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

Abstract

Abstract. One of the great challenges in glaciology is the ability to estimate the bulk ice anisotropy in ice sheets and glaciers, which is needed to improve our understanding of ice-sheet dynamics. We investigate the effect of crystal anisotropy on seismic velocities in glacier ice and revisit the framework which is based on fabric eigenvalues to derive approximate seismic velocities by exploiting the assumed symmetry. In contrast to previous studies, we calculate the seismic velocities using the exact c axis angles describing the orientations of the crystal ensemble in an icecore sample. We apply this approach to fabric data sets from an alpine and a polar ice core. Our results provide a quantitative evaluation of the earlier approximative eigenvalue framework. For near-vertical incidence our results differ by up to 135 m s −1 for P-wave and 200 m s −1 for S-wave velocity compared to the earlier framework (estimated 1 % difference in average P-wave velocity at the bedrock for the short alpine ice core). We quantify the influence of shearwave splitting at the bedrock as 45 m s −1 for the alpine ice core and 59 m s −1 for the polar ice core. At non-vertical incidence we obtain differences of up to 185 m s −1 for P-wave and 280 m s −1 for S-wave velocities. Additionally, our findings highlight the variation in seismic velocity at non-vertical incidence as a function of the horizontal azimuth of the seismic plane, which can be significant for non-symmetric orientation distributions and results in a strong azimuth-dependent shear-wave splitting of max. 281 m s −1 at some depths. For a given incidence angle and depth we estimated changes in phase velocity of almost 200 m s −1 for P wave and more than 200 m s −1 for S wave and shear-wave splitting under a rotating seismic plane. We assess for the first time the change in seismic anisotropy that can be expected on a short spatial (vertical) scale in a glacier due to strong variability in crystalorientation fabric (±50 m s −1 per 10 cm). Our investigation of seismic anisotropy based on ice-core data contributes to advancing the interpretation of seismic data, with respect to extracting bulk information about crystal anisotropy, without having to drill an ice core and with special regard to future applications employing ultrasonic sounding.

A full Stokes ice-flow model to assist the interpretation of millennial-scale ice cores at the high-Alpine drilling site Colle Gnifetti, Swiss/Italian Alps

“…Likewise, anisotropic ice rheology is so far not being included. The presence of anisotropic ice at CG has been revealed by seismic surveys (Diez and others, 2013), and was recently further investigated by ice-core measurements (Kerch and others, 2018). We tested the non-linear General Orthotropic Flow Law (GOLF) (Gillet-Chaulet and others, 2005; Ma and others, 2010) available within Elmer/Ice, running steady-state simulations with a prescribed fabric distribution (Licciulli, 2018).…”

Crystallographic preferred orientations of ice deformed in direct-shear experiments at low temperatures

“…Furthermore, the CPO controls elastic anisotropy in ice and, through this, the anisotropy of sound wave velocity (Kohnen and Gow, 1979;Diez et al, 2015;Vaughan et al, 2017). Seismic data can be used to constrain bulk CPOs (e.g., Bentley, 1972;Smith et al, 2017;Kerch et al, 2018), and understanding how CPOs relate to deformation kinematics and conditions is valuable in limiting the range of possible CPO solutions in a field seismic experiment (Picotti et al, 2015;Vélez et al, 2016).…”

“…In this contribution, we adapt the direct shear method, applied in rock deformation studies (e.g., Schmid et al, 1987;Dell'angelo and Tullis, 1989;Zhang and Karato, 1995;Heilbronner and Tullis, 2006;Kohlstedt and Holtzman, 2009), to polycrystalline ice. By confining our samples with gas pressure, we are able to apply relatively high differential stresses without causing brittle fracture of the samples, allowing us to shear ice to large strains at much lower temperatures than have been applied before.…”

On the Limitations of Using Polarimetric Radar Sounding to Infer the Crystal Orientation Fabric of Ice Masses

“…As noted in the introduction, shear-wave splitting provides an alternative seismic approach for determining the crystal fabric of ice masses and of the mineral aggregate in the upper mantle. The elastic plane-wave equation in momentum space is an eigenvalue problem similar to Equation 5 (Diez & Eisen, 2015;Hellmann et al, 2021;Kerch et al, 2018; see also Supporting Information S1 for details): Qu − ρω 2 u = 0, where u is the displacement field of the plane wave, ρ is the mass density, and Q is the acoustic tensor. At first glance, it would therefore seem that our methodology could be adapted for elastic (seismic) problems, too.…”