Velocity and AVO analysis for the investigation of gas hydrate along a profile in the western continental margin of India |
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| Authors: | Pawan Dewangan T Ramprasad |
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| Institution: | (1) Geological Oceanography, National Institute of Oceanography, Dona Paula, Goa, 403004, India |
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| Abstract: | 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. |
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| Keywords: | Gas hydrates BSRs Velocity analysis AVO analysis Wave-equation datuming Error analysis Tomographic velocity analysis |
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