Refraction FWI of a circular shot OBN acquisition in the Brazilian pre-salt region

Abstract

We develop a workflow based on full-waveform inversion (FWI) to estimate P-wave velocities in a deepwater Brazilian pre-salt field using the recently introduced circular shot ocean bottom node (OBN) acquisition geometry. Such a geometry comprises a source vessel sailing in large radius concentric circular trajectories and seismic signals are recorded by OBN arrays. The circular shot OBN survey provides mostly refracted waves separately from reflected waves, so the FWI process is mainly driven by diving waves. We introduce a new FWI workflow to analyze non-preprocessed OBN refraction data, which includes automated steps such as data selection solving an Eikonal equation, estimation of a source signature that accounts for ghost and bubble effects, and gradient preconditioning using a non-stationary filter and seismic illumination. We consider two objective functions based on the $L^1$ and $L^2$ norms. The FWI results demonstrated that using our proposed workflow with the $L^1$ norm objective function and the circular OBN survey can lead to an improvement in pre-salt velocity models. Furthermore, using these improved models we construct reverse-time migration (RTM) images of the conventional OBN dataset, showing significant improvements in the salt stratification, the base of salt, and the lateral resolution of the pre-salt area. The Brazilian pre-salt case study demonstrated that the circular shot OBN acquisition maximizes the illumination of deep reservoirs through the ultra-long offset and full-azimuth coverage that prioritizes the recording of diving waves.