GNGTS 2019 - Atti del 38° Convegno Nazionale

552 GNGTS 2019 S essione 3.1 domain and allows us to obtain coherent information from seismic noise recorded by a stations pair. For each stations pairs a daily c-c function is calculated over several days and added together resulting in a stacked c-c function. It is expected to observe the propagation of surface waves at all pairs of stations symmetrically with respect to zero time because of the noise isotropy with respect to the seismic network. For a stations pair the presence of a coherent signal and its stability with time in different ranges of frequency is analysed. Data and processing . We have analysed the continuous noise recorded by stations of the local seismic network installed in Wysin in the framework of the EU project SHEER (SHale gas Exploration and Exploitation Induced Risks). The network consists of 6 surface broadband stations (CHRW, GLOD, PLAC, SKRZ, STEF, SZCZ) and 3 borehole broadband stations (GW3S, GW4S, GWS1; Fig. 1). Data are available from 1st January 2016 to 19th July 2017. This period includes the HF operations (López Comino et al. , 2018): 9th-18th June 2016 (Wysin-2H well) and 20th-29th July 2016 (Wysin-3H well). Before computing the cross-correlation functions, the single station ambient noise records were processed following the method proposed by Bensen et al. (2007). The data were decimated at 50 Hz and band pass filtered between 0.1 and 2 Hz. The mean and the trend were removed. Then, we applied the one-bit normalization to equalize the recordings in time domain and to reduce the effects of transient signals (as spikes, earthquakes and anthropogenic events). Results. We computed the c-c between the stations pairs of two months of data (1st June – 1st August 2016) in which the HF operations took place. We analysed different frequency bands within the range 0.1-2.0 Hz. The results show that a coherent phase is well identified in the band 1.5-2.0 Hz. The phase is present at all couple of stations and it propagates with a velocity of about 400 m/s (see Fig. 2a). A loss of coherence in the signal is evident before and after the HF operations and occurs at almost regular time intervals (Fig.2b and 2c). We also computed c-c functions and stacks using the seismic noise acquired in different time windows. We observe that in the frequency band 1.5-2.0 Hz there is always a coherent signal except in the winter time. We suppose, therefore, that there are seasonal factors influencing the ambient seismic noise in the study area. Fig. 2 - Two-month analysis for the Wysin site in the 1.5-2.0 Hz range. a) Cross-correlation stacks organized for increasing interstation distance. b) and c) Cross-correlations for two pairs of stations (GWS1-SZCZ and GWS1- CHRW) as a function of time. The stack of all the cross-correlation functions is shown on top of the related panel.