Elastic RTM: anisotropic wave‐mode separation and converted‐wave polarization correction

Abstract

In this study, we investigate the impact of different wavemode separation approaches on TTI anisotropic elastic reverse time migration (RTM). We tested the cheap but less accurate Helmholtz decomposition method as well as the expensive but more accurate pseudo-derivative method on a simple 2D model. Our study shows that the stacking operator associated with the RTM algorithm can naturally remove most of the leaking-mode artifacts generated by the less accurate Helmholtz decomposition method in anisotropic media. Therefore, for the purposes of kinematic imaging, it is practically acceptable to use the Helmholtz decomposition for mode separation in elastic anisotropic RTM. Another issue in elastic RTM is that converted PS wavefields have mixed polarity depending on the P-wave incidence angle. As a result, a given event has opposite signs in different shot images, causing the event to be destroyed after stacking over all shots. We propose a correction approach in which the angle-domain commonimage gathers are computed at every imaging point and the polarity is corrected in the angle domain before stacking. The polarization correction allows the same event to be stacked constructively yielding a more consistent PS image.