Velocity Analysis in Complex Subsurface Structures Using Prestack Reverse-Time Depth Migration.

Migration velocity analysis is investigated to obtain a long-wavelength velocity model for a complex structure. Residual wavefront curvature analysis is combined with prestack reverse-time depth migration to update the velocity model as well as the image of the subsurface structure. The imaging point for a given velocity error is solved rigorously in the case of a dipping layer. Convergence behavior is examined by three synthetic examples in an iterative scheme in which the depth and velocity are modified interactively. A stable solution can be obtained by smoothing the model space with an appropriate window length. It was found that the velocity is modified from the shallower part to the deeper part gradually, and that the number of iterations that is necessary for convergence is correlated with the ratio between the target depth and window length of the moving average. In a field example of land survey conducted around a buried fault zone, lateral variation of velocity was successfully detected. With the final velocity model and final seismic section, this method provides more reliable interpretations than the CMP stacking method.