Aspects of Residual Moveout after Downward Continuation

The case is considered that an approximate prestack depth migration with an inaccurate velocity model and a subsequent horizon analysis for residual moveout have been applied to a seismic survey. A Solution is developed in which the parameters required for the inversion are determined in the vicinity of the reflected events and in terms of travel times which have been previously determined from residual moveout analyses. The validity of the suggested algorithm is demonstrated for the inversion of synthetic data exhibiting dipping structures and laterally inhomogeneous velocities. For moderate offsets the approximations predict the residual moveout of migrated reflections and are related to the spreading functions for an exploding reflector in the true velocity model.

For the case of a vertically inhomogeneous medium expressions are obtained for the exact determination of residual moveout after downward continuation. The case is considered that for a reservoir structure the velocity is constant beyond a certain depth and residual moveout is to be preserved: an estimate is provided for the increase in offset as a function of the increase in thickness of the overburden.