Gas hydrates are solids comprised of a hydrogen-bonded water lattice with entrapped guest molecules of gas. There are convincing arguments that vast amounts of methane gas hydrate are present in sediments under the world's oceans as well as in on-shore sediments in the Arctic. This hydrate is possibly the largest carbon and methane pool on earth. As such, methane hydrate may be the principal factor in global climate balancing. One may also treat this methane pool as a potential energy source. These considerations ignite the scientific and business community’s interest in quantifying the amount of methane hydrate in the subsurface.
　　Gas hydrate reservoir characterization is, in principle, no different from the traditional hydrocarbon reservoir characterization. Similar and well-developed remote sensing techniques can be used, seismic reflection profiling being the dominant among them.
　　Seismic response of the subsurface is determined by the spatial distribution of the elastic properties. By mapping the elastic contrast, the geophysicist can illuminate tectonic features and geobodies, hydrocarbon reservoirs included. To accurately translate elastic-property images into images of lithology, porosity, and the pore-filling phase, quantitative knowledge is needed that relates rock’s elastic properties to its bulk properties and conditions. Specifically, to quantitatively characterize a natural gas hydrate reservoir, we must be able to relate the elastic properties of the sediment to the volume of gas hydrate present and, if at all possible, the permeability. One way of achieving this goal is through rock physics effective-medium modeling.
　　Our effective-medium model for sediment with gas hydrate is based on an approach which relates the elastic moduli of soft unconsolidated clastic sediment to the porosity, pore fluid compressibility, mineralogy, and effective pressure. The model assumes that at the critical porosity of 30%-40%, the effective elastic moduli of the dry mineral framework of the sediment can be calculated using the Hertz-Mindlin contact theory for elastic particles. This end point is connected with the zero-porosity, pure mineral, end point by the modified lower Hashin-Shtrikman (HS) bound which is appropriate for the description of uncemented rock. For porosity above the critical porosity, the critical porosity end point is connected with the 100% porosity end point (zero elastic moduli) by the modified upper HS bound. Once the dry-frame elastic moduli are known, those of the saturated sediment are calculated using Gassmann’s fluid substitution. The hydrate is simply treated as part of the load-bearing frame, i.e., its presence acts to reduce the porosity and, at the same time, alter the elastic properties of the composite solid matrix phase. The net effect is an increase in the P- and S-wave velocity in water-saturated rock where part of the pore space is filled with gas hydrate.
　　We present data that validate the model and show how this model can be used to calculate the porosity and methane hydrate concentration in a reservoir from seismic-derived elastic properties.