3-D multiparameter full-waveform inversion for ocean-bottom seismic data using an efficient fluid–solid coupled spectral-element solver

Abstract

SUMMARYOcean-bottom seismic acquisition systems deployed on the seabed give access to three-component geophone data and hydrophone data. Compared with conventional streamer acquisitions, the separation of sources and receivers makes it possible to increase the maximum offset and azimuth coverage for improving the illumination at depth. Furthermore, the three-component geophones naturally capture elastic wave propagation effects. While this information is mostly overlooked up to now, reconstructing jointly P- and S-wave velocities would significantly improve the subsurface characterization. To achieve a 3-D high-resolution multiparameter reconstruction, we design an efficient 3-D fluid–solid coupled full waveform modelling and inversion engine. In this engine, fluid and solid domains are divided explicitly and handled with the acoustic and elastic wave equations, respectively. The numerical implementation is based on a time-domain spectral-element method (SEM) with a flexible 3-D Cartesian-based hexahedral mesh, which contributes to an accurate coupling of the acoustic and elastic wave equations and high computational efficiency through domain-decomposition based parallelization. We select the best acoustic–elastic coupled formulations among 4 possibilities with criteria based on numerical accuracy and implementation efficiency. Moreover, we propose a specific hybrid approach for the misfit gradient building so as to use a similar modelling solver for both forward and adjoint simulations. Synthetic case studies on a 3-D extended Marmousi-II model and a 3-D deep-water crustal-scale model illustrate how our modelling and inversion engine can efficiently extract information from ocean-bottom seismic data to simultaneously reconstruct both P- and S-wave velocities within a full waveform inversion framework.