Radio/Electromagnetic Detection essentially comprises a receiver/amplifier connected to an antenna that detects the electromagnetic field created when a current passes through a buried cable. The precise location of the cable is identified by passing the receiver over the ground in the area above the cable; the peak strength of the electromagnetic field indicates when the receiver is closest to the cable, which is usually directly vertically above it. The survey is started over a wider area and narrowed as the signal is pinpointed. The scan is then repeated at 90 degrees to the first pass to verify the location. Certain equipment may also provide an estimate of the depth at which the cable runs. This process is repeated at intervals along the potential route of the cable establishing its route in plan and the depth variations along its run.
Where the service being searched for is not a power carrying cable but has the facility to carry a generated signal, for example a metal water pipe, a signal generator may be attached to it at an accessible chamber to establish an electromagnetic field which can then be followed using the same survey technique.
These tools are usually referred to as Cable Avoidance Tools (or CAT) systems; the artificial signal generation system is often referred to as a CAT & Genny.
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