Estimation of ice fabric within Whillans Ice Stream using polarimetric phase-sensitive radar sounding

Abstract

Here we use polarimetric measurements from an Autonomous phase-sensitive Radio-Echo Sounder (ApRES) to investigate ice fabric within Whillans Ice Stream, West Antarctica. The survey traverse is bounded at one end by the suture zone with the Mercer Ice Stream and at the other end by a basal ‘sticky spot’. Our data analysis employs a phase-based polarimetric coherence method to estimate horizontal ice fabric properties: the fabric orientation and the magnitude of the horizontal fabric asymmetry. We infer an azimuthal rotation in the prevailing horizontal c-axis between the near-surface (z ≈ 10–50 m) and deeper ice (z ≈ 170–360 m), with the near-surface orientated closer to perpendicular to flow and deeper ice closer to parallel. In the near-surface, the fabric asymmetry increases toward the center of Whillans Ice Stream which is consistent with the surface compression direction. By contrast, the fabric orientation in deeper ice is not aligned with the surface compression direction but is consistent with englacial ice reacting to longitudinal compression associated with basal resistance from the nearby sticky spot.

Rapid and accurate polarimetric radar measurements of ice crystal fabric orientation at the Western Antarctic Ice Sheet (WAIS) Divide deep ice core site

“…When ice deforms solely by vertical uniaxial compression, such as at the centre of an ice dome, the c-axes rotates towards the vertical and forms a cluster fabric; where lateral tension exists from flow extension, such as at an ice divide, the c-axes orient in a vertical girdle distribution orthogonal to the direction of strain extension (Alley, 1988). Following previous studies (Fujita et al, 2006;Drews et al, 2012;Brisbourne et al, 2019;Jordan et al, 2019Jordan et al, , 2020b, we assume that the E 3 eigenvector is aligned in the vertical direction, and the E 1 and E 2 eigenvectors are parallel to the horizontal plane.…”

“…Divide. We then calculate the fabric asymmetry using the polarimetric coherence methods outlined in Jordan et al (2019Jordan et al ( , 2020b).…”

“…Recently, Jordan et al (2019) developed a polarimetric coherence framework that extends existing methods (Dall, 2010;Fujita et al, 2006) to quantify COF eigenvalues through estimating the relative phase between orthogonal co-polarised measurements. A follow-up study by Jordan et al (2020b) demonstrated the validity of this framework, albeit with a coarse azimuthal resolution via the azimuthal rotational setup, from measurements obtained using an autonomous phase-sensitive radio-echo sounder (ApRES). The ApRES is a phase-sensitive frequency-modulated continuous-wave (FMCW) ground-based radar system (Brennan et al, 2014) that has been gaining traction over the last five years, not only to investigate englacial polarimetry (Brisbourne et al, 2019;Jordan et al, 2020b), but also in wider radioglaciological investigations involving englacial deformation (Gillet-Chaulet et al, 2011;Kingslake et al, 2014Kingslake et al, , 2016Nicholls et al, 2015;Young et al, 2019), englacial meltwater content (Kendrick et al, 2018;Vaňková et al, 2018), basal melting (Corr et al, 2002;Davis et al, 2018;Jenkins 2 https://doi.org/10.5194/tc-2020-264 Preprint.…”

“…Although polarimetric radar sounding data analysis has been implemented to detect horizontally-asymmetric COF for almost half a century (e.g., Hargreaves, 1977), it has not yet been widely implemented, with the majority of radar studies that measure COF variations in ice being conducted within the last fifteen years at coincident ice-coring sites for comparative analysis (Drews et al, 2012;Eisen et al, 2007;Fujita et al, 2006;Jordan et al, 2019Jordan et al, , 2020aLi et al, 2018;Matsuoka et al, 2003Matsuoka et al, , 2009Matsuoka et al, , 2012. Moreover, the majority of previous polarimetric radar studies infer COF at a coarse azimuthal resolution that is limited by the number of observations made along an acquisition plane that rotates around an azimuth centre (Brisbourne et al, 2019;Doake et al, 2002Doake et al, , 2003Jordan et al, 2020b;Matsuoka et al, 2003Matsuoka et al, , 2012. Because the maximum azimuthal resolution that can be achieved is subject to human error in measuring the angles between each acquisition plane, there is a coarse limit to the precision of the orientation of fabric asymmetry that can be achieved through this acquisition method.…”

Five decades of radioglaciology

“…This information can be used to constrain the depth distribution of the crystal orientation fabric along survey lines, enabling the investigation of processes occurring in ice masses, comparison to ice-dynamic models, and interpretation of particle-astrophysical observations (e.g. Jordan and others, 2019, 2020a,b; Shoemaker and others, 2020).…”