Abstract. We offer a first-principle-based effective medium model for elastic-wave velocity in unconsolidated, high porosity, ocean bottom sediments containing gas hydrate. The dry sediment frame elastic constants depend on porosity, elastic moduli of the solid phase, and effective pressure. Elastic moduli of saturated sediment are calculated from those of the dry frame using Gassmann's equation. To model the effect of gas hydrate on sediment elastic moduli we use two separate assumptions: (a) hydrate modifies the pore fluid elastic properties without affecting the frame; (b) hydrate becomes a component of the solid phase, modifying the elasticity of the frame. The goal of the modeling is to predict the amount of hydrate in sediments from sonic or seismic velocity data. We apply the model to sonic and VSP data from ODP Hole 995 and obtain hydrate concentration estimates from assumption (b) consistent with estimates obtained from resistivity, chlorinity and evolved gas data.

.[ 1 ] We used ultrasonic pulse transmission to measure compressional, P, and shear,S , wave speeds in laboratory-formed polycrystalline ice Ih, sI methane hydrate, and sII methane-ethane hydrate. From the wave speed'sl inear dependence on temperature and pressure and from the sample'sc alculated density,w ed erived expressions for bulk, shear,and compressional wave moduli and Poisson'sratio from 20 to 5 Cand 22.4 to 32.8 MPa for ice Ih, 20 to 15 Cand 30.5 to 97.7 MPa for sI methane hydrate, and 20 to 10 Ca nd 30.5 to 91.6 MPa for sII methane-ethane hydrate. All three materials had comparable Pa nd Sw ave speeds and decreasing shear wave speeds with increasing applied pressure. Each material also showed evidence of rapid intergranular bonding, with ac orresponding increase in wave speed, in response to pauses in sample deformation. There were also key differences. Resistance to uniaxial compaction, indicated by the pressure required to compact initially porous samples, was significantly lower for ice Ih than for either hydrate. The ice Ih shear modulus decreased with increasing pressure, in contrast to the increase measured in both hydrates.

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Simultaneous measurements of compressional and shear wave speeds through polycrystalline methane hydrate have been made. Methane hydrate, grown directly in a wave speed measurement chamber, was uniaxially compacted to a final porosity below 2%. At 277 K, the compacted material's compressional wave speed was 3650 ± 50 m/s. The shear wave speed, measured simultaneously, was 1890 ± 30 m/s. From these wave speed measurements, we derive Vp/Vs, Poisson's Ratio, bulk, shear and Young's moduli.

We offer a first‐principle‐based, effective medium model for elastic‐wave velocity in unconsolidated, high porosity, ocean bottom sediments containing gas hydrate or free gas. The dry sediment frame elastic constants depend on porosity, elastic moduli of the solid phase, and effective pressure. Elastic moduli of saturated sediment are calculated from those of the dry frame using Gassmann's equation. To model the effect of gas hydrate on sediment elastic moduli we use two separate assumptions: (a) hydrate modifies the pore fluid elastic properties without affecting the frame and (b) hydrate becomes a component of the solid phase, reducing porosity and modifying the elasticity of the frame. The goal of the model is to predict the amount of hydrate in sediments from sonic or seismic velocity data. We apply the model to sonic and VSP data from ODP Hole 995 and obtain hydrate concentration estimates from assumption (b) that are consistent with estimates obtained from resistivity, chlorinity, and evolved gas data.