Wavefield tomography using reverse time migration backscattering

Abstract

Reverse time migration backscattering contains kinematic information that can be used to constrain velocity models. The backscattering results from the correlation between forward scattered and backscattered wavefields from sharp interfaces, i.e., sediment-salt interfaces. The synchronization between these wavefields depends on the velocity of the sediment section and the correct interpretation of the sharp boundary. We have developed an optimization workflow in which the sediment velocity and the sharp boundary are updated iteratively. The presence of sharp boundaries in the model lead to high- and low-wavenumber components in the objective function gradient; the high-wavenumber components correspond to the correlation of wavefields traveling in opposite directions, whereas the low-wavenumber components correspond to the correlation of wavefields traveling in the same direction. This behavior is similar to reverse time migration in which the high-wavenumber components represent the reflectors (the signal) and the low-wavenumber components represent backscattering (noise). The opposite is true in tomography: The low wavenumber components represent changes to the velocity model, and the high-wavenumber components are noise that needs to be filtered out. We use a directional filter based on Poynting vectors during the gradient computation to preserve the smooth components of the gradient, thus spreading information away from the sharp boundary. Our tests indicated that velocity models are better constrained when we include the sharp boundaries (and the associated backscattering) in wavefield tomography.