An Offshore Full Waveform Inversion with Automatic Salt Flooding: OBN Case Study

Full waveform inversion (FWI) is one of several techniques used to estimate high resolution velocity fields by employing an optimization approach. For the success of FWI using diving waves, low-frequency components and large offset traces are essential. A 3D ocean bottom node (OBN) survey had been performed with offsets up to 18 km and an airgun source capable of generating frequencies as low as 1.8 Hz. The survey area is geologically complex due to the existence of islands and halite salt bodies. In this case, uneven illumination and heterogeneous velocity distributions may hinder FWI from producing a proper velocity model. In addition, preconditioning procedures are required to suppress noise and balance amplitudes of the data, which can also alter the amplitude information. To overcome these problems, we weighted the kinematic information in the data. We also employed automatic salt flooding because refraction events were not sufficient to define the bottom of the salt. For stabilizing automatic salt flooding, we smoothed the gradient vector before the velocity update during FWI and employed a new imaging condition capable of suppressing migration terms in the gradient kernel.