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Subsalt Imaging: An Integrated Approach to Image Improvement of Vintage Data


Reservoir objectives trapped under allochthonous salt bodies remains one of the most interesting plays for oil and gas exploration. Accurate imaging of the geological structures is a key factor to reduce uncertainty of the potential prospect. Pre-stack depth migration has been known to help improve the imaging around complex targets with sharp lateral velocity variations. The generic workflow to tackle this problem involves several iterations of depth migration and manual picking of the top and base of the salt body. Velocity model building and seismic imaging of subsalt sedimentary systems remains a challenge in the petroleum industry. Thus, in this study, we have taken a step forward to improve subsalt imaging by combining: 1) state-of-the-art pre-processing techniques of long-offset vintage data such as efficient demultiple, diffraction attenuation and deghosting; 2) iterative seismic interpretation of allochthonous salt bodies; and 3) cutting edge velocity model building technique such as multi-parameter full waveform inversion (FWI) and ray-based tomography. The result of this approach largely improves the salt geometry and the resolution of the velocity model, thus leading to improving the final migrated subsalt image. The final migrated image using Reverse Time Migration (RTM) with surface offset gathers benefitted from low frequency boost due to the deghosting applied during pre-processing, giving rise to better imaging of deeper sediments. In conclusion, with the difficulties of subsalt imaging, we were able to show the value of a multi-faceted approach to improving subsalt image. For old acquired seismic datasets, new pre-processing techniques such as deghosting, can add some value. Combining this pre-processing approach with iterative seismic interpretation of allochthonous salt bodies and multi-parameter FWI, helps achieve a global improvement of the image of subsalt sediments leading to better de-risking of reservoir prospects.