ALBERT

Description: Full waveform inversion (FWI) is a powerful tool used to quantify the elastic properties of the subsurface from seismic data. Because of very high computational cost, the technique has so far been used for either 2-D full elastic or 3-D acoustic media while the extension to 3-D elastic media to a realistic model size is still a challenging task. However, the Earth being 3-D, elastic and highly heterogeneous, one would require a full 3-D elastic wave equation for accurate modelling of amplitudes and phases within the inversion process. The acoustic approximation could significantly impact the final waveform inversion results, mainly due to the amplitude variation with offset effect. This effect becomes extremely important in the presence of strong contrasts in S -wave velocity and density, specifically when long-offset reflection data are included for waveform inversion. Recent increase in computer power allows for more efficient parallel computing using thousands of processors simultaneously thus making 3-D elastic waveform inversion feasible today. In this paper we consider a synthetic study based on a 3-D elastic medium for inversion of both P - and S -wave velocities using multicomponent, ocean-bottom cable seismic data. Both the forward modelling part and the inversion part are carried out in the time domain. The inverse problem is parametrized in terms of P - and S -wave velocities, while the density, being difficult to reconstruct, is not inverted and is linked to the P -wave velocity. Among several synthetic examples, a successful experiment on a small part of a 3-D SEG/EAGE overthrust model is presented, demonstrating the feasibility of inverting and accurately quantifying both P - and S -wave velocities. The resolution analysis of the waveform inversion is tested using a checkerboard model. Our results show that 3-D elastic FWI of sparsely spaced sources can retrieve P - and S -wave velocities accurately.