Gabon Offshore - New Technology Enhancing Imaging of Complex Subsurface in Gabon

Offshore Gabon has several challenges for seismic exploration. The major difficulty is obtaining a detailed and accurate velocity model. Complicated salt structures with overhangs, variability within the salt and carbonate rafts with Karst features pose difficult challenges to conventional velocity model building. Moreover, these introduce illumination issues which are not addressed by traditional migration. This is particularly important in the pre-salt areas where the target generally lies. With this paper we show how the recent technological advances in velocity model building through Full Waveform Inversion (FWI) and particularly Time-Lag FWI (TL-FWI), followed by imaging through the Least-Squares (LS) migration have provided a step change improvement in the pre-salt image of narrow azimuth data acquired in the West African Atlantic margin of Gabon. To overcome cycle-skipping and amplitude discrepancy between synthetic and recorded data in the presence of sharp velocity contrasts and large scale geo-bodies, typical for Gabon context, we propose the use of a robust FWI cost function like the one employed in TL-FWI. The alignment of the resultant estimated velocity model with the geology of the margin allows the generation of considerable imaging uplifts. Additionally, the LS migration mitigates the noise generated by the migration operator and illumination deficiencies.