Multiparameter elastic FWI of 4C OBN data with spectral elements: First inferences from the Gorgon FWI case study

Abstract Evolving from acoustic full-waveform inversion (AFWI) to elastic anisotropic FWI of long-offset, wide-azimuth multicomponent ocean-bottom node data is one of the most topical issues in exploration geophysics. The goal is to tend toward reservoir characterization by accounting for elastic effects in FWI. In this framework, two methodological issues are the computational overhead generated by elastic wave modeling and the management of the four components during multiparameter FWI. Although the finite-difference method on regular Cartesian grids is widespread in AFWI, this choice may be questioned in elastic FWI (EFWI), where complex bathymetry and low shear waves near the sea bottom may require unstructured meshing of the medium to reach the best trade-off between accuracy and computational cost. In this study, we perform a first assessment of the spectral element method (SEM) on unstructured hexahedral meshes as the forward engine of time-domain EFWI with a preliminary application on a low-frequency narrow-azimuth subset of the 2015 Gorgon OBN data set, offshore Australia. We show that EFWI can be applied to the pressure component to update both compressional and shear wave speeds (VP and VS) and that multiparameter inversion leads to better convergence than monoparameter inversion for VP. The good data fit achieved after FWI, the structural consistency between the long-wavelength EFWI model and the 25 Hz AFWI model reconstructed during a previous study, and the consistent VP/VS ratio in the reservoir with empirical relationships relating VP and VS for sand and shale lithologies provide a first validation of our SEM-based EFWI workflow. The next step is the inversion of the four components for joint reconstruction of the density and the full elasticity tensor.