HR: 0800h
AN: NS51B-04    [Abstracts]
TI: Effects of Attenuation of Gas Hydrate-bearing Sediments on Seismic Data: Example from Mallik, Northwest Territories, Canada
AU: * Bellefleur, G
EM: gbellefl@nrcan.gc.ca
AF: Geological Survey of Canada, 615 Booth St., Ottawa, On K1A0E9, Canada
AU: Riedel, M
EM: mriedel@eps.mcgill.ca
AF: Earth and Planetary Sciences, McGill University, 3450 University St., Montreal, Qc H3A2A7, Canada
AU: Brent, T
EM: tbrent@nrcan.gc.ca
AF: Geological Survey of Canada, 3303 - 33 St. N.W., Calgary, Ab T2L2A7, Canada
AB: Wave attenuation is an important physical property of hydrate-bearing sediments that is rarely taken into account in site characterization with seismic data. We present a field example showing improved images of hydrate- bearing sediments on seismic data after compensation of attenuation effects. Compressional quality factors (Q) are estimated from zero-offset Vertical Seismic Profiling data acquired at Mallik, Northwest Territories, Canada. During the last 10 years, two internationally-partnered research drilling programs have intersected three major intervals of sub-permafrost gas hydrates at Mallik, and have successfully extracted core samples containing significant amount of gas hydrates. Individual gas hydrate intervals are up to 40m in thickness and are characterized by high in situ gas hydrate saturation, sometimes exceeding 80% of pore volume of unconsolidated clastic sediments having average porosities ranging from 25% to 40%. The Q-factors obtained from the VSP data demonstrate significant wave attenuation for permafrost and hydrate- bearing sediments. These results are in agreement with previous attenuation estimates from sonic logs and crosshole data at different frequency intervals. The Q-factors obtained from VSP data were used to compensate attenuation effects on surface 3D seismic data acquired over the Mallik gas hydrate research wells. Intervals of gas hydrate on surface seismic data are characterized by strong reflectivity and effects from attenuation are not perceptible from a simple visual inspection of the data. However, the application of an inverse Q-filter increases the resolution of the data and improves correlation with log data, particularly for the shallowest gas hydrate interval. Compensation of the attenuation effects of the permafrost likely explains most of the improvements for the shallow gas hydrate zone. Our results show that characterization of the Mallik gas hydrates with seismic data not corrected for attenuation would tend to overestimate thicknesses and lateral extent of hydrate-bearing strata and hence, the volume of hydrates in place.
DE: 0714 Clathrate
DE: 0915 Downhole methods
DE: 0935 Seismic methods (3025, 7294)
DE: 5144 Wave attenuation
SC: Near-Surface Geophysics [NS]
MN: 2007 Joint Assembly