Geophysical Science and Education in Bulgaria


Ludmil Christoskov , Gotze Tenchov, Vera Ivanova , Zheljo Zhelev , Petar Stavrev, Ivan Petkov, Toma Dobrev . Dimitar Mishev


Introduction

The geophysical science in Bulgaria has a history of more than 100 years. Measurements of the magnetic declination on our lands were made as early as the end of the 18th century. The development of two basic geophysical sciences in Bulgaria - meteorology and seismology - was started in 1890-1891 by academician Spass Vatsov. The first Central Meteorological Station in Sofia was established, where later on in 1905 the first Seismic Station with two mechanical seismographs was set up. Thanks to the efforts made by Acad. Spass Vatsov, the first systematic geophysical observations and their new-style dating were organized. In 1902 the first book of the series "Earthquakes in Bulgaria" was published and in 1907 the Seismological Bulletin of the Sofia Station was printed and circulated.
Among the first geophysical surveys it is necessary to mention the absolute measurements of the magnetic field in 1911 at Sofia, Plovdiv, Bourgas and Nova Zagora, carried out by the Carnegie Institute in Washington. The first totally Bulgarian geomagnetic measurements of declination, inclination and horizontal intensity were made by Acad. K. Popov in the period 1917-1920. The initial gravity surveys were carried out by Prof. I. Kovachev around the salt stock in Provadia. The first systematic gravimetric surveys date back to 1941 and were performed by the State Geographical Institute under the supervision of Acad. V. Hristov and Prof. I. Petkov. of Acad. V. Hristov and Prof. I. Petkov. In the middle of the century important steps were taken towards the further development and expansion of geophysical surveys as well as the training of specialists in geophysics. A general idea of the location of the main geophysical institutions and observation stations on the territory of the country can be obtained from the attached figure.
In 1946 a department of Meteorology and Geophysics was established at the Physical Faculty of the University of Sofia. During l951-1952 academic year a specialization in meteorology and geophysics was organized within the physics speciality. The training of students was entrusted to the department then headed by Acad. L. Krastanov. When the University of Mining and Geology was founded in 1953, a department of Geophysical Exploration Methods was set up and training of students was started in the speciality "Geophysical Exploration Methods". This speciality was later renamed "Applied Geophysic" which reflects more fully the content of the wide education profile formed. Exploration geophysics was also given a strong impetus. At the end of the 1940's and during the 1950's, the basic geological and geophysical exploration structures were built within the framework of the Committee of Geology. This activity is discussed in detail in the paper "Exploration geophysics in Bulgaria- development, achievements, perspectives".
The Geophysical Institute of the Bulgarian Academy of Sciences was founded during 1959- 1960 on the initiative of Acad. L. Krastanov. The institute has the task of carrying out research and observations on the earthquakes, magnetic and gravitational field, ionosphere and some physical processes in the atmosphere. The seismic, magnetic, gravimetric (Earth tides) and ionospheric observation networks were reorganized and gradually expanded. The series of Proceedings of the Geophysical Institute started to be published which in 1975 developed into the Bulgarian Geophysical Journal.
The National Committee of Geodesy and Geophysics (established in 1959) and the Bulgarian Geophysical Society (established in 1990) have considerable contributions to the successful development of geophysics, to the realization of internal and international co- operation. The bases of the international relations and co-operation in this field were laid as early as the beginning of the century. In 1903 Bulgaria became a member of the International Seismological Association. Regular exchange of meteorological and seismological bulletins and data began with international and national centres including all neighbouring countries. Bulgarian publications were used by authors from other countries and foreign scientists began to investigate our geophysical phenomena and fields. The first publication is probably the one by G. Grablovitz (1904) in Italy on the destructive earthquake in the Strouma valley on 4 April 1904.
Now Bulgaria is a member of the International Union of Geodesy and Geophysics (IUGG), of the International Associations of Seismology and Physics of the Earth Interior (IASPEI), Geomagnetism and Aeronomy (IAGA), Meteorology and Atmospheric Physics (IAMAP), Hydrological Sciences (IAHS) of the European Seismological Commission (ESC), European Geophysical Society (EGS), European Association of Geoscientists and Engineers (EAGE), Balkan Geophysical society (BGS), etc. Bulgarian geophysical data are sent to and used continuously by all world centres such as WDC A (Boulder, Colorado), World Centre B (Moscow-Obninsk), ISC (England), NEIC (Boulder, Colorado), EMSC (Strasbourg), etc.

