GNGTS 2018 - 37° Convegno Nazionale

GNGTS 2018 S essione 3.2 687 Study area. The Bowditch Pond (elevation: 2901 m a.s.l., area: 2405 m 2 ) is located in a small valley depression in the uppermost area of the Cimalegna glacial plateau, in the Piemonte Region, close to the border with the Valle d’Aosta Region (Fig. 1). The pond has no persistent surface inflows while a surface outflow is present, although it usually disappears during the ice-free season (August). Permafrost is likely to exist in the basin (Boeckli et al. , 2012). The water basin is mainly surrounded by coarse debris deposits of various rock types outcropping in the surroundings, belonging to different overlapping tectono-metamorphic units, and varying from micaschists and gneisses, with acid composition and schistous and massive structure, to eclogites, amphibolites, and other rock types with basic composition. In particular, the northern shore of the pond is characterized by garnet-bearing micaschists with embedded eclogitic lenses and a big quartz vein. To the NW of the pond, an elongated ridge of compact rock is present. The latter is composed of garnet-bearing micaschists. Eclogites are outcropping to the East and micaschists with no evident garnet content are present along the south-western shore. The complex geo-structural background of the pond surroundings is complicated and partially hidden by the presence of fractured rocks and coarse debris covers. Data acquisition. Extensive waterborne GPR profiles and ERT lines were acquired on the Bowditch Pond. An IDS 200-MHz bistatic antenna placed at the bottom of a plastic inflatable boat and controlled by an IDS K2 digital acquisition unit was used for waterborne GPR profiling. The boat was driven by an electric engine to maintain a constant speed and an Ublox EVK- 5T GPS was used to track the survey position. GPR profiles were densely acquired following different directions in order to sufficiently cover the pond area in two subsequent campaigns. In the first survey, 19 profiles were acquired with a time increment of 0.48 ns and 1024 samples per trace, for a total time of 500 ns; in the second one, 17 profiles were recorded with the same time increment, 2048 sample per trace and a trace duration of 1000 ns. On the whole, we acquired 1331.2 m of GPR profiles (Fig. 2). By contrast, waterborne electrical profiling with towed floating cables was not possible due to the small area of the pond. As a consequence, Fig. 2 - Location of the geophysical survey and bathymetry contour map of the pond reconstructed from GPR data. The pond perimeter is marked with a black line. Green lines trace the waterborne GPR profiles. Yellow dots refer to the electrode locations of the 10 ERT lines (9 lines with 24 electrodes at 3-m spacing + 1 line with 48 electrodes at 1-m spacing).