electromagnetic methods (EM)
In the frequency domain method (FDEM), the transmitter emits a sinusoidal current at a specific frequency. The electromagnetic force induced in the ground will be shifted in phase with respect to the primary field. At the receiver, the secondary field generated by the eddy currents in the ground will show the same amount in phase shift. The in-phase component is used to detect the presence of ferrous objects beneath the surface and the quadrature component is related to ground conductivity.
FDEM methods produce maps of conductivity and in-phase component that are widely used for detecting geologic transitions between depositional facies, soil salinity, mapping lateral transitions in the type soil or studying leachate plumes in waste dumps and landfills.