Hydrate distribution off Vancouver Island from multifrequency single-channel seismic reflection data

On the northern Cascadia margin, single channel seismic data clearly image the bottom-simulating reflector (BSR) that marks the base of the hydrate st ability zone. Airguns with three distinct source frequencies (30, 75, and 1 50 Hz) were recorded along several coincident lines, and at all frequencies the BSR appears as a single pulse. BSR reflection coefficients are slightl y larger for the 30-Hz data than for the 75- or 150-Hz data. Multifrequency analysis indicates that the BSR is due to a single sharp interface at the base of the hydrate zone, whereas the velocity structure away from the inte rface must be gradationally-varying so as to produce no separate reflection and no tuning effects at the 75- and 150-Hz frequencies. In contrast, the seafloor reflection in some areas has significantly larger amplitudes for d ata recorded with the 75-Hz source, relative to the 30-Hz source. Synthetic seismogram analysis of the two data sets shows that the high amplitudes ca n be produced by a 2-m-thick high-velocity layer at the seafloor, with velo cities and densities corresponding to those of carbonate. Using the 75-Hz s ource, seafloor and BSR amplitudes and reflection coefficients were measure d over a tight grid of lines, nominally spaced at 200 m. Maximum BSR reflec tion coefficients of 0.15-0.18 were observed beneath topographic highs, whi le maximum seafloor reflection coefficients of up to 0.5-0.6 were found on the flanks of the topographic highs. This suggests that topography provides a major control on the flow of methane-bearing fluids. Amplitude measureme nts, converted to values of velocity above the BSR, were used to estimate h ydrate concentrations, which reaches a maximum of 15-18% of the total sedim ent volume with mean values near 5-10%. Over the 100 km(2) survey area, the volume of methane gas in the hydrate reservoir is estimated as 6.4 x 10(10 ) m(3), or 2.1 trillion cubic feet.