Rozkład prędkości średnich w utworach permomezozoicznych NW Polski

Henryk Jankowski




So far various methods have been used to transform the seismic time sections for West Pomerania into depth sections and consequently considerable discrepancies exist between data interpreted in different years. A uniform version of the average velocities distribution in the Pomerania swell and in a part of the Pomerania trough has been completed by the Geophysical Department of the Geological Institute.
The velocity distribution here discussed is a modification of previous interpretations and it is based on average velocity measurements in deep boreholes. The differences in solutions for various parts of West Pomerania result from an uneven distribution of drilling data and from certain logical facts recognized by the analysis of individual average velocity measurements in relation to the geological structure of the area.
Following the analysis of vertical holographs for deep boreholes seven average velocity areas have been distinguished: 1 ­– the area of applicability of the Świdwin 2 curve, 2 – the area of applicability of the Świdwin 3 curve, 3 – the Szczecinek – Korytowo area, 4 – the Babilon – Stobno area, 5 – the Biały Bór – Brda area, 6 – the Bobolice area and 7 – the Gozd area.
The applicability of the Świdwin 2 curve has been confined to a small Triassic outcrops area directly adjacent to the borehole. Average velocities in this area have the highest values.
The applicability area of the Świdwin 3 curve is in the middle part of the Pomerania swell i. e. in the Jurassic outcrops zone.
Io the Szczecinek – Korytowo area a different solution has been adopted on the gounds of a bigger amount of average velocity measurements in deep boreholes and the fact that reflection boundaries corresponding to successive stratigraphic series have been recorded on the time sections. For precise transformation of the time into the, depth values vertical hodographs to the top of the individual series have been established. On the basis of, interval velocities within the beds the velocity variations have been defined for regional stratigraphic complexes.
The velocity solutions for the Babilon – Stobno, Blały Bór – Brda, and Gozd area have been, based on mean vertical hodographs. The boundaries between the areas have been traced out according to similar changes of the average velocities.
Linear transition zones (Fig. 1) have been defined between the individual areas.
The average velocity distribution pattern served to compile the average velocity map of the Z reflection horizon that enables to define the depth of this horizon on any site of the area discussed.

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