Frequency-domain full-waveform inversion of OBS wide-angle seismic data
Résumé
In deep offshore, crustal-scale seismic imaging is classically performed by traveltime tomography applied to wide-angle seismic data recorded by a network of Ocean Bottom Seismometers (OBS). The resulting velocity model is of limited resolution. If the wide-angle experiment is multifold thanks to densely sampled OBSs, this large-scale velocity model can be used as a starting model for full-waveform inversion. Considering the full wavefield recorded over a broad range of apertures is expected to provide a significant resolution improvement in the velocity models. We applied 2D frequency-domain full-waveform inversion to wide-angle data recorded by 100 OBSs deployed perpendicular to the trench axis of a subduction zone, offshore Japan. The full-waveform modelling/inversion are entirely implemented in the frequency domain. Thirteen frequencies ranging from 3 to 15 Hz were inverted sequentially with a weighted least-square method, the velocity model obtained for each frequency being used to start the inversion of the next one. The resolution of the velocity models inferred from traveltime and full waveform tomographies were estimated to be ~10 km and 500 m respectively at 10 km depth suggesting a resolution improvement by an over order of magnitude. The relevance of the structures was assessed by ray tracing and full-waveform modelling.