Trehu, Anne M. et al. (2005): Near-offset vertical seismic experiments during Leg 204

Leg/Site/Hole:
ODP 204
Identifier:
2007-035113
georefid

10.2973/odp.proc.sr.204.120.2006
doi

Creator:
Trehu, Anne M.
Oregon State University, College of Oceanic and Atmospheric Sciences, Corvallis, OR, United States
author

Bangs, Nathan L.
Christian-Albrechts-Universitaet zu Kiel, Federal Republic of Germany
author

Guerin, Gilles
Joint Oceanographic Institutions, United States
author

Identification:
Near-offset vertical seismic experiments during Leg 204
2005
In: Trehu, Anne M., Bohrmann, Gerhard, Torres, Marta E., Rack, Frank R., Bangs, Nathan L., Barr, Samantha R., Borowski, Walter S., Claypool, George E., Collett, Timothy S., Delwiche, Mark E., Dickens, Gerald R., Goldberg, David S., Gracia, Eulalia, Guerin, Gilles, Holland, Melanie, Johnson, Joel E., Lee, Young-Joo, Liu, Char-Shine, Long, Philip E., Milkov, Alexei V., Riedel, Michael, Schultheiss, Peter, Su Xin, Teichert, Barbara, Tomaru, Hitoshi, Vanneste, Maarten, Watanabe, Mahito, Weinberger, Jill L., Boetius, Antje, Brockman, Fred J., Deyhle, Annette, Fehn, Udo, Flemings, Peter B., Girguis, Peter R., Heesemann, Martin, Joye, Samantha B., Lorenson, Thomas D., Mills, Christopher T., Musgrave, Robert J., Popa, Radu, Ussler, Bill, Wilkes, Heinz, Winckler, Gisela, Winters, William J., Proceedings of the Ocean Drilling Program; scientific results; drilling gas hydrates on Hydrate Ridge, Cascadia continental margin; covering Leg 204 of the cruises of the drilling vessel JOIDES Resolution; Victoria, British Columbia, Canada, to Victoria, British Columbia, Canada; Sites 1244-1252; 7 July-2 September 2002
Texas A&M University, Ocean Drilling Program, College Station, TX, United States
204
Three successful vertical seismic profiles (VSPs) were acquired during Ocean Drilling Program (ODP) Leg 204 at South Hydrate Ridge. The data confirm earlier results from ocean bottom seismometer data and analysis of moveout from common midpoint reflection data that the average velocity between the seafloor and the bottom-simulating reflector (BSR) is <1600 m/s throughout the region and is lowest near the summit, where the amount of hydrate is greatest. This result supports the conclusions that free gas and hydrate coexist beneath the summit and that the average amount of gas hydrate present elsewhere is low. The data also indicate that low-velocity zones (LVZs) resulting from free gas beneath the BSR must be thin and stratigraphically controlled. The only LVZ resolvable from traveltime analysis of the VSP data is associated with Horizon A, which has been interpreted to be the primary conduit transporting free gas to vents at the summit of South Hydrate Ridge. Thin LVZs associated with Horizons B and B', however, are indicated by sonic logs as well as by strong negative polarity reflections in the multichannel seismic data. This limited distribution of sub-BSR free gas contrasts with previous results at North Hydrate Ridge (Leg 146) and Blake Ridge (Leg 164), which indicate the presence of free gas zones several hundred meters thick that result in distinct LVZs in the VSP data from those earlier ODP legs.
English
Coverage:Geographic coordinates:
North:44.3500
West:-125.0900East: -125.0400
South:44.3400

Applied geophysics; acoustical logging; bottom-simulating reflectors; Cascadia subduction zone; common-depth-point method; East Pacific; geophysical methods; geophysical profiles; geophysical surveys; Hydrate Ridge; Leg 204; low-velocity zones; North Pacific; Northeast Pacific; Ocean Drilling Program; ocean floors; Pacific Ocean; reflection methods; seismic methods; seismic profiles; surveys; velocity; vertical seismic profiles; well-logging;

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