Understanding The Mesozoics Beyond Basalt: A Case Study Of Sub-Basalt Imaging

The Deccan Volcanic Province of India is considered as one of the largest basalts-covered regions in the world, formed due to extensive outpouring of basaltic lavas during Deccan volcanism (∼65 Ma). The sedimentary sequence below the flood basalt is mainly characterized by Mesozoic strata with a varying thickness of 1000 m to 2500 m. It is considered that requisite heat generation due to Deccan Trap volcanism soon after the Cretaceous sedimentation may have acted as a catalyst in hydrocarbon potential in this area (Vardhan et al. 2008). However, it is essentially unexplored because of the limitations of conventional marine streamer P-wave seismic acquisition in imaging the structures both intra-basalt and sub-basalt. The major challenges can be considered as follows: Strong reflections due to high impedance contrasts at the top (and bottom) of the basalts leading to significant loss of transmitted seismic energy; Scattering of energy due to large acoustic impedance contrast at top and bottom of the basalt; Generation of multiples, both surface-related and interbed, from the top and bottom of the basalt, and intra-basalt boundaries, masking genuine primary reflections at the pre-basasediments; Significant attenuation of seismic energy in the basaltic sequences due to its complex internal structure generally causing weak sub-basasignal; Low signal-to-noise ratio creating ambiguity in estimating accurate velocity model of subsurface. This case study demonstrates that, even with legacy marine streamer surveys, an appropriate workflow of combining suitable advanced technologies can help to overcome the long-standing challenges of sub-basalt imaging. The reprocessed data show clear uplift in the sub-basalt imaging and the inversion results validate the quality of the new data in relation to the well logs.