Two‐dimensional acoustic anisotropic (VTI) full waveform inversion: The Valhall case study

Abstract

Anisotropy can have a significant footprint in seismic imaging from surface wide‐aperture data, because of the difference between vertical and horizontal velocities in vertical transversely isotropic (VTI) media. A key issue in anisotropic full waveform inversion (FWI) is to define a suitable parametrization of the subsurface model, and the number of parameter classes that can be reliably reconstructed for the chosen parametrization. We address this issue with an application of frequency‐domain acoustic isotropic/anisotropic FWI to real surface wide‐aperture data recorded by an ocean bottom cable in the Valhall field. Isotropic FWI results show that the horizontal velocity is mainly reconstructed in the upper structure, dominantly constrained by diving waves and supercritical reflections. This can lead to depth stretching and/or overestimated velocity perturbations in the deep velocity structure, mainly controlled by short‐aperture reflections. Monoparameter anisotropic FWI shows a reliable reconstruction of either the vertical, the horizontal or the NMO velocity, when the model is parametrized by one wave speed and the Thomsen's parameters δ and ε. For such mono‐parameter FWI, the smooth δ and ε background models are kept constant during FWI iterations. Alternatively, the vertical and the horizontal velocities can be jointly reconstructed by multiparameter FWI, when the subsurface model is parametrized by the two wave speeds and δ, this latter is kept fixed during FWI iterations.