VLF-EM surveying is a geophysical technique using Very Low Frequency radio transmissions. The continuous, high power, low frequency electromagnetic transmission ranges from 3 kHz to 24 kHz. When the waves come in contact with a conductive material in the subsurface, they generate a secondary electromagnetic field around it.
These instruments monitor a number of different VLF frequencies simultaneously to measure these secondary electromagnetic fields induced by the distinct VLF transmitters . Once these conductors are identified in the data, the orientation, tilt-angle and/or dip-angle of both primary and secondary fields will determine its position.
As the instrument passes perpendicular over a conductive material that is standing vertical, the VLF response will change from a maximum on one side and a minimum on the other. This point, where it changes from positive to negative is called the ‘crossover point' and is directly above the conductor. If the conductive material dips, then the shape of the VLF response will be distorted to be either positive or negative.
Once a survey is completed, the data acquired normally consists of three parameters relating to the secondary field. These three parameters include; the amplitude of the field, its quadrature and in-phase components in relation to the primary electromagnetic field. Once examined they are typically presented as profiles over distance.
Further filtering is performed to produce a clearer and well defined location of the crossover point which aids in the interpretation of the anomaly. Even more advanced filtering of the real component provides clearer details and provides clues to the degree of conductivity of the body. This advanced filtering can also provide an indication of the depth of the source of the anomaly.
Additionally, the inverse of a conductivity measurement can be made with the Matrix VLF-EM survey. If a weak secondary electromagnetic field is measure, it can indicate the presence of a non-conductive or resistive body.
VLF-EM data provides a very unique set of data that can be used to create imagery of structure and even stratigraphy, where most other types of airborne geophysical surveys cannot, as it can map contrasts in conductivity and resistivity of the subsurface. While VLF-EM data has not been commonly used in exploration, it is becoming a more attractive option with the development of new VLF-EM systems.
An example of new technology is that of Terraquest Ltd of Markham, ON who is generating a great deal of excitement with its proprietary airborne Matrix VLF-EM system which uses three over-sized orthogonal coils to map geological structures from the air. The Matrix VLF-EM technology maps fine grain, low conductivity and more disseminated structures providing a more comprehensive database for geologists trying to sort our geological complexities. Consequently, this technology is suited to mapping structure related to any exploration commodity.
Specialists in Airborne Geophysical 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 surveys, airborne gravity surveys, airborne radiometric surveys, and airborne electromagnetic surveys 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.
Ed. GMH