Sub-Basaltic Sediment Imaging with Teleseismic Earthquakes using a Transdimensional Bayesian Approach

The thick Deccan Traps in the Saurashtra Peninsula pose significant challenges for sub-basaltic sediment imaging using conventional seismic methods. To offer a reliable alternative, we employed a transdimensional Bayesian joint inversion of teleseismic P-wave polarizations and receiver functions. Using Bayesian inversion, which offers greater flexibility in incorporating data variance into the objective function due to its probabilistic framework, we obtain 1-D velocity models beneath five broadband stations. The resulting 1-D shear-wave velocity models indicate sub-basaltic sediment thicknesses of ~1.3 km at KHER, 1.6 km at MANK, and 1.4 km at TANA, overlain by high-velocity shear-wave layers (Vs ~2.8–3.0 km/s) with thicknesses of ~0.7–0.9 km. In contrast, no evidence of sub-basaltic sediments was observed at SONT and MORK. The exceptionally low near-surface Vs (~0.85 km/s) and the gradual increase in Vs at SONT suggest the presence of unconsolidated thick sediments (~2.3 km) overlying high-velocity basement rocks (Vs ~3.4 km/s), likely corresponding to exposed Mesozoic formations with no indication of basaltic traps. Meanwhile, MORK exhibits relatively higher near-surface Vs (~2.2 km/s), indicating more compacted sediments with a thinner sediment layer (~0.8 km)  overlying ~1.4 km thick volcanic rocks (Vs ~3.1 km/s). This study highlights the potential of passive seismic exploration in imaging sedimentary formations hidden beneath thick volcanic rock layers, offering a cost-effective alternative to conventional geophysical methods.