Electromagnetic Induction (EMI)

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 induce a secondary current in underground conductors, which results in an alternating secondary magnetic field that is sensed by the receiving coil. Unlike conventional resistivity techniques, no ground contact is required. This eliminates direct electrical coupling problems and allows much more rapid data acquisition.

Electromagnetic induction instruments are used for many different types of geologic, engineering, and environmental investigations. These include shallow soils mapping, soil-salinity mapping, groundwater investigations, and the detection and delineation of waste pits and associated subsurface contaminants from acids, salts or volatile organic contaminants (VOC’s). They have also been used extensively for the detection of conductive geologic media such as clays and ferrous mineral deposits, as well as the detection of resistive geologic materials such as sand and gravel deposits. In addition, the systems are used for near-surface archaeological investigations, the detection of buried foundations, and the detection of buried metallic objects such as drums, tanks, large diameter utilities and other non-descript metallic objects.

The GSSI Profiler EMP-400 is a portable, digital multi-frequency electromagnetic induction sensor. The User can collect from one (1) to three (3) frequencies simultaneously. The system bandwidth extends from 1 KHz to 16 KHz, in 1 KHz steps. The system’s primary data output is the In-Phase and Quadrature components of the mutual coupling field ratio of the transmitted field to the induced field in parts per million (PPM) at all frequencies, and the apparent conductivity (σa) at 15 kHz. The magnitude of the In-Phase and Quadrature components of the induced secondary field, as well as the apparent conductivity are stored for each reading along with a time stamp and user supplied survey grid information. GPS data, in the form of a NMEA 0183 GGA string, is also recorded if the internal GPS is enabled or if an external GPS system is connected with the PDA via the PDA RS-232 serial port.

The Geonics, Inc. 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 (12’ offset) 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.

The Geonics, Inc. EM-61 instrument is a high resolution, time-domain device for detecting buried conductive objects. A powerful transmitter generates a pulsed primary magnetic field, which induces eddy currents in nearby conductive objects. The decay of the eddy currents, following the input pulse, is measured by the coils, which in turn serve as receiver coils. The decay rate is measured for two (2) coils, mounted concentrically, one above the other. By making the measurements at a relatively long time interval after termination of the primary pulse, the response is nearly independent of the electrical conductivity of the ground. Thus, the instrument is a super-sensitive metal detector. Due to its unique coil arrangement, the response curve is a single well defined positive peak directly over a buried conductive object. This facilitates quick and accurate location of targets. Conductive objects (e.g. drums, USTs) can be detected to a maximum depth of approximately twenty (20) feet.