Inversion for Salt Flank Geometry Using Transmitted P- and S-Wave Travel Times

Accurate delineation of the salt flank is important for oil and gas exploration in areas with salt intrusion. We describe the application of deformable-layer tomography (DLT) to invert for the geometry of salt flank using travel times of transmitted P- and S-waves from surface sources to downhole receivers. The DLT allows us to take advantage of a common situation that the salt velocity is known and generally invariant, but we need to determine the variable geometry of salt flank. We demonstrate our new method using a physical model experiment mimicking the setup of a walkaway vertical-seismic-profiling (VSP) survey. We first use picked P-wave arrivals to invert for the salt flank geometry; the sinusoid shape of the salt flank is estimated fairly by the DLT due to the uneven P-wave raypath coverage. As an improvement, we further incorporate the S-wave arrivals in the DLT. The picking of S-wave arrivals is assisted with modeled S-wave arrivals based on the P-wave DLT model. The DLT solution using both P- and S-wave arrivals delineates the salt flank geometry more accurately than that using P-wave arrivals alone. The salt flank delineation using DLT can complement with the conventional salt proximity and migration methods. It could also serve as constraints or the initial model for the full-waveform inversion of salt geometry.