Problemy zastosowania sejsmicznych metod refleksyjnych w Polskich Karpatach Wschodnich w świetle dotychczasowych wyników

Z. Mitera

Abstract


Problems of application of seismic reflection methods in the Polish Eastern Carpathian Mountains in the light of prezent investigations

Application of geophysical methods of prospecting in the region of Carpathian Mountains is rather difficult, due to the complexity of the geological structures encountered in this area. Among these methods some possibilities offers the refraction and reflection seismology. The determination of elastic properties of the rocks involved in the investigated area is very important for selection of suitable method and field technique of seismic prospecting. From several hundred records, obtained during the seismic work conducted by the Pionier Company in 1934 and 1935 in Poland, it was possible to tabulate the velocities of elastic waves for most typical Carpathian formations (Table No. 1). The examination of these velocities indicates, that, with exception of the Cretaceous beds, other strata do not show great differences in the magnitude of their velocities. For this reason majority of the Carpathian rocks do not form favorable conditions for the reflection work, especially when they contain a number of intercalated high speed beds. Besides that, steep dips, which are usually found iii the Carpathian geologic structures, represent further difficulty and limitation in application of reflection seismology in this area. The same refers to the application of refraction method, which is still more limited, due to small dimensions of geologic structures in comparison with the necessary length of refraction profiles, which should be laid down during the measurements. In the subsequent part discusses the author the results obtained in different localities in the Eastern Carpathians. Figure 1 represents the seismic reflection profile together with the geologic section of the so called deep element of Nahujowice, west of Borysław. Majority of the recorded reflections occur in the Polanica beds (Oligocene). The Menilitic formation, together with the hornstones does not produce any outstanding reflections. Probably they are crushed by the tectonic movements and therefore do not represent, in spite of their high hardness, favorable conditions for the reflection of elastic waves. Fig. 2 reproduces the record obtained in vicinity of the well „Millie 1“ in Nahujowice. Further investigations carried out in the region of the deep element of Borysław have shown, that also in this area it is difficult to obtain reliable reflections, which could serve for the correlation of certain beds. Fig. 6 represents a trial correlation of recorded reflections with known geologic strata in Borysław, and fig. 3 shows the typical reflections obtained in this region. Better conditions for reflections seam to exist in the Carpathian overthrust in the region of Orów and Mraźnica. In fig. 4 are reproduced the records obtained there. Another experiments have been conducted in the vicinity of Bitków, about 100 kilometers southeast of Borysław, where oil and gas horizons are in folded tectonic elements of the type of Borysław. Figures 7 and 8 give the general view of this part of Carpathian Mountains and show the type of an instrument trailer used in this difficult and inaccessible area. The results obtained there are reproduced in fig. 9, which shows the seismic profile through the deep folds of Bitków. In fig. 10 are demonstrated both the seismic and geologic profiles through Bitków. Much better conditions for application of seismic reflection method exist in the Sub-Carpathian Depression, between the Carpathian Range and the Podolian Plateau. In most cases there are soft beds of clays, shales and sands in contact with much harder formations, which represent good reflection horizons. A typical reflection record from this area is shown in Fig. 5. The outstanding deep reflection occuring on this record, can be identified with hard gypsum beds in the depth of about 1100 meters. These beds may be followed continually over large areas in the Carpathian Foreland. Therefore in conclusion it may be stated, that not the Carpathian Mountains but the Foreland offers most possibilities for successful application of reflection seismology.

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