The allocthonous salt bodies found on the Gulf of Mexico Shelf (GOM Shelf) act as complex 3D lenses that severely distort wave propagation. Correct salt and subsalt imaging require 3D depth migration before stack (3D MBS), but many 3D MBS images contain areas below salt which are devoid of reliable reflections. Although inadequate seismic processing and incorrect velocities are often cited as causes of poor subsalt imaging, subsalt shadow zones more often are representative of poor illumination with conventional surface P-wave seismic.

We use normal incidence ray-tracing, offset 3D ray-tracing, and finite difference seismic modeling to understand critical angle effects and acquisition limits common to surveys in regions with high salt-sediment velocity contrast and complex geologic structure. The ray-tracing data are captured on illumination maps; subsalt horizons containing gaps in areas from which no rays can reach the surface. Using interpretive workstations, we find that structural attributes controlling the critical angle (base of salt dip, and change in subsalt isopach) correlate extremely well with modeled illumination maps.

We further evaluate the quality of our 3D depth migrations by comparing the images with migrated finite difference models. If subsalt shadow zones observed on real data correlate with modeled illumination gaps, then we gain confidence in the depth image and ignore questionable events in the shadow zones. Poor correlation suggests we evaluate other causes, such as processing errors, migration parameters, velocity problems, or rock property effects.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah