Depth migration and prestack time migration require an accurate velocity model for optimum imaging. An important feature of Equivalent Offset prestack time migration is the convergence of velocities. The common scatter point (CSP) gathers may be formed from a gross velocity approximation, and then analyzed to produce a more accurate estimate of the velocity. The sensitivity of the input velocity, verses output velocity, is expressed as a ratio, and shows large variations on the input velocity have little effect on the output velocity.
Examples of semblance diagrams are shown which start with an infinite velocity, and converging to a reasonable estimate in a few iterations. A panel of one percent velocity increments is also included
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