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.
- Al-Chalabi M (1979) Velocity determination from seismic reflection data. In: Fitch AA (ed) Developments in geophysical exploration methods, vol 1. Applied Science Publishers, London, pp 1–68Google Scholar
- Holbrook WS, Hoskins H, Wood WT, Stephen RA, Lizarralde D, Leg 164 Science Party (1996) Methane hydrate and free gas on the blake ridge from vertical seismic profiling. Science 273:1840–1843Google Scholar
- Hyndmann RD, Spence GD (1992) A seismic study of methane hydrate marine bottom simulating reflectors. J Geophys Res 97:6683–6698Google Scholar
- Markl RG, Bryan GM, Ewing JI (1970) Structure of the Blake-Bahama outer ridge. J Geophys Res 75:4539–4555Google Scholar
- McKenzie DP, Sclater J (1971) The evolution of Indian Ocean since the late Cretaceous. Geophys J R Astron Soc 25:437–528Google Scholar
- Sheriff RE, Geldart LE (1982) Exploration seismology vol 1, history, theory, data acquisition. Cambridge University Press, New YorkGoogle Scholar
- Shipley TH, Houston MH, Buffler RT, Shaub FJ, McMillen KJ, Ladd JW, Worzel JL (1979) Seismic evidence for widespread possible gas hydrate horizons on continental slopes and rises. Am Assoc Petrol Geol Bull 63:2204–2213Google Scholar
- Sloan ED (1998) Clathrate hydrates of natural gases. Marcel Dekker Inc, New YorkGoogle Scholar
- Veerayya M, Karisiddaiah SM, Vora KH, Wagle BG, Almeida F (1998) Detection of gas charged sediments and gas hydrate horizons along the western continental margin of India. In: Henriet JP, Mienert J (eds) Gas hydrates: relevance to world margin stability and climate change. Geol Soc London Spec Publ 137:239–253Google Scholar
- Zoeppritz K (1919) On the reflection and propagation of seismic waves. Gottinger Nachrichten 1:66–84Google Scholar