Study on Propagation Characteristics of Extremely Low-Frequency (ELF) Electric Fields in Terrestrial Channels

Extremely low-frequency (ELF) electromagnetic waves, distinguished by their long wavelengths and strong penetration capabilities, are extensively utilized in strategic communications, geophysical exploration, industrial and rescue operations, and scientific research. Consequently, investigating their propagation characteristics is of significant importance. To validate the existence of terrestrial waveguide channels for ELF electromagnetic waves in an air–ground stratified medium, a field experiment was conducted using a horizontal electric dipole (HED) source. The propagation attenuation and phase velocity of the ELF electric field were measured. By integrating theoretical analysis with experimental data, the attenuation law of ELF waves in the air–ground multilayer medium and the presence of subsurface channels were verified. The experimental results indicate that within the test area, a distinct attenuation “inflection point” occurs at approximately 10 km. For transmitter–receiver offsets less than 10 km, the electric field amplitude decays rapidly with increasing distance, whereas for offsets beyond 10 km, the attenuation rate diminishes significantly. Phase velocity measurements at 1 Hz yield values of 54–55 km/s, substantially lower than the speed of light in free space. The corresponding subsurface conductivity, σ ≈ 0.0033 S/m, falls within the typical range of terrestrial near-surface conductivity (0.001–1 S/m), thereby confirming the contribution of subsurface waveguide channels to ELF electric field propagation over land.