GNGTS 2022 - Atti del 40° Convegno Nazionale

GNGTS 2022 Sessione 3.2 439 and understand the boundaries and the geometries of the EM facies and horizons on GPR data. We focused on amplitude-, phase- and frequency-related attributes, integrating them during data interpretation. Results and discussions. Spatial distribution of debris inside and on the Sforzellina Glacier encompasses all the different cases, being both supraglacial, englacial and subglacial. In GPR profiles, debris appears generally with a high amplitude scattered facies. The main issue for the identification of surficial debris on GPR data is the maximum optical resolution limit of a GPR antenna, which is roughly equal (at maximum) to a quarter of the dominant wavelength. So, for a 250 MHz central frequency antenna, the maximum attainable resolution corresponds to about 15 cm for a EM velocity of 10 cm ns -1 . Such velocity value resulted from the calibration of GPR data with direct measurements of debris thickness allowing to calculate a mean EM velocity. So, surficial debris cannot be imaged and properly identified if debris thickness is lower than at least 15 cm since it interferes with the direct ground wave. A high scattered zone imaged in GPR profiles can be associated to a mixture of ice and englacial debris, which causes scattering phenomena of the EM wave, as described in Forte et al., 2021. In the Sforzellina Glacier, a high scattered facies is clearly imaged along profiles in the accumulation area, with a peculiar shape and extension (Fig. 2A). Similarities with the glacier discussed in Forte et al., 2021, especially in terms of morphologies, relation with the ice-rock interface and response to the GPR attribute analysis, allowed to assess with a high level of confidence that also the high scattered zone inside the Sforzellina Glacier is related to englacial debris. In addition, englacial debris can be found in the shear zones, as expression of ice dynamics and englacial debris movements. As a matter of fact, in Fig. 2B-C we can easily note many high amplitude- reflections, crossing the entire ice thickness with a peculiar geometry due to the downward movement of the glacier, which encounters an obstacle in the bedrock undulating morphology. Usually this consideration is left on the background or even completely disregarded, but both bedrock depressions and highs highly affect ice thickness and dynamics and, as a consequence, the internal glaciological structures behaviour, as it is apparent in Fig. 2B. Shear zones take charge of debris from the base of the glacier and release it close to or on the surface. The glacier bottom can be defined as either ice-bedrock or ice-debris interface. This specification is needed because in some GPR profiles, below the ice, a peculiar EM facies has been noted. It is characterized by a very high amplitude and continuous reflection on top (white line in Fig. 2C), some local scattering and layering, and a low amplitude mainly continuous reflection at the base (red dotted line in Fig. 2C). This facies could be associated with a ground moraine with material deposited by moving ice and dropped by the glacier when it becomes too heavy to move. The aforementioned shear zones could take charge of the smallest debris materials and releasing them on the surface, while creating denser zones of debris cover on the surface. Fig. 1 - (A) Location map of the Sforzellina glacier inside the Stelvio National Park (B) Ortophoto of the glacier in black and white colour, with superimposed the GPR profiles (black lines) and their starting points (yellow dots).