Electromagnetic Techniques
Very Low Frequency (VLF) Surveys
Detail
Very Low Frequency (VLF) surveying is a continuous-wave (frequency domain) electromagnetic technique that utilises high power, low frequency radio transmissions as the source. Eleven major transmitters located across the globe generate these transmissions, providing a range of frequencies from 3kHz to 24kHz. Their primary use is in communication with submerged submarines and for long-range radio positioning.
Sensed at a distance greater than a few tens of kilometres the EM transmissions act as plane waves propagating outwards horizontally. When these waves intersect a buried conductor they induce eddy currents that generate a secondary magnetic field concentric around the source of the currents. The strength of the eddy currents is greatest when the long axis of the conductor is oriented parallel to the direction of propagation (i.e. on a radial from the active transmitter). In this orientation the magnetic vector is acting tangentially. Modern VLF instruments enable measurements to be carried out at a number of different frequencies in sequence in order to ensure optimum secondary field signal strength. However, the orientation of the survey lines still has to be chosen to lie perpendicular to the expected orientation of the targets.
VLF surveys involve measuring the orientation (tilt-angle/dip-angle) of the vector summation of the primary (horizontal) and secondary magnetic field vectors. As the instrument passes perpendicularly over a vertical target the vector orientation changes from a maximum on one side to a minimum on the other side. The point at which the reading changes from positive to negative is termed the 'cross-over' point and lies directly above the conductor. If the conductor dips then the anomaly shape will be distorted in either the positive or negative sense (depending on dip direction).
The VLF method is primarily used in mineral exploration work but has also been successfully applied in engineering and groundwater surveys to detect conductive fault zones and other sub-vertical conductors.
