The method is based on the emission of radio-frequency electromagnetic pulses by a transmitting antenna. As these waves pass through materials, they are partially reflected and diffracted at physical discontinuities (changes in dielectric constant or electrical conductivity) and detected by a receiving antenna. By analyzing the time and intensity of the reflected signal, information on the nature, depth, and geometry of the objects or layers passed through can be obtained.

The system consists of two coils: a transmitter (Tx), which generates an alternating electromagnetic field of known frequency, and a receiver (Rx), which measures the ground response. When the primary field passes through the ground, it induces electric currents in the conductive materials. These currents, in turn, generate a secondary field, the intensity of which depends on the electrical conductivity of the medium.

The method allows two components to be measured:

• The quadrature component, used to calculate the apparent electrical conductivity;
• The in-phase component, which is sensitive to magnetic susceptibility and the presence of metallic materials.

FDEM is a rapid technique suitable for detailed mapping of large portions of land.

These electromagnetic variations, which propagate into the Earth’s interior, allow investigation of the distribution of electrical properties (resistivity) in subsurface materials. The classical magnetotelluric (MT) survey is mainly used for studies at great depths, due to the ability of low-frequency waves to penetrate up to tens of kilometers.

Its high-frequency variant, known as audiomagnetotelluric (AMT), on the other hand, allows for greater resolution at shallower depths, generally between a few hundred meters and a few tens of meters. The high depth of investigation, combined with the absence of artificial sources and virtually zero environmental impact, makes the MT-AMT method an extremely effective and noninvasive tool for geophysical exploration in both regional and local settings.