Recent advances in imaging mean that we no longer need to ignore seismic velocity anisotropy. Laboratory and field studies provide compelling evidence that shales exhibit intrinsic transverse isotropy (TI). In this symmetry class, the seismic velocity parallel to the laminations is greater than that perpendicular to the layering, and the difference can be as high as 30%. In flat-layered clastic sequences, such as in the Western Canada sedimentary basin, the TI axis of symmetry is essentially vertical, and isotropic depth migration of reflection seismic data tends to overestimate the true depths of reflectors. This occurs because the imaging velocity, based...

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