov,
K.Boyadjieva, I.Petkov, 1979). On the heat flow map of Europe, a geothermal zone
characterized by low surface density values (30-50 mW/m2) is expressed in the Moesian plate.
Another zone having sub-meridian orientation and values in the range of 50 to 70 mW/m2 is
well distinguished in the Fore-Balkan and parts of the Balkan Tectonic zone. Then, the heat
flow is decreasing again in Central and East Sredna Gora (30-50 mW/m2). A geothermal zone
having sub-meridian orientation and characterized by relatively high surface density values
(60-80 mW/m2) is located in the central part of the Rhodope Massif.
The first map of surface heat flow density on the territory of Bulgaria elaborated on the base
of precise measured values of the thermal conductivity coefficient was published in 1986
(T.Velinov, 1986). This map comprises heat flow values calculated in 127 points – 51 of them
in North Bulgaria, and 76 in the rest of the country. A higher density of observation points
characterizes the western part of North Bulgaria, the region along Strouma valley, the central
part of Rhodope Massif, and the Bourgas trough. On the contrary, low density of
measurement points is observed in the areas of North-Bulgarian uplift, West and East Balkan
zones, western part of Rhodope Massif, and Kraishte region. The heat flow values in South
Bulgaria are calculated for different depth intervals – 300-600 m and 300-1100 m
respectively. On the presented map, the values of surface heat flow density in Bulgaria are
varying in a wide range – from 41 mW/m2 in the vicinity of Koprinka Dam up to 186 mW/m2
in Erma Reka geothermal region.
The second map of surface heat flow density on the territory of Bulgaria is published in 1991
(P.Petrov et al., 1991). In the section describing the methods of investigation of thermal field
and geothermal regime in Bulgaria it is mentioned that the deepest level of temperature
measurement in North Bulgaria is 5200 m in exploration well Aglen R-2 (164C). According
to this paper, a temperature at a maximum depth of 3000 m is registered in exploration well
Pomorie R-1 in South Bulgaria (84C). This heat flow map of Bulgaria is more integral and
representative than the first one. It is composed according to heat flow density data for 148
points of the whole country – 63 of them are in North Bulgaria and 85 in South Bulgaria. It is
stated that heat flow values are over 100 mW/m2 in Strouma valley and North Rhodope
Massif, and even above 200 mW/m2 in Erma Reka region.
A major conclusion can be drawn of the presented review of published geothermal data
concerning heat flow in Bulgaria. The applied depth interval (maximum 1100 m) and the
local near-surface hydrothermal sources in South Bulgaria (more than 230) have led to a
substantial increase in geothermal gradient values. For that reason higher surface heat flow
density values are calculated. The heat flow field is highly disturbed and characterized by
magnified values in areas where thermal water is present. Both discussed heat flow maps
(T.Velinov, 1986; P.Petrov et al., 1991), regardless of the existing discrepancy in some local
values, are giving reliable and convincing information about geothermal section and character
of thermal regime only for the near-surface part of Earth’s crust in Bulgaria.
Schematic map of surface heat flow density in Bulgaria developed after applying an
averaging window of 400 km2.
One of the objectives of the presented study is to estimate the resources of geothermal energy
in the upper part of earth’s crust that can be exploited by application of modern technologies.
The quantitative determinations of surface heat flow density on the territory of Bulgaria
presented in the quoted paper (T.Velinov, 1986) are used in order to obtain information
concerning the above-mentioned problem. They are supplemented with heat flow data from
the Bulgarian Black Sea shelf (Ch.Dachev, 1986; A.Douchkov and S.Kazancev, 1985). A
combined heat flow map is developed on the basis of this quantitative information.
Arithmetical averaging is applied over the composed map in order to emphasize on the
regional component of heat flow field and to reduce the influence of local near-surface
anomalies having non-stationary nature. Averaging windows of 400 km2 and 1600 km2 are
utilized respectively.
