Tomographic approach for gas hydrate investigation in Kerala-Konkan region,India

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 Kerala-Konkan 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.