Wave-equation migration velocity analysis. Ⅱ. Subsalt imaging examples

摘要

Subsalt imaging is strongly dependent on the quality of the velocity model. However, rugose salt bodies complicate wavefield propagation and lead to subsalt mul-tipathing, illumination gaps and shadow zones, which cannot be handled correctly by conventional traveltime-based migration velocity analysis (MVA). We overcome these limitations by the wave-equation MVA technique, introduced in a companion paper, and demonstrate the methodology on a realistic synthetic data set simulating a salt-dome environment and a Gulf of Mexico data set. We model subsalt propagation using wave paths created by one-way wavefield extrapolation. Those wave paths are much more accurate and robust than broadband rays, since they inherit the frequency dependence and multipathing of the underlying wavefield. We formulate an objective function for optimization in the image space by relating an image perturbation to a perturbation of the velocity model. The image perturbations are defined using linearized prestack residual migration, thus ensuring stability, relative to the first-order Born approximation assumptions. Synthetic and real data examples demonstrate that wave-equation MVA is an effective tool for subsalt velocity analysis, even when shadows and illumination gaps are present.