GNGTS 2021 - Atti del 39° Convegno Nazionale

199 GNGTS 2021 S essione 2.1 CHARACTERIZATION OF VLF/LF SIGNATURES RELATED TO SEVERE WEATHER AND STRONG EARTHQUAKES FOR GEOHAZARD RISK REDUCTION C. Fidani 1 and M. Siciliani 2 1 Central Italy Electromagnetic Network, Fermo, Italy 2 “A. Bina” Seismological Observatory, Perugia, Italy Radio transmitters in the Very Low Frequency (VLF) and Low Frequency (LF) ranges, from 3 kHz to 300 kHz, which are typically reflected from the lowest ionospheric layers, are useful for a wide range of applications in addition to radiocommunication. In particular, the stability of propagation in the VLF range (3-30 kHz) makes them useful for navigation and time comparison purposes. Transmitters in VLF range are used for standard frequency and time broadcasts (Blair et al., 1967) and wide range of communications and navigational applications (Casselman et al., 1959). The stability of transmission on this range permits frequency comparison to within a few parts in 10 12 , thus permitting long range navigation utilizing phase comparison between spaced atomic frequency-controlled or phase locked transmissions. Navigation and location accuracies of 1-3 nautical miles are achieved by the Omega navigation system (Beukers, 1974). The mode theory has been developed for calculating signal levels at VLF, and may be used efficiently at LF up to at least 60 kHz. It is a full wave theory that includes diffraction and surface wave propagation, where the waves are considered to propagate between the Earth and the ionosphere as normal modes (Wait, 1964). For LF the wave-guide is many wavelenghts, and many propagation modes must be considered for short distances. Whereas, for VLF and distances greather than 1000 km Fig. 1 - The spectrogram recorded at Perugia station of the “A. Bina” Seismic Observatory for VLF band.