Abstract:
A high resolution multichannel seismic survey was carried out using the Deep-Tow Acoustics/Geophysics System (DTAGS) to investigate deep sea gas hydrates in the vicinity of Ocean Drilling Program Site 889/890 on the continental slope off Vancouver Island. The near-bottom DTAGS geometry and high frequency (250-650 Hz) source provide vertical and horizontal resolutions of the order of approximately 2 m and approximately 25 m, respectively, that allow much higher resolution of the structure in the upper approximately 400 m sediments than can be obtained from standard surface-towed multichannel data. The DTAGS high resolution sections resolve fine scale layering as thin as a few meters within the hydrated zone and below the Bottom Simulating Reflector (BSR), small scale faults, and vertically oriented zones with no reflectors (blanking) that may represent channels for upward expulsion of fluid or gas. The BSR is well defined in low frequency data at a depth of about 250 mbsf over much of this region, but it has a very low amplitude at the higher DTAGS frequencies. Interval velocities determined from the seismic data indicate values of about 1500 m/s to depths of 100 mbsf, increasing to about 1850 m/s at the BSR. Combined with the results of previous lower frequency seismic surveys in this area, the new DTAGS data provide the means to determine the frequency dependence of the BSR over a broad band from 15 to 650 Hz. Based on all the survey data, a model was inferred for the velocity profile BSR that is consistent with the reflectivity over the band. The model consists of a 6-m negative gradient layer at the BSR in which the velocity decreases by 250 m/s. The profile is gradational over vertical scales that are small compared to the acoustic wavelengths at low frequencies, but large with respect to the wavelengths at the DTAGS frequencies. Consequently the model predicts reflectivity from essentially a single interface at low frequencies but a much weaker reflection at high frequencies.