On the map obtained after applying an averaging window of 400 km2 (Figure 1), the surface
regional heat flow values are varying in the range of 40-80 mW/m2. A geothermal zone (I) is
detached along the line Lom – Veliko Turnovo – Aitos. It has a sub-parallel orientation and is
characterized by medium activity (heat flow density values in the range of 60-70 mW/m2).
This zone occupies the area of Lom depression, West and Central Fore-Balkan (southern edge
of Moesian plate) and northern periphery of East Sredna Gora. One less active geothermal
anomaly is expressed in the zone’s western part near the town of Montana. The East Balkan
zonal anomaly I’ (Sliven – Aitos anomaly) preserves its relatively high deep geoenergy
potential.
The maximum heat flow values for the country (in the range of 60 to 80 mW/m2) are forming
the well-distinguished South-Western geothermal zone (II), characterized by high activity.
This zone has a sub-meridian orientation and a well-expressed crust-mantle geoenergy nature
determined by active crustal fault and block structures. Two anomalies can be separated
inside this zone by a slight transition along its spread - the Sofia anomaly, having an elliptical
shape and characterized by the highest heat flow density values for the country, and the vaster
Rila-Pirin geothermal anomaly to the south, respectively. The form and the spread of Sofia
geothermal anomaly are showing that its contemporary activity is a result of the interaction of
at least two deep faults – the Strouma and the Maritza one respectively. These deep fault
systems are accompanied by a bundle of normal-slip faults, grabens and magmatic block
structures characterized by high values for the thermal conductivity coefficient (T.Dobrev,
V.Ivanova and R.Radkov, 1989; S.Kostyanev, T.Dobrev and E.Spasov, 1988). The Strouma
fault and its interaction with the bordering magmatic and metamorphic block structures cause
the activity of Rila-Pirin geothermal anomaly.
The second most important geoenergy zone is the Central Rhodope one (III). This zone has an
endogenetic character and heat flow density values from 60 up to over 75 mW/m 2. Its
epicentral part is well expressed in the Erma Reka – Madan region and continues in Greece.
To the west (towards the Bratsigovo-Dospat depression) this zone is limited by a high
gradient of the heat flow density. The heat flow field expansion to the north up to the Maritza
deep fault and its gradual transition toward the eastern part of the Rhodope Massif are proving
the massive geoenergy potential of this geothermal zone. This potential is created and
recharged by the activity of the Central Rhodope crust-mantle fault zone (T.Dobrev,
J.Shcukin, 1974) and by the tectonic position of the Central Rhodope block-like anticline
developed in the region. The neotectonic geodynamic activity of the fault zone is the main
reason for the presence of large disjunctive structures having normal-slip fault and block
origin and characterized by high values for the thermal conductivity coefficient. Increased
heat flow density values can be observed in the belt where the Central Rhodope fault zone
crosses the Maritza fault and its southern satellite structures. These tectonic and magmatic
structures are gravitating towards the periphery of the Rhodope Central Massif.
Analysis of regional heat flow field in South Bulgaria (Rhodope Central Massif and its
surroundings) is showing without ambiguity that both, the localization of well-distinguished
geothermal zones and heat flow anomalies, and their precise position, have predominantly an
endogenetic crust-mantle origin.
The South-Western (II) and the Central Rhodope (III) zones are separated from the Lom –
Veliko Turnovo – Aitos zone (I) through the Berkovitsa – Etropole – Elhovo belt. This belt,
representing the fourth geothermal zone (IV), is mapped by minimal values of the heat flow
density (in the range of 45 to 55 mW/m2). The belt is traced in West Balkan, Sredna Gora
Anticlinorium, Upper-Thrace Depression and Sakar Massif. Limited quantities of geothermal
data are available for some parts of this relatively negative zone. The thermal loss in this
diagonally elongated zone is obviously connected to the tectonic and petrologic
thermoshielding peculiarities, and to the near-surface hydrogeologic conditions in the region.
