Geophysical Technology

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Electromagnetic (EM) System

Utilities Technical Support employs Electromagnetic Systems to analyze subsurface contents. Electromagnetic profiling is done by using a time-varying electromagnetic field (known as the primary field) to induce eddy currents in subsurface conductors. Using a receiver coil on the EM instrument, the secondary magnetic field caused by these currents is measured along with the original transmitted signal. The secondary field is then segregated into two orthogonal components; the real and imaginary (quadrature) components. The quadrature components represent the vector components 90 degrees out of phase with the primary field, while the real components represent the vector components of the primary field’s in-phase. The quadrature (out of phase) component provides a measure of the apparent ground conductivity while the real (in-phase) component is responsive to buried metallic objects.

During survey operations, the EM system is carried above ground without making any direct contact with it. As a result of their operational nature, EM surveys are faster and more cost effective than conventional resistivity surveys. The transmitting coil gives out a primary electromagnetic field, which in turn creates a secondary electromagnetic field in the ground. The magnitude of the secondary field is measured using the receiving coil. The ratio of magnitude between primary and secondary fields is called the in-phase component. Quadrature fields are directly related to ground conductivity. Ground conductivity, in turn, is affected by changes in lithology and shallow groundwater. Since, buried metal produces a strong secondary field; the in-phase component is a useful indicator of buried metal targets, if they are present in the subsurface.