Characterizing PS Imaging Conditions of Elastic Reverse Time Migration Through an Angle-Domain Comparative Analysis

The elastic reverse time migration (ERTM) gradually becomes a powerful tool for imaging complex structures, especially for using converted waves. In the common ERTM workflow, the relatively linear scattering/reflection process in the subsurface local domain can be reconstructed through accurate injection of multicomponent records and extrapolation of vector wavefields. Subsequently, imaging conditions (ICs) are applied to construct meaningful depth images. One of the classical challenges in PS imaging is polarity reversal at normal incidence, which significantly impacts the stacking of images obtained from multishot experiments. To address this issue, various ICs have been developed. Specifically, operations that combine the source-side P wave and receiver-side S wave can introduce implicit angle-dependent weighting factors, which help to improve the quality of PS depth images. In this study, we conduct a comparative analysis of four specific PS ICs of ERTM: the angle-domain IC, the divergence- and curl-based IC, the vector IC, and the impedance gradient term from full-waveform inversion (FWI). We link the ICs with the PS Born scattering process to investigate the implicitly embedded angle-dependent weighting factors. By using the accurate local plane wave decomposition (LPWD) approach for angle-domain wavefield decomposition, various numerical experiments are carefully designed to characterize the PS ICs. Their abilities for solving the polarity reversal at normal incidence are evaluated. The influences of the angle-dependent weighting factors on imaging resolutions are also quantified. Under the constructed angle-domain investigation frame, we can not only analyze existing ICs but also explore potential new ICs toward enhanced elastic imaging.