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

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 similar to 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., L-1 interface (0.2328 Hz to 0.3862 Hz), L-2 interface (0.7843 Hz to 2.5123 Hz) and L-3 interface (6.2477 Hz to 27.1119 Hz). The geology and stratigraphic records supplement correlation for L1,L2 and L-3 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 (L-1) is 281-466 m and for DT-T boundary (L-2)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. (C) 2018 Elsevier B.V. All rights reserved

Subsurface profiling along Banni Plains and bounding faults, Kachchh, Western India using microtremors method

The present article is a maiden attempt to map shallow subsurface rheological interfaces laterally across the Banni Plains and to decode geometry of the antecedent faults associated with the Kachchh Mainland Fault using the microtremor method. We conducted microtremor data acquisition for thirty-one sites along N-S transect from Loriya in Mainland Kachchh to Bhirandiara towards Patcham Island. Results from H/V spectral ratio technique show presence of two distinct rheological interfaces characterised by the resonant frequency (f(r)) ranges 0.23-0.27 Hz and 0.8252-1.5931 Hz respectively. The above frequency ranges are correlated with the depths of the Mesozoic-Basement (M-B) interface and the Quaternary-Tertiary (Q-T) interface. Using either the velocity (Vs) of seismic waves at the M-B and Q-T interfaces (calculated as 1830 m/s and 411 m/s respectively) or the standard non-linear regression relationship derived for the Banni Plains (h = 110.18f(r)(-1.97)) we estimate the depth range for M-B interface to be 1442-1965 m and for Q-T interface to be 44-160 in. The subsurface profile across the Banni Plains educe cluster of four faults that develop an array of imbricate faults at the forefront of the Kachchh Mainland Fault within the Banni Footwall Syncline. The geometry of the faults suggests a `positive flower structure' indicating step-overs and strain restraining bends displaying push-ups resulting from localized shortening between converging bends of Kachchh Mainland Fault and the South Wagad Fault. The Banni Footwall Syncline preserves evidence of two episodes of deformations. The initial deformation event led to subsidence within the Kachchh Mainland Fault Zone bringing Mesozoic sequence juxtaposed to the basement rocks, whereas the later event is dominated by an uplift developing a positive flower structure in the Kachchh Mainland Fault Zone. Finally, the present study provides a mechanism to investigate faults and fault geometries correlating surface structural grains with subsurface structures