VLF uses very low frequency (VLF) communication signals (15–30 kHZ) generated from military transmitters around the world used to provide communication with submarines deep under water. VLF is a quick reconnaissance method used to locate water-bearing fractures and other conductivity contrasts in the subsurface that may be indicative of mineralization, contaminant plumes, and gross variations in lithology. Several companies manufacture VLF instruments; one of the most popular is the ABEM Wadi as it has an integrated software package for processing and displaying collected data.
VLF transmitters are powerful and induce electric currents in conductive bodies thousands of miles away that generate secondary magnetic effects measured by the instrument. Localized conductors, such as water-filled fractures, cause angular disturbances in this signal which can be measured. The depth of investigation is dependent on the subsurface conductivity but survey depths of several hundred feet are possible. Unfortunately military transmitter stations are not always a reliable signal source because of maintenance and budget constraints, and it may be preferable to use one on the several portable VLF transmitters that are available.
VLF has two important limitations; the first is cultural interference from nearby electric lines, metal fences and underground utilities, the second is that ideal data can only collected along profiles perpendicular to signal orientation. Cultural interference cannot be overcome and may eliminate the use of VLF as a potentially effective site tool; it is best applied in open undeveloped areas. The azimuth of effective profiles is constrained by the location of the signal source with maximum instrument response obtained along profiles oriented perpendicular to the transmitter azimuth, while maximum signal occurs along features that are linear and oriented parallel to the direction of the signal. As a result transmitter location, profile direction, and anomaly orientation are key considerations when designing a VLF survey. Topography also influences VLF data and should be considered during data reduction.
VLF interpretation is highly qualitative and subjective, but can be refined when constrained by the results of intrusive data and a sound understanding of the underlying geology. Personnel at Delta Geophysics have conducted VLF surveys in varied geologic terrains since 1987 and have the experience with this method necessary to determine its efficacy at a site and obtain optimum results from its application.
VLF surveys can be conducted quickly and provide a large data set at an inexpensive cost. Under the proper circumstances VLF is an excellent reconnaissance tool for high-grading an area in preparation for more detail geophysical investigations or test drilling, and can make an important contribution to an integrated investigation effort.