GNGTS 2013 - Atti del 32° Convegno Nazionale

transitions among different media could be detected: “soil” – snow, on the banks, and “soil” - water - ice - snow over the lake. To this aim we used: the resistivimeter Electra, the seismometer SoilSpy Rosina and five tromographs Tromino. The average stratigraphy on the lake, known from an in situ probe, was: 0.7 m snow, a thin water layer (0.07 m), 0.45 m thick ice and finally 16 m of water. Constrained by the environmental conditions, room at disposal, and the necessity to made all the measurements in a short time lapse, the electrical acquisition sensors were disposed in the snow with a interval of 3 m (as the sensor for the multichannel seismics). For the geoelectrics this step would allow a certain depth of investigation and, at the same time, a vision, albeit crude, of the snow level. Some operational difficulties made impossible to achieve more detailed data (e.g., inter-electrode space 0.5 m). Interferences of water with the electrical parts forced us to work on shorter a profile (93 m), limited to 32 electrodes. Despite the operative and technical conditions and the strong contrast between the various media, to which the air in small crevasses (cracks evident also on the snow surface) has to be added, the measures are reasonably repeatable and meaningful. The measured range of apparent resistivity is very wide, ranging from low values (around 50 Ω·m) corresponding to more conductive means, referable to water and melting snow, up to extremely high values (thousands of Ohm meter), which represent the presence of practically insulating media (probably air inside the surface cracks). An intermediate resistive response is associated with what is most likely the compact substrate; in particular a stratigraphic transition (probably with the substrate) is clearly marked at the lake bottom (roughly 40 m from the lake bank) by a sharp increase in resistivity (around 1300 Ω·m). Although the amount of data collected was less than what was planned to adequately describe the site, it was also possible to recognize high resistive anomalies at the small crevasses in the first part of the array (corresponding to the bank of the lake). The presence of snow, water, ice and water again, is the cause of a complex response characterized by heterogeneous and also abrupt steps. The wide range of resistivity observed and the jumps among lateral and vertical resistivities do not help in the inversion processes with usual software, which results in models with high error. Regarding the seismics, both active and passive acquisitions, in single-station and in array, were carried out, consisting of: active MASW (Multichannel Analysis of Surface Waves), passive ReMi (Refraction Microtremor), passive ESAC (Extended Spatial AutoCorrelation) and mainly 11 acquisitions (20’ at 128 Hz) of microtremor in free field (along a line of 100 m). The surveys allow to estimate the velocity of shear waves (Vs) of the snow (around 80 m/s) from the velocity of the surface waves (Rayleigh in the case of this study); to measure the resonance frequencies of the site and to rebuild a seismic stratigraphy in 1D and in 2D. For the 2D reconstruction, the individual HVRS (Horizontal to Vertical Spectral Ratio) curves have been converted from the frequency domain to the depth domain, exploiting the results of the tests in arrays as a constraint. It can be observed at the extremes of the seismic section (Fig. 3) the seismic bedrock outcrops, while in the central part there is a large inversion velocity linked to the presence of water under the ice surface (the thin red level close to the surface). The data seem to suggest a thickness of the lake in order of 20 m and then an anomaly in the middle of the profile, as in the electrics, that would require further investigation. The seismics and electrics results are coherent, in relation to the common depth of investigation. Conclusion. We have tested the performance of geoelectrical, seismic and remote sensing prospection in the detection of the characteristics of the first subsoil in glacial and periglacial environmental as in the Presena Glacier area. The greatest effort was addressed towards the collection of local data about the physical status of the snowpack. In particular, for detailed 215 GNGTS 2013 S essione 3.3