Imaging of subvolcanic Mesozoics in the Saurashtra peninsula of India using traveltime inversion of wide-angle seismic data

Abstract

Low-velocity Mesozoic sediments exposed in the northeastern part of the Saurashtra peninsula of western India are assumed to be below the high-velocity volcanic rock cover. Conventional seismic refraction and reflection profiling experiments often fail to map hidden low-velocity formations caused by inherent limitations of these methods. However, wide-angle seismics involving both refractions and reflections is an effective tool for imaging in such a geological setting. Inversion of wide-angle seismic traveltime data from a 45 km long profile reveals a three-layered velocity structure above the basement with Mesozoic sediments sandwiched between exposed volcanic rock and an underlying basalt flow. The derived velocity model closely matches the lithology of the nearby deep Lodhika well, demonstrating the potential of wide-angle seismics in mapping formations hidden below volcanic rock cover. The volcanic rock cover is almost flat, ˜1.4 km thick, with an average velocity of ˜5.1 km s−1. The Mesozoic sediments have a velocity of 4.3 km s−1 and thickness variations from 0.9 to 1.6 km. The underlying basalt flow has a velocity of 5.15 km s−1 and varies in thickness from 1 to 2 km. The basement, with a velocity of 5.80–5.95 km s−1, is at an average depth of 3.9 km. Shear wave analysis yielded velocities varying between 2.65–2.75 and 3.30–3.40 km s−1 for the exposed volcanic rock and the subsurface basement, respectively, corresponding to Poisson ratios of 0.31 and 0.26. The high Poisson ratio in the volcanic rock cover is a result of rock fracturing as indicated by the fast decay of seismic energy with offset.