Up/Down Image Separation in Elastic Reverse Time Migration
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Pure and Applied Geophysics
Up/Down Image Separation in Elastic Reverse Time Migration PENG GUO1
and GEORGE A MCMECHAN2
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. Keywords: Elastic wave imaging, reverse time migration, propagation direction, image separation.
1
Deep Earth Imaging, The Commonwealth Scientific and Industrial Research Organisation (CSIRO), Perth, WA 6151, Australia. E-mail: [email protected] 2 Center for Lithospheric Studies, The University of Texas at Dallas, 800 W Campbell Road (ROC21), Richardson, TX 75080, USA. E-mail: [email protected]
1. Introduction Advanced seismic imaging techniques using full acoustic and elastic wave equations, such as reverse time migration (RTM) (Baysal et al. 1983; Whitmore 1983; McMechan 1983; Zhang and Zhang 2011; Hu and Wang 2015; Duan et al. 2017; Guo and McMechan 2018), are able to include wave propagation in all available spatial directions, and thus has been used for imaging complicated geological structures, such as steep-dips and overturned reflectors with strong lateral velocity variations. However, the use of full wave equations also generates back-scattering reflections during both the source and adjoint receiver wavefield propagations, where there are high
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