Up/Down Image Separation in Elastic Reverse Time Migration

Abstract

Reverse time migration (RTM) using a two-way wave equation is able to generate high-quality images of the subsurface. Besides the ability to image complicated geological features, image artifacts can be observed in elastic P and S wave images, where high velocity contrasts/gradients in the velocity model produce back-scattering reflections during wavefield extrapolations. Separating RTM images into components based on wavefield propagation directions is able to effectively remove such image artifacts. We first compare image separation methods, including up/down image separation during, and post, imaging. The post-imaging method is easy to implement using existing RTM software, as it requires only simple operations applied to the stacked RTM images. The separated up/down images are similar to those separated during imaging, but there are also differences. The differences come from the different physical meanings of the up/down separated images. The during-imaging methods separate RTM image into its up/down components using propagation directions of the individual source/receiver wavefields, while the post-imaging method separates images based on the vector sum of the source and receiver wavefield directions. Inspired by these differences, we develop a composite workflow that combines a cost-efficient up/down imaging condition and a post-imaging separation step, which provides an alternative solution for obtaining up/down separated images from full waveform imaging. Numerical example using an elastic version of the Sigsbee 2A model indicates that the up/down separated images from the proposed workflow are effectively equivalent to those obtained using the during-imaging separation methods. More interestingly, we show the pros and cons in the imaging results from different up/down separation methods, especially in converted P–S images.