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Electromagnetic Method
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Electromagnetic (EM) methods include frequency domain EM methods, such as EM induction, EM utility locator/metal detection methods, and very low frequency (VLF) EM, as well as, time domain EM methods. FPM Geophysical and UXO Services geophysicists have successfully utilized a wide variety of EM methods for hydrogeologic, engineering, and environmental investigations.
EM Method (frequency domain) EM induction and EM utility location are the most common EM techniques applied to shallow engineering and environmental investigations. EM induction surveys are often conducted using the Geonics EM31, EM34, and EM38 terrain conductivity meters. The EM31 consists of a transmitter coil mounted at one end and a receiver coil mounted at the other end of a 12-foot long plastic boom. Apparent conductivity and in-phase field strength are measured and stored along with line and station numbers in a digital data logger. The EM31 can explore to depths of about 18 feet, but is most sensitive to materials about 3-4 feet below ground surface. The EM38 has a 1-meter coil spacing and is designed for characterizing the upper 1.5 meters. The EM34 consists of a large transmitter and receiver coil connected by a reference cable. It measures conductivity at coil separations of 10, 20, or 40 meters and is used for exploration to depths up to 60 meters. EM Method (time domain) The most commonly used example of a time domain EM instrument is the Geonics EM61. In general, the EM61 generates a pulsed primary magnetic field that induces electrical currents to flow, temporarily, in nearby metallic objects. The induced currents are detected and measured by two receiver coils mounted on the EM61 assembly. The EM61-MK2 can measure the decaying eddy currents at up to four different time gates, facilitating the detection of small targets and improved target characterization compared to the standard EM61. In addition, the MK2 uses an improved data logger that can be fully integrated with a Global Positioning System (GPS) so that geographic coordinates can be recorded along with the EM61 measurements. Ferrous and non-ferrous metals are detected by the EM-61, an important capability for UXO detection, as some ordnance components may be made out of non-ferrous metals such as aluminum and brass. EM Utility Location Method EM utility locators are used for tracing metallic pipes and utility cables and clearing drilling and excavation locations. These utility locators consist of a separate transmitter and receiver. The transmitter emits a radio frequency EM field that induces secondary fields in nearby metallic pipes and cables. The transmitter can also be directly connected to a target line for more accurate discrimination of the target utility. The receiver detects these fields and accurately locates and traces the pipes, often to distances over 200 feet from the transmitter. Modern utility locators are also capable of providing approximate depth estimates of the pipes and sweeping areas for 60 Hz signal emanating from live electrical lines. VLF EM Methods VLF surveys measure various components of the VLF EM fields in the frequency range of 15 to 30 kHz. These low frequency EM fields are generated by submarine communications stations scattered around the world. Long linear conductors give rise to localized anomalies in these VLF fields, which make the VLF method an effective tool for mapping conductive fault and fracture zones, especially water-bearing fracture zones in hard rock environments. VLF is often used in conjunction with EM, electrical and/or seismic methods for sighting high yield water wells and locating preferential contaminant pathways in bedrock, but VLF can be used as a stand alone method. 
EM61 survey to locate buried drums. |