Velocity-model building with a good understanding of anisotropy is one of the important elements in a prestack depth migration (PSDM) project. During a recently conducted PSDM project, we observed that (1) the application of negative delta/epsilon is necessary to achieve reasonable depthing and gather flatness, and (2) azimuthal variation in gather flatness exists in the interval where negative delta/epsilon are necessary. A geologic explanation of these interesting observations was necessary to justify the velocity-model-building result. Investigation of stress magnitude/orientation and theoretical anisotropy of unconsolidated sandstone under nonhydrostatic stress revealed that these observations can be explained by stress-induced anisotropy. This concept may be applicable to other fields, and the existence of azimuthal anisotropy and value of anisotropy parameters can be roughly estimated by the method described.

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