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Investigation of gas hydrate along the western continental margin
Gas hydrate is an ice-like non-stoichiometric crystalline solid formed under high pressure and low temperature when gas concentration exceeds the saturation limit. Gas hydrates are widely present in continental slope environments where a deep water column exerts enough pressure to stabilize gas hydrate. The occurrence of gas hydrates can be inferred from the presence of Bottom-Simulating Reflectors (BSRs) on stacked reflection seismic data. A BSR is identified on seismic sections based on its distinct characteristics i.e., (i) mimicking the sea-floor topography, (ii) an opposite polarity with respect to the seafloor, (iii) typical association with amplitude blanking in the hydrate stability zone, and (iv) crosscutting the existing geological horizons because it represents a phase transition. Seismic studies on several active and passive margins demonstrate that a high amplitude BSR is primarily due to the presence of free gas at the base of hydrate stability zone. The occurrence of gas hydrates has been inferred from the presence of BSRs along the western continental margin of India (WCMI). The presence of several gas escape features has also been observed in shallow seismic records on the margin. Interval velocity and Amplitude Versus Offset (AVO) analysis of the multi-channel seismic (MCS) reflection data collected along the WCMI show the existence of free gas below the BSR and suggests a hydrates/free gas mechanism for the BSR formation along the margin. The scientists of NIO assessed spatial and vertical distribution of gas hydrates by analyzing the interval velocities and AVO responses obtained from MCSs. The hydrate cements the grains of the host sediment, thereby increasing its velocity, whereas the free gas below the base of hydrate stability zone decreases the interval velocity. Conventionally, velocities are obtained from the semblance analysis on the Common Mid-Point (CMP) gathers. Here, we used waveequation datuming to remove the effect of the water column before the velocity analysis. We show that the interval velocities obtained in this fashion are more stable than those computed from the conventional semblance analysis. The initial velocity model thus obtained is updated using the tomographic velocity analysis to account for lateral heterogeneity. The resultant interval velocity model shows large lateral velocity variations in the hydrate layer and some low velocity zones associated with free gas at the location of structural traps. The reflection from the base of the gas layer is also visible in the stacked seismic data. Vertical variation in hydrate distribution is assessed by analyzing the AVO response at selected locations. AVO analysis is carried out after applying true amplitude processing. The average amplitudes of BSRs are almost constant with offset, suggesting a fluid expulsion model for hydrate formation. In such a model, the hydrate concentrations are gradational with maxima occurring at the base of hydrate stability zone. For more information, please read: Dewangan, P.; Ramprasad, T. Velocity and AVO analysis for the investigation of gas hydrate along a profile in the western continental margin. Mar. Geophys. Res.: 28(3); 2007; 201-211.
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