The territory of North East Bulgaria (geothermal zone V), and especially the North-Bulgarian
Uplift, are characterized by low fluctuations in the heat flow density values and a
predominantly non-active background. The weak positive Shabla thermal anomaly is not
expressed on the field. The obtained in this region heterogeneous structure of the heat flow
field is principally due to the near-surface “cold” Valanginian-Jurasic carbonate aquifer
complex, to the dislocation of the Paleozoic complex, and to the block disjunction of the
Baikal crystalline fundament. The insufficient surface density of the input thermal data has
also influenced the obtained result for the heat flow distribution in the North-Bulgarian Uplift
and its northern periphery (T.Velinov, K.Bojadgieva, 1981; P.Petrov et al., 1991).
The heat flow field on the territory of the Bulgarian Black Sea shelf is characterized by the
absence of well-distinguished thermal anomalies. At the same time it has to be mentioned that
the West Black Sea shelf zone (VI), according to its surface heat flow density values (in the
range of 50 to 55 mW/m2), is more like the “cold” regions on the territory of Bulgaria,
characterized by low thermal activity. Similar conclusions are also reached in some other
studies of heat flow in the western part of Black Sea (A.Douchkov, S.Kazancev, 1985;
A.Kondjurin, V.Sogelnikov, 1983; V.Kopzar, A.Mitropolskii, 1983).
An attempt is done to utilize information about heat flow field distribution in other Balkan
countries in order to supplement thermal data in the near-border regions and to match the
composed map to those of the neighbouring countries. Unfortunately, there is no fit between
the heat flow map of Bulgaria and those of the neighbouring countries as they are generally
showing higher heat flow density values. This is due to the existing differences in the initial
assumptions and the applied methods for determining the surface heat flow density values.
For this reason, we are going to mention only some regional elements of heat flow field in the
neighbouring countries, connected to large-scale geologic units that are spread also in
Bulgaria.
A vast low activity thermal zone (values in the range of 60 to 70 mW/m2) is well
distinguished on the heat flow map of Romania to the north of Danube River (Geothermal
map S.R.Romania, Scale 1:1 000 000, 1985). No local anomalies can be observed in this
zone. Its spread and morphology are mapping the location and reflecting the plate-like
behaviour of the Moesian lithosphere block. The heat flow nature in Doubrudja is completely
different. The heat flow field in this region has a mosaic character and is expressed by several
relatively high active (values in the range of 70-100 mW/m2) and low active (values in the
range of 30-50 mW/m2) thermal anomalies. These anomalies are forming local zones striking
in NW-SE direction. This orientation is typical for the different according to their age and
genesis fault and block structures in Doubrudja.
The surface heat flow density isolines on the territory of Serbia close to the Bulgarian border
are striking predominantly in NW-SE direction (Heat flow density in Yugoslavia, Scale 1:2
500 000, 1987). This orientation corresponds to the alignment of the folded Karpatian-Balkan
structural-tectonic zone. The heat flow values in the near-border area decrease from NW
towards SE (from 100 to 60 mW/m2).
The thermal anomalies on the territory of FYROM close to the Bulgarian border are showing
relatively high activity (Heat flow density in Yugoslavia, Scale 1:2 500 000, 1987). Surface
heat flow density values in this area are in the range of 80 to 120 mW/m2. These anomalies
are connected to the major tectonic elements of the Serbian-Macedonian Massif.
The surface heat flow density on the territory of Turkey is determined utilizing a constant
value for the thermal conductivity coefficient (=const) in the entire area under study (Heat
flow density distribution in Turkey, Scale 1:2 500 000, 1986). The heat flow field in the
European part of Turkey and its neighbouring Black Sea shelf has values in the range of 70 to
80 mW/m2 and is characterized by isolines striking predominantly in NW-SE direction. Local
disturbances in the heat flow orientation and regional characteristics can be observed only to
the west of Bosphorus and the town of Chorlu.
Schematic map of surface heat flow density in Bulgaria developed after applying an
averaging window of 1600 km2.
