Toward target-oriented FWI: An exact local wave solver applied to salt boundary inversion

Seismic full waveform inversion (FWI) uses the gradient of the objective function for computing model updates. This requires computation of the forward and adjoint wavefields on the current model estimate. Calculating the gradient on the full computational domain is wasteful when it is only required in a limited region of interest, as is the case in 4D seismic and salt boundary estimation, for example. In this paper, a local solver is introduced that accurately computes, up to machine precision, all the wavefield interactions between model updates restricted to a region of interest and inhomogeneities in the background model outside. The local solver therefore generates exactly the same forward and adjoint wavefields in the region of interest that a full domain solver would have generated. In this paper, the exact local gradient at the boundary of a salt body is computed from these exact local wavefields. A level set method uses this gradient to automatically update the local salt boundary estimate.