Electromagnetic (TEM) surveying methods make us of the response of the ground to the propagation of electromagnetic field. This response vary according to the conductivity of the ground (in S/m).
Primary TEM fields are generated using a alternating current in a loop wire (coil) or a natural TEM source.
The reponse of the ground is the generation of a secondary TEM field. The resultant field is detected by the alternating currents that thery induce in a receiver coil.