Depth migration after stack

Abstract

The conventional methods for migrating a seismic section, e.g., the finite‐difference method and the Kirchhoff summation method, are inadequate in the presence of significant lateral variations in velocity. For this type of velocity distribution, the basic migration output should be in true depth, although for practical purposes it may be preferable to display it with a nonlinear depth scale. A finite‐difference method has been implemented for obtaining migrated depth sections. The concept underlying this involves all the usual assumptions of a dip line and primary reflections only, with the seismic section considered as the surface measurement of an upcoming wave field which we process with downward continuation in small increments of depth, rather than the customary increments of traveltime. The specified velocity variation laterally along a thin layer results in transmission time changes which must be corrected by a small static time shift applied to each seismic trace. This additional operation within the migration algorithm can be difficult and expensive to implement and is the main reason for its prior omission. Results are given of depth migration applications to both synthetic and real seismic data.