GNGTS 2015 - Atti del 34° Convegno Nazionale

138 GNGTS 2015 S essione 3.3 for determining the H/V spectral ratio were acquired (recording time 20 min; sampling rate 64 Hz) with the same 3C geophone used for the active HS acquisitions. The source considered for the active data was a standard 8-kg sledgehammer and for the HF acquisition we used a simple and classical wooden beam. As previously stated, for the sake of brevity here we will consider just three components: the HVSR, the Rayleigh-wave effective dispersion curve (i.e. the phase velocities defined via MAAM) and the Love wave group velocities (from the active dataset). As also pointed out by Cho et al. (2013), during the analyses (being a pilot project data were acquired using various seismographs) we verified that the Signal-to-Noise (S/N) ratio determined by the adopted acquisition system , reveals as crucial in the acquisition of the data then used for the Miniature Array Analysis of Microtremors . The noise inevitably introduced in the data by non-optimizedA/D units, can in fact represent a serious problem that cause incorrect (lower) phase velocities in particular in the low-frequency range (see the theoretical description in Cho et al. , 2013), being the noise compensation procedure capable of compensating the noise only when its level does not reach extreme values. Following the procedure suggested by Cho et al. (2013), we divided the data into small windows and, in order to remove the possibly-pernicious effects of large-amplitude transients, removed all the segments having an average amplitude larger than a threshold fixed on the basis of the mean root-mean-square ( rms ) values of the acquired traces. While the highest frequency which can be analyzed is limited by mere spatial aliasing effects and is then related to the adopted radius of the array [its value can be estimated in about 3.5 times the wavelength λ as defined in Cho et al. (2013): see the 3.5-λ line reported in Fig. 2b], the lowest frequency that can be soundly determined depends also on the quality of the adopted acquisition system (that determines the amount of electronic noise that can significantly pollute the data and analyses, especially in the low-frequency range). For the present pilot survey, we actually acquired 2 MAAM data sets (according to two different radii - see acquisition parameters reported in Tab. 1): while the 2-m radius allowed to retrieve the dispersion curve in the 4÷16 Hz frequency range (see Fig. 2b), the 5-m radius array provided reliable phase velocities approximately in the 2÷10 Hz range. Tab. 1 - MAAM acquisition parameters (see also Fig. 2a). sampling rate 4 ms (Nyquist frequency 125 Hz) acquisition length 30 min radius 2 m; 5 m sensors four vertical 2Hz geophones Tab. 2 - HS acquisition parameters. sampling rate 1 ms (1000 Hz) acquisition length 1 s offset 40 m one 3-component 2Hz geophone (used also for the acquisition sensor of the microtremor data used to define the HVSR) stack 4 For validation purposes, MAAM analyses were compared with the results obtained from standard passive multi-channel bidimensional array (L-shaped 18-channel configuration), processed according to the ESAC methodology and the obtained dispersion curves resulted perfectly consistent.