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GEOPHYSICAL RESEARCH LETTERS, VOL. 30, NO. 14, 1733, doi:10.1029/2003GL017477, 2003

Seismic anisotropy in gas-hydrate- and gas-bearing sediments on the Blake Ridge, from a walkaway vertical seismic profile

Ingo A. Pecher

University of Texas, Austin, Texas, USA


W. Steven Holbrook

University of Wyoming, Laramie, Wyoming, USA


Mrinal K. Sen

University of Texas, Austin, Texas, USA


Daniel Lizarralde

Georgia Institute of Technology, Atlanta, Georgia, USA


Warren T. Wood

Naval Research Laboratory, Stennis Space Center, Mississippi, USA


Deborah R. Hutchinson

U.S. Geological Survey, Woods Hole, Massachusetts, USA


William P. Dillon

U.S. Geological Survey, Woods Hole, Massachusetts, USA


Hartley Hoskins

Woods Hole Oceanographic Institution, Woods Hole, Massachusetts, USA


Ralph A. Stephen

Woods Hole Oceanographic Institution, Woods Hole, Massachusetts, USA


Abstract

We present results from an analysis of anisotropy in marine sediments using walkaway vertical seismic profiles from the Blake Ridge, offshore South Carolina. We encountered transverse isotropy (TI) with a vertical symmetry axis in a gas-hydrate-bearing unit of clay and claystone with Thomsen parameters ɛ = 0.05 ± 0.02 and δ = 0.04 ± 0.06. TI increased to ɛ = 0.16 ± 0.04 and δ = 0.19 ± 0.12 in the underlying gas zone. Rock physics modeling suggests that the observed TI is caused by a partial alignment of clay particles rather than high-velocity gas-hydrate veins. Similarly, the increase of TI in the gas zone is not caused by thin low-velocity gas layers but rather, we speculate, by the sharp contrast between seismic properties of an anisotropic sediment frame and elongated gas-bearing pore voids. Our results underscore the significance of anisotropy for integrating near-vertical and wide-angle seismic data.

Received 13 April 2003; accepted 21 May 2003; published 17 July 2003.

Index Terms: 0915 Exploration Geophysics: Downhole methods; 0910 Exploration Geophysics: Data processing; 0994 Exploration Geophysics: Instruments and techniques; 3025 Marine Geology and Geophysics: Marine seismics (0935); 5102 Physical Properties of Rocks: Acoustic properties.

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Citation: Pecher, I. A., W. S. Holbrook, M. K. Sen, D. Lizarralde, W. T. Wood, D. R. Hutchinson, W. P. Dillon, H. Hoskins, and R. A. Stephen (2003), Seismic anisotropy in gas-hydrate- and gas-bearing sediments on the Blake Ridge, from a walkaway vertical seismic profile, Geophys. Res. Lett., 30(14), 1733, doi:10.1029/2003GL017477.