Deep‐tow Seismic Investigations of Methane Hydrates

High-resolution deep-tow multichannel seismic technology, developed by the Naval Research Laboratory, has been used to study natural gas hydrates in the marine environment. The 48-channel deep-tow system uses a 250-650 Hz source; the source and receiver array are towed approximately 300 m above the, seafloor. The combination of the geometry achieved and source frequency allows us to study the geologic structure and compressional velocities within the Hydrate Stability Zone (HSZ) with significantly greater resolution than any other available technique. The data taken with this system have revealed features that require modifications of simple models for the HSZ. For example, extensive faulting has been resolved that extends from the seafloor to the base of the HSZ. Further, compressional velocity estimates obtained from these data suggest vertical zones of elevated velocity that appear to be associated with these faults. This is consistent with chemical/hydrologic models that indicate hydrates are created preferentially where fluid flux rates are high enough to exceed saturation. Data acquired at two locations on the Blake Ridge off the east coast of the U. S. and on the Cascadia margin off the Canadian west coast exhibit significantly different seismic character, implying different manifestations of flux through the HSZ. The high-resolution seismic data indicate the need for a 3-D, dynamic model of the HSZ to more fully explain the generation and dissociation of natural gas hydrates.