GNGTS 2019 - Atti del 38° Convegno Nazionale

654 GNGTS 2019 S essione 3.2 or relocation of crevasses and diffuse instabilities in the frontal sectors, it was not possible to follow the same profiles in each survey operating in safe conditions. In addition, due to the shape of both low-frequency antennas and the encountered topographic conditions, it was not possible to direct drag the antennas on the glacier surface to maximize the electromagnetic coupling. In each survey, the antenna was consequently maintained a few centimeters above the thin layer of snow partially hiding the cover of blocks and debris of the glacier. Abasic processing procedure was adopted: (i) start time of each trace was shifted to delete samples before the main bang and obtain exact zero time; (ii) low-frequency components were removed (dewow); (iii) high- pass horizontal filtering was applied to remove horizontally coherent components (background removal); (iv) geometrical spreading correction was applied, to gain signal amplitude with depth (divergence compensation). Manual picking of the ice bottom reflections was finally performed on the processed time sections. Due to the reduced snow cover during the surveys, the picking of the reference glacier topography (corresponding to the top of the debris cover) was neglected. Auniform ice velocity of 0.17 m/ns was considered for time to depth conversion, disregarding the top debris cover. GPR results. Retrieved data quality was very poor if compared with the typical S/N ratio of GPR data acquired on other glaciers. High scattering is noticed along the profiles, with extremely high attenuation of the EM signals with depth. Not always clear readability in the processed profiles was observed for the 70-MHz surveys. The globally observed low data quality was attributed to the presence of the debris cover, the absence of direct coupling between antenna and glacier surface and the probable widespread presence of debris and/or small-scale water bodies within the ice column. The best results were obtained with the 40- MHz antenna. In these radargrams (example in Fig. 2a), clearer reflections could be spatially Fig. 2 - Exemplificative 40-MHz GPR profile close and perpendicular to the N lobe terminus (no. 12, in red in Fig. 1c): (a) processed radargram; (b) processed radargram with overlapping tracking of the main reflectors. (c) Interpolated glacier bottom map from GPR results. (d) 3D reconstruction of the N lobe topography and bottom morphology.