Ultralong-offset data acquisition can complement full-waveform inversion and lead to improved subsalt imaging

Full-waveform inversion (FWI) has proved itself as a valuable tool for updating and improving the accuracy of subsurface velocity models. For optimum performance, particularly in complex geological environments such as subsalt targets in the Gulf of Mexico (GOM), FWI requires 3D seismic data containing low frequencies and long offsets. For onshore seismic surveys, it is relatively straightforward and cost-effective to record long-offset data; however, for offshore surveys it can incur the additional cost of ocean-bottom cable (OBC) acquisition or multivessel towed-streamer techniques. The recent introduction of dual-coil-shooting multivessel full-azimuth acquisition has provided a highly efficient method of delivering marine 3D seismic data sets with long offsets. Tests using synthetic data and a real GOM dual-coil acquisition data set with full-azimuth distribution and long (up to 14.3-km) offsets demonstrate significant potential for improving the accuracy of velocity models and the reliability of the resulting depth images. The results indicate that the long offsets and low frequencies provided by the dual-coil acquisition method are of particular benefit in helping FWI to enhance the shallow section of the model. Furthermore, lower frequencies and longer offsets mitigate the sensitivity of the inversion process to the initial velocity model by enabling FWI to update the low-wavenumber component of the velocity model.