Near-offset vertical seismic experiments during Leg 204

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doi: 10.2973/odp.proc.sr.204.120.2006
Author(s): Tréhu, Anne M.; Bangs, Nathan L.; Guèrin, Gilles
Ocean Drilling Program, Leg 204, Shipboard Scientific Party, College Station, TX
Author Affiliation(s): Primary:
Oregon State University, College of Oceanic and Atmospheric Sciences, Corvallis, OR, United States
Other:
Christian-Albrechts-Universität zu Kiel, Federal Republic of Germany
Joint Oceanographic Institutions, United States
University of Texas at Austin, United States
University of Leicester, United Kingdom
Eastern Kentucky University, United States
U. S. Geological Survey, United States
Lamont-Doherty Earth Observatory, United States
Volume Title: 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
Volume Author(s): Tréhu, 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.; Gràcia, Eulàlia; Guèrin, 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.
Source: 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, Anne M. Tréhu, Gerhard Bohrmann, Marta E. Torres, Frank R. Rack, Nathan L. Bangs, Samantha R. Barr, Walter S. Borowski, George E. Claypool, Timothy S. Collett, Mark E. Delwiche, Gerald R. Dickens, David S. Goldberg, Eulàlia Gràcia, Gilles Guèrin, Melanie Holland, Joel E. Johnson, Young-Joo Lee, Char-Shine Liu, Philip E. Long, Alexei V. Milkov, Michael Riedel, Peter Schultheiss, Su Xin, Barbara Teichert, Hitoshi Tomaru, Maarten Vanneste, Mahito Watanabe, Jill L. Weinberger, Antje Boetius, Fred J. Brockman, Annette Deyhle, Udo Fehn, Peter B. Flemings, Peter R. Girguis, Martin Heesemann, Samantha B. Joye, Thomas D. Lorenson, Christopher T. Mills, Robert J. Musgrave, Radu Popa, Bill Ussler, Heinz Wilkes, Gisela Winckler and William J. Winters; Ocean Drilling Program, Leg 204, Shipboard Scientific Party, College Station, TX. Proceedings of the Ocean Drilling Program, Scientific Results (CD ROM), Vol.204, 23p. Publisher: Texas A&M University, Ocean Drilling Program, College Station, TX, United States. ISSN: 1096-2514
Note: In English. Available only on CD-ROM in PDF format and on the Web in PDF or HTML; includes appendix. 14 refs.CD-ROM format, ISSN 1096-2514; WWW format, ISSN 1096-7451; illus., incl. sects., 2 tables, sketch map
Summary: 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.
Year of Publication: 2005
Research Program: ODP Ocean Drilling Program
Key Words: 20 Geophysics, Applied; 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
Coordinates: N443400 N443500 W1250400 W1250900
Record ID: 2007035113
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