Abstract
ABSTRACTWe describe methods for measuring crystal orientation fabric with sonic waves in an ice core borehole, with special attention paid to vertical-girdle fabrics that are prevalent at the WAIS Divide. The speed of vertically propagating compressional waves in ice is influenced by vertical clustering of the ice crystal c-axes. Shear-wave speeds – particularly the speed separation between fast and slow shear polarizations – are sensitive to azimuthal anisotropy. Sonic data from the WAIS Divide complement thin-section measurements of fabric. Thin sections show a steady transition to strong girdle fabrics in the upper 2000 m of ice, followed by a transition to vertical-pole fabrics below 2500 m depth. Compressional-wave sonic data are inconclusive in the upper ice, due to noise, as well as the method's inherent insensitivity to girdle fabrics. Compared with available thin sections, sonic data provide better resolution of the transition to pole fabrics below 2500 m, notably including an abrupt increase in vertical clustering near 3000 m. Our compressional-wave measurements resolve fabric changes occurring over depth ranges of a few meters that cannot be inferred from available thin sections, but are sensitive only to zenithal anisotropy. Future logging tools should be designed to measure shear waves in addition to compressional waves, especially for logging in regions where ice flow patterns favor the development of girdle fabrics.
Highlights
A better understanding of the physics governing ice deformation is critical both for predicting the dynamic response of ice sheets to climate forcing (e.g. Vaughan and Arthern, 2007), and for interpreting paleo-climate records from ice cores (e.g. Alley and others, 1997; Waddington and others, 2001) and ice-sheet internal layer measurements (e.g. Waddington and others, 2007; Martin and others, 2009)
By measuring sonic waves in a borehole, it is possible to constrain the types of fabric that prevail in the surrounding ice and to determine parameters describing that fabric
With results from sonic and thin-section studies at the West Antarctic Ice Sheet Divide ice core site (WAIS-D), we demonstrate the relationship between sonic wave speeds and several fabric parameters, and present a framework for interpreting existing sonic data, as well as a prescription for future developments in sonic logging
Summary
A better understanding of the physics governing ice deformation is critical both for predicting the dynamic response of ice sheets to climate forcing (e.g. Vaughan and Arthern, 2007), and for interpreting paleo-climate records from ice cores (e.g. Alley and others, 1997; Waddington and others, 2001) and ice-sheet internal layer measurements (e.g. Waddington and others, 2007; Martin and others, 2009). By measuring sonic waves in a borehole, it is possible to constrain the types of fabric that prevail in the surrounding ice and to determine parameters describing that fabric. This method is readily implemented at sites where boreholes remain open after core drilling, and uniquely complements other methods for measuring COF. With results from sonic and thin-section studies at the West Antarctic Ice Sheet Divide ice core site (WAIS-D), we demonstrate the relationship between sonic wave speeds and several fabric parameters, and present a framework for interpreting existing sonic data, as well as a prescription for future developments in sonic logging
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