Multi-channel and multi-polarization radar measurements around the NEEM site

Abstract

Abstract. Ice properties inferred from multi-polarization measurements, such as birefringence and crystal orientation fabric (COF), can provide insight into ice strain, viscosity, and ice flow. In 2008, the Center for Remote Sensing of Ice Sheets (CReSIS) used a ground-based VHF (very high frequency) radar to take multi-channel and multi-polarization measurements around the NEEM (North Greenland Eemian Ice Drilling) site. The system operated with 30 MHz bandwidth at a center frequency of 150 MHz. This paper describes the radar system, antenna configurations, data collection, and processing and analysis of this data set. Within the framework derived from uniaxial ice crystal model, we found that ice birefringence dominates the power variation patterns of co-polarization and cross-polarization measurements in the area of 100 km 2 around the ice core site. The phase shift between ordinary and extraordinary waves increases nonlinearly with depth. The ice optic axis lies in planes that are close to the vertical plane and perpendicular or parallel to the ice divide depending on depth. The ice optic axis has an average tilt angle of about 11.6 • vertically, and its plane may rotate either clockwise or counterclockwise by about 10 • across the 100 km 2 area, and at a specific location the plane may rotate slightly counterclockwise as depth increases. Comparisons between the radar observations, simulations, and ice core fabric data are in very good agreement. We calculated the effective colatitude at different depths by using azimuth and colatitude measurements of the c axis of ice crystals. We obtained an average effective c axis tilt angle of 9.6 • from the vertical axis, very comparable to the average optic axis tilt angle estimated from radar polarization measurements. The comparisons give us confidence in applying this polarimetric radio echo sounding technique to infer profiles of ice fabric in locations where there are no ice core measurements.

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

“…Although the cross-polarised power anomaly has a 90 • ambiguity (Li et al, 2018), this ambiguity can be resolved if the corresponding co-polarised measurements detect anisotropic scattering (Brisbourne et al, 2019). We find that the symmetry axis orientation observed in our measurements aligns exactly with the observed present-day surface strain regime rather than the orientation of flow, at 250 • ±7 • (Fig.…”

“…By tracing the azimuthal minima in the cross-polarised power anomaly profiles through depth ( Fig. 2c) (Li et al, 2018) and resolving the observed 90 • ambiguity by identifying the azimuthal location of the centre of the quadrant patterns ( https://doi.org/10.5194/tc-2020-264 Preprint. Discussion started: 28 September 2020 c Author(s) 2020.…”

“…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.…”

Five decades of radioglaciology

“…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.…”

Large-scale englacial folding and deep-ice stratigraphy within the West Antarctic Ice Sheet

“…As the depth of R1 rules out ice-density fluctuations, we attribute the unit to comprise a strong ice-crystal fabric, noting also that significant variability in crystal fabric is observed in the lower parts of West Antarctic ice cores (e.g. Gow and Williamson, 1976;Fitzpatrick et al, 2014), and that radar reflection anisotropy has been verified by ice core evidence from Antarctica and Greenland (Eisen et al 2007, Drews et al 2012Li et al 2018). The anisotropic layers cannot be due to directional roughness of layer reflectivity, as the anisotropy is unique to specific layers (Figures 6a and Supplementary Figures 1-4).…”