Tomographic approach for gas hydrate investigation in Kerala-Konkan region, India
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ORIGINAL RESEARCH PAPER
Tomographic approach for gas hydrate investigation in Kerala-Konkan region, India Naresh Kumar Thakur Æ Pasupuleti Prasada Rao Æ N. Vishwanath Æ Sanjeev Rajput Æ Bhaskarabhatla Ashalatha
Received: 21 December 2006 / Accepted: 18 December 2007 / Published online: 19 January 2008 Ó Springer Science+Business Media B.V. 2008
Abstract Seismic tomography is an effective means of estimating velocity and structure from multichannel seismic (MCS) reflection data. In this study we have followed a 2D approach to arrive at the probable velocity field configuration from multichannel seismic data and infer the presence of gas hydrates/free-gas in the offshore KeralaKonkan region, along the eastern part of a seismic line on which a bottom simulating reflector (BSR) has previously been identified. Tomographic modeling consists of the identification of reflection phases and picking of respective travel times for various source-receiver positions. These picks were then utilized to arrive at a 2D velocity field following a forward and inversion approach using a ray tracing technique. The modeling for the first time brought out the finer scale velocity structure under the region of investigation. Modeling through the 2D approach shows lateral variation in velocity field along the studied segment of the seismic line. The results indicate a thin (*50–60 m) sedimentary cover with velocity ranging from 1,770 to 1,850 m/s. A sedimentary layer with high P-wave velocity 1,980–2,100 m/s below the sea floor was interpreted as the hydrate layer. The thickness of this layer varies between 110 and 140 m. The hydrate layer is underlain by a low-velocity layer having velocities in the range 1,660–1,720 m/s. This low velocity may represent a free gas layer, whose thickness varies between 50 and 100 m located below the hydrated layer. The investigation suggests the occurrence of gas hydrate underlain by free gas in some parts of the Kerala-Konkan offshore region.
N. K. Thakur (&) P. P. Rao N. Vishwanath S. Rajput B. Ashalatha Gas Hydrate Group, National Geophysical Research Institute, Cyber Building, Uppal Road, Hyderabad, AP 500007, India e-mail: [email protected]
Keywords Tomography Gas hydrates 2D Velocity field Forward and inversion approach
Introduction The occurrence of hydrate is often inferred from the identification of a bottom simulating reflector (BSR) on seismic reflection profiles (e.g. Shipley et al. 1979). The presence of gas hydrate alters the physical properties of sediments by filling voids in pore water molecules with methane hydrates, which in turn by reduces the effective pore space and permeability (Lee et al. 1996; Helgerud et al. 1999; Jakobsen et al. 2000; Gei and Carcione 2003; Chand and Minushull 2003), and thereby increases the acoustic velocity (Chand et al. 2004). If the hydrated layer is underlain by gas or brine-saturated sediments, an appreciable seismic velocity drop is observed in the transition to this lower layer. This change in the impedance contrast across the hydrate
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