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

Friday, 19 December 2014
Dan J Kluskiewicz1, Edwin D Waddington2, Michael McCarthy3, Sridhar Anandakrishnan4, Donald Voigt4 and Kenichi Matsuoka5, (1)University of Washington Seattle Campus, Seattle, WA, United States, (2)University of Washington, Seattle, WA, United States, (3)Earth & Space Sciences, Seattle, WA, United States, (4)Pennsylvania State University Main Campus, University Park, PA, United States, (5)Norwegian Polar Institute, Tromsø, Norway
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.