The schematic map of the surface heat flow density developed after applying an averaging
window of 1600 km2 (Figure 2) is giving a global picture of the deep crust-mantle structures
distribution according to their geothermal characteristics. Three “hot” (thermally positive)
zones are well expressed on the presented map – I’, II, and III. They represent the main
geothermal resource of Bulgaria. A relatively “cold” (negative geothermal background) area
surrounds these zones.
The East Balkan (Sliven – Aitos) zone I’ is characterized by medium thermal activity (surface
heat flow density values in the range of 60 to 65 mW/m2). Its origin is connected to
geodynamic processes taking place in the eastern parts of Fore-Balkan and Stara Planina
structural chain. The existence of East Balkan zone is showing that in depth this area is
disconnected from the sub-equatorial belt (values in the range of 50-60 mW/m2) revealing the
location and the present tectonic stability of West and Central Fore-Balkan (southern edge of
Moesian plate). The Moesian block is expressed by relatively negative values on the regional
heat flow field. It is mapped by the low active North-Bulgarian geothermal zone V (values of
about 55 mW/m2) that remains open to the north.
The South-Western zone (II) is characterized by high surface heat flow density values (in the
range of 60 to 70 mW/m2). It reflects the geothermal activity of West Sredna Gora and West
Rhodope granitized megablock and their peripheries. The deep thermal roots of this zone are
represented by the two closely connected positive anomalies – the Sofia one (values in the
range of 65-70 mW/m2), and the Rila-Pirin one (values of about 65 mW/m2), respectively. It
can be stated that the biggest part of the geothermal energy on the territory of Bulgaria is
concentrated in the South-Western zone.
The Central Rhodope geothermal zone (III) is characterized by surface heat flow density
values from 60 up to over 70 mW/m2. It has a well-expressed concentric form with its
epicentral part located in the Erma Reka – Madan. The vast spread out of this zone is
revealing its considerable geothermal resources.
The Etropole - Karlovo – Elhovo geothermal zone (IV) has relatively low heat flow values (in
the range of 50 to 55 mW/m2). It separates the East Balkan (Sliven – Aitos) zone of medium
activity (I’) from the South-Western (II) and the Central Rhodope (III) zones. The minimal
heat flow density values in this zone are mapping the arch and the periphery of Sredna Gora
Anticlinorium and a region in the eastern part of Upper-Thrace Depression (T.Dobrev,
J.Shcukin, 1974; T.Dobrev, V.Ivanova and R.Radkov, 1989). The zone continues to the
southwest and reaches the relatively “cold” Sakar-Strandja tectonic region.
Conclusions
The schematic maps of the surface heat flow density developed after applying averaging
windows of 400 km2 and 1600 km2 are fulfilling and making more detailed the available
information about the distribution of this important geothermal parameter. The density of the
regional heat flow on the territory of Bulgaria is in the range of 40-80 mW/m2. Five
geothermal zones are well distinguished on both composed maps.
A geothermal zone (I) is detached along the line Lom – Veliko Turnovo – Aitos on the map
obtained after applying an averaging window of 400 km2 (Figure 1). This zone has a subparallel orientation and is characterized by medium activity (heat flow density in the range of
60-70 mW/m2).
The maximum heat flow values for the country (in the range of 60 to 80 mW/m2) are forming
the well-distinguished South-Western geothermal zone (II), characterized by high activity.
This zone has a sub-meridian orientation and a well-expressed crust-mantle geoenergy origin.
Two anomalies can be separated inside this zone by a slight transition along its spread - the
Sofia anomaly, characterized by the highest heat flow density values for the country, and the
vaster Rila-Pirin geothermal anomaly to the south, respectively.
The second most important geoenergy zone is the Central Rhodope one (III). This zone has an
endogenetic character and heat flow density values from 60 up to over 75 mW/m 2. Its
epicentral part is well expressed in the Erma Reka – Madan region and continues in Greece.
The South-Western (II) and the Central Rhodope (III) zones are separated from the Lom –
Veliko Turnovo – Aitos one (I) through the Berkovitsa – Etropole – Elhovo belt. This belt,
representing the fourth geothermal zone (IV), is mapped by minimal values of the heat flow
density (in the range of 45 to 55 mW/m2).
