Advancing interpretation of incoherent scattering in ice-penetrating radar data used for ice core site selection

Abstract

Below the coherent layering in ice-penetrating radar data collected in Antarctica and Greenland, incoherent scattering is common. This scattering is signal, not noise, and has the potential to inform our understanding of the structure and dynamics of the bottom 20 % of glaciers and ice sheets. Here, we present a comparison between radar imagery and ice core properties for 16 ice core sites across Antarctica and Greenland to identify possible sources for incoherent scattering and evaluate its use in ice core site selection. We find that incoherent scattering is commonly coincident with either gradual changes in crystal orientation fabric or rapidly fluctuating fabrics in deep ice, where strain is localized by strength differences associated with ice grain size. Macro-scale deformation and layer folding at scales below the range resolution of radar do not seem to result in incoherent scattering or induce an echo-free zone as has been previously hypothesized. Where incoherent scattering is laterally homogeneous in intensity, layering is typically undisturbed in nearby ice cores. But where incoherent scattering is laterally heterogeneous in intensity and the pattern does not appear conformal with subglacial topography, we find multi-meter-scale folding and associated discontinuities in nearby ice core records. Future higher-resolution sampling of fabric in ice cores would allow for more quantitative interpretation of incoherent scattering and its amplitude, but we show that the qualitative nature of incoherent scattering has the potential to inform us about the continuity of climate records at prospective ice core sites and should be considered when evaluating the nature and quality of basal ice.