In radio communications terminology, VLF means Very Low Frequency, typically about 15 to 25 kHz. VLF-EM is a geophysical survey technique that measures VLF signals from very powerful radio transmitters that induce electric currents in conductive bodies thousands of kilometres away. These powerful transmitters are located around the world, and are used primarily for military communication purposes with submarines. Signals from these transmitters act as primary fields that are capable of energizing conductive bodies (such as graphite, metallic minerals and structures) in the ground. Once energized, the current within these bodies emits a secondary field which can be measured by VLF-EM receivers. VLF-EM surveys can be done with ground or airborne instruments, and data from these surveys can then be analyzed and interpreted to assist with mapping of conductive materials within the survey area.
VLF-EM Transmitter Stations
In North America there are four transmitters; a very powerful one in Cutler, Maine (24.0 kHz), another of medium power in LaMoure, North Dakota (25.2 kHz), one in Jim Creek, Washington (24.8), and another in Puerto Rico (40.8 kHz). Around the world, additional VLF stations exist in Iceland, UK, Germany, Norway, Italy, Turkey, India, South Korea, Japan, Australia and Hawaii.
| Frequency (kHz) | Call Sign | Power | Location | Latitude | Longitude |
| 16.3 | VTX1 | Southern India | +08.387015 | +077.752762 | |
| 16.4 | JKN | 45 kW | Novik, Norway | +66.974353 | +013.873617 |
| 17.0 | VTX2 | Southern India | +08.387015 | +077.752762 | |
| 18.2 | VTX3 | Southern India | +08.387015 | +077.752762 | |
| 19.2 | VTX4 | Southern India | +08.387015 | +077.752762 | |
| 19.58 | GBZ | 500 kW | Anthorn, UK | +54.911643 | -003.278456 |
| 19.8 | NWC | 1000 kW | Exmouth, Australia | -21.816328 | +114.165586 |
| 20.27 | ICV | 20/43 kW | Tavolara, Italy | +40.923127 | +009.731011 |
| 21.4 | NPM | 566 kW | Pearl Harbor, Hawaii | +21.420166 | -158.151140 |
| 22.1 | GQD | 500 kW | Skelton, UK | +54.731799 | -002.883033 |
| 22.2 | NDT | 200 kW | Ebino, Japan | +32.082084 | +130.827960 |
| 23.4 | DHO38 | 800 kW | Rhauderfehn, Germany | +53.078900 | +007.615000 |
| 24.0 | NAA | 1000 kW | Cutler, Maine | +44.644936 | -067.281639 |
| 24.8 | NLK | 250 kW | Jim Creek, Washington | +48.203487 | -121.916827 |
| 25.0 | unid25 | Mokpo, South Korea | +34.679068 | +126.445383 | |
| 25.2 | NML | 500 kW | La Moure, North Dakota | +46.365990 | -098.335638 |
| 26.7 | TBB | Bafa, Turkey | +37.412725 | +027.323342 | |
| 37.5 | NRK/TFK | 100 kW | Grindavik, Iceland | +63.850365 | -022.466773 |
| 40.8 | NAU | 100 kW | Aguada, Puerto Rico | +18.398762 | -067.177599 |
Airborne VLF-EM Surveys
Terraquest’s Matrix VLF-EM system by Magenta Inc. is a newly developed digital frequency specific VLF system as well as broadband capability. It utilizes three orthogonal coils mounted in the aircraft stinger, coupled with a receiver-console. The primary signals originate from up to four VLF frequencies, and full parametrization of these VLF-EM signals are recorded. The system typically responds to variations in overburden conductivity, to large faults or shear zones, and to graphitic formational conductors. Because of these characteristics, Matrix VLF-EM can be useful as a geological structure mapping tool, particularly when combined with magnetics and radiometrics.
Terraquest has recently added optional inversion modelling to create resistivity products which further enhance the Matrix VLF-EM data to focus on project specific geological features. Using these resistivity products, further interpretations from the VLF-EM data may include the following:
- characterize rock types by their inherent resistivity
- enhance the resolution of contacts between lithologies, independently from magnetics
- augment the conductor axis provided by the total field amplitude
- identify resistivity lows commonly associated with contact zones, alteration and mineralized structures
- downward continuation using progressive depth slices
- discriminate and map surficial conductivities separately from the deeper bedrock responses
Specialists in Airborne VLF-EM Surveys
Terraquest was established in 1984 and since then, our dedicated team has flown over 1,500 airborne geophysical surveys using both fixed wing and helicopter platforms. Our professional crews provide significant experience having carried out airborne magnetic, gravity, gamma ray spectrometry, and electromagnetics on five continents. Our quality data sets have been utilized in the exploration for base and precious metals, kimberlite, hydrocarbons, uranium, rare earth minerals and water. The company has performed exemplary surveys for both small and large exploration groups as well as many government agencies. References can be provided upon request. Visit www.terraquest.ca.
