Abstract

The Bikaner-Nagaur Basin in Rajasthan, western India, presents significant challenges for seismic imaging due to its complex geology. A thick shallow carbonate unit comprising dolomites, limestone, interbedded shales, and evaporites overlies sandstone reservoir intervals and exhibits high velocity and density contrasts. Compounded by near-surface salt layers, topographic variations from sand dunes, a total subsurface column of 1000-1500 m and further carbonates in the deeper section, conventional Pre-Stack Time Migration (PSTM) yielded suboptimal results and struggled to produce a geologically consistent velocity model. To overcome these limitations, a Pre-Stack Depth Migration (PSDM) workflow was implemented. Initial attempts at grid-based tomography using PSTM-derived models failed to converge for the carbonate formation with very high velocity. A revised strategy incorporated well sonic logs and time-migrated horizons 1D map migrated into depth domain to generate a robust starting model. This model was integrated with a near-surface velocity model from advanced refraction statics and refined through iterative tomography. In areas lacking well control, biases from PSTM derived seismic velocity were also introduced. A total of five (05) tomographic iterations, including two (2) anisotropic (VTI) updates, and one (1) pass of well constrained tomography updates were carried out (Woodward et al., 2008). Kirchhoff PSDM was employed for both interim model updates and final migration (Biondi, 2006). The approach delivered significant improved imaging fidelity over time migration, consistency with well markers, and a geologically consistent velocity model, establishing its effectiveness in tackling imaging challenges in complex basinal settings.