Offset plane waves vs. common-azimuth migration for sub-salt imaging

Offset plane wave migration and common-azimuth migration are among the most promising wave-equation migration methods for efficiently imaging 3-D marine data sets. Offset plane wave migration has some computational advantages over common-azimuth migration, but common-azimuth is more accurate. Sub-salt images produced by commonazimuth migration are better focused than the corresponding images produced by offset plane wave migration. These differences can be attributed to an approximation intrinsic to offset plane wave migration: the offset plane wave components are downward continued separately, instead of being allowed to mix as they should be in media with laterally varying velocity. Offset plane wave migration also approximates the cross-line offset ray parameter of the plane-waves with zero. A theoretical analysis of the migration errors caused by this approximation is confirmed by the imaging results. The shallow reflectors dipping at 45 degrees with respect to the acquisition axes are poorly imaged by offset plane wave migration.