Development of the main trends in the geophysical investigations of the solid Earth Seismology

As was mentioned above the beginnings of the systematic study of the earthquakes in Bulgaria were laid in 1891 by Acad. Spass Vatsov. The subsequent development is connected with the names of St. Staykov, K. Kirov, K. Yankov, I. Petkov, Ek. Grigorova, etc. Three main stages can be identified in the development of Bulgarian seismology: - First stage - non-instrumental (1891-1904); - Second stage - of initial instrumental observations (1905- 1959); - Third stage - of extensive scientific research and upgrading of the observation network (since 1960). These three stages are naturally differentiated and have the following given below characteristics. During the first stage Acad. S. Vatsov organized a correspondence network for collection of data on the earthquakes which have occurred in Bulgaria and ensured the accumulation and processing of the macroseismic data. The publication of the first books and bulletins on the earthquakes in Bulgaria began. After the largest earthquake in Bulgaria on 4 April 1904 in the river Strouma valley with a magnitude of 7.8 and an intensity of X, the construction of seismic stations was accelerated.
The beginning of the second stage was marked by the setting up of the first seismological station (1905) and the installation of the seismoscopes at Sofia, Petrohan, Rila Monastery and Kazanluk. On 14 June 1913 a damaging earthquake occurred in Gorna Oryahovitsa - Veliko Turnovo where St. Staykov carried out the first field investigation of the region and prepared his book "Materials on the seismography of Bulgaria, Thracia and Macedonia". On April 14th, 18th and 25th, 1928 damaging earthquakes occurred in the Maritsa valley which became the subject of many investigations for Bulgarian and foreign scientists. The outstanding seismologist Ch. Richter cites the unique levelling before and after the earthquakes and uses it as one of the few examples in the world at the time. A modern 1000 kg mechanical seismograph Wiechert was installed in Sofia Station (1934).
The seismic section was separated from the meteorological department and in 1951 it was transferred to the Committee of Geology. The first attempt for seismostatistical zoning was realized in the country and the first Code for Construction in Earthquake Regions (1957) was enforced.
The third stage of extensive research and modernization of the observation network is related to the activity of the Seismology Section of the Geophysical Institute at BAS. The main directions of its activity are as follows: seismological monitoring, seismicity and seismic zoning, physics of the seismic waves and their sources, deep structure, earthquake prediction and impact. New important results have been obtained on the seismicity of the country, original methods, algorithms and software have been developed, the instrumental observations have been highly improved. The following results can be estimated as being very important. In 1980, under the supervision of L. Christoskov, corresponding member of BAS, the National Operative Telemetric System for Seismological Information (NOTSSI) was established thus achieving a high level of registration and automation in data processing. Fig. 1 shows the location of the seismological stations in the country. In addition to NOTSSI, the local networks "Provadia" and "Kozlodouy" were put into operation and the construction of network "Belene" is underway. A prognostic seismic zoning of the country has been completed under the supervision of Acad. E. Bonchev. The zoning map for a period of 1000 years serves as a basis for the new building codes in Bulgaria (1987). Precise magnitude scales have been designed for evaluating the earthquake intensity. A 3-D model of the lithosphere for the Central Balkans has been developed. The first earthquakes precursors have been documented and a program has been developed for establishing a prediction testing side "Sofia-Plovdiv". An automated system has been elaborated for assessing of the consequences of future large earthquakes in the country.

