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.
- Planimetric layouts with measurement of apparent resistivity contrasts
- Punctual high-frequency mapping of anomalies up to a maximum depth of 12 m from the sensor
- Area mapping of large areas of the presence of aquifers by induction of a secondary low-frequency electromagnetic field.
- Conductive materials such as metal, pipes, barrels, cables, concrete reinforcements, drains
Location of faults and karstic or highly fractured areas (location of aquifers)
- Mapping of buried structures and presence of bedrock
- Mapping of aquifers
- EMFAD® electromagnetic sensor system – UG12 DS/WL. It works in passive reception mode of external Radio Stations and in active mode through two electromagnetic wave transmitters
- External frequencies: 0-160kHz
- TRX2 and TRX3: 5, 7, 9, 19, 24, 33, 70, 90 and 124 kHz