Sejsmiczne modelowanie pokrywy osadowej wzdłuż profilu przez platformę prekambryjską i paleozoiczną

Abstract

Przedstawiono wyniki jedno- i dwuwymiarowego modelowania struktury pokrywy osadowej na platformie prekambryjskiej i paleozoicznej w Polsce. Wykorzystano materiały eksperymentalnych prac sejsmicznych z profili refrakcyjnych M-9 (3-X-73) i I-VI-66. Wielowarstwowe modele dwuwymiarowe zostały dobrane metodą prób i błędów, przy użyciu metody dynamicznego śledzenia promienia w złożonych ośrodkach (metoda ray tracing). Porównanie eksperymentalnego pola falowego z hodografami teoretycznymi i sejsmogramami syntetycznymi wykazuje zgodność podstawowych grup fal refrakcyjnych występujących w pierwszych impulsach. Dla obydwu profili określono model struktury osadów, w tym również utworów podpermskich, a także prześledzono granicę podłoża. Uzyskane wyniki porównano z wcześniejszymi opracowaniami.

Seismic models of sedimentary cover of the Precambrian and Palaeozoic Platforms in Poland

Results of one and two-dimensional modelling of sedimentary cover structure of the Precambrian and Palaeozoic platforms in Poland are presented. The modelling was based on materials of seismic studies along refraction profiles M-9 and I-VI-66 and seismic profiling in boreholes. In two-dimensional modelling, a method of dynamical ray tracing in complex media was applied. The models of sediment structure was determined for both profiles, including pre-Permian formations, and the boundary of the basement. On profile 1-VI -66 the depth of the crystalline basement ranges from about 3 km in the platform region to more than 9 km in the marginal synclinorium (Fig. 3). The mean velocity in the overburden varies, respectively, from 3200 to 4200 m/s, and the refracted wave boundary velocity in the basement from 6050 to 6300 ml s. In the sedimentary cover model we distinguished Cainozoic-Mesozoic and Cambrian-Silurian complexes, separated by a higher velocity layer (some 5100 ml s) associated with the Permian formations. On profile M-9 the depth of basement of 5900-6000 m/s velocity increases in the NE direction from about 3 to 12 km. The mean velocity in the sedimentary complex changes from 3900 to 4900 m/s, respectively. Underneath the Zechstein formations, having a velocity of some 5000 m/s, we detected the presence of boundaries in the Older Palaeozoic, having velocities of some 5300 and 5500 m/s, and also layers of lower velocity (Fig. 9).

As a result of modelling made for the two profiles we obtained two-dimensional models of the sedimentary cover and its basement. Theoretical travel-times show a very good agreement with experimental ones (of the order of 0.05-0.1 s). Also the synthetic seismograms show a qualitative agreement in amplitudes of the main groups of refracted and reflected waves. The two-dimensional modelling with the use of the ray tracing method in a complex medium made it possible to determine and identify refraction boundaries and low-velocity layers in the Older Palaeozoic. The results obtained were compared with ealier studies.