GNGTS 2022 - Atti del 40° Convegno Nazionale

438 GNGTS 2022 Sessione 3.2 NOT ONLY CLEAN ICE INSIDE ALPINE GLACIERS: A CASE STUDY ON THE SFORZELLINA GLACIER (CENTRAL ITALIAN ALPS) I. Santin 1 , E. Forte 1 , M. Guglielmin 2 1 Department of Mathematics and Geosciences, University of Trieste, Trieste, Italy 2 Department of Theoretical and Applied Sciences, Insubria University, Varese, Italy Introduction. The Sforzellina Glacier is a small debris-covered glacier in the South-East sector of the Stelvio National Park. Its evolution has been monitoring since 1950 by analysing its front retreat and since 1990 by geophysical techniques (Guglielmin et al., 1995) in order to evaluate the total ice mass of the glacier, and, consequently, the total stored water. The Sforzellina Glacier is in almost constant retreat and shrinking since the Little Ice Age (LIA), except a small advance around 1980 (Cannone et al., 2008), as proved by the complex morainic system located on the glacier foreland and the progressive debris accumulation on the Glacier surface (Tarca and Guglielmin, 2022). Debris-covered glaciers are characterized by peculiar features, driven by processes that are largely absent from their clean-ice counterparts (Miles et al., 2020). In fact, debris accumulated on the surface is only the most evident deposition zone, as the spatial distribution of debris inside and on the glacier, known as glacial debris cascade, is the result of the combination among debris sources, transport paths and deposition zone (Kirkbride, 2011). As debris is added on the accumulation area, it is progressively buried by snow or entrained in crevasses and follows englacial paths until the emergence in the ablation zone. On the contrary, debris supplied in the ablation zone tends to remain on the surface or emerging on the glacier surface along the flow lines. Glacier downward movements drive the transport of englacial debris across the ice thickness and affect the deposition of basal debris, in relation to the bedrock physical properties, such as roughness, morphology and thermal conditions. The properties and transport of debris and its direct effect on the surface insulation have been well studied and modelled by Reznichenko et al., 2010, Anderson and Anderson, 2016, and Giese et al., 2021, but geophysical techniques, especially Ground Penetrating Radar (GPR), can provide high resolution imaging and accurate spatial reconstruction of the debris distribution. As a matter of fact, GPR can not only provide a detailed imaging of the internal structure of glaciers, but also characterize different frozen materials. In this work, we aim at imaging the distribution of the debris inside the Sforzellina Glacier exploiting GPR data and highlighting strengths and limitations of such a technique. Methods. A ground-coupled GPR survey was carried out in September 2021 on the Sforzellina Glacier, resulting in a 2 km-long dataset of GPR profiles, located both outside and inside the actual glacier boundary (Fig.1). A Malå Geoscience ProEx GPR system, equipped with 250 MHz shielded antennas, triggered by an electro-mechanical odometer, was used to perform the survey. Even with the good quality of the raw GPR data, processing was essential to increase the signal/noise ratio and to improve the imaging while maintaining the original data signature and preserving the amplitude contrasts further used for different glacial facies characterization. The processing flow includes drift removal (zero- time correction), bandpass filtering (corner frequencies are 20–80–350–650 MHz, with a typical not symmetrical trapezoidal shape to limit Gibbs phenomena), background removal, exponential amplitude recovery, topographic (static) correction, depth conversion and migration. For the last three steps, we considered a simplified velocity field with velocity values equal to 17 cm ns -1 for ice and to 13 cm ns -1 for the debris, on the basis of dedicated diffraction hyperbolas analyses and correlations with direct data on selected locations. Both migrated and not migrated data are essential for interpretation and facies definition. In particular, scattering is very helpful to discriminate between different glaciological units. GPR interpretation was supported by GPR attribute analysis, which allowed to better image