Preprints
https://doi.org/10.5194/tc-2021-7
https://doi.org/10.5194/tc-2021-7

  04 Feb 2021

04 Feb 2021

Review status: this preprint is currently under review for the journal TC.

Acoustic velocity measurements for detecting the crystal orientation fabrics of a temperate ice core

Sebastian Hellmann1,2, Melchior Grab1,2, Johanna Kerch3,4, Henning Löwe5, Andreas Bauder1, Ilka Weikusat3,6, and Hansruedi Maurer2 Sebastian Hellmann et al.
  • 1Laboratory of Hydraulics, Hydrology and Glaciology (VAW), ETH Zurich, Zurich, Switzerland
  • 2Institute of Geophysics, ETH Zurich, Zurich, Switzerland
  • 3Alfred-Wegener-Institut Helmholtz-Zentrum für Polar- und Meeresforschung, Bremerhaven, Germany
  • 4GZG Computational Geoscience, Georg-August University, Göttingen, Germany
  • 5WSL Institute for Snow and Avalanche Research SLF, Davos, Switzerland
  • 6Department of Geosciences, Eberhard Karls University, Tübingen, Germany

Abstract. The crystal orientation fabrics (COF) in ice cores provides detailed information, such as grain size and distribution and the orientation of the crystals in relation to the large-scale glacier flow. These data are relevant for a profound understanding of the dynamics and deformation history of glaciers and ice sheets. The intrinsic, mechanical anisotropy of the ice crystals causes an anisotropy of the polycrystalline ice of glaciers and affects the velocity of acoustic waves propagating through the ice. Here, we employ such acoustic waves to obtain the seismic anisotropy of ice core samples and compare the results with calculated acoustic velocities derived from COF analyses. These samples originate from an ice core from Rhonegletscher, a temperate glacier in the Swiss Alps. Point-contact transducers transmit ultrasonic p-waves with a dominant frequency of 1 MHz into the ice core samples and measure variations of the travel times of these waves for a set of azimuthal angles. In addition, the elasticity tensor is obtained from laboratory-measured COF and calculate the associated seismic velocities. We compare these COF-derived velocity profiles with the measured ultrasonic profiles. Especially in the presence of large ice grains, these two methods show significantly different velocities since the ultrasonic measurements examine a limited volume of the ice core whereas the COF-derived velocities are integrated over larger parts of the core. This discrepancy between the ultrasonic and COF-derived profiles decreases with an increasing number of grains that is available within the sampling volume and both methods provide concise results in presence of a similar amount of grains. We also explore the limitations of ultrasonic measurements and provide suggestions for improving their results. These ultrasonic measurements could be employed continuously along the ice cores. They are suitable to support the COF analyses by bridging the gaps between discrete measurements, since these ultrasonic measurements can be acquired within minutes and do not require an extensive preparation of ice samples when using point-contact transducers.

Sebastian Hellmann et al.

Status: open (extended)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • CC1: 'Comment on tc-2021-7', David Prior, 01 Apr 2021 reply
  • CC2: 'Comment on tc-2021-7', Sridhar Anandakrishnan, 14 Apr 2021 reply
  • CC3: 'Comment on tc-2021-7', Sridhar Anandakrishnan, 14 Apr 2021 reply

Sebastian Hellmann et al.

Sebastian Hellmann et al.

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Short summary
In this study, we analyse whether ultrasonic measurements on ice core samples could be employed to derive information about the particular ice crystal orientation in these samples. We discuss if such ultrasonic scans of ice core samples could provide similarly detailed results as the established methods, which usually destroy the ice samples. Our geophysical approach is minimally invasive and could support the existing methods with additional and (semi-)continuous datapoints along the ice core.