Updating velocity models for complex pre-salt targets in Santos basin

Pre-salt oil discoveries in the Santos basin have made it one of the most prospective petroleum provinces in the world. The prospects that these plays provide are not without challenge. Complex geological features and deep targets complicate the exploration and evaluation process. Limited illumination of available narrow-azimuth (NAZ) seismic data further add to the challenge. Full waveform inversion (FWI) is a natural choice to update complex overburdens. The recently proposed formulation by Zhang et al. (2018) provides a reliable approach for the complex geologies of the Santos basin. For deeper updates, waveequation migration velocity analysis (WEMVA) relies on reflections through the wave-equation, making it a potential candidate for salt and pre-salt model updates. While still susceptible to the limitations imposed by NAZ data, Donno et al. (2019) introduced a regularization to reduce some of these adverse effects. Iterating these two flavors of waveequation based inversions, we demonstrate that reasonable velocity updates can be achieved in some of the complex areas of the Santos basin using NAZ data. Salt delineation and scenario testing are still required in some places to improve unresolved areas. Results are promising with a significant uplift in the pre-salt layer.