Target-oriented refraction waveform inversion: A Brazilian pre-salt case study

Abstract

The ocean-bottom-node (OBN) technology has been increasingly used to characterize the Brazilian pre-salt reservoirs. The independence between the geometries of seismic sources and receivers, enabled by an OBN survey, permits designing wide-aperture acquisition geometries that favour the recording of refracted waves from ultra-deep reservoirs. In this context, we present a target-oriented full-waveform inversion (FWI) methodology to evaluate the inversion of refracted waves simulated using a realistic velocity model inspired by typical Brazilian pre-salt oil fields. In our tests we compare three acquisition geometries, using the same node configuration on the seabed, but different source geometries: (i) circular shot geometry, (ii) conventional OBN geometry, (iii) long-offset OBN geometry. We perform a wavepath analysis to investigate the illumination of the target region promoted by the refracted waves of a circular shot OBN survey. The wavepath analysis suggests that Brazilian pre-salt reservoirs may be well illuminated using the much fewer source points in a circular shot acquisition geometry. The FWI results with synthetic data show that refracted waves bring important information from the ultra-deep reservoirs which are indispensable for obtaining a reliable final velocity model. We show that adding a low-cost circle shot geometry to a conventional OBN survey delivers improvements from the long-offset data that are comparable to those obtained from the much more expensive long-offset OBN geometry. Furthermore, these results suggest that the long-offset and multi-azimuth dataset has great potential to improve the reservoir characterization in deep-water exploration marine settings.