Anisotropic Scattering in Radio-Echo Sounding: Insights from Northeast Greenland

Abstract. Anisotropic scattering and birefringence-induced power extinction are two distinct mechanisms affecting the azimuthal power response in Radio Echo Sounding (RES) of ice sheets. While birefringence is directly related to the crystal orientation fabric (COF), anisotropic scattering can, in principle, have various origins. Although both mechanisms can appear separately, they often act jointly, complicating efforts to deduce the COF strength and orientation from RES signals. In this study, we assess the relative importance of anisotropic scattering and birefringence using quad-polarized ground-based RES measurements collected in the Northeast Greenland Ice Stream (NEGIS). We employ curve-fitting techniques to analyze the synthesized full azimuthal response, revealing insights into the dominance and orientation of the two different mechanisms at various depths between 630 m and 2500 m. We find that anisotropic scattering clearly dominates the radar signal in most depths larger than 1000 m, while birefringence effects are only important at shallower depths and in the vicinity of the ice- stream shear margins. We further find that the co-polarized power difference follows the ice-sheet stratigraphy with a notable transition in strength and/or direction at the Wisconsin-Holocene transition and in folded ice outside the ice stream, possibly indicating inverted stratigraphy in these folded units. We conclude that small-scale fluctuations in the horizontal COF eigenvalues is the most likely mechanism responsible for the anisotropic scattering observed in our survey area. Mapping the strength and orientation of scattering in quad-polarized measurements thus have the potential to provide independent estimates of the COF orientation and distinguish ice units with different scattering properties, e.g. from different climatic periods.