Geomagnetism

Since 1960 the basic studies of Earth magnetism were concentrated at the Geophysical Institute of BAS particularity at the Section of Earth Magnetism and Gravimetry which in 1961 involved the Magnetic Observatory at Panagyourishte (MOP with the code PAG on Fig. 1). They were preceded by a number of fundamental geomagnetic surveys. The first magnetic declination map for epoch 1921.0 was drawn according to measurements made by Acad. K. Popov. Detailed measurements of the declination. inclination and horizontal intensity were made in the period 1934-1938 by Hr. Kalfin. During 1937·1939 Acad. K. Popov, Prof. M. Venedikov and Prof. M. Andreichin carried out measurements of the declination in the southern part of the Black Sea coast. In 1937 MOP was set up with a variation house and an absolute house equipped with magnetic theodolites Schulze and variometers Askania. Later on an additional absolute house was built to the observatory (1943), a second absolute house (1981), etc. New equipment was subsequently provided: an Earth inductor, variometers and theodolites Mating and Wiesenberg, quartz H-magnetometers QHM-57, variometers Bobrov, proton magnetometers, including component magnetometer, etc. An annual magnetic bulletin has been published since 1965. Since 1966 the absolute magnetic measurements have been made three times a month.
From MOP three general magnetic surveys were made: 1937-1947 (Chr. Kalfin), 1958- 1961 (K. Kostov) and 1978-1980 (I. Buchvarov). As a result, declination maps were drawn for epochs 1940.0, 1950.0, 1960.0, l965.0, 1970.0 and 1980.0. Maps were also drawn of the basic components of the magnetic field and anomaly field for Bulgaria. At the same time, new and original methods were developed and applied for estimating the elements of the Earth magnetic field.
A special contribution of the Section of Earth Magnetism and Gravimetry is laying the foundations of the paleomagnetic (Sen. Res. Assoc. P. Nozharov, 1965) and archeomagnetic surveys (Sen. Res. Assoc. M. Kovacheva, 1967) in Bulgaria. A paleomagnetic and archeomagnetic laboratory was set up at the section where the first studies were made on volcanic, old and young sedimentary rocks, as well as on samples of archaeological excavations. The age of plutons, such as the Vitosha one, was defined, as well as the horizontal movements and rotation of various geostructures, the inversion of the geomagnetic field during different geological epochs. Now the longest series of archeomagnetic determinations are available and curves have been plotted of the secular variations in the direction and intensity of the ancient geomagnetic field. Over the past ten years theoretical studies have been carried out on the origin of the Earth magnetic field (A. Anoufriev).

Gravimetry

During 1959-1960 the absolute value of the gravity force was determined for the first time at the Sofia Gravimetric Station. A gravimetric connection with pendulum instruments was effected with the World gravimetric point at Potsdam simultaneously by a German (K. Elstner) and a Bulgarian teams (I. Petkov).
For plotting the state gravimetric map on a scale of 1:200,000, during 1960-1961 high precision measurements were made with a static airborne gravimeter and a first-rate gravimetric grid of the country, was created (I. Petkov. G. Georgiev, P. Petrov with the active support of Acad. Vl. Hristov, Bulgarian Academy of Sciences). It serves as a basis for solving geodetic and special problems and provides absolute points for the gravimetric surveys on the territory of Bulgaria. As a result, one of the best maps of the anomalous gravity field in Europe was compiled by a large team of geophysicists.
Since 1960 the researchers at the Section of Earth Magnetism and Gravimetry have been studying actively the Earth tides (Cor. Mem. A. Venedikov). An original method has been developed for analysing Earth tide data which is known and used world-wide. In 1980, the first observations of the Earth tides in the country were made in collaboration with the Department of Meteorology and Geophysics (V. Stanchev) and the Czechoslovak Academy of Sciences. Results were obtained which confirm the liquid phase of the Earth's outer core and useful effects were noticed when searching for earthquake precursors. Inverse problems of the potential fields
As early as in the 1960's at the Section of Earth Magnetism and Gravimetry Sen. Res. Assoc. D. Zidarov, DSc, began systematic work on solving the inverse problems of the potential fields. A method was developed for solving these problems with a finite number of elementary sources (point masses, current loops, etc.). Over 100 papers and 3 monographs have been published. Under the supervision of D. Zidarov, this topic was developed by other section associates such as Sen. Res. Assoc. Zh. Zhelev, Sen. Res. Assoc. T. Petrova, etc. Also during the 1960's the topic of solving the inverse problems of the potential fields was taken up successfully by Prof. I. Nedyalkov with his associates from Bulgarian Academy of Sciences. Theoretical and methodological investigations on the inverse gravimetric and magnetic problems were carried out by Prof. L. Dimitrov and Assoc. Prof. P. Stavrev from the Department of Applied Geophysics at the University of Mining and Geology. Prof. P. Burnev from the Mathematical Institute of Bulgarian Academy of Sciences and other investigators have also contributed to that topic. On the basis of the methods, algorithms and software developed, the sources of potential fields can be determined in terms of location and approximate configuration, which is of particular importance for studying the physics of the Earth interior and for mineral prospecting.

