GNGTS 2019 - Atti del 38° Convegno Nazionale

GNGTS 2019 S essione 3.2 619 Ekström G., Tromp J. and Larson E. W. F.; 1997: Measurements and global models of surface wave propagation . J. Geophys. Res., 102 , no. B4, 8137-8157. Gouedard P., Yao H., Ernst F. and Van der Hilst R. D.; 2012: Surface wave eikonal tomography in heterogeneous media using exploration data . Geophys. J. Int., 191 , 781–788. Kästle E. D., El-Sharkawy A., Boschi L., Meier T., Rosenberg C., Bellahsen N., Cristiano L. and Weidle C.; 2018: Surface Wave Tomography of the Alps Using Ambient-Noise and Earthquake Phase Velocity Measurements . J. Geophys. Res., Solid Earth, 123 , no. 2, 1770-1792. Kennett B. L. N., Sambridge M. S. and Williamson P. R.; 1988: Subspace methods for large inverse problems with multiple parameter classes. Geophysical Journal, 94 , 237-247. Lin F.-C., Ritzwoller M. H. and Snieder R.; 2009: Eikonal tomography: surface wave tomography by phase front tracking across a regional broad-band seismic array . Geophys. J. Int., 177 , 1091–1110. Rawlinson N. and Sambridge M.; 2005: The fast marching method: An effective tool for tomographic imaging and tracking multiple phases in complex layered media. Explor. Geophys., 36 , 341–350. Ritzwoller M. H. and Levshin A. L.; 1998: Eurasian surface wave tomography: Group velocities . J. Geophys. Res., 103 , no. B3, 4839-4878. Shapiro N. M., Campillo M., Stehly L. and Ritzwoller M. H.; 2005: High-Resolution Surface-Wave Tomography from Ambient Seismic Noise . Science, 307 , 1615-1618. Trampert J. and Woodhouse J. H.; 1996: High resolution global phase velocity distributions . Geophys. Res. Lett., 23 , no. 1, 21-24. THE USE OF FREQUENCY DOMAIN ELECTRO-MAGNETOMETER FOR THE CHARACTERIZATION OF PERMAFROST ACTIVE LAYER: CASE STUDIES IN THE SWISS ALPS J. Boaga 1 , M. Phillips 2 1 Dipartimento di Geoscienze, Università di Padova, Italy 2 Federal Research Insitute WSL-SLF, Davos, Switzerland The characterization of permafrost active layer (AL) is crucial for several reasons, from the consequences on potential slope instability to the monitoring of the climate change effects in the periglagical environments. Permafrost is the well-known ground layer with a temperature remaining at or below 0°C for at least two consecutive years. Permafrost interests one quarter of the Northern Hemisphere and 17% of the entire Earth (Biskaborn et al., 2019), and is intensively studied from decades in the polar regions and in the high mountain environments (Phillips et al., 2009). Geophysical prospecting can usefully help to extend the punctual borehole information to wider area, increasing the characterization of the permafrost zones (Hauck 2001; Scott et al. 1990). During the Summer 2019 we performed several geophysical tests in different permafrost sites already monitored with boreholes log temperature in the Swiss Alps. In particular we focused on electrical resistivity tomography (ERT) and Frequency Domain Electro-magnetic techniques (FDEM), in order to compare the methods and test the applicability of FDEM to characterize the active layer, in terms of layer thickness and lateral continuity (Fig. 1). ERT can provide in fact an electrical imaging of the subsoil able to discern active layer thickness, ice detection and geological features of the subsoil (Marescot et al., 2003). Contactless method, such the frequency domain electro-magnetometer (FDEM), should be preferable form the logistic point of view since: i) it can provide electrical properties of the subsoil with no need of physical electrical contact with the soil; ii) it can cover wider area of exploration respect to ERT, iii) it is faster and has easier data collection than ERT. In this work we present the application of FDEM in Swiss Alps permafrost case studies, coupled with the classical ERT measurements and the temperature borehole log information. FDEM, ERT and boreholes temperature log results are in general agreement (Fig. 2), suggesting promising advances for future active layer monitoring with contactless quick technique as the FDEM method.