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 km2 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 km2 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.
Highlights
Radio echo sounding of ice sheets is a decades-old technique (Robin et al, 1969)
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
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 km2 around the ice core site
Summary
Radio echo sounding of ice sheets is a decades-old technique (Robin et al, 1969). Ice thickness profiles, internal layers, bed topography, and basal conditions revealed by radar echo sounding are widely used for analysis and modeling of ice mass balance, temperature, flow, and dynamics (Shepherd et al, 2012; MacGregor, 2015; Bamber et al, 2013; Hughes et al, 2016). Many previous studies show that ice properties, such as the birefringence and crystal orientation fabric (COF), could be inferred from multi-polarization radar measurements. This paper first describes the surface-based radar system and antenna configurations used in the data collection of multi-polarization measurements, and gives details of the data processing techniques and analysis. It compares the measurements with simulation results according to the theory of plane wave propagation in a birefringent medium. This study provides a comparison between radio echo sounding measurements and ice core observations with discussions and conclusions on the ice COF and birefringence properties around the NEEM ice core site
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