MULTICOMPONENT COMMON‐RECEIVER GATHER MIGRATION OF SINGLE‐LEVEL WALK‐AWAY SEISMIC PROFILES1

Abstract

AbstractSeismic data are usually separated into P‐waves and S‐waves before being put through a scalar (acoustic) migration. The relationship between polarization and moveout is exploited to design filters that extract the desired wavetype. While these filters can always be applied to shot records, they can only be applied to a triaxial common‐receiver gather in special cases since the moveout of scattered energy on the receiver gather relates to path differences between the surface shots and the scatterer while the polarization is determined by the path from scatterer to downhole geophone. Without the ability to separate wavefields before migration, a ‘vector scalar’ or an elastic migration becomes a necessity.Here the propagation of the elastic wavefield for a given mode (e.g. P‐S) is approximated by two scalar (acoustic) propagation steps in a ‘vector scalar’ migration. ‘Vector’ in that multicomponent data is migrated and 'scalar’ in that each propagation step is based on a scalar wave equation for the appropriate mode. It is assumed that interaction between the wavefields occurs only once in the far‐field of both the source and receiver. Extraction of the P, SV and SH wavefields can be achieved within the depth migration (if one assumes isotropy in the neighbourhood of the downhole receiver) by a projection onto the polarization for the desired mode. Since the polarization of scattered energy is only a function of scatterer position and receiver position (and not source position), the projection may be taken outside the migration integral in the special case of the depth migration of a common‐receiver gather. The extraction of the desired mode is then performed for each depth migration bin after the separate scalar migration of each receiver gather component.This multicomponent migration of triaxial receiver gathers is conveniently implemented with a hybrid split‐step Fourier‐excitation‐time imaging condition depth migration. The raytracing to get the excitation‐time imaging condition also provides the expected polarization for the post‐migration projection. The same downward extrapolated wavefield can be used for both the P‐P and P‐S migrations, providing a flexible and efficient route to the migration of multicomponent data.The technique is illustrated on a synthetic example and a single‐level Walk‐away Seismic Profile (WSP) from the southern North Sea. The field data produced images showing a P‐P reflector below the geophone and localized P‐P and P‐S scatterers at the level of the geo‐phone. These scatterers, which lie outside the zone of specular illumination, are interpreted as faults in the base Zechstein/top Rotliegendes interface.