Migration before stack—Procedure and significance

Abstract

Common‐depth‐point (CDP) stacking is based on the assumption that reflection points are coincident and situated midway between the respective source and receiver locations. If the reflector is structurally deformed, the reflection points move updip from the midpoint. As the structural dip increases, the reflection points for a CDP group of traces are farther removed from each other and normal stacking procedures [i.e., reflection apparent velocities for horizontal reflectors used for normal moveout (NMO) corrections] become increasingly inaccurate. Under such circumstances prestack migration is desirable, particularly when high frequencies are to be preserved. One published approach to prestack migration (Sattlegger and Stiller, 1973) involves the generation of substacks of adjacent traces followed by migration and summation of individual substacks. While adequate in many instances, cases exist where even substacks are degraded by the reflection time differential between component traces. This paper discusses an alternative technique to prestack migration without recourse to substacks. Common‐source traces, after gain recovery and static time corrections but before NMO corrections, are migrated using Kirchhoff summation. Aperture is computed for each sample according to specified maximum dips. Traces are simultaneously migrated and stacked, then output sequentially in sets of 12. The method is economical and provides enhanced reflection continuity and reliability in comparison to poststack migration. Moreover, the collapsing of diffractions is more effective.