Advanced analysis of the sub-glacial environment using radar echo sounding simulations

Abstract

Sub-glacial conditions beneath large ice masses, especially the sub-glacial hydrological system, exert control on ice velocity and melting near the grounding zone. Despite the importance, the hydrological system beneath the Earth's polar ice sheets remain one of the greatest uncertainties when projecting climate-induced sea level rise. Analyzing variations in airborne ice penetrating Radar Echo Sounding (RES) reflectivity is an established technique for investigating sub-glacial topography. The high dielectric contrast between glacier ice, liquid water, and other substrate materials is exploited, with observations of elevated RES reflectivity commonly interpreted as sub-glacial water. Although this technique is powerful and enables wide geographic coverage, RES analysis is subject to several uncertainties which can lead to data misinterpretation. In this work, a radar backscattering simulator is adapted to model RES reflections from sub-glacial water structures within defined topography. In the second chapter, the methodology is established, with outputs demonstrating that RES reflections from flat sub- glacial canals will be much brighter than from rounded Rothlisberger channels. Simulated outputs are then compared to actual reflectivity from an RES flight line collected over Thwaites Glacier in West Antarctica. Ultimately, the study demonstrates consistency between a bright reflector in the actual RES data and simulated flat canals or a sub-glacial lake. In the third chapter, we combine the same simulation method with a 2-dimensional finite difference model of englacial temperatures. This study evaluates whether sub-glacial channels can exist in canyons identified beneath Devon Ice Cap in Nunavut, Canada (DIC). Results indicate that observed along-track variations in bed reflectivity could be induced by topography instead of extensive sub-glacial water, and basal temperatures could not support connected freshwater canals. Finally, the fourth chapter applies the simulation methodology to over 400km of RES observations across the downstream region of Thwaites Glacier. Results demonstrate wide variability in the fit quality between actual and simulated RES reflectivity. These differences are interpreted as material homogeneity or heterogeneity in the glacial bed, correlating geospatially with some previous hypotheses for the Thwaites hydrological system. The simulation method may ultimately reduce ambiguity in RES interpretations by distinguishing between bed reflectivity variations induced by topography vs. material transitions.