Revealing Complex Sub-Basalt Structures Offshore India Through Advanced Seismic Processing

Sub-basalt seismic imaging is very challenging due to large impedance contrasts at sediment-basalt interfaces. The impedance of basalt usually gives a strong reflection coefficient at the top of basalt, and thus generates strong multiples. Offshore western India, this issue is compounded by short-period seabed multiples generated by the shallow sea floor. Moreover, the presence of the basalt layer limits the angle of reflections from sub-basalt structures, making velocity modeling difficult. The combination of strong, complex multiples and the challenges of obtaining a reliable velocity model gives rise to poor imaging beneath and within the basalt. In this study, a comprehensive pre-migration demultiple flow was devised to tackle the strong surface and interbed multiples. For velocity model building, full-waveform inversion (FWI) was applied for the shallow velocity update and non-linear scanning tomography was then utilized to update the velocity within and beneath the basalt layer. Due to the poor initial velocity model, an enhanced dynamic-warping FWI approach was used to mitigate the cycle-skipping issue, and the maximum FWI frequency was extended to 20 Hz. With the benefits from the comprehensive demultiple process and advanced velocity model building, imaging of the complex sub-basalt structures in this area was improved.