Abstract

On the Piedmont of the southeastern United States, seismic reflection statics at different points in a typical common-depth-point (CDP) gather can differ from one another by more than 50 msec because of topographic relief in excess of 50 m, variations in the thickness of the weathered zone that can extend deeper than 50 m, and variation of velocity in the weathered zone of more than 100 m/sec. The ABCD method for computing statics is introduced to account for these velocity and thickness variations as well as the topographic effect. This method combines elevations and positions of source-receiver points with times of first arriving refracted waves read from reflection correlograms or seismograms. It was tested in central Virginia where typical piedmont conditions are encountered. At four locations, ABCD statics are close to control values determined independently from refraction experiments. At 99 source-detector points, ABCD statics differ by an average of 4 msec, (maximum of 19 milliseconds) from conventional elevation statics that do not account for local velocity and thickness variations in the weathered zone. Where source-receiver points were in line, modified ABCD statics were obtained from first arrival traveltimes without using elevation and position data. In this metamorphic terrane where clear reflections are difficult to record, ABCD statics appear to be more effective than conventional elevation statics for enhancing reflections on a seismic record section.

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