The VLF instrument utilizes the electromagnetic components of VLF waves (very low frequency) produced by VLF transmitters around the world. The VLF transmitters are used by military personnel to communicate with submarines. The frequency of the VLF waves emitted from these transmitters ranges from 15 to 30 Kilohertz (KHz). These VLF waves can propagate many thousands of kilometers and can penetrate deep into the Earth’s surface.
For a proper VLF survey to be performed a VLF transmitter must be located approximately on strike or trend to the geology in the study area. As with magnetic surveys, it is important to know the structural pattern of the bedrock in order to plan the flight lines in a perpendicular orientation to the geology.
An incoming VLF wave will create eddy currents in subsurface structures that are either conductive or in low in electrical resistance compared to the surrounding rock. A secondary magnetic field, in opposition to the primary field, is then created by these eddy currents. The resulting measurement by the instrument is the ratio (%) of the strengths of the vertical and horizontal fields. The amplitude and phase components of the particular wavelength in VLF are referred to as in-phase (real) and quadrature (imaginary).
Frequencies can be recorded from VLF transmitter stations, which have the best orientation relative to the local geology. The three most common stations used in Canada are Culter, Maine (24.0 khz); LaMour, North Dakota (25.2 khz) and Jim Creek, Washington (24.8 khz). Both in-phase and quadrature components are useful measurements for identifying possible ore bodies.
VLF total field grid during a survey
As with all electromagnetic methods, VLF surveys are susceptible to cultural interference such as powerlines, therefore, advance knowledge of any utilities in the survey area is important. In addition, VLF data should not be collected during periods of increase solar activity or electromagnetic storms due to the interference cause in the atmosphere between VLF transmitter stations and the receiver.
GEM offers Ground and Airborne VLF systems. For more information and pricing please contact [email protected].