Sources of backscatter at 5.3 GHz from a superimposed ice and firn area revealed by multi-frequency GPR and cores

Abstract

We investigate the major sources of backscatter at 5.3 GHz, within the superimposed ice and firn areas of a polythermal glacier. Two ground-penetrating radar systems, an 800 MHz impulse system and a polarimetric 5.3 GHz frequency-modulated continuous-wave system, are used to acquire along-glacier profiles in the accumulation area of Kongsvegen, Svalbard. The 800 MHz response is used to map reflection horizons in the glacier. Using cores from the superimposed ice and firn areas, the causes of these reflection horizons, in terms of snow, firn and ice layers, are investigated. Superimposing the reflection horizons on the co-polarized and cross-polarized 5.3 GHz profile, we are able to determine how the 5.3 GHz frequency responds to the different media. Scattering at rough interfaces and volume scattering occur in the superimposed ice area and are apparently caused by air-bubble number, size and distribution. In the firn the strongest return originates from below the previous summer surface, consistent with previous findings. At approximately the same depth, strong incoherent scattering begins. The rapid decrease in coherent reflections indicates the significance of scattering in the firn.