HR: 0800h
AN: GP11A-0813    [Abstracts]
TI: Marine Controlled Source Electromagnetics for Gas Hydrate Evaluation on the Cascadia Margin: Correlation Between Resistivity Anomalies and Seismic Blank Zones
AU: * Schwalenberg, K
EM: katrin@physics.utoronto.ca
AF: University of Toronto, Department of Physics 60 St. George St, Toronto, ON M5S 1A7 Canada
AU: Edwards, R N
EM: edwards@core.physics.utoronto.ca
AF: University of Toronto, Department of Physics 60 St. George St, Toronto, ON M5S 1A7 Canada
AU: Willoughby, E C
EM: ele.willoughby@nrcan.gc.ca
AF: Pacific Geoscience Centre, Geological Survey of Canada 9860 West Saanich Road, Sidney, BC V8L4B2 Canada
AU: Mir, R
EM: rmir@physics.utoronto.ca
AF: University of Toronto, Department of Physics 60 St. George St, Toronto, ON M5S 1A7 Canada
AU: Yuan, J
EM: jian@physics.utoronto.ca
AF: University of Toronto, Department of Physics 60 St. George St, Toronto, ON M5S 1A7 Canada
AU: Spence, G
EM: gspence@uvic.ca
AF: University of Victoria, School of Earth and Ocean Science, Victoria, BC V8W 2Y2 Canada
AB: The gas hydrate deposits on the Cascadia Margin have been the focus of a vast number of projects and marine experiments to investigate the distribution and concentration of this potential future energy resource. Gas hydrate or frozen gas consists mainly of methane and water molecules. It is stable at low temperatures and high pressures and forms in pore space within the hydrate stability zone in marine sediments. Hydrate itself is electrical insulating and replaces conductive pore fluid, which subsequently increases the bulk resistivity of a hydrate formation. Accurate measurements of the seafloor resistivity can be a useful tool for hydrate estimation, which is essential for resource and environmental hazard evaluation. The instrument - a development of the University of Toronto - is basically an inline dipole-dipole configuration dragged along the seafloor. It has been successfully applied in previous experiments on the Cascadia Margin and on the Chilean Margin. Here we present a promising new data set collected in summer 2004. Measurements have been conducted along three profiles. For the first time data have been collected with the marine controlled source electromagnetic method in water depths shallower than required for hydrate stability (i.e. below 500m). These data are important as a reference site for hydrate assessment. The second profile covers the bullseye, a vent site in vicinity of ODP site 889B that correlates with a region of seismic blanking and recovered hydrate outcrops. The 7km long profile also covers another series of seismically identified vent sites. The average amplitudes of the measured electric fields and thus the related apparent resistivities along the first profile (no hydrate) are smaller than along the second profile (vent sites). This is consistent with the idea of an increased resistivity in hydrate rich zones. Two pronounced anomalies occur along the second profile in spatial agreement with the bullseye and the other series of vent sites. The third profile intersects the second profile at the bullseye. However, beside of a careful lowering procedure, the array was likely tangled on the seafloor for most parts of the deployment which complicates or even inhibits the interpretation, but points out the importance of a careful coordination of ships navigation and instrument deployment as well as weather and sea conditions for a successful experiment.
UR: http://www.physics.utoronto.ca/~edwards/
DE: 3000 MARINE GEOLOGY AND GEOPHYSICS
DE: 0600 ELECTROMAGNETICS
DE: 0930 Oceanic structures
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2004 AGU Fall Meeting