GNGTS 2017 - 36° Convegno Nazionale

652 GNGTS 2017 S essione 3.2 the peak amplitude exhibits a minimum that correlates well with the orientation of typical crevasse patterns in a glacier. In addition, the H/V curves often exhibit the peak/trough structure (e.g. the Pian di Neve, Lobbia and Forni glaciers), suggesting that the propagation of Rayleigh waves drives the shape of the H/V spectra. However, in some cases the peak/trough structure is less evident (e.g. the Aletsch glacier), indicating a predominance of Love waves, which should be taken into account when width and depth of the investigated glacier are comparable. The analysis of three-component seismic data acquired on the WIS (west Antarctica) showed that the results of this technique have to be carefully evaluated when the seismometers are coupled with a thick firn layer at the surface, or when highly deformable sediments are present underneath the ice body. In case the propagation of Rayleigh surface waves is dominant and the glacier thickness can be constrained, ellipticity inversion of the H/V spectra allows us to infer the seismic properties of the basement in the presence of 1-D structures and a rigid bedrock. This work shows that the HVSR technique provides a cost-effective alternative to active-source geophysical methods, that have been used as the standard for several years to map glacier thicknesses and to estimate bedrock properties. The minimal logistics involved in the HVSR measurements allow for mass-balance studies also in extreme environments (e.g. Himalaya and Karakoram glaciers), where the common methods are generally impractical. A single operator in a working day could easily carry console and seismometer and sample glacier geometry at several locations. In addition seismic, geoelectric and RES methods often generate ambiguous or unsatisfactory results for different reasons, for example, coupling problems, wave scattering, high amplitude attenuation in the shallow layers and when the target depth is beyond the imaging capability. In these cases, the HVSR methodology may provide reliable thickness estimates and resolve ambiguities, notwithstanding that the experimental error could become large for thicknesses exceeding 700-800 m. Finally, when the ice thickness is known from other sources, the method allows easy discrimination between a rigid bedrock and a deformable basement (or water) beneath the ice body. Acknowledgements We dedicate this paper to the memory of Marco Mucciarelli, who encouraged us to perform this study. This work was supported by the Italian National Program of Antarctic Research (PNRA–WISSLAKE Project). References Bard P. Y. and Bouchon M.; 1985: The two-dimensional resonance of sediment-filled valleys . Bull. Seismol. Soc. Am., 75 , 519-541. Bonnefoy-Claudet S., Köhler A., Cornou C., Wathelet M. and Bard P. Y.; 2008: Effects of Love waves on microtremor H/V ratio . Bull. Seism. Soc. Am., 98 , 288-300. Carcione J. M., Picotti S., Francese R., Giorgi M. and Pettenati F.; 2017: Effect of soil and bedrock anelasticity on the S-wave amplification function. Geophysical Journal International, 208 (1), 424-431. Fäh D., Kind F. and Giardini D.; 2001: A theoretical investigation of average H/V ratios . Geophys. J. Int., 145 , 535–549. Konno K. and Ohmachi T.; 1998: Ground-motion characteristics estimated from spectral ratio between horizontal and vertical components of microtremor . Bull. Seism. Soc. Am., 88 (1), 228–241 Nakamura Y.; 2000: Clear identification of fundamental idea of Nakamura’s technique and its applications . Proc. 12 th World Conf. on Earthquake Engineering, paper #2656. Nogoshi M. and Igarashi T.; 1971: On the amplitude characteristics of microtremor (part 2) . J.. Seism. Soc. Japan, 24 , 26-40. Picotti S., Francese R., Giorgi M., Pettenati F. and Carcione J. M.; 2017: Estimation of glaciers thicknesses and basal properties using the horizontal-to-vertical component spectral ratio (HVSR) technique from passive seismic data. Journal of Glaciology, 63 , 229-248. Tuan T.T., Scherbaum F. and Malischewsky P.G.; 2011: On the relationship of peaks and troughs of the ellipticity (H/V) of Rayleigh waves and the transmission response of single layer over half- space models . Geophys. J. Int., 184 (2), 793–800 Wathelet M.; 2008,: An improved neighborhood algorithm: parameter conditions and dynamic scaling . Geophys. Res. Lett., 35 , L09301. Wathelet M., Jongmans D. and Ohrnberger M.; 2004: Surface wave inversion using a direct search algorithm and its application to ambient vibration measurements . Near Surf. Geophys., 2 , 211–221.