A number of advanced 3-D prestack depth migration methods have been developed and applied to seismic imaging processing in recent years. But we still fail to obtain good images of some particular subsurface events such as sub-salt structures, even if the velocity models are true. We propose target-oriented visibility analysis (TOVA) approach to demonstrate why it is difficult to image these particular events. Two examples, a horizontal layered model and a SEG/EAGE salt model, are shown in this paper. We see that TOVA depends on overburden velocity distribution, acquisition geometry, dip angle and reflectivity of the inter face to be migrated. TOVA is a useful tool to evaluate how various factors affect image qualities, and as well, to improve acquisition geometry and enhance subsalt image.

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