Exploration geophysics in Yugoslavia: the roots are deep

Among the worldwide geophysical community Yugoslavia is renowned for having produced several scientists who have made enormous contributions to modern geophysics. This is the appropriate opportunity to pay tribute to them. Rudjer Bošković (1711-1787) was born and educated in Dubrovnik, became a Fellow of the Royal Society (of London) and a professor in Paris and Milan. His contribution on sunspot phenomena (1922) is worthwhile reading for everyone interested in the geomagnetic field and related geophysics. Although his contributions in astronomy and geophysics are of the highest rank, Yugoslavia decided to name one of its nuclear research institutes (Zagreb) after him. Nikola Tesla (1856-1943) is far better known for his accomplishments in electrical engineering, especially in the field of electromagnetic phenomena, but every student of geophysics learns his name on first acquaintance with the earth's magnetic field. Milutin Milanković (1879-1958) formulated the mathematical basis of polar-wandering and continental drift, which exactly explained Alfred Wegener's descriptive hypothesis of the phenomenon. Although Milanković's theory of the insolation effect on the ice-age cycles (1941) is not of direct significance for exploration geophysics, it is important in geophysics for understanding one of the basic phenomena, that of the sunearth interaction. Andrija Mohorovičić (1857-1936) made a discovery of much closer relevance to the field of exploration geophysics, since we are prospecting within the earth's crust whose lower boundary is known as Mohorovičić's discontinuity. The earthquake on 8th October 1906, whose epicentre was 50 km southeast of Zagreb, the home city of Mohorovičić, was a challenge for him to study the phenomenon thoroughly. Mohorovičić's (1910) results were so fascinating that it took some time to appreciate them. It was many years later that the crust's lower boundary was recognised as Mohorovičić's discontinuity. Please note that it is neither the Moho nor the M-discontinuity: it is only correct to refer to it as Mohorovičić's discontinuity. Over a period of many years af ter 1906, the earth's crustal thickness was determined at isolated locations, but it was not determined systematically until the 1960s! The importance of the earth's crustal thickness was not appreciated until an understanding of palaeomagnetism, polar wandering, continental drift and plate tectonics became essential even for guiding the prospecting for mineral resources. During the 1960s the two Yugoslav geophysicists Milanković and Mohorovičić were among the most frequently quoted authors. At that time the two Yugoslav geophysical exploration companies (Geofizički Institut from Belgrade, and Geofizika from Zagreb) started systematic investigation of the earth's crustal thickness over Yugoslavia. The method applied was Deep Seismic Sounding (DSS), which is essentially seismic refraction. The thickness of the crust was determined along eight profiles generally oriented perpendicular to the Dinarides. According to Dragašević (1973) and Dragašević & Andrić (1974) the shotpoint-to-geophone distances were up to 600 km, and when joint refraction shooting was organised together with Soviet, Polish, Hungarian and Yugoslav geophysicists, the distances were as much as 1500 km with charges of up to 5000 kg. Some of the results of DSS are shown in Fig. 1 along a profile running from the Adriatic sea (the shotpoint at Petrovac) to the Danube (at Negotine, a city on the Yugoslav-Rumanian border). The DSS was technically a demanding challenge for Yugoslav geophysicists, but the results were rewarding.