Converted-wave guided imaging condition for elastic reverse time migration with wavefield separation

Abstract

Elastic reverse time migration (ERTM), which is capable of using multicomponent seismic data, provides not only an improvement of the P-P image compared to the one derived from acoustic RTM, but also more opportunities to understand the subsurface through converted wave images (P-S, S-P, and S-S images). However, the polarity reversals in P-S and S-P images and cross-talk noises generated in S-P and S-S images degrade the converted wave images of ERTM. To overcome these problems, we derive a new P-S converted wave imaging condition for 2D ERTM based on wavefield separation techniques. The proposed imaging condition, called converted-wave guided (CWG) imaging condition, incorporates an extra term that represents the sign and wavelength of S-waves converted from source wavefields into the zero-lag cross-correlation imaging condition for P-S imaging. The extra term compensates for the polarity reversal of separated S-waves from receiver wavefields because the converted S-waves from source wavefields also have the change in polarity. In addition, since this CWG imaging condition produces images where P- and S-waves from source wavefields and S-waves from receiver wavefields coincide, image resolution is enhanced without generating spurious events. Our approach is motivated by the specific feature of ERTM that generates converted waves at the reflection points (conventional imaging points) when proper elastic models are used. Through a numerical experiment with a simple elastic model, we demonstrate that the proposed CWG imaging condition successfully corrects the polarity reversal and provides higher image resolution. We also test our migration algorithm on a synthetic ocean bottom cable (OBC) dataset created using the Marmousi-II model. The P-S image obtained from CWG imaging condition shows continuous events and improved image resolution.