| Methane hydrate is an ice-like solid, a non-stochiometric mixture of water and the gas methane. It occurs worldwide in offshore sediments under appropriate temperature and pressure conditions. Marine methane hydrate is a possible hydrocarbon resource, a hazard to drilling and a source of a major greenhouse gas. Quantitative estimates of hydrate concentrations in marine sediment are difficult to obtain by conventional geophysical methods. While the base of hydrate deposits may stand out clearly on seismic sections as the Bottom Simulation Reflector (BSR), the upper boundary is not well delineated. Nor is there a signature from within the hydrate stability zone itself. Detection of hydrate deposits may therefore be accomplished through seismic surveys, but estimation of the total mass requires additional information.; In this study, a novel transient electric dipole-dipole technique was specifically designed for assessment of offshore methane hydrate deposits. The method is based on the assumption of a reduced electrical conductivity in hydrate rich zones. The apparatus includes one or more self-contained receivers, capable of measuring small variations in the electric field, linked in a linear array with a remote transmitter. The array can be suspended vertically in the water column or towed in contact with a relatively flat, sedimented seafloor. Field trials of the apparatus on the northern Cascadia margin have proven successful. In all three surveys, excellent data were collected.; Differential methods to reduce systematic errors were developed to analyse the data. Apparent resistivities on three lines in the vicinity of ODP site 889 demonstrate that the resistivity of the seafloor is remarkably uniform over the whole area. The average hydrate concentration, deduced with the aid of a reference model based on the electrical logs of ODP sites 888 and 889, is about 17–26% of pore space (9–13% of sediment volume) in the 100 m interval above the BSR. The values are consistent with those obtained by other analyses. The presence of hydrate is predicted in a region to the east of ODP hole 889B where there is no visible BSR. Further, significant variations in apparent resistivity are observed in an area near seismic line 89-05. |
