NATURAL-GAS HYDRATES ON THE SOUTHEAST US MARGIN - CONSTRAINTS FROM FULL-WAVE-FORM AND TRAVEL-TIME INVERSIONS OF WIDE-ANGLE SEISMIC DATA

Strong bottom-simulating reflectors (BSR) have been mapped over a regi on of approximately 50,000 km(2) on the southeastern U.S. margin and h ave been associated with possible abundance of natural gas hydrates. I n June 1992, coincident single-channel seismic and wide-angle ocean bo ttom seismic data were acquired in the region, focusing on the Blake R idge and the Carolina Rise. Wide-angle reflections from the BSRs were clearly observed at offsets up to similar to 6 km. Joint travel time i nversion was conducted with wide-angle and vertical-incidence data in order to explore possible regional variation, and the resultant two-di mensional average velocity models imply higher background velocities o n the Carolina Rise. Full waveform inversion was then performed to det ermine the seismic origin of the BSRs. The best fit model shows a simi lar low velocity (similar to 1.4 km/s) beneath the BSR at both sites, indicating trapped free gas with low saturation (<10%). The inversion results also indicate that a thin, high-velocity wedge, with a maximum velocity of similar to 2.3 km/s, is present just above the Blake Ridg e BSR. Sediment reflectivities were also calculated, and higher reflec tivities are observed on the Carolina Rise, An increase in reflectivit y below the BSR seems to correspond to the gas-bearing zone at both si tes. Concentration of hydrates were estimated based on these velocity models. Whereas average hydrate concentration of 3% of the total sedim ent volume is suggested for the lower half of the hydrate stability zo ne at the Blake Ridge, only a very low average concentration of hydrat e can be expected at the Carolina Rise. The hydrate seem to be concent rated near the base of the hydrate stability zone, and the maximum hyd rate concentration is estimated as similar to 20% at the Blake Ridge a nd similar to 7% at the Carolina Rise, both of which are too high to b e explained by in situ biogenic activity only and require some seconda ry accumulation mechanism. It is suggested that hydrate recycling caus ed by the stability field migration may have effectively condensed hyd rates at both sites. Additional enhancement by upward fluid expulsion may also be viable for the Blake Ridge.