Very Low Frequency Technique:

VLF measures perturbations in the magnetic field of radio waves. The perturbations are cause by geologic subsurface features. Thus by examining the nature of the magnetic field's perturbations we can identify anomalous areas in the subsurface.

VLF is used in several applications. Aquifers and groundwater and can be located. Leachate can be identified for environmental investigations. Geotechnical applications include fault and overburden mapping. Also, likely mineral deposits can be discovered for mining surveys.

VLF techniques utilizes a plane-wave radio signal (15-30 kHz) emanating from one of several worldwide radio transmitters used for submarine communications. Military transmitters provide radio signal coverage for all of North America. The magnetic field of the military's radio signal (primary field) causes electric currents to be induced in conductive geologic units or structures. Those eddy currents in turn create a secondary magnetic field. This secondary magnetic field interacts with the primary magnetic field emanating from the military transmitters creating one perturbed magnetic field. The perturbed field may be phase shifted and oriented in a different direction than the primary field depending on the shape or geometry of the conductor, the orientation of the conductor, and the conductivity contrast with the surrounding material (e.g., the host rock).

This perturbed magnetic field is measured by the VLF receiver. Because certain information is known about original magnetic field created by the military transmitters, perturbations can be assessed and attributed to subsurface eddy currents. Because these perturbations are caused be subsurface features, the nature of the perturbations enable us to deduce things about the subsurface.

All VLF instruments measure two components of the magnetic field or equivalently the “tilt angle” and ellipticity of the field. Some instruments also measure the third magnetic component and/or the electric field. The electrical field is measured by inserting two probes in the ground, spaced about 5 meters apart, and measuring the potential difference at the transmitter frequency. The electric field provides additional information about the overburden thickness and conductivity.

The advantages of VLF techniques are that a large area can be covered quickly and only a one person crew is required. This makes VLF very economical. However, although VLF can identify anomalous areas, it is limited in its ability to provide detailed information about the nature of the geologic bodies causing the anomalies. For these reasons, VLF is generally used a as a reconnaissance tool for identifying areas requiring further investigation.