Induction principle. A primary magnetic field generates eddy currents in the ground that produce a secondary magnetic field recorded at the receiver
Electromagnetic methods (EM) exploit the close relationship between electric and magnetic fields. A transmitter releases a time-varying electric current into a transmitter coil. The current in the transmitter coil generates a primary magnetic field. The magnetic field induces an electromotive force in the ground that generates eddy currents. These currents induce a secondary magnetic field opposing the primary magnetic field. The receiver coil measures two fields: the primary from the transmitter and the secondary from the eddy currents in the ground.
There are two kind of EM methods known as time-domain (TDEM) and frequency-domain (FDEM). The first one measures the decay of the induced EM pulse whereas in the second, the transmitter current varies sinusoidally with time at a fixed frequency that is selected on the basis of the desired depth of exploration of the measurement (high frequencies for shallower depths).
Electromagnetic methods (EM)