Very-low-frequency (VLF) Methods
Overview
VLF survey methods use very-low-frequency, radio communication signals to determine electrical properties of shallow bedrock and near-surface soils, primarily as a reconnaissance tool. VLF profiles can be run quickly and inexpensively to identify anomalous areas warranting further investigation by other surveys, drilling or sampling.
The technique is especially useful for mapping steeply dipping structures such as faults, fractures and shallow areas of potential mineralization. Depth of investigation varies from 4-5 meters in conductive soils to 40-60 meters in highly-resistive soils. more
VLF techniques measure the perturbations in a plane-wave radio signal (15-30 kHz) emanating from one of several worldwide radio transmitters maintained for submarine communications. Military transmitters in Bangor, Maine; Seattle, Washington; Annapolis, Maryland; and Lualualei, Hawaii provide adequate coverage for this purpose for all of North America.
VLF instruments measure two components of the magnetic field: the “tilt angle” and ellipticity. Some instruments also measure a third magnetic component and/or the electrical field.
Logistics
VLF instruments are backpack portable and operated by one person. Productivity depends on the terrain and vegetation, but generally, several kilometers of line may be covered in a good day. State-of-the-art instruments include software to store the data and survey coordinates which are dumped to a laptop computer at the end of the day. more
Magnetic field measurements do not require ground contact and can be made in less than a minute at each station. Station spacing may vary from 5 to 20 meters (15 to 60 feet) depending on the objective.
The electrical field is measured by inserting two probes about 5 meters apart into the ground and measuring the potential difference at the transmitter frequency, which provides added information about the overburden thickness and conductivity.
Results
VLF interpretation is generally subjective in nature (in contrast to quantitative modeling which requires high data density and a well-constrained model). Anomalous areas are identified and a gross characterization attached to the anomaly (e.g., steeply dipping conductor or thickening conductive overburden).
Considerations
Some weaknesses of VLF surveys include:
- sensitivity to cultural interference from pipelines, utilities, fences, and other linear, conductive objects
- bias from topographic effects that are difficult to remove from the data
- unfavorable ionospheric conditions can sometimes compromise data quality
- military VLF transmitters being subject to outages