Vector-based elastic reverse time migration

Abstract

Prestack elastic reverse time migration (RTM) of multicomponent seismic data requires separating PP and PS reflections before, or as part of, applying the image condition, and using image conditions that preserve the angle and amplitude information. Both of these requirements are best achieved when all operations are on vectors. We have created a new 2D migration context for isotropic, elastic RTM, which included decomposition of the elastic source and receiver wavefields into P- and S-wave vectors by decoupled elastodynamic extrapolation, which retained the same stress and particle velocity components as the input data. Then, the propagation directions of the incident and reflected P- and S-waves were calculated directly from the stress and particle velocity definitions of the P- and S-wave Poynting vectors. An excitation-amplitude image condition that scaled the receiver wavelet by the source vector magnitude produced angle-dependent images of PP and PS reflection coefficients with the correct polarities, polarization, and amplitudes. It thus simplified the process of obtaining PP and PS angle-domain common-image gathers (ADCIGs); it was less effort to generate ADCIGs from vector data than from scalar data. We found that the resulting prestack elastic images were nearly identical to the corresponding source-normalized crosscorrelation images and had improved resolution because the wavelet broadening that resulted from the crosscorrelation was not present.