Preprints
https://doi.org/10.5194/tc-2021-382
https://doi.org/10.5194/tc-2021-382
 
11 Jan 2022
11 Jan 2022
Status: a revised version of this preprint is currently under review for the journal TC.

Ultrasonic and seismic constraints on crystallographic preferred orientations of the Priestley Glacier shear margin, Antarctica

Franz Lutz1, David J. Prior1, Holly Still2, M. Hamish Bowman1, Bia Boucinhas3, Lisa Craw4, Sheng Fan1, Daeyeong Kim5, Robert Mulvaney6, Rilee E. Thomas1, and Christina L. Hulbe2 Franz Lutz et al.
  • 1Department of Geology, University of Otago, Dunedin, New Zealand
  • 2School of Surveying, University of Otago, Dunedin, New Zealand
  • 3Antarctica New Zealand, Christchurch, New Zealand
  • 4Institute for Marine and Antarctic Sciences, University of Tasmania, Hobart, TAS, Australia
  • 5Division of Earth Sciences, Korea Polar Research Institute, Incheon, Republic of Korea
  • 6British Antarctic Survey, Natural Environment Research Council, Cambridge, United Kingdom

Abstract. Crystallographic preferred orientations (CPOs) are particularly important in controlling the mechanical properties of glacial shear margins. Logistical and safety considerations often make direct sampling of shear margins difficult and geophysical measurements are commonly used to constrain the CPOs. We present here the first direct comparison of seismic and ultrasonic data with measured CPOs in a polar shear margin. The measured CPO from ice samples from a 58 m deep borehole in the left lateral shear margin of the Priestley Glacier, Antarctica, is dominated by horizontal c-axes aligned sub-perpendicular to flow. A vertical seismic profile experiment with hammer shots up to 50 m away from the borehole, in four different azimuthal directions, shows velocity anisotropy of both P-waves and S-waves. Matching P-wave data to the anisotropy corresponding to CPO models defined by horizontally aligned c-axes gives two possible solutions for c-axis azimuth, one of which matches the c-axis measurements. If both P-wave and S-wave data are used, there is one best fit for azimuth and intensity of c-axis alignment that matches well the measurements. Azimuthal P-wave and S-wave ultrasonic data recorded in the laboratory on the ice core show clear anisotropy that matches that predicted from the CPO of the samples. With good quality data, azimuthal increments of 30° or less will constrain well the orientation and intensity of c-axis alignment. Our experiments provide a good framework for planning seismic surveys aimed at constraining the anisotropy of shear margins.

Franz Lutz et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • CC1: 'Comment on tc-2021-382', Nicholas Rathmann, 23 Feb 2022
    • AC1: 'Reply on CC1', Franz Lutz, 08 May 2022
  • RC1: 'Comment on tc-2021-382', Thomas S Husdon, 23 Mar 2022
    • AC2: 'Reply on RC1', Franz Lutz, 08 May 2022
    • AC4: 'Reply on RC1', Franz Lutz, 08 May 2022
  • RC2: 'Comment on tc-2021-382', Chao Qi, 30 Mar 2022
    • AC3: 'Reply on RC2', Franz Lutz, 08 May 2022
    • AC5: 'Reply on RC2', Franz Lutz, 08 May 2022

Franz Lutz et al.

Franz Lutz et al.

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Short summary
Ice crystal alignment in the sheared margins of fast flowing polar ice is important as it may control ice sheet flow rate, from land to the ocean. Sampling shear margins is difficult because of logistical and safety considerations. We show that crystal alignments in a glacier shear margin in Antarctica can be measured using sound waves. Results from a seismic experiment on the 50m scale and from ultrasonic experiments on the decimetre scale match ice crystal measurements from an ice core.