Electromagnetic induction (EM) uses the principle of induction to measure the electrical conductivity of the subsurface. A primary alternating electric current of known frequency and magnitude is passed through a sending coil creating a primary magnetic field in the space surrounding the coil, including underground. The eddy currents generated in the ground introduce a secondary current in underground conductors which results in an alternating secondary magnetic field, that is sensed by the receiving coil. The secondary field is distinguished from the primary field by a phase lag. The ratio of the magnitudes of the primary and secondary currents is proportional to the terrain conductivity. The depth of penetration is governed by the coil separation and orientation. Unlike conventional resistivity techniques no ground contact is required. This eliminates direct electrical coupling problems and allows much more rapid data acquisition.
For shallow profiling a Geonics, Inc. EM-31 Terrain Conductivity meter is used. One person can collect up to 10,000 data points per day with this instrument. The EM 31 uses an alternating electromagnetic field, which fills the space below and above ground, surrounding the transmitting coil. When the electromagnetic field couples with a conductor, for example a steel pipe under the ground, AC eddy currents are induced to flow in the pipe. This generates a secondary magnetic field, which is sensed by the co–planar receiver coil. Due to phase lag the computer on board can discriminate between the primary and secondary fields and outputs the measurements of the secondary field (thus, a conductive zone is sensed by the induced secondary magnetic field).
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