Głębokie sondowania sejsmiczne i ich rola w kompleksie metod geofizycznych w badaniach budowy skorupy Ziemi

Jan Skorupa

Abstract


DEEP SEISMIC SOUNDINGS AND THEIR ROLE IN THE COMPLEX OF GEOPHYSICAL METHODS DURING INVESTIGATIONS OF THE EARTH’S CRUST STRUCTURE

Summary

The paper deals with the importance of the progress in recognition of the Earth’s structure, particularly of its near-surface layers, to understand and to appreciate the laws of development of tectonical and magmatic processes and presents the subdivision of the earth interior into zones according to elasticity properties. Pointing to the method of their distinguishing (use of remote earthquakes) the author pays attention to other seismological techniques applied in the research of the structure of the shallower Earth’s layers. Surface waves yield a possibility of obtaining the “mean” structure of great units, and only a net of special stations makes possible to trace the thickness of the crust within smaller areas. The research of near earthquakes allows to interpret in detail the near-surface beds of the Earth. By means of this method A. Mohorovičič has distinguished the discontinuity accepted to-day as the bottom of the Earth’s crust, and V. Conrad has established the further discontinuity within the Earth's crust. The lartter discovery has contributed to introduction of the Earth’s crust model, subdivided into “layers”, or “stages”, which are “granitic”, or “basaltic”. On the basis of seismological data some authors have distinguished also “low velocity layers” within the Earth’s crust. The opinions on the interpretation of the seismological data are divergent because the materials may be interpreted by means of various velocity presumptions. To give to the reader a chance of knowing the possibility, or impossibility of the Earth’s crust models, the present author discusses the existing distributions of hydrostatic pressure, temperature and density of the near-surface layers of the Earth. In addition, he also takes into consideration the results of the researches concerning velocity of elastic waves. The researches were made on rock samples and the results obtained for the estimated values of pressure and temperature. The discussed researches point rather to an unreality of existence of the beds characterized by the decreased velocity, or to the existence of a great vertical gradient of velocity. Nevertheless, they do not yield evidences of existing two-layer model of the crust. The author points to a difference in opinions of many authors as to the existence of Conrad’s discontinuity, “universal” for all continents. Up to the thirties, the quantity of information was so scarce that one generally accepted the Mohorovičič’s discontinuity to be for all the continents at a stable depth from 30 to 40 km. However, already at that time one paid attention to an anomalous structure of the mountainous areas. At present, the knowledge of the problems considerably increased and we know that the Moho discontinuity is from 3-7 km under the oceans, to about 60-75 km under the young mountains. Moreover, the “granite” layer is lacking under the oceans. The existing “basaltic” layer would rest then on the “ultrabasites” there. The author also discusses the significance of such conventional determinations and pays attention to the great difference in density of the individual stages of the Earth’s crust, a fact allowing to use gravimetric and hypsometric data for determination of the crust thickness. The period of a large methodical progress in the investigations of the Earth’s crust after the World War II was characterized by a general introduction of modern apparatuses both in the methods using artificial explosions and in the seismological methods. In the methods using artificial explosions were applied both refractions and reflection works. As far as these latter are concerned one used near-critical reflections. Nevertheless, this method is not used at present on a greater scale. Great progress may be noted in refraction method, particularly developed in the USSR, where correlation method of refracted waves has particularly been developed and completed by the near-critical reflections. The works made in this way have been named “deep seismic soundings”. After discussing the methods and the results obtained in various countries by means of refraction method, the author illustrates the development and methodical possibilities of the “deep soundings” and describes various kinds of these latter. Moreover, the author points to the separable possibilities of the method and stresses further possibilities of their increasing. On the other hand he takes into account also deficiencies of the method, existing particularly in the presumptions taken for interpretation, as well as points to a necessity of introducing model, theoretical and experimental investigations. In spite of the “detailed” results that may be obtained, the method itself is expensive. Therefore, a possibility is also discussed to use other geophysical methods, although these methods give mainly generalized results. Thus, the results of magnetotelluric method are discussed, the correlation of which with geological pictures presents some difficulties; in addition the results of gravimetry are also considered. As far as the latter method is concerned, the author stresses that its accuracy is low (e.g. the errors concerning the thickness of the Earth’s crust amounts, according to R. M. Demienicka, to ± 4 km), the accuracy of the elaborated maps of large areas may, however be increased by means of “deep seismic soundings”. This technique was applied to elaborate the map of Moho surface morphology in the USSR. Moreover, seismical data were used f.ex. by Demienicka to elaborate the map of Moho horizon morphology on the earth scale on the basis of gravimetrical and hypsometrical data in the areas, where gravimetric materials were lacking. The general characteristic of the Moho surface morphology for the USSR has been cited by the present author according to A. A. Borisov, as the example illustrating the results obtained through the connection of seismic and gravimetric data. The most striking feature is that the amplitude of alteration of the Moho surface height is several times higher than the alteration of height of the consolidated crust. Locally, the rules cannot be established. The present author demonstrates that the way leading to recognize the structure of the Earth’s crust “from general to detailed data” and from large to small units is hardly effective. In the light of the facts hitherto observed, we may see at present a different way - i.e. detailed investigations on typical units to facilitate synthetical conclusions for small units. As an example the author cites one profile of deep seismic soundings made in Ukraine, and once more refers to other possibilities of the interpretation discussed above. Ending this paper the author points that the interpretation restrictions concern mainly middle horizons, since the investigation of the Moho surface morphology appears to be mastered. The restrictions may be removed and we may expect that in the works concerning the base problems of tectonics, magmatic processes, and even prognosis of some mineral raw materials, a marked qualitative progress will undoubtedly be reached.


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