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Abstract

Prestack depth migration is widely used as the best tool for imaging and velocity model QC and refinement, but is mostly applied isotropically, even when there is evidence of anisotropy. We outline a methodology of model-building and imaging for use in anisotropic contexts and apply it to a line of data from offshore West Africa dominated by massive shales. The results show that the anisotropy in the shale increases with depth, along with the velocity. The prestack migrated data are sufficiently sensitive to the anisotropy that a thin layer of sand showing little or no anisotropy can be detected, and they must be included in the model to get flat common image gathers everywhere. This suggests that the use of anisotropy as a lithology-discriminating attribute may be feasible on a depth scale of only a few wavelengths. In any case anisotropy determination is important for precise application of other lithoseismic methods such as AVO.

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