A Polarimetric Coherence Method to Determine Ice Crystal Orientation Fabric From Radar Sounding: Application to the NEEM Ice Core Region

Abstract

Ice crystal orientation fabric (COF) records information about past ice-sheet deformation and influences the present-day flow of ice. Polarimetric radar sounding provides a means to infer anisotropic COF patterns due to the associated birefringence of polar ice. Here we develop a polarimetric coherence (phase-based) method to determine horizontal properties of the COF. The method utilizes the azimuth and depth-dependence of the vertical gradient of the hhvv coherence phase to infer the dielectric principal axes and birefringence which are then related to the second order fabric orientation tensor. Specifically, under the assumption that one of the orientational eigenvectors is vertical, we can determine the horizontal eigenvectors and the difference between the horizontal eigenvalues (a measure of horizontal fabric asymmetry). The method exploits single-polarized data acquired with varying antenna orientation. It applies to ground-based 'multi-polarization' surveys and is demonstrated using data acquired by CReSIS (Center for Remote Sensing of Ice Sheets) using MCRDs (Multi Channel Coherent Radar Depth Sounder) from the NEEM ice core region in Greenland. The analysis is validated using a combination of polarimetric matrix backscatter simulations and comparison with COF data from the NEEM ice core. The results are consistent with a conventional model of ice deformation at an ice divide where a lateral tension component is present, with minor horizontal COF asymmetry and the greatest horizontal concentration of crystallographic axes orientated near-parallel to the ice divide.

Five decades of radioglaciology

“…Challenges remain in combining disparate datasets, conditioning data for comparison with modeling and utilizing radiometric, interferometric and polarimetric information in modeling (e.g. Hindmarsh and others, 2009; Schroeder and others, 2016c; Castelletti and others, 2017, 2019; Winter and others, 2017, 2019; Chu and others, 2018b; Jordan and others, 2019). Finally, data–model interaction is a two-way street: testable hypotheses produced by theory and models may suggest new observational targets or provide new reasons to tap the rich radioglaciological archive (e.g.…”

“…Vaughan and others, 2006; Dall and others, 2010). These systems allow for the analysis of crystal-fabric orientation from polarization information (Doake and others, 2002; Fujita and others, 2003; Matsuoka and others, 2003; Eisen and others, 2007; Drews and others, 2012; Li and others, 2018; Wang and others, 2018; Jordan and others, 2019). 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.…”

“…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).…”

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

“…The polarimetric coherence method provides a way to measure the polarimetric phase shift from sounding data and hence infer horizontal anisotropy in the ice fabric. A full description of the method is given by Jordan and others (2019), with Dall (2010) providing the initial proof-of-concept.…”

“…When applied to multi-polarization plane data, the polarimetric coherence method pairs separate co-polarized measurements at 90 degrees, and considers the azimuthal behavior of the phase shift to infer the principal axes. Following Jordan and others (2019) we assume the convention that when θ = 0° the h polarization plane is aligned with the x -axis and the v polarization plane is aligned with the y -axis (i.e. bins 1 and 5 in Fig.…”