GNGTS 2019 - Atti del 38° Convegno Nazionale

GNGTS 2019 S essione 3.2 653 from the time delay and an interpretation of the bottom morphology was retrieved from data interpolation along the 9 cross-sections. In the upper part of the glacier (W and SW of Zamboni Zappa Mountain Hut) both radar and seismic results seemed to locate the bedrock at depths of 200–250 m. Progressing northwards, major discrepancies in ice bottom determination arose between the two techniques. Particularly, radar bottom reflectors were found to be located approximately 100 m higher than the seismic reflectors in the central part of the glacier. The two techniques, sensitive to different physical changes in the subsurface, seemed therefore to identify different interfaces. This difference in ice bottom location was interpreted as due to the presence of a layer of subglacial deposits with relevant thickness (up to more than 100 m in the central part of the glacier) between ice and bedrock. In the light of these unclear results, new geophysical prospections were carried out on site between 2016 and 2018. New GPR surveys. In October 2016, 29 GPR profiles were acquired with an air-coupled 70-MHz GPR monostatic antenna, for a total surveyed length of about 2200 m (Fig. 1c). The antenna was connected to an IDS K2 TR200 acquisition unit and manually transported above the ground surface. Traces were recorded with a time window of 1000 ns and a sampling rate of 1024 samples/trace; they were georeferenced by means of a GPS Ublox EVK-5T system. Asecond survey was performed inMarch 2018: 22 radar profiles were acquired with the same air-coupled 70-MHz antenna transported on skis, for a total length of 2700 m of acquisitions with trace window of 2000 ns and 2048 samples/trace (Fig. 1c). Finally, in December 2018 a lower frequency air-coupled antenna was manually moved along 15 profiles focused on the terminal sector of the northern lobe of the glacier, for a total length of 2200 m of acquisitions with a time window of 1200 ns, 1024 samples/trace (Fig. 1c). All the surveys were performed with a snow cover, from absent to a few tens of centimeter thick, on the debris cover. Due to the rough and rapidly evolving topography of the glacier along the years, the opening and/ Fig. 1 - (a) Geographic location and (b) aerial view of the north-eastern glaciers of the Monte Rosa Massif. The main glacial masses are delimited by light blue areas (modified from the Italian Glaciological Committee (CGI-CNR), http://www.glaciologia.it/i-ghiacciai-italiani) . (c) GPR surveys. Green lines: 70-MHz GPR profiles (October 2016); blue lines: 70-MHz GPR profiles (March 2018); red lines: 40-MHz GPR profiles (December 2018). GPR profiles are labelled with a progressive number for each survey. The black dashed line highlights the approximate glacier perimeter in October 2016 (first GPR campaign) with respect to the background orthophoto (AGEA 2015, WMS service at www.geoportale.piemonte.it) .