Seismic Studies of Methane Gas Hydrate, offshore Vancouver Island

Abstract

A seismic program in June 1993 on the continental slope off Vancouver Island focused on the bottom simulating reflector (BSR) formed at the base of a methane hydrate layer. Wide-angle ocean bottom seismometer (OBS) data and single-channel seismic data were collected in the vicinity of Hole 889B of the Ocean Drilling Program. Near-vertical to wide-angle arrivals from a 120-in.3 air gun were recorded during two deployments of five OBSs. A strong BSR was observed to offsets of about 3.6 km. Traveltime inversions were conducted for arrivals at four OBS sites, and the results show that the average velocity above the BSR is about 1.70-1.78 km/s in the 90 (±30) m interval above the BSR. Ray tracing constrained the vertical velocity gradient to about 1.0 km/s/km. Velocities above the BSR were considered higher than those of unhydrated sediments at equivalent depths, and provided estimates for hydrate saturations of 11 %-20% above the BSR. Approximately 1050 km of single-channel seismic data were digitally recorded from the 120-in.3 air gun, plus 110 km recorded from a 40-in.3 air gun. A grid of lines was acquired with the larger gun, at a nominal line spacing of 200 m. A map of BSR reflection amplitudes over this grid indicates the detailed variation in hydrate or free-gas distribution in the region of the drill hole. A major control on their distribution appears to be topography, with the strongest BSR occurring beneath a ridge formed by an anticlinal uplift of accretionary wedge sediments. The BSR response for different air gun sizes, including a large air gun array recorded during a previous multichannel survey, was compared along coincident lines. With increasing source frequency, the BSR continuity became poorer and reflection coefficients became smaller; as well, only a single BSR reflection was observed on all seismic sections. A thin layer with large hydrate or free-gas concentrations, a few meters in thickness, is thus unlikely to be present at the BSR. The different sources, which have different lateral resolutions or Fresnel zone widths, also indicate that hydrate may be discontinuous laterally at a scale of 200-400 m.