Members of the U.S. Geological Survey (USGS) and our colleagues have participated in a number of gas-hydrate field programs in order to study the relationship between the occurrence of natural gas hydrate and physical properties of surrounding sediments. USGS personnel are also simulating in situ conditions during laboratory gas hydrate experiments performed on: (a) reconstituted sediment samples, and (b) natural samples recovered from wells drilled in the Canadian Arctic and offshore.

　　A number of factors, including gas type and quantity, pore water pressure and salinity, and geothermal characteristics influence the type, location, and amount of gas hydrate formed in a region. Physical properties of the host sediment or reservoir also exert great influence on natural gas hydrate formation and as such, dictate whether deposits in a particular area may be economically recoverable.

　　In addition to determining acoustic velocity, shear strength, permeability, and index properties of sediments containing natural and laboratory-formed gas hydrate, we are analyzing the capabilities of different laboratory techniques to mimic natural behavior in sediment. We have formed hydrate in both initially water-saturated and partly water-saturated, sieved Ottawa sand specimens at a pore pressure of 12 MPa and typically at a 250 kPa confining stress. Acoustic velocity, shear strength, and permeability of samples with different initial water saturations are significantly affected by the amount of hydrate present and its distribution. Acoustic P-wave velocities increased from a baseline 1.7 km/s to 4.0 km/s using different hydrate and sample formation techniques. Although acoustic properties are significantly affected by hydrate present at grain boundaries, the mere presence of hydrate in pores (but not necessarily at grain contacts) causes strength to significantly increase because dilation during undrained shear results in greater negative pore pressures. Recently determined strengths of three Ottawa sand samples are related to the degree of water saturation, bulk density, and hydrate content.

　　With financial support from the U.S. Dept. of Energy, the Institute Polaire Francais (IPEV) and the USGS jointly conducted a cruise aboard the Marion Dufresne to collect giant piston cores for use in determining the deep subbottom gas hydrate distribution in the northern Gulf of Mexico. Surficial (within 8 m of the seafloor) hydrates recovered during the cruise did not appear to be lithologically controlled and were either disseminated or comprised massive layers. Numerous core properties have been determined. Early results show that water content typically decreases rapidly to a subbottom depth of about 8 to 9 m, but then decreases at a lower rate below that depth. Because gas hydrate formation requires a source of water, observations such as these are important facets of gas hydrate research.