Fabric beats in radar data across the NEGIS ice stream

Abstract

<p>Crystal anisotropy of ice causes slight birefringence for electromagnetic waves. At the same time, the mechanical anisotropy amounts to several orders of magnitude, thus making fabric properties highly-relevant for internal deformation. To date, bulk anisotropy of glaciers and ice sheets can be determined by geophysical methods, such as polarimetric radar, or direct sampling from ice cores. A shortcoming has been so far that changes of bulk anisotropy could mainly be inferred at single point observations, but less so as continuous profiles. Here, we exploit the effect of birefringence caused by bulk anisotropy in co-polarized airborne radar data to determine the horizontal anisotropy across the North-East Greenland Ice Stream. We base our analysis on the fact that birefringence causes a second-order effect on radar amplitudes, which leads to a beat frequency in the low and medium frequency range (O(100 kHz)), which is proportional to the horizontal anisotropy. Complementing our radar analysis with direct fabric and dielectric property observations we can constrain the range of all three fabric eigenvalues as a function of space across and along the ice stream. Finally, we assess the effect of the inferred fabric distribution on the overall ice rheology in the context of ice stream dynamics. Our overall approach has the advantage that it can be applied to co-polarized radar systems, as commonly used in profiling surveys, and does not require dedicated cross-polarized radar set-up. This provides the opportunity to revisit older data, especially from Greenland and Antarctica, to map fabric anisotropy in ice-dynamically interesting regions.</p>

Airborne ultra-wideband radar sounding over the shear margins and along flow lines at the onset region of the Northeast Greenland Ice Stream

“…The radar data furthermore allow a systematic analysis of the return power of internal layers and the bed and could potentially reveal information about the englacial temperature regimes and properties of the bed and basal hydrology (e.g., Franke et al, 2020Franke et al, , 2021c as well as provide crucial boundary conditions for ice flow models (Gerber et al, 2021). In addition our data hold the potential to map the crystal orientation fabric (COF) and especially the horizontal anisotropy from the birefringence effects (Eisen et al, 2021;Young et al, 2021), e.g., at the shear margins or at intersecting radar profiles of different polarization directions. Because ice stream regimes have a strong effect upon the COF (Lilien et al, 2021), it is another important, but yet poorly constrained parameter in ice flow modeling.…”

“…The Northeast Greenland Ice Stream (NEGIS) efficiently drains a large area of the Greenland Ice Sheet and is a crucial component of the ice sheet mass balance (Fahnestock et al, 1993;Rignot and Mouginot, 2012). It extends from the central ice divide over more than 600 km towards the northeastern coast, where it discharges ice through the three marineterminating glaciers (79 • North glacier, Zachariae Isbrae, and Storstrømmen glacier).…”

Airborne ultra-wideband radar sounding over the shear margins and along flow lines at the onset region of the Northeast Greenland Ice Stream

“…The radar data furthermore allow a systematic analysis of the return power of internal layers and the bed and could potentially reveal information about the englacial temperature regimes and properties of the bed and basal hydrology (e.g., Franke et al, 2020Franke et al, , 2021c as well as provide crucial boundary conditions for ice flow models (Gerber et al, 2021). In addition our data hold the potential to map the crystal orientation fabric (COF) and especially the horizontal anisotropy from the birefringence effects (Eisen et al, 2021;Young et al, 2021), e.g., at the shear margins or at intersecting radar profiles of different polarization directions. Because ice stream regimes have a strong effect upon the COF (Lilien et al, 2021), it is another important, but yet poorly constrained parameter in ice flow modeling.…”

“…In addition our data hold the potential to map the crystal orientation fabric (COF) and especially the horizontal anisotropy from the birefringence effects (Eisen et al, 2021;Young et al, 2021), e.g., at the shear margins or at intersecting radar profiles of different polarization directions. Because ice stream regimes have a strong effect upon the COF (Lilien et al, 2021), it is another important, but yet poorly constrained parameter in ice flow modeling.…”

“…The Northeast Greenland Ice Stream (NEGIS) efficiently drains a large area of the Greenland Ice Sheet and is a crucial component of the ice sheet mass balance (Fahnestock et al, 1993;Rignot and Mouginot, 2012). It extends from the central ice divide over more than 600 km towards the northeastern coast, where it discharges ice through the three marineterminating glaciers (79 • North glacier, Zachariae Isbrae, and Storstrømmen glacier).…”