Various reverse time migration (RTM) angle gathergeneration techniques have been developed to address poorsubsalt data quality and multiarrival induced problems ingathers from Kirchhoff migration. But these techniquesintroduce new problems, such as inaccuracies in 2D subsurfaceangle gathers and edge diffraction artifacts in 3D subsurfaceangle gathers. The unique rich-azimuth data setacquired over the Shenzi field in the Gulf of Mexico enabledthe generally artifact-free generation of 3D subsurface anglegathers. Using this data set, we carried out suprasalt tomographyand salt model building steps and then produced 3Dangle gathers to update the subsalt velocity. We used tiltedtransverse isotropy RTM with extended image conditionto generate full 3D subsurface offset domain common imagegathers, which were subsequently converted to 3D anglegathers. The angle gathers were substacked along the subsurfaceazimuth axis into azimuth sectors. Residual moveoutanalysis was carried out, and ray-based tomography wasused to update velocities. The updated velocity modelresulted in improved imaging of the subsalt section. We alsoapplied residual moveout and selective stacking to 3D anglegathers from the final migration to produce an optimizedstack image.
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