The propagation model and characteristics for extremely low frequency electromagnetic wave in earth-ionosphere system and its application in geophysics prospecting

This paper employs the finite-difference time-domain method (FDTD) to model and study the propagation characteristics and distribution features for extremely low frequency (ELF) in Earth-ionosphere (EI) system. For electromagnetic pulse source whose bandwidth is 10 Hz, the ELF electromagnetic wave forms standing wave. When the bandwidth is 100 Hz, there exists 180° phase reversals. Due to EI system, the electromagnetic field generated by horizontal dipole antenna is mainly vertical electric field and horizontal magnetic field when the receiver is away from the source antenna and locates on the surface of the earth. The distribution is mainly determined by the polarization direction of the source antenna other than the topographic feature and the conductivity. The ELF electromagnetic field can be used as incident field to detect conductivity anomaly underground. Thus, this paper may provide valuable information for long distance and large-scale underground detection, showing potential applications in geophysics prospecting.