In radio communications terminology, VLF means very low frequency, about 15 to 25 kHz. Relative to frequencies generally used in geophysical exploration, these are actually very high frequencies. The radiated field from a remote VLF transmitter, propagating over a uniform or horizontally layered earth and measured on the earth’s surface, consists of a vertical electric field component and a horizontal magnetic field component each perpendicular to the direction of propagation.
The VLF method uses relatively simple instruments and can be a useful reconnaissance tool. Potential targets include tabular conductors in a resistive host rock such as faults in limestone or igneous terrain. The depth of exploration is limited to about 60% to 70% of the skin depth of the surrounding rock or soil. Therefore, the high frequency of the VLF transmitters means that in more conductive environments, the exploration depth is quite shallow; for example, the depth of exploration might be 10 to 12 m in 25-Ωm material. Additionally, the presence of conductive overburden seriously suppresses response from basement conductors, and relatively small variations in overburden conductivity or thickness can themselves generate significant VLF anomalies. For this reason, VLF is more effective in areas where the host rock is resistive and the overburden is thin.
Applications of VLF method:
- Aquifer description
- Detection and mapping layers of clay
- Identification of fractures and fissures
- Mapping of sand and gravel deposits
- Determination of the sliding plane
- Identification of buried household objects
- Identify wells