Non-artifact vector P- and S-wave separation for elastic reverse time migration

Abstract

Elastic reverse time migration (RTM) uses the elastic wave equation to extrapolate multicomponent seismic data to the subsurface and separate the elastic wavefield into P- and S-waves. P- and S-wave separation is a necessary step in elastic RTM to avoid crosstalk between coupled wavefields. However, the current curl-divergence operator-based separation method has a polarity reversal problem in PS imaging, and vector separation methods often have separation artifacts at the interface, which affects the quality of the imaging stack. We propose a non-artifact P- and S-wave separation method based on the first-order velocity-strain equation. This equation is used for wavefield extrapolation and separation in the first-order staggered-grid finite-difference scheme, and the storage and calculation amounts are consistent with the classical first-order velocity-stress equation. The separation equation does not calculate the partial derivatives of the elastic parameters, and thus, there is no artifact in the separated P- and S-waves. During wavefield extrapolation, the dynamic characteristics of the reflected wave undergo some changes, but the transmitted wavefield is accurate; therefore, it does not affect the dynamic characteristics of the final migration imaging. Through numerical examples of 2D simple models, part SEAM model, BP model, and 3D 4-layer model, different wavefield separation methods and corresponding elastic RTM imaging results are analyzed. We found that the velocity-strain based elastic RTM can image subsurface structures well, without spike artifacts caused by separation artifacts, and without polarity reversal phenomenon of the PS imaging.