Very Low Frequency Electromagnetic Method
Very low frequency (VLF) electromagnetic methods are used primarily for surface and near-surface investigations of known conductive targets such as mines, pipelines, caves, and shipwrecks. VLF-EM instruments send low-frequency signals to the ground which are reflected by changes in the electrical properties of the subsurface. The VLF-EM instrument measures the strength, polarization, attenuation and phase of the reflected signal. This information can then be used to delineate subsurface structure and depth to the target feature.
In general, VLF-EM methods are low cost, easy to use and they provide a fast and effective way to detect conductive objects and subsurface anomalies. Many of these advantages stem from the use of relatively low transmitter power, which is limited by national electrical safety regulations and considerations. Low frequencies are also preferred since they tend to penetrate deeper into the ground than higher frequency signals, reducing the need for long cables and complicated multiple transmitter-receiver arrangements. The resolution of the received signals is, however, limited by the long wavelength of the VLF signal.
VLF-EM surveys are typically conducted with a handheld transmitter and receivers spread out in a linear array along the survey line. The transmitter is set up so that its antenna is oriented parallel to the survey line. This allows for increased sensitivity for a target that is perpendicular to the survey line. An alternative approach is called a “fishbone” survey, which is used to more effectively detect off axis targets. When conducting a VLF-EM survey, the instrument operators systematically traverse over a grid in order to create an electromagnetic map of the surveyed area.
VLF-EM techniques have many applications in the fields of geophysical exploration, military reconnaissance, archaeological mapping, and remediation engineering. In mineral exploration and mining, VLF-EM methods are used to detect and map buried ore deposits and petrochemicals as well as bunkers, tunnels and other underground infrastructure. In the field of military reconnaissance, the low-power VLF-EM signals can be used to detect heavily buried targets that would be undetectable with higher-powered equipment. Archaeologists use VLF-EM data to map hidden underground structures, while in the remediation engineering field, it can be used to detect buried contamination and leaking underground storage tanks.
In recent years, several improvements have made VLF-EM surveying easier and more effective than ever. Automated data acquisition systems now allow for faster data collection and real-time display of results. Additionally, software packages have been developed which allow for the integration of VLF-EM data with other geophysical data sets and seismic images. These advances have made it easier and faster to detect and characterize hidden objects and anomalies.
Overall, the VLF-EM method provides an inexpensive, intermediate-depth survey technique for locating conductive targets. Beyond its numerous applications, it is especially valuable for its ability to detect targets buried deeply enough to be invisible to surface sensors and yet too shallow for seismic surveys. The combination of increased sensitivity, automated data acquisition, and the deep penetration of VLF signals make this an ideal tool for environmental and geological investigations alike.
