GNGTS 2013 - Atti del 32° Convegno Nazionale

cylinder up right on the left of Fig. 2) with eigenfrequency 1 Hz, upper corner frequency 100 Hz, and a dy- namic range > 136 dB. The Quan- terra Q330 (http://www.kinemetrics . com/p-78-Q330.aspx) is a low-power 3-6 channel remote broad-band seis- mic system: it includes Quanterra’s ultra-low-power delta-sigma 24-bit A/D, a DSP/RAM module contain- ing 8Mb, a GPS receiver, power con- version, sensor control, and a telem- etry management module. For these characteristics the FADA station was connected via GPRS modem trans- mitting data in real time to the OGS seismic data centre in Udine. A sim- ple Short Time Average/Long Time Average (STA/LTA) detector after high pass filtering the data at 20 Hz tagged events of interest in real time, automatically sending alarm messag- es via email and Short Message Sys- tem (SMS) to OGS and Civil Defence of the Veneto Region personnel. The FADA station, being the only one in the network sending data in real time, was also equipped with an iTEm infrasonic sensor model “prs 0110a” (right in Fig. 2), with a sen- sitivity of 25 mV/Pascal @ 1 Hz, ± 100 Pa full scale range, frequency response 0.01 – 100 Hz -3 dB@ 0.02 Hz, and self noise -52 dB relative to 1 Pa. iTEM is a consulting and system design company spin off from Florence University based in Florence (Italy) working in the Earth Sciences and Geophysics (http:// www.item-geophysics.it/ ). Data analysis. After a first analysis of several months of combined seismic and infrasound recordings in the Fadalto area, after automatically pre-selecting of interesting event time windows based on high pass filter above 20 Hz and STA/LTA detector on vertical component of the seismic signal, we categorized events of interest in several categories, including: 1. Seismic and acoustic recordings of local earthquakes: seismic and acoustic signals share the same high frequency content (10-50 Hz), suggesting that the infrasonic signals is produced by ground-coupled waves. 2. Signals not associated with earthquakes or rumbles generated by the same high frequency and resonant source (Fig. 3): similar signals have been recorded by Florence University with an infrasound sensor in the Valle d’Aosta site during helicopter over flights. MacAyeal et al. (2008) however suggested in a personal communication that these signals might express stick and slip of land masses, perhaps premonitory to a large landslide. Conclusions. No direct evidence in the recordings were found of the rumbles. A deeper seismic analysis of the phenomena however indicated that the rumbles can be associated to variation of underground water levels which in turn causes: Fig. 3 – Infrasound (black top and blue bottom) and seismic (red bottom) waveforms of observed event with infrasound spectrogram (top). 199 GNGTS 2013 S essione 3.3