Two-dimensional Anisotropic Visco-elastic Full Waveform Inversion of Wide-aperture 4C OBC Data from the Valhall Field

In this study, we assess the feasibility of 2D vertical transverse isotrope (VTI) visco-elastic full waveform inversion (FWI) for the reconstruction of highly resolved models of the vertical P-wave and S-wave velocities (Vp0 and Vs), density and attenuation (Qp) from 4C OBC wide-aperture data recorded in the Valhall oil field. The Thomsen parameters delta and epsilon are kept constant during the FWI: they are previously determined by reflection traveltime tomography. FWI is performed in the frequency domain following a multiscale approach. For performing reliable multi-parameters inversion of the multi-components data, we propose a visco-acoustic & elastic FWI workflow, which proceeds hierarchically over data components and parameter classes. First, our strategy relies on the reconstruction of the dominant parameters (Vp0 followed by density and Qp) by visco-acoustic FWI before considering the updating of the second-order parameter (Vs) by visco-elastic FWI. Second, data components (hydrophone versus geophones, wide apertures versus short apertures) are progressively introduced in the inversion, such that the long wavelengths of each parameter class are reconstructed before the shorter ones, hence, introducing a second level of data preconditioning in our multiscale approach. The FWI models are validated against sonic logs, seismic modeling, and source wavelet estimation.