Subsurface imaging of brown coal bearing Tertiary sedimentaries - Deccan Trap interface using microtremor method

Abstract

We propose an application of the microtremor (ambient noise) H/V spectral ratio technique to identify significant rheological boundaries at shallow depths, estimate thickness of both lignite bearing Tertiary sedimentary sequence and late Cretaceous Deccan basalt flows and comprehend basinal geometry of Umarsar Basin (Babia syncline). Forty-six stations were gauged in a grid format at ~250 m resolution during the microtremor survey. The microtremor H/V spectral ratio technique takes into account the frequency of the ratio between the horizontal (NS + EW) and vertical components of persistent Rayleigh waves in the area. Depth estimates are made using Mean_Vs (433.69 m/s) from three borehole records (MMA_Vs (431.6 m/s), MMB_Vs (406.86 m/s) and MMC_Vs (462.6 m/s) using standard relationship between depth and velocity. In the present study, we recognize three rheological interfaces viz., L1 interface (0.2328 Hz to 0.3862 Hz), L2 interface (0.7843 Hz to 2.5123 Hz) and L3 interface (6.2477 Hz to 27.1119 Hz). The geology and stratigraphic records supplement correlation for L1, L2 and L3 interfaces with Mesozoic–Deccan Trap (M-DT) boundary, Deccan Trap-Tertiary (DT-T) boundary, and boundaries between shale‑carbonate hardpans within sediment sequence belonging to Naredi Formation respectively. The estimated depth range for M-DT boundary (L1) is 281–466 m and for DT-T boundary (L2) is 43–138 m. The subsurface image acquired from the frequency records advocates for a palaeo high in SE portion that retains its entity over the present landscape. The frequency records advocate for 369 m to 206 m thick Deccan basalt and 85 m to 18 m thick lignite bearing Naredi Formation. The ambient noise seismic study further suggests NW-SE trending basin geometry of Babia syncline comprising three distinct depressions having six local depocenters. Finally, we propose the microtremor H/V spectral ratio technique as a tool to develop economical borehole plan with realistic reserve estimate and a step forward towards rapid economical assessment covering large mining lease areas complementary to local geological studies.