Marine gas hydrates, prevalent in offshore sediments in Canada and Japan, are a possible hydrocarbon resource, a hazard to drilling and a source of a major green-house gas. Quantitative estimates of hydrate concentrations in deep sea sediment are difficult to obtain by conventional methods. We have sought novel techniques specifically designed for assessment such as transient electric dipole-dipole electromagnetics. The latter method is based on the assumption of a reduced electrical conductivity in hydrate rich zones. Field trails of new apparatus on the Cascadia margin have proven successful. Excellent data were collected and analyzed using a differential phase method to reduce systematic error. Apparent resistivities collected on three lines demonstrate that the resistivity of the seafloor is remarkably uniform over the whole survey area. The average hydrate concentration, deduced with the aid of a reference model based on the electrical logs of ODP holes 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. Further, the presence of hydrate is predicted in a region to the east of 889B where there is no visible BSR. ¿ 2000 American Geophysical Union