Forward modeling for “earth-ionosphere” mode electromagnetic field

A fixed artificial source (greater than 200 kW) was used and the source location was selected at a high resistivity region to ensure high emission efficiency. Some publications used the “earth-ionosphere" mode in modeling the electromagnetic (EM) fields with the offset up to a thousand kilometer, and such EM fields still have a signal/noise ratio of 10–20 dB. This means that a new EM method with fixed source is feasible, but in their calculation, the displacement in air was neglected. In this work, some three-layer modeling results were presented to illustrate the basic EM fields’ characteristics in the near, far and waveguide areas under “earth-ionosphere” mode, and a standard is given to distinguish the boundary of near, far and waveguide areas. Due to the influence of the ionosphere and displacement current in the air, the “earth-ionosphere” mode EM fields have an extra waveguide zone, where the fields’ behavior is very different from that of the far field zone.