Applied geophysics

Scientific researches in the field of applied geophysics are being carried out at all institutions developing geophysical activity in Bulgaria. They comprise the specific theoretical and methodological basis of the observations, processing and interpretation of geophysical data for the purposes of geology prospecting, exploration and mining of mineral and energy resources, civil engineering, ecology, archaeology, etc. An essential part of the studies are directed towards investigating the deep structure and development of the Earth's crust in Bulgaria and the structural divisions of the Moesian plate, Rhodope massif, Srednogorie, Kraishtidite, the Black Sea shelf. It is necessary to note the achievements in studying the oil and gas perspectiveness of North Bulgaria and the metallogenic prognosis in South Bulgaria. Rational methods have been developed for complex interpretation of geology-geophysical data. A software package has been designed for solving prediction problems by the image recognition methods. 25 years ago computer transformations and modelling of gravitational and magnetic anomalous fields was performed. Detailed studied have been made of the petrophysical characteristics and general regularities of their distribution for the main rock varieties in the country. The scientific research in the field of borehole geophysics involves studies on the properties of terrigenous and fracture-cavernous reservoirs and on the optimisation of methods for interpretation of the complex of borehole geophysical data. Borehole measurements have given data on the geothermal field and its anomalous zones in Bulgaria. Maps have been drawn of the heat flow density and the temperature distribution at different depths.
Original methods and equipment have been developed for radiowave inspection, electrical resistivity methods, express x-ray radiometric and kapametric rock sampling for the purposes of exploration, mining and processing of minerals. By using geophysical methods, a great number of discoveries were made from the Thracian and early Bulgarian cultures which are of particular significance for archaeology. In the field of ecology, studies are being carried out on the geophysical and geological factors having environmental impact, engineering seismo-geological characteristics, air and water pollution, protection of the biosphere against geophysical anomalous phenomena, etc. In this direction, new possibilities are revealed for expanding the applied geophysical studies.

Space geophysics and remote sensing of the Earth from Space

This is the youngest and most promising trend in geophysics which in Bulgaria started to develop successfully during the last 20-25 years with the active participation and under the supervision of Acad. D. Mishev. Simultaneously with the research work performed mainly at the Central Laboratory of Solar-Terrestrial influences of Bulgarian Academy of Sciences, specialists are trained at Sofia University "St. Kliment Ohridski", Technical University of Sofia and University of Mining and Geology "St. Ivan Rilski". Special attention is paid to the participation in priority national and international research projects. On board the satellites, automatic interplanetary stations and manned space stations, including the project "Bulgaria 1300", unique apparatus and systems, designed at the Bulgarian Academy of Sciences showed perfect operation and were used for measuring important geophysical parameters: the basic components of the Earth magnetic and electric fields, gamma and x-ray radiation, airflow, solar-terrestrial influences, etc.