Multiple-Component Crystal Fabric Measurements from Acoustically-Generated Normal Modes in Borehole

Dan J Kluskiewicz, University of Washington Seattle Campus, Seattle, WA, United States, Edwin D Waddington, University of Washington, Department of Earth and Space Sciences, Seattle, WA, United States, Michael McCarthy, University of Washington, Department of Earth and Space Sciences, Seattle, United States, Sridhar Anandakrishnan, Pennsylvania State University, Department of Geosciences and Earth and Environmental Systems Institute, University Park, United States, Donald Voigt, Pennsylvania State University Main Campus, University Park, PA, United States and Kenichi Matsuoka, Norwegian Polar Institute, Glaciology and Geology, Tromsø, Norway
Abstract:
Sound wave velocities in ice are a proxy of crystal orientation fabric. Because p- and s-waves respectively travel faster and slower in the direction of an ice crystal c-axis, the velocities of these waves in a fabric are related to the clustering of ice crystal c-axes in the direction of wave propagation. Previous sonic logs at Dome C, NGRIP, WAIS, and NEEM have inferred a single component fabric description from the velocities of vertically-propagating p-waves around each ice core borehole. These records supplement thin-section measurements of crystal fabric by sampling larger numbers of crystals in a depth-continuous log.

Observations of azimuthally anisotropic vertical-girdle fabrics at ice-core sites such as WAIS, NGRIP, and EDML underly a benefit for logging methods that are sensitive to such fabrics. We present a theoretical framework for using borehole flexural modes to measure azimuthal crystal-fabric anisotropy, and describe ongoing efforts to develop a sonic logging tool for this purpose. We also present data from p-wave logs and thin section measurements at the WAIS Divide, and describe how a flexural wave log could supplement the existing measurements.

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