The crystal fabric of ice from full‐waveform borehole sonic logging

Abstract

[1] In an ice sheet, a preferred crystal orientation fabric affects deformation rates because ice crystals are strongly anisotropic: shear along the basal plane is significantly easier than shear perpendicular to the basal plane. The effect of fabric can be as important as temperature in defining deformation rates. Fabric is typically measured using analysis of thin sections under the microscope with co-polarized light. Due to the time-consuming and destructive nature of these measurements, however, it is difficult to capture the spatial variation in fabric necessary for evincing ice sheet flow patterns. Because an ice crystal is similarly elastically anisotropic, the speed of elastic waves through ice can be used as a proxy for quantify anisotropy. We use borehole sonic logging measurements and thin section data from Dome C, East Antarctica to define the relations between apparent fabric and borehole measured elastic speeds (compressional V P and vertically polarized shear V SV ). These relations, valid for single maximum fabrics, allow in-situ, depth-continuous fabric estimates of unimodal fabric strength from borehole sonic logging. We describe the single maximum fabric using a 1 : the largest eigenvalue of the second-order orientation tensor. For ice at À16C and a 1 in the 0.7-1 range the relations are V P = 248 a 1 3.7 + 3755 m s À1 and V SV = À210a

Deriving micro- to macro-scale seismic velocities from ice-core <i>c</i> axis orientations

“…Although the interpretation of these data is not straightforward (Maurel et al, 2015), it is currently the only technique that is capable of a continuous fabric measurement. However, a sonic pulse samples the volume around the borehole (∼ 2 m 3 , Gusmeroli et al, 2012), which means the method is not azimuth-sensitive. While it cannot provide the two-dimensional microstructure nor exact and highly resolved fabric information, it can help to bridge the gap between laboratory-based interval fabric measurements and large-scale seismic data.…”

“…To this end, ultrasonic methods can be applied in ice-core boreholes (Bentley, 1972;Gusmeroli et al, 2012) to infer crystalorientation fabric in situ. Although the interpretation of these data is not straightforward (Maurel et al, 2015), it is currently the only technique that is capable of a continuous fabric measurement.…”

“…The motivation of this study is to improve the interpretation of seismic data by connecting the micro-and the macro-scale using the elastic properties of ice. Early work to this end was accomplished by Bennett (1968); Bentley (1972); Blankenship and Bentley (1987), and more recent approaches include Gusmeroli et al (2012), Maurel et al (2015), and . Specifically, the starting point of this paper is the study by , who establish a connection between the commonly reported fabric parameter of second-order orientation tensor eigenvalues and the elasticity tensor describing the polycrystalline medium to calculate seismic velocities from ice-core fabric data.…”

Monitoring the temperature-dependent elastic and anelastic properties in isotropic polycrystalline ice using resonant ultrasound spectroscopy

“…We minimize χ 2 for all our inversions to a narrow level of tolerance (12)(13)(14) to improve the comparability of our measurements. Starting values of c αβ for polycrystalline ice are taken from Gammon et al (1983) and Gusmeroli et al (2012).…”

“…Much focus has recently been placed on understanding the physical properties of ice that influence seismic wave propagation (Maurel et al, 2015). Of particular interest are the relationships of seismic wave attenuation (Peters et al, 2012;Gusmeroli et al, 2010Gusmeroli et al, , 2012 to the ice temperature.…”