Can polarimetric wide-angle radar surveys teach us more about ice fabric anisotropy?

The fabric anisotropy of ice and its flow dynamics are co-dependent. Parameters used in models that solve the evolution of ice fabric are currently unconstrained, for which comparisons with observations are needed. In observations and models, the ice fabric can be represented by a crystal orientation tensor, describing the spatial distribution of ice crystal orientations. Because ice crystals are not only mechanically, but also dielectrically anisotropic, the fabric anisotropy causes birefringence and anisotropic scattering and can be inferred by polarimetric radar surveys. In recent years, the advancement of polarimetric radar methods has resulted in a surge of available observational data. However, all existing methods are performed with a nadir-looking radar geometry. As a consequence, these approaches are only sensitive to horizontal fabric anisotropy, making the assumption necessary that one eigenvector of the crystal orientation tensor is aligned in vertical (nadir) direction. We aim to develop an approach to measure the actual orientation of this eigenvector. Here, we present the results of a polarimetric wide-angle common midpoint (CMP) survey conducted on Ekstrm Ice Shelf, Dronning Maud Land, Antarctica, using the Autonomous phase-sensitive Radio Echo Sounder (ApRES). Our CMP survey had a maximum antenna offset of 200 m, with an ice shelf thickness of 250 m. For several englacial reflectors, we observe offset-dependent phase shifts between orthogonal antenna orientations. We explore these phase variations by modelling the off-nadir radio wave propagation in the birefringent ice. These wide-angle radar surveys have the potential to infer the full crystal orientation tensor, required for a constitutive paramerization of glacial flow.