Abstract:
A detailed single channel seismic reflection survey of a region on the Juan de Fuca continental margin, 50 km offshore Vancouver Island, with a clear gas hydrate bottom-simulating reflector has been analyzed and interpreted. The results have been integrated with previously obtained multichannel seismic and Ocean Drilling Program borehole geophysical data. Seismic data includes tightly spaced single channel data collected in the immediate vicinity of Site 889 during Pacific Geoscience Centre (Geologic Survey of Canada) Cruise PGC93002 in June, 1993, and 100% coverage versions of multichannel lines collected in 1989. Borehole data includes sonic and density logs, drill core density estimates, and vertical seismic profile velocity estimates from Ocean Drilling Program Site 889. Approximately 610 km of single channel data were acquired in two tightly spaced grid patterns, GRID A and GRID B, with survey lines in each grid nominally spaced 200 m and 100 m apart. respectively. GRID A consists of 41 lines. 12 km long ( approximately 500 km total), and covers approximately 100 km (super 2) . GRID A used a 1.97 L (120 in. (super 3) ) airgun as its acoustic source which generated a dominant frequency of 75 Hz. GRID B consists of 28 lines, 4 km long ( approximately 110 km) and covers approximately 11 km (super 2) . These data were acquired with a 0.65 L (40 in. (super 3) ) airgun, yielding a dominant frequency of 150 Hz. The 100% coverage lines were acquired with an airgun array of total volume 128.2 L (7820 in. (super 3) ). This array produced a dominant acoustic frequency of 30 Hz. The seismic survey target was the bottom simulating reflector (BSR) which corresponds to the base of the methane hydrate stability field, estimated at less than 240 mbsf in the study area. The BSR response to varying source frequencies has been examined and clearly suggests that the required negative impedance contrast is derived from a combination of high-velocity hydrated sediments above the BSR, and low velocity gas-bearing sediments below. Tuning width synthetic seismogram modeling has shown that these layers can not be thin (i.e., <25 m), and that each layer must have a gradational boundary away from the BSR, since no sharp top to the hydrated sediment layer or sharp bottom to the gas-bearing sediment layer is observed on seismic data of any available frequency. Reflection coefficient analysis shows a high degree of variability in reflector strength across the study area, and has established that local highs in seafloor and BSR reflectivity are closely correlated to local maxima in seafloor topography. The strongest reflections occur beneath a ridge formed by an anticlinal uplift of accretionary wedge sediments. Reflection coefficients as high as 0.20 for the BSR, and as high as 0.60 for the seafloor have been calculated in these areas. Synthetic models suggest that the high seafloor reflection coefficients are associated with regions where methane-bearing fluids reaching the seafloor have reacted with seawater to produce a thin layer of carbonate pavement. High reflection coefficient values for the BSR have been interpreted as regions where increased sediment hydration has occurred due to an inferred increased fluid flux. Hydrate concentration has been estimated from the BSR reflection coefficients and suggests that the hydrated sediments above the BSR, for the 100 km (super 2) of GRID A, contain approximately 3.92X10 (super 8) m (super 3) of methane hydrate, which yields a methane gas volume at standard temperature and pressure of 6.43X10 (super 10) m (super 3) (2.07 tcf).