“… a European radio network for studying the VLF/LF radio precursors of earthquakes, called the INFREP network.” (Biagi et al., 2019)
General:
VLF/LF waves from terrestrial narrowband transmitter propagate in the so-called Earth-ionosphere waveguide, established by the Earth’s surface (land and surface of the sea) as lower boundary, the D/E-region of the ionosphere as upper boundary (day/night, i.e., approx. 60-90 km altitude), and the great circle path between transmitter and receiver as second (latitudinal/longitudinal) dimension.
Electric- and magnetic-field variations between transmitter and receiver have a characteristic pattern, e.g., stong attenuation during terminator times or higher variations during night time (ionospheric E-region). Superposed on this pattern are waveguide modulations caused by external influences, e.g., solar x-ray flares or gamma-ray bursts on the upper boundary (the lower ionosphere), and modulations on the surface or induced by the lithosphere – among them natural sources like strong earthquakes, volcanic eruptions or artificial (man-made) sources.
A single INFREP station receives simultaneously up to 10 transmitter (equal to 10 paths), and the whole INFREP network (several spatially distributed receivers) enables the sampling of paths (electromagnetic links) in a “spiderweb”-like spatial structure. This setting pinpoint (local) sub-ionopsheric waveguide modulating sources, and of course the radio links between transmitter and receiver shall cross earthquake prone areas over Europe. In practical terms these are electric field strength variations measured with the INFREP antennas (plus the analogue and digital electronics with the data recording). In general, INFREP has the potential for applications in natural hazards investigations via VLF/LF radio wave propagation.