Enhanced reverse time migration of walkaway VSP Data
Jing Wang, Kristopher A. Innanen
Elastic reverse time migration is a vector wave theory-based depth domain migration algorithm. Wavefields separation is a key step to remove crosstalk artifacts and improve the imaging quality. This operation allows us to produce vector P- and S-wavefields with the same phases and amplitudes as the input coupled wavefields, while significantly reducing computational costs. The test results illustrate that the amplitude and phase of the separated wavefields are changed and the polarity is reversed after the Helmholtz decomposition, so that further correction are needed. Aiming to solve these problems, decoupled elastic wave equations are utilized to simulate the wavefields propagation, which can maintain the vector property of original wavefields without polarity reversal. With the separated vector wavefields, we implement a modified dot-product imaging condition for elastic reverse time migration, which produces migrated images with accurate amplitudes. Several numerical examples are used to demonstrate its feasibility and robustness for imaging complex subsurface structures. In this report, we firstly review the Helmholtz decomposition and analyze its shortcomings. Next, we utilize the decoupled elastic wave equations to obtain vector P- and S-wavefields with the same phases and amplitudes as the input coupled wavefields. And then, a dot-product imaging condition and corresponding elastic reverse time migration workflow are presented to produce PP- and PS-images. Finally, several numerical examples are used to illustrate the performance of this wavefield decomposition strategy and the corresponding workflow. Besides, the numerical simulation methods for the forward and backward wavefields’ extrapolation, the effects of multiple waves are also discussed in this report to further improve the imaging efficiency and accuracy.