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Winters, W. J., W. F. Waite, and David H. Mason,
Effects of Methane Hydrate on the Physical Properties of Sediments
William J. Winters,1 William F. Waite,2 David H. Mason3
1U.S. Geological Survey, Woods Hole, Massachusetts, U.S.A.
2U.S. Geological Survey, Woods Hole, Massachusetts, U.S.A.
3U.S. Geological Survey, Woods Hole, Massachusetts, U.S.A.
The authors thank L. Poppe and E. Sundquist for their helpful reviews. This work was supported by the Coastal and Marine Geology Program and Energy Program of the U.S. Geological Survey, and partial funding was provided by the Gas Hydrate Program of the U.S. Department of Energy.
Grain size, pore content, and arrangement of pore constituents have a profound effect on acoustic and strength properties of sediments. We tested specimens containing gas hydrate, methane, and water in the pore space of coarse- and fine-grained sediments to simulate the marine environment and of frozen coarse-grained sediment to simulate permafrost conditions.
The measured compressional wave velocity (Vp) changes with the extent to which the pore material cements sediment grains. Hence, for equal effective stresses, Vp is lowest in gas-charged sediments, increases for water-saturated sediments, then increases significantly for hydrate-bearing sediments because of sediment cementation provided by hydrate. Frozen sediment, effectively fully saturated and fully cemented sediment, exhibits the highest Vp.
Sediment strength follows the same pattern but also shows a strong dependence on sediment grain size. For consolidation stresses associated with the upper several hundred meters of subbottom depth, pore pressures decreased during shear in coarse-grained sediments containing gas hydrate, thereby increasing strength, whereas pore pressure in fine-grained sediments typically increased during shear, which decreased strength. The presence of free gas in pore space damped the pore-pressure response during shear and reduced the strengthening effect of gas hydrate in sands.
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