The territory of North East Bulgaria (geothermal zone V), and especially the North-Bulgarian
uplift, are characterized by low fluctuations in the heat flow density values.
The schematic map of the surface heat flow density developed after applying an averaging
window of 1600 km2 is giving a global picture of the deep crust-mantle structures distribution
according to their geothermal characteristics. Three “hot” (thermally positive) zones are well
expressed on the presented map – I’, II, and III. They represent the main geothermal resource
of Bulgaria. A relatively “cold” (negative geothermal background) area surrounds these
zones.
References
Bojadgieva, K. and Gasharov, S., 2001. Catalogue of geothermal data of Bulgaria,
GorexPress, Sofia, p. 163.Dachev, H., 1986. Structure of Earth’s crust in Bulgaria,
Technika, Sofia, p. 334. (in Bulgarian)
Dobrev, T. and Shcukin, J., 1974. Geophysical fields and seismisity in the eastern part of the
Karpatian-Balkan region, Nauka, Moscow, p. 170. (in Russian)
Dobrev, T., Ivanova, V. and Radkov, R., 1989. Integrated geophysical problems, Technika,
Sofia, p. 319. (in Bulgarian)
Dobrev, T., Dimovski, S. and Kostyanev, S., 2004. Level of study of the geothermal field in
Bulgaria and a methodical approach towards investigating its depth distribution,
Annual of the University of Mining and Geology, Vol. XLVII, part 1, Geology and
Geophysics, pp. 251-258. (in Bulgarian)
Duchkov, A. and Kazantsev, S., 1985. Heat flow through the western part of the Black Sea
floor, Geology and Geophysics Journal, No.8, Nauka, Siberian Dept., Novosibirsk,
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Geothermal map S.R.Romania, Scale 1:1 000 000, 1985, Atlas geologic foala, No.15, Institute
de Geologie si Geofizica.
Heat flow density distribution in Turkey, Scale 1:2 500 000, 1986.
Heat flow density in Yugoslavia, Scale 1:2 500 000, 1987, GZL.
Heat flow map of Europe, 1979, Spriner-Verlag, Berlin-Heidelberg, Editors V.Cermak and
E.Hurtig (Co-editors for Bulgaria: K.Bojadgieva, D.Gueorgiev, I.Petrov, P.Petrov and
T.Velinov).
Kondjurin, A. and Sogelnikov, S., 1983. Heat flow in the western part of Black Sea,
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floor sediments, Geological Journal, Vol. XLIII, No.3. (in Russian)
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seismicity in the Earth’s crust of Bulgaria, Proc. XXI Gen.Assembly, Sofia.
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Figure captions
Figure 1. Schematic map of surface heat flow density in Bulgaria developed after applying an
averaging window of 400 km2
Heat flow zones: I – Fore-Balkan (Lom – Veliko Turnovo – Aitos) medium activity geothermal zone (60-70
mW/m2); II – South-Western high activity geothermal zone (60-80 mW/m2); III – Central Rhodope high activity
geothermal zone (60-75 mW/m2); IV – Berkovitsa – Etropole – Elhovo negative low activity geothermal zone
(45-55 mW/m2); V – North Bulgaria negative low activity geothermal zone (50-55 mW/m2); VI – West Black
Sea shelf negative low activity geothermal zone (50-55 mW/m2)
Figure 2. Schematic map of surface heat flow density in Bulgaria developed after applying an
averaging window of 1600 km2
Heat flow zones: I’ – West Balkan (Sliven – Aitos) medium activity geothermal zone (60-65 mW/m2); II –
South-Western high activity geothermal zone (60-70 mW/m2); III – Central Rhodope high activity geothermal
zone (60-75 mW/m2); IV – Etropole – Karlovo – Elhovo negative low activity geothermal zone (50-55 mW/m2);
V – North Bulgaria negative low activity geothermal zone (50-55 mW/m2)