Building a multiple layer model for near-surface static corrections

Abstract

The first 100 m of near-surface geology may be very complex and challenging to model for static corrections. The topography in arid environments includes sand dunes outcrops, dry flash flood valleys (wadis) and salt flats (sabkhas). The near-surface base of weathering and the velocity of the unconsolidated material are highly variable. Below the unconsolidated weathering layer the rock strata often becomes more uniform and regular. The single layer velocity model (Buck, 1996), assumes that the base of weathering (low velocity material) is above the datum and that the upholes adequately sample and define the near-surface velocity field. Where there are shallow upholes the single layer velocity model may only model the weathering velocity not its thickness and ignores the higher velocity consolidated layer below the base of weathering. The uphole survey is a real physical data source for near-surface models. In Saudi Arabia, upholes of 100m nominal depth were usually acquired spaced 4 km along the 2D seismic acquisition line grid. The quality of the time/depth picks is variable because of inaccuracies in picking of the first arrival times and in the depth positioning of the geophone in the uphole. A multiple layer model is a comprehensive solution that will accommodate all position of the datum relative to the base of weathering. Having modeled the static corrections for the 2D seismic lines, a single effective layer velocity can be calculated. This single velocity would generate the same static, thus it is not a physical velocity. Where the datum is above surface and there are positive static corrections there will be negative single effective velocity.