The electromagnetic methods (EM) are applied for environmental, geotechnical or hydrogeological purposes.
There are two types of electromagnetic (EM) survey: the first one, based on frequency domain, measures the amplitude and phase of an EM induced field (FDEM), the second one, based on the time domain, measures the decay time of an electromagnetic pulse induced by a transmitter (TDEM).
The investigations in the frequency domain electromagnetic (FDEM) involve the shallow part of the underground (5-10 m). The method allows to obtain, in a short time, surface maps values of apparent electrical conductivity and magnetic susceptibility of the subsoil.
The frequency-domain electromagnetic method is used in the following fields of application:
The survey method TDEM (time domain electromagnetic survey) is applied to reconstruct the distribution of real resistivity of the subsurface at depth (down to 300 - 400 m).
The typical setup for investigation of resistivity survey consists of a transmitter connected to a coil of electrical cable (square, rectangular or circular) placed on the ground and a receiver coil, of equal size, connected to a receiver via a cable. The size of the coil is proportional to the depth of investigation (the greater the area of the coil, the greater the depth of investigation).
The TDEM surveys are used to evaluate: