Seismic velocities on the Nova Scotian margin to estimate gas hydrate and free gas concentrations |
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| Institution: | 1. Natural Resources Canada, Geological Survey of Canada (Atlantic), NS, Canada;2. College of Marine Sciences, Shanghai Ocean University, Shanghai, China;3. Earth and Environmental Sciences, Memorial University of Newfoundland, NL, Canada;4. Tragsa-SGM, Julian Camarillo 6b, 28037 Madrid, Spain;1. U.S. Geological Survey, Woods Hole Coastal and Marine Science Center, 384 Woods Hole Road, Woods Hole, MA 02543-1598, USA;2. U.S. Geological Survey, St. Petersburg Coastal and Marine Science Center, 600 Fourth Street South, St. Petersburg, FL 33701, USA;3. Center for Marine and Wetland Studies, Coastal Carolina University, 1270 Atlantic Avenue, Conway, SC 29526, USA |
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| Abstract: | This article provides new constraints on gas hydrate and free gas concentrations in the sediments at the margin off Nova Scotia. Two-dimensional (2-D) velocity models were constructed through simultaneous travel-time inversion of ocean-bottom seismometer (OBS) data and 2-D single-channel seismic (SCS) data acquired in two surveys, in 2004 and 2006. The surveys, separated by ～5 km, were carried out in regions where the bottom-simulating reflection (BSR) was identified in seismic reflection datasets from earlier studies and address the question of whether the BSR is a good indicator of significant gas hydrate on the Scotian margin. For both datasets, velocity increases by 200–300 m/s at a depth of approximately 220 m below seafloor (mbsf), but the results of the 2006 survey show a smaller velocity decrease (50–80 m/s) at the base of this high-velocity layer (310–330 mbsf) than the results of the 2004 survey (130 m/s). When converted to gas hydrate concentrations using effective medium theory, the 2-D velocity models for both datasets show a gas hydrate layer of ～100 m thickness above the identified BSR. Gas hydrate concentrations are estimated at approximately 2–10% for the 2006 data and 8–18% for the 2004 survey. The reduction in gas hydrate concentration relative to the distance from the Mohican Channel structure is most likely related to the low porosity within the mud-dominant sediment at the depth of the BSR. Free gas concentrations were calculated to be 1–2% of the sediment pore space for both datasets. |
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