Wave‐equation migration velocity analysis with extended common‐image‐point gathers

Abstract

Wave-equation migration velocity analysis (WEMVA) is an image-domain velocity model building technique based on band-limited wave propagation and designed especially for complex subsurface environments. It exploits the coherency of reflection events measured in extended images produced by a cross-correlation imaging condition with non-zero lags. Conventional approaches use either space-lags or time-lag common image gathers, in which only partial information of the extended images is used for velocity updates. We propose an WEMVA approach using the complete information from both space-lags and time-lags of extended images. With this approach, the velocity model building benefits both from the robustness of using the time-lag information and from the high resolution of using the space-lags information. Such an implementation is facilitated by using extended common-image-point gathers (CIPs) constructed sparsely along reflections and defined jointly for spaceand time-lags. These CIPs avoid the bias towards nearly-horizontal reflectors so that steeply dipping events are well preserved in the gathers and the corresponding information related to velocity can be used. Also, the computation of the extended images can be avoided in areas where the velocity is known, e.g., inside salt bodies, or areas where the signal-to-noise ratio is too low, e.g., in shadow zones. Using CIPs for WEMVA can reduce the cost of constructing extended images and offer flexibility for the velocity model building.