Gas hydrates are known to exist at several deep ocean-floor locations across the continental margin of the eastern United States. Hydrates are stable in sediments having sufficient gas saturations at suitable pressures and temperatures. These conditions usually confine the hydrate to the uppermost few hundred meters of sediment below the sea floor. At greater depths and increased temperatures, free gas occurs and is probably trapped by the overlying hydrate zone. The transition boundary between the hydrate zone and free-gas zone is a relatively large impedance contrast to seismic sound waves and, hence, produced a high-amplitude reflection on multichannel seismic data.

Regional multichannel seismic reflection profiles have been collected by the U.S. Geological Survey along the continental shelf from Cape Hatteras to the Blake Outer Ridge area. These profiles reveal several major and several minor seismic-amplitude anomalies that parallel the sea floor. Depth maps of the high-amplitude reflections were constructed to determine whether free gas is trapped by an impermeable gas hydrate layer, or by local structural or stratigraphic traps. In some places, derivation of interval velocities has produced abnormally low velocities for the free gas layer. Investigation of the high-amplitude reflectors shows that they are not distinct, but consist of several interfering phases which complicate the selection of correct velocities.

Two-dimensional seismic modeling was used as an aid for determining the proper velocity estimating technique. Depth maps of the high amplitude reflections demonstrate the distribution of these anomalous features and strongly suggest the formation of gas hydrates.

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