Variable Fluid Flow Along The Nova Scotian Slope - A Study Of Gas Hydrates With Ocean- Bottom Seismic Data

Bottom-simulating seismic reflectors (BSRs) as indicators of gas hydrate have been observed in the Scotian Basin (eastern continental margin of Canada), although the presence of hydrate is comparatively rare. Former studies near the Mohican Channel (100 miles offshore Halifax) show a BSR in that area approx. 350m below seafloor with an underlying low-velocity zone of generally 150 m thickness. Models of hydrate and gas as part of the sediment frame give concentrations of 2 to 6 % hydrate and less than 1 % free gas (LeBlanc et al., 2007). Where a BSR is present, these values are considered representative of the hydrate and gas concentrations in this passive-margin environment. In 2006, a joint project by the Geological Survey of Canada and Dalhousie University targeted gas hydrate near the Mohican Channel. In this area, a clear BSR near the side-wall of the channel disappears in a direction away from the channel and parallel to slope. However, there are no obvious changes in geology that would cause the distributions of the BSR to vary strongly in this direction. To try and detect lateral changes in physical properties (particularly those controlling upward fluid flow), and to determine why the BSR is so localised in this environment, 19 ocean-bottom-seismometers (OBS) were deployed along a 20 km line from the area of the BSR to the area with no BSR. Data from a 3D seismic cube (EnCana Ltd.) over the OBS locations and the nearby Torbrook geotechnical borehole show that the BSR dissipates where the density of near-vertical polygonal faulting increases. Lateral variation in hydrate and gas distributions implied by the change in the BSR, and the coincident change in polygonal faulting, points to a variable upward fluid flow along slope. With the wide-angle data we attempt to ascertain velocities above and below the BSR at different distances from the Mohican Channel, and to use the inferred hydrate and gas concentrations to calculate the variation in upward fluid flow along the slope. This variation is linked to shear-wave splitting estimated from the OBS data by relating changes in shear-wave splitting to changes in the density of cracks.