Superimposed ice accumulation zone as mapped by GPR and satellite SAR

Ground-Penetrating Radar (GPR) profiles have been recorded annually along the centerline of Kongsvegen, Svalbard since 1999, and a dense grid was obtained in the upper ablation and firn areas in Spring 2003. The GPR operates at 500 MHz, and grid profiles reach depths equivalent to about 24 and 19 m in firn and ice respectively. GPR profiles reveal distinct layering in the firn area, starting upglacier from around the mean equilibrium line altitude, and increasing in thickness upglacier. These easily detected layers correspond to previous years' summer surfaces, and can be correlated in the firn area with the net balance measured at stakes as part of the mass balance monitoring program. Below the firn-ice transition (FIT) the layers are due to superimposed ice formation. Here the layers are more irregular, are characterized by weaker back-scatter, and appear as prograding deposits, the latter consistent with the mechanism of superimposed ice formation in which snowpack water flowing downglacier is frozen onto the glacier in thin sheets. The FIT as imaged by GPR coincides in location with a transition from medium to high back-scatter observed in satellite SAR imagery. We compare the medium backscatter zone in the SAR imagery to a thickness map of the superimposed ice layers obtained from the GPR grid, and to ice cores. From the area extent derived by SAR imagery and the depth distribution imaged by GPR, and a DEM of the glacier surface, it seems that the accumulation of superimposed ice is governed strongly by the surface topography. Furthermore, time-series of SAR images and GPR centreline profiles show that the location of the FIT is retreating, in response to a record number of years with negative mass balance.