Where does a glacier end ? GPR measurements to identify the limits between the slopes and the real glacier area. Application to the Austre Lovénbreen, Spitsbergen -- 79°N

Glacier limits are usually mapped according to a spatial discrimination based on color of remote sensing images or aerial photography. What appears like ice (white or light colored areas) at the end of the ablation period (end of summer) corresponds to the glacier, while what appears as rock (dark areas) is identified as the slope. This kind of visual discretization seems to be insufficient in the case of small arctic glaciers. Indeed, the slopes have been described as very unstable parts of glacial basins. Debris are generated by the inclination of the slopes, and reach the glacier surface. Thus, the visible limit does not correspond to the ice extension: a significant amount of ice is potentially covered by rock debris, enlarging the actual glacier surface with respect to the observed area. Hence, we apply Ground Penetrating Radar (GPR) measurements for mapping, beyond the central parts of the glacier, the steep slopes of the Austre Lovénbreen (Spitsbergen, 79°N). The aim is to assess the discrepancy between the limits extracted from remote sensing methods -- aerial photography, satellite images and derived digital elevation models -- and the GPR data which exhibit significant ice thickness at locations considered outside the glacier itself. The ice is observed to extend typically from 25 to 30 meters, and up to 100~meters, under the slopes. These measurements allow for a new determination of the rock/ice interface location following criteria beyond the visual and morphological characteristics seen from the surface, as obtained by remote sensing techniques or in-situ observations.