2D elastic full-waveform inversion of wide-angle seismic data to characterize a 70 Ma old crust and Moho in the equatorial Atlantic Ocean

Full waveform inversion (FWI) is a high-resolution seismic imaging technique that uses waveforms from the entire seismic data for determining the elastic parameters of the subsurface and has been a massive success in both the academic and industrial fields. The inversion scheme of FWI iteratively minimizes the misfit function using linearized model updates by computing its gradient using the adjoint-state method. The complex behaviour of the misfit function due to the non-linearity of the inverse problem makes FWI dependent on an appropriate initial model. This dependency makes FWI a locally optimized technique while providing a high-resolution image of the subsurface upon successful convergence. We apply 2D elastic full-waveform inversion to an ocean bottom seismometer data (OBS) from a 70 Ma old crustal segment, north of the St. Paul fracture zone near 18 W parallel to the Mid-Atlantic Ridge in the equatorial Atlantic Ocean. The OBS spacing is 14.3 km, and the shot interval is 300 m. Apart from crustal Pg, mantle Pn, and wide-angle reflections from the Moho, we also observe the converted S-wave arrivals, such as Sg, Sn, and SmS, which allows us to determine both P and S-wave velocity structure of the sub-surface. We first perform travel time inversion to get starting velocity model for both P and S-waves and then perform the elastic full-waveform inversion. We have used a multistep inversion process using the layer-stripping strategy with increasing frequencies and offsets to overcome the challenges of non-linearity of the seismic data. Our objective is to determine the detailed structure of the lower crust, Moho transition zone and upper mantle. We are particularly interested in imaging the possible presence of layering in the lower crust and at the Moho transition zone.