Inferring glacier ice-crystal orientation fabric from oblique polarimetric radar

Abstract

Downward-looking radar systems are widely used to infer the structure and basal conditions of glaciers and ice sheets. Oblique propagation, in contrast, is rarely considered even though polarimetric traveltime tomography is sensitive to the full crystal orientation fabric, which exerts important viscous control on ice masses. This study presents a polarimetric common midpoint (CMP) framework for ice sheets that accounts for the dielectric anisotropy arising from preferred crystal orientations and preferred bubble shapes in firn, including refractive bending due to density variations. Traveltimes are modelled along oblique ray paths using a Maxwell–Garnett effective medium formulation coupled to a single-stage Herron–Langway density model with a power-law bubble eccentricity profile, validated against Antarctic firn cores from Dome Fuji. An optimization problem is proposed for simultaneously inferring the crystal fabric profile and bubble close-off (BCO) depth of any firn–ice column, which is shown to be robust to noise in observed traveltimes and may require limited acquisition obliquities of up to 30 degrees. The inverse problem is applied to the Ekström ice shelf, Antarctica, revealing a girdle-type fabric that strengthens with depth despite restricted CMP offset. Finally, shortcomings and potential improvements are discussed, including suggestions for designing CMP surveys that aim to infer crystal fabrics.