GNGTS 2019 - Atti del 38° Convegno Nazionale

298 GNGTS 2019 S essione 2.1 years. Magnetic detection for CIEN started in 2012 at Chieti Station, then it was extended at Città di Castello, Avigliano Umbro, Perugia, Narni, Norcia, Gubbio, L’Aquila and Fermo by exploiting monitoring sites for the electric detection. Presently, there are six CIEN stations working with a magnetic sensor: Chieti, Perugia, Norcia, Gubbio, L’Aquila and Fermo. Other four stations equipped with a magnetic detector were already installed and going to be activatred in Città di Castello, Avigliano Umbro, Narni and Colfiorito. Two new magnetic detectors are ready to be installed at Camerino and Santa Maria degli Angeli stations (Fig. 1). Fig. 1 - The geographical distribution of the twelve CIEN stations. Active stations in red, still inactive in orange and planned in yellow. The chosen frequency bands for electric components were in the Extremely Low Frequency (ELF) and the Very Low Frequency (VLF) ranges while, for the magnetic detector, it was chosen the ELF band only. The magnetic detector consists in a electric shielded loop oriented along the Appennine belt, NW/SEward, located underground and inside the station protected from atmospheric phenomena. After several years of experimentation, the Chieti station monitored the 1 Hz – 450 Hz band and this standard was also adopted for other CIEN stations with different types of low frequency wide band amplifiers. Different loop typologies were also tested concerning big loop and squares on air as well as small coils with iron and mu-metal cores. Magnetic detectors are shielded with respect to electric fields and connected to PC sound cards. The signals are sampled at 900 Hz with a 16 bit A/D conversion, permitting a dynamical range of about 96 dB. “Spectrum Laboratory” free software (DL4YHF, 2019) is used to implement the recording parameters and to analyze the signals utilizing Fast Fourier Transform (FFT). Data are stored in a DVD for further analysis allowing for a further comparison of the measurements. FFT analysis is saved for each band in compressed jpeg image. FFT is calculated at every 4.096 seconds with an input size of 16,384 and a Hann window function as for the electric components. The power amplitude range was chosen to cover −106 dB to −10 dB with the to_many_colors legend, which allows us to distinguish about 20 different colors and shades, permitting to evaluate 5 dB amplitude differences. Color images were saved with a resolution of 1,278 × 972 pixels. A logarithm frequency scales was used to plot the magnetic component in order to highlight low perturbations below the power supply network at 50 Hz. Time was identified on the spectrograms by dashed vertical lines, one every 300 seconds, see Fig. 2.