Explicit High-contrast Surface Refinement Using Full Waveform Inversion

Full waveform inversion (FWI) provides a powerful mechanism for resolving high-resolution structure within complex seismic velocity models. However, many models used in FWI contain complex bodies (e. g. salt intrusions) that generate a large, sharp contrast in seismic properties. Refining these with FWI can prove extremely difficult when they are represented implicitly within a conventional gridded velocity model. The constraints required to optimize convergence at large contrasts are different from those required elsewhere. It is difficult to allow for this when the surfaces have no explicit representation. This paper describes a new approach to refining complex 3-D surfaces directly using FWI. An explicit representation of the surface is updated during the inversion. The gradient required is obtained by projecting a conventional FWI gradient (for seismic parameters) to a position and shape parameter gradient for the 3-D surface. The method relies upon accurate gridded model generation in the vicinity of the surfaces and a linearization of this process to enable the gradient projection. We present 2-D and 3-D synthetic examples that resolve complex structure within a Gulf of Mexico top salt contrast. The examples demonstrate very rapid and stable convergence that resolves complex features beyond the resolution of the finite-difference grid.