Velocity and AVO analysis for the investigation of gas hydrate along a profile in the western continental margin of India
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The occurrence of gas hydrate has been inferred from the presence of Bottom-Simulating Reflectors (BSRs) along the western continental margin of India. In this paper, we assess the spatial and vertical distribution of gas hydrates by analyzing the interval velocities and Amplitude Versus Offset (AVO) responses obtained from multi-channel seismics (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 wave-equation 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.
KeywordsGas hydrates BSRs Velocity analysis AVO analysis Wave-equation datuming Error analysis Tomographic velocity analysis
The authors wish to thank the Director, National Institute of Oceanography (NIO) for giving the permission to publish the paper. Our gratitude goes to the members of Gas Hydrate Team, NIO, for useful discussions. We would also like to thank Mr. Priyank Jaiswal, Rice University, for his comments/suggestions in implementing RayInvr code in NIO. We thank the anonymous reviewers and the associate editor for their invaluable comments and suggestions that has drastically improved the quality of the paper. The seismic data used in this paper was provided by Gas Authority of India Limited (GAIL) through National Gas Hydrate Program (NGHP) from Oil and Natural Gas Commission (ONGC). This is NIO contribution no. 4270.
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