Isotropic elastic reverse time migration using the phase- and amplitude-corrected vector P- and S-wavefields

Abstract

In elastic reverse time migration (RTM), wavefield separation is an important step to remove crosstalk artifacts and improve imaging quality. State-of-the-art techniques for wavefield separation in isotropic elastic media include using the Helmholtz decomposition and introducing an auxiliary wave equation. Although these two approaches produce pure-mode vector wavefields with correct amplitudes, phases, and physical units, their computational costs are still high under current computational capability, especially for 3D large-scale problems. Based on the P- and S-wave dispersion relations, we have developed an efficient wavefield separation strategy for elastic RTM. Instead of solving a vector Poisson’s equation in the Helmholtz decomposition, we modify the phases of source wavelet as well as multicomponent records and scale the amplitudes of extrapolated wavefields with the squares of P- and S-wave velocities. 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. With the separated vector wavefields, we implemented a modified dot-product imaging condition for elastic RTM. In comparison with the previously proposed dot-product imaging condition, this modified imaging condition enables us to eliminate the effects of multiplication with a cosine function and hence produces migrated images with accurate amplitudes. Several 2D and 3D numerical examples are used to demonstrate the feasibility and robustness of our method for imaging complex subsurface structures.