Sonic velocities in an active gas hydrate system, Hydrate Ridge

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doi: 10.2973/odp.proc.sr.204.124.2006
Author(s): Guèrin, Gilles; Goldberg, David S.; Collett, Timothy S.
Ocean Drilling Program, Leg 204, Shipboard Scientific Party, College Station, TX
Author Affiliation(s): Primary:
Lamont-Doherty Earth Observatory, Palisades, NY, 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
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, 38p. 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. 25 refs.CD-ROM format, ISSN 1096-2514; WWW format, ISSN 1096-7451; illus., incl. 1 table, sketch map
Summary: During Ocean Drilling Program Leg 204, sonic logs were recorded in six holes across the southern summit of Hydrate Ridge, to measure in situ the sonic velocities of the sediments in an active gas hydrate system. In addition, vertical seismic profiles were acquired in four of these holes to help the integration of the borehole data within the high-resolution three-dimensional seismic survey of the area. Synthetic seismograms generated from the velocity and the density logs confirm the depth and the nature of the main reflectors identified in the seismic survey. In particular, the bottom-simulating reflector (BSR) was reached in five of the wells and is adequately reproduced by the synthetic seismograms. The most prominent horizons under the crest of Hydrate Ridge, Horizons A, B, and B', are also reproduced by the synthetics for the wells where they were present. The lower amplitudes of the synthetic reflections suggest that part of the free gas carried by these horizons, which have been identified as conduits to the Hydrate Ridge reservoir, escaped during drilling operation. We use a model for the cementation of grains by hydrate formation to estimate gas hydrate saturations above the BSR and the Gassmann model with fluid substitution to estimate free gas saturation below the BSR. These elastic models suggest that gas hydrate occupies ∼10%-20% of the pore space over the BSR in the holes closest to the crest (Holes 1245E, 1247B, and 1250F) and that gas hydrate saturations further on the flank are <10%. These results agree well overall with saturations derived from resistivity logs by Archie's method but indicate a less heterogeneous gas hydrate distribution than the resistivity logs. The Gassmann model identifies up to 2% of free gas in Horizons A, B, and B' but also indicates pervasive presence of free gas below the BSR in the holes closest to the crest, 1245E, 1247B, and 1250F. In all the holes, the presence of gas hydrate or free gas results in the reduction of sonic logging waveform amplitudes, most prominently the high-frequency dipole waveforms.
Year of Publication: 2005
Research Program: ODP Ocean Drilling Program
Key Words: 07 Marine Geology and Oceanography; 20 Geophysics, Applied; Acoustical logging; Bottom-simulating reflectors; Cascadia subduction zone; East Pacific; Gas hydrates; Geophysical methods; Geophysical profiles; Geophysical surveys; Hydrate Ridge; Leg 204; North Pacific; Northeast Pacific; Ocean Drilling Program; Ocean floors; Pacific Ocean; Seismic methods; Seismic profiles; Seismograms; Surveys; Synthetic seismograms; Velocity; Vertical seismic profiles; Waveforms; Well-logging
Coordinates: N443400 N443500 W1250400 W1250900
Record ID: 2007035112
Copyright Information: GeoRef, Copyright 2018 American Geosciences Institute.