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VELOCITY ANALYSIS OF LWD AND WIRELINE SONIC DATA IN HYDRATE-BEARING SEDIMENTS ON THE CASCADIA MARGIN

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Title: VELOCITY ANALYSIS OF LWD AND WIRELINE SONIC DATA IN HYDRATE-BEARING SEDIMENTS ON THE CASCADIA MARGIN
Author: Goldberg, David; Guerin, Gilles; Malinverno, Alberto; Cook, Ann
Subject Keywords sonic logging;LWD;marine sediments;CH4 hydrate saturation;methane hydrate saturation;ICGH 2008;International Conference on Gas Hydrates 2008
Issue Date: 2008-07
Publicly Available in cIRcle 2008-09-03
Citation: Goldberg, David; Guerin, Gilles; Malinverno, Alberto; Cook, Ann. 2008. VELOCITY ANALYSIS OF LWD AND WIRELINE SONIC DATA IN HYDRATE-BEARING SEDIMENTS ON THE CASCADIA MARGIN. Proceedings of the 6th International Conference on Gas Hydrates (ICGH 2008), Vancouver, British Columbia, CANADA, July 6-10, 2008.
Abstract: Downhole acoustic data were acquired in very low-velocity, hydrate-bearing formations at five sites drilled on the Cascadia Margin during the Integrated Ocean Drilling Program (IODP) Expedition 311. P-wave velocity in marine sediments typically increases with depth as porosity decreases because of compaction. In general, Vp increases from ~1.6 at the seafloor to ~2.0 km/s ~300 m below seafloor at these sites. Gas hydrate-bearing intervals appear as high-velocity anomalies over this trend because solid hydrates stiffen the sediment. Logging-while-drilling (LWD) sonic technology, however, is challenged to recover accurate P-wave velocity in shallow sediments where velocities are low and approach the fluid velocity. Low formation Vp make the analysis of LWD sonic data difficult because of the strong effects of leaky-P wave modes, which typically have high amplitudes and are dispersive. We examine the frequency dispersion of borehole leaky-P modes and establish a minimum depth (approx 50-100 m) below the seafloor at each site where Vp can be accurately estimated using LWD data. Below this depth, Vp estimates from LWD sonic data compare well with wireline sonic logs and VSP interval velocities in nearby holes, but differ in detail due to local heterogeneity. We derive hydrate saturation using published models and the best estimate of Vp at these sites and compare results with independent resistivity-derived saturations.
Affiliation: OtherOther
URI: http://hdl.handle.net/2429/1619
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