Electromagnetic Induction

The Electromagnetic technique is most commonly used for locating metallic underground utilities such as pipes, wires and cables.

Electromagnetic Methods –

Conductive Trace (direct connect)
This method involves applying a signal directly to a desired metallic target. A receiving antenna is then used to detect the signal along the pipe or cable allowing for horizontal location. Connecting a signal directly to a target is considered to be the most accurate method for identifying metallic pipes and cables. Above and below ground surface features such as: meters, hydrants, valves, dispensers, manholes, hand holes, junction boxes, pedestals, risers and drops are considered good sources for this application.

Inductive Trace
The transmitted signal is indirectly applied atop the ground surface within close proximity of the suspected path of a known metallic utility system. The magnetic field from the transmitter will induce a current onto the target conductor. This method is very effective when direct connect is not possible.

Inductive Sweep (wide scan)
The “two man sweep” is another effective method for scanning medium to large sized areas. Two individuals walk parallel to one another at a minimum distance of thirty feet (30’), one with the transmitter and the other with the receiving antenna. As the individual with the transmitter approaches a metallic utility system, the individual with the receiving antenna notices a signal response.

Inductive Search (narrow scan)
Often referred to as a “split box” technique. This operation can be performed by one individual. The receiver and transmitter are tethered by a pole at a fixed distance. Once the equipment has been calibrated to the local environment, the operator can walk at a comfortable pace. Large ferrous objects (those comprised of iron or steel) create distortion within the Earth’s own magnetic field. The equipment will sense changes within its own magnetic field set-up between the receiver and transmitter once over top of a large ferrous object.

Inductive Coupler
Similar to the inductive method, a split ring aerial (clamp or coupler) is attached to the transmitter then applied directly around a pipe or cable. The magnetic field set-up by the coupler sends current onto the target conductor. The receiving antenna detects the transmitted signal.

Passive Detection
Naturally occurring magnetic fields are often present on certain types of utility systems. For example, a current carrying electric cable produces a 50hz/60hz signal. Receiving antennas with the corresponding frequency tuner can detect these naturally present signals. Passive VLF signals are often present on conductive utility systems and also can be detected by a receiving antenna.

Sonde
Portable, small, lightweight transmitters can be attached to a cable and pushed/pulled through a conduit or pipeline. These tiny transmitters emit a detectable signal which can be identified by a receiving antenna above ground enabling precise delineation.