The detection of the natural electromagnetic field inside the magneto-ionospheric cavity is of great relevance for two reasons. On the one hand it is the basis of studies for a better understanding of the phenomena relating to the physics of the magnetosphere and the ionosphere, on the other it is the basis of the research of possible emissions of ULF-VLF signals coming from the internal structures of the Earth.
The frequencies of the investigated signals range from the lowest region of the spectrum where a network of magnetic sensors for ULF frequencies and detection stations for electromagnetic signals up to frequencies of 25 kHz is active. The effects of lightning, spherics, whistlers and tweeks also fall within this range, which are typical examples of electromagnetic emissions triggered by energy pulses inside the Earth-Ionosphere cavity.
The new Cassandra experimental network for electromagnetic monitoring in the VLF band joins the IRON network for monitoring radon gas in a systematic and comparative study campaign of seismic precursors which will also employ neural network algorithms. The network includes 4 sites distributed along the Apennines.
In laboratory experiments, on rocks subjected to rupture with uniaxial compression, monitoring of the very low frequency electromagnetic spectrum (ELF – VLF) highlighted the existence of impulsive electromagnetic emissions associated with the microfracturing necessary for the enucleation of the fracture. A similar emission process was also found on the limestone slopes immediately after the quarry fronts were demolished: this time under conditions of stress relief during the settlement process of the slopes. These experimental experiences give hope for the possibility of identifying very low-frequency electromagnetic emissions in nature spontaneously generated by the mechanical process of earthquake preparation. Basically it would be a precursor associated, like many others, with the dilatancy of the rock but in this case it would be a radiation generated directly and systematically by the dilatancy.
Therefore, the continuous monitoring of VLF electromagnetic radiation as a possible seismic precursor appears promising. However, as with any other precursory phenomenon, one must not be impressed by the name: in the current state of research, an operational use for the purpose of forecasting is not yet possible. The best strategy, on the other hand, is the comparative study of as many potential precursors as possible.
Radio signals in the VLF band have wavelengths of hundreds of kilometers. The active antenna used in the monitoring network is only one meter long and works as a field receiver of the electric field component only. The working band extends from 20Hz to 20kHz, with a smooth response in the range between 1 and 13kHz and sensitivity of 1μV. Monitoring occurs continuously at the sampling frequency of 44100 Hz, which is the same frequency used in music CDs.
The working band of the VLF monitoring stations corresponds to that of the acoustic frequencies and the electromagnetic signal, converted into an electrical signal, is analyzed through acoustic spectrograms or "sonograms".