A stable and effective strategy for angle gather extraction from reverse time migration using traveltime gradient

Abstract

ABSTRACTThe generation of angle-domain common imaging gathers (ADCIGs) from reverse time migration (RTM) is essential for velocity updates, migration quality control and amplitude variation with angle (AVA) inversion. However, computational stability and efficiency challenges remain with ADCIGs extracted from RTM. Here, we develop a stable and effective strategy to produce ADCIGs during RTM based on the traveltime gradient. The first arrival traveltime is calculated efficiently using the dynamic programming scheme, and the source-side propagating direction of the local plane-wave can be obtained using the traveltime field gradient. Compared with propagation directions calculated using the local slowness analysis method, the traveltime gradient can also provide stable and accurate source-side directions. Benefitting from the migrated images, we can collect normal direction information for the reflector from the stacked images effectively using the omnidirectional plane-wave destruction (OPWD) method. Construction of subsurface scattering angles is straightforward via combination of the source-side propagation and reflector normal directions. Subsequently, we apply an effective imaging condition based on the excitation criterion in the angle domain to generate ADCIGs. Thanks to conversion from a wavefield-based to a traveltime-based calculation, the proposed method provides more stable and reliable wave propagation directions in complicated areas compared with the Poynting vector (PV) method which is based on the full wavefield. Because traveltime can be calculated efficiently beforehand, the proposed method can be adapted economically to the conventional RTM imaging frame, which reveals significant implementation potential for practical seismic exploration. We demonstrate the stability and effectiveness of this approach for both synthetic and field data.