GNGTS 2024 - Atti del 42° Convegno Nazionale

Session 3.3 GNGTS 2024 Sample frequency is equal to 100 Hz so, as to investgate the whole range of frequencies of the sensor, we employ a signal windowing of 16.384 samples. We thus calculated the so-called Power Spectral Density (PSD, Fig. 2), the amplitude spectra and the spectral rato (Nakamura, 1989) (Fig. 3d), as their temporal variability identfes the main noise sources at the recording sites, defnes the noise levels of a staton and assesses the earthquakes detecton capability. Fig. 2 shows the PSDs calculated for the three components and the seasonal ones of the vertcal component, for the whole period (308 days analysed). Comparing our results with the NLNM and NHNM reference curves (Peterson, 1993) we observed that the noise spectra levels are generally high and exceed the upper limit for period below 2 s and above 10-20 s. In the microseisms band (0.1-10 s), we observed the typical peaks, the so-called single (SF) and double frequency (DF), as a result of the interacton between atmosphere, sea surface, and seafoor (Webb, 1998). Fig. 2 – Seasonal and whole period PSD’s for the three components. All the curves are reported in grey, while the mean one is color-coded. Peterson reference curves in black. Because it is not always possible to check the arrangement of the instrumentaton during the deployment, is fundamental the a posteriori establishment of the eventual correcton to apply to the horizontal channels signals as to bring them to the conventonal Cartesian reference. Among the several approaches known in the literature, we choose that proposed by Doran and Laske (2017) which is based on the measurement of the Rayleigh wave arrival angles. The result highlighted a deviaton from north of 107°; therefore, by applying this correcton we observed that the signal is almost constantly polarized towards SSE-NNW (Fig. 3c).