GNGTS 2017 - 36° Convegno Nazionale

GNGTS 2017 S essione 3.2 649 Ambient vibration recordings and the horizontal-to-vertical component spectral ratio (HVSR) technique used to map glaciers thickness and basal properties S. Picotti 1 , M. Giorgi 1 , R. Francese 2 , F. Pettenati 1 1 OGS – Istituto Nazionale di Oceanografia e di Geofisica Sperimentale, Trieste, Italy 2 Dipartimento di Fisica e Scienze della Terra, Università di Parma, Italy Introduction. The application of geophysical techniques on glaciers can be very useful to study the overall impact of climate changes on the terrestrial cryosphere. The multichannel seismic, geoelectric and GPR (ground penetrating radar) methods are the most popular and widely geophysical techniques used in glaciology. Besides enabling to monitor the glacier thickness and then the ice mass balance, these methods can provide useful information on the basal sediment properties, which influence the overall glacier flow. However, these methods require considerable economic and operational efforts, and can be used only where the logistical difficulties are easily overcome. Passive seismics can help to solve this problem, by using broadband seismometers and the HVSR (Horizontal to vertical spectral ratio) method. Microtremor measurements and the Nakamura’s HVSR technique, generally used for site-effect studies as well as to determine the thickness of soft sediment layers, can be effectively used to map the glaciers thicknesses. The application of the HVSR technique on glaciers, requiring less economic and operational efforts, may allow to avoid most of the logistical difficulties characterizing the active seismic and GPR methods in these extreme environments. The great advantage of passive seismic tools, in addition to the reduced size and weight, is that there is no need of an active source of elastic waves. In fact, taking advantage of the ambient noise, they does not require the use of artificial sources such as explosives, which are usually detonated in boreholes to be drilled using hot water. Here we resume the work of Picotti et al. (2017), who performed several tests regarding the reliability of the HVSR method on ice. These tests were carried out on some Alpine glaciers and in the Antarctic continent. Methods and theory. We used different analytical techniques for the different methodology adopted to image the glacier bottom and validate the ice thickness obtained from the HVSR technique. In particular, for the active seismic method, the traveltime inversion of P and S diving waves in the firn allowed us to determine the vertical velocity gradient down to approximately 10 m depth, where the medium is pure ice. Below the firn, an iterative imaging technique involving P-wave velocity analysis, tomography and pre-stack depth migration (Picotti et al. , 2017) has been used to produce a vertical seismic section of the glacier, basal sediments and bedrock. The HVSR method was originally introduced by Nogoshi and Igarashi (1971) and first applied by Nakamura (1989). The method is based on the frequency spectrum obtained by dividing the horizontal component by the vertical component, either displacement, particle velocity or acceleration, since the results are equivalent. The source can be ambient noise, earthquakes or active sources of different nature. It has been shown that for Rayleigh waves propagating in a layer over a half space, the method yields the fundamental resonance frequency and the related amplitude. In general, the HVSR peak corresponds to Rayleigh-wave, Love- wave and/or S-wave resonances (���������������� Bonnefoy-Claudet et al. , 2008�� ����� �������������� ������� ). Other investigations suggest that the H/V ratio provides the site S-wave transfer function. 2D ���������� ��� ������ �� ��� �������� �� ���� ����� �� ���� ������� ����� ��������� � resonances are caused by the trapping of body waves in soft layered media overlying a solid bedrock. The fundamental S-(body) wave resonance frequencies corresponding to a 2D basin of half-width w and thickness h are (1)