Radar Attenuation Tomography for Mapping Englacial Temperature Distributions Using Off-Nadir Airborne Radio-Echo Sounding

Englacial temperature and water content play critical roles in glacier dynamics, both within ice sheets and mountain glaciers. As radio wave attenuation is sensitive to both of these properties, radio-echo sounding (RES) serves as a useful tool for mapping out their distributions within glaciers. Ground-based bistatic surveys, in which multi-offset measurements are taken, provide a large diversity in bed incidence angles and travel-path lengths. Provided the anomaly of interest is sufficiently sampled, these measurements can be exploited to perform attenuation tomography, thereby recovering the distribution of englacial radio wave attenuation from which englacial temperature can be estimated. Extensive RES surveys have been carried out over Antarctica using airborne radar; however, due to the monostatic geometry, methods for estimation of englacial radio wave attenuation and basal roughness have relied primarily on nadir returns. These estimates are often derived from 2D spatial correlation of basal return power and ice thickness or by employing layer-tracking methods. These techniques are limited in that the former uses echoes from a large spatial footprint, preventing the detection of small-scale anomalies, while the latter assumes a known, spatially invariant reflectivity for tracked layers. However, by considering returns from off-nadir in airborne surveys, techniques from multi-offset surface surveys can be modified and extended to perform airborne attenuation tomography. While not reaching the range of path diversity achievable in surface-based surveys due to limitations imposed by total internal reflection at the ice-air interface, airborne off-nadir returns contain valuable information about subglacial and englacial conditions that is often ignored. Thus, we propose a method for estimating englacial attenuation and basal roughness using the drop in power from the peak to tail of hyperbolic scattering events in unfocussed radargrams associated with the rough bed surface. The travel-paths of the bed returns across a given hyperbolic event vary in both length and bed incidence angle. Thus, the drop in return power across a hyperbolic event gives insight into both the integrated attenuation along a travel-path, as well as the scattering function at the bed. Specular reflections from internal layers with varying dips similarly provide diversity in travel-path lengths, allowing the derivation of a relationship between path length and return power without the complications brought about by diffuse scattering at rough surfaces. Using the diverse path lengths and angles through the ice, a tomographic inversion to map the spatial distribution englacial attenuation anomalies can be implemented. This technique is applied to synthetic data, as well as data collected using the British Antarctic Survey’s Polarimetric-radar Airborne Science Instrument (PASIN), specifically to lines collected over the Eastern Shear Margin of Thwaites Glacier. This location was chosen as constraining bed conditions and identifying expected englacial thermal anomalies are critical to understanding the history and modelling the future of Thwaites.