Five years of surveying with the Very Low Frequency—Electro magnetics method

Abstract

The VLF-E.M. (Very Low Frequency-Electro Magnetics) method was first used for geophysical surveying in 1964. Since then many hundreds of surveys have been carried out; several variations on the original design have been introduced; and the method has become airborne. The VLF instrument is a directional radio receiver, which can be tuned to distant radio transmitters in the 15–25 kHz band. The first VLF instrument and the one most widely used to date measures the vertical real and imaginary components of the magnetic field vector. The imaginary component has been found useful in distinguishing between conductors lying at different depths below ground. Ground conductivity is also interpretable. Relative conductivity-thickness of a buried conductor affects the VLF response but is harder to interpret. The VLF-E.M. method has established its usefulness in geophysical surveying for massive sulphides and for geological mapping. Field results suggest that it may also play a useful role in exploring for the more disseminated ores, where suitable conductivity contrasts at these frequencies exist.