Electromagnetic surveys

The Very Low Frequency method is based on the principle of electromagnetic induction and is particularly suitable for the investigation of rapidly submerging (pseudo-vertical) electrically conductive and elongated bodies. The primary field, in this case, is induced by military broadcasting stations positioned at strategic points around the planet, which were used to transmit with submarines. These signals, being transmitted from far away from the survey points, can be considered to have a predominantly horizontal component and, permeating the subsoil, induce electric currents inside the buried conductive bodies which in turn generate vertical secondary electromagnetic fields. So the principle is the same as for induced electromagnetic methods, only that in this case, the instrument used is only a receiver, can be used kilometers away from the transmitter (therefore the greater depth of investigation), and operates at lower frequencies.

At buried pseudo-vertical conductive bodies the resulting electromagnetic field (primary + secondary) will have a positive peak before the body, will be zero at the body and will have a negative peak immediately after.

The resistivity values that are obtained vary according to the type of material examined, the presence of soil or rock, the presence of cavities, the thickness and type of debris cover and the humidity conditions.