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

Abstract

S U M M A R YLow-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.

Sub-basalt sediment imaging — The efficacy of magnetotellurics

“…Subsequent to the magnetotelluric studies in the Saurashtra region (Sarma et al, 1992), a wide-angle reflection and refraction study, instead of conventional seismic reflection and refraction profiling, was carried out to image the sub-basalt lithology (Sain et al, 2002) along a 40 km long north-south profile, that runs close to one of the MT traverses occupied earlier in the region (see Fig. 1).…”

Feasibility of using the magnetotelluric method for sub-basalt imaging at Kachchh, India

“…Mesozoic sediments of economic importance are buried underneath the lava flows of the Cretaceous-Tertiary boundary. A series of experiments have been conducted in the past to image the subbasalt targets in this region (Sain et al, 2002;Patro et al, 2005) and many more employing multiple geophysical techniques are expected. Biswas, 1982) and various MT sites along the southern part of Kachchh.…”

The effect of intra-trappean heterogeneities on seismic data: A case study from the Deccan Traps

“…The log data were available for depths down to 2.7 km. Beyond this depth in the model, velocity was assumed to vary between 4.5 km/s and 5.2 km/s (at 5 km depth) based on the wide-angle seismic data modelling results from Deccan Traps (Pandey, 2003;Sain et al 2002). The log data were smoothed at every 3 m depth interval.…”