Electromagnetic and Radiofrequency Line Locators
Electromagnetic and Radiofrequency line locators operate either in “passive” mode by locating a background signal or in “active” mode by locating a signal introduced into the utility line using a transmitter. Three sources of background signals exist as follows:
- Background signals due to the flow of electrons in a conductor acting like an antenna. These oscillate producing fields which can be detected by an electromagnetic receiver (Radiofrequency method);
- Current carrying conductors producing a 60Hz signal can be detected by an electromagnetic receiver (Electromagnetic method); and Utilities in close proximity to power lines or used as grounds may also be picked up with a receiver.
Signals introduced into a utility line can be either indirectly or directly induced. Introducing a signal indirectly consists of generating and transmitting a magnetic field from a transmitter above ground. For optimum results the transmitter should have the same general orientation as the utility line which can be obtained through trial and error. Alternatively, a signal can be introduced directly using an induction clamp, a circular clamp, which induces a signal only in the particular conductor that it is clamped around.
The best possible tracing signals are those which are generated using the direct connect method. By virtue of the closed current loop, there is very little chance of the resulting signals being distorted. This is the preferred method of tracing a utility when and where possible by the private utility locator.
The main weakness of this method is its inability to trace non-metallic objects, although this can be overcome by using an “in-pipe transmitter” sonde or a trace wire. When access can be gained by the utility locator to a pipeline, a flexible insulated trace wire can be fed into the conduit. The transmitter is connected to the trace wire and the signal in the wire can be traced as before. The need to insert a trace wire into the pipeline could be, at least partially, overcome if all new non-metallic pipes incorporated tracer wires. Alternatively, an “in-pipe transmitter” sonde can be used in the same manner. Sondes are small, self-contained radio transmitters, which create an electromagnetic field of their own. In both cases the receiver is passed over the approximate location of the pipeline and the location with the highest signal strength is marked by the utility locator as the horizontal location of the pipeline. By rotating the receiver until the highest signal strength is received the approximate orientation of the pipeline is determined. Finally, the depth of the pipeline can be estimated, but this measurement is prone to error. A particular area of error is the situation where a pipe, situated near the target pipe, possesses a strong magnetic field causing extraneous currents to be induced thus leading to misinterpretation. This would indicate a single utility with greater depth than the actual depth due to the combined field. Conversely, the presence of a conductor above the pipeline being traced could lead to a shallower than the true depth being recorded.
Used to locate buried pipes, cables, and sewers, this type of device detects the alternating magnetic fields that surround a conducting metallic line. As a result, it can’t locate nonmetallic lines, such as plastic pipe, unless it is installed with tracer wires. This technology works in all soil conditions, even under water.
There are several major manufacturers of electromagnetic locators. The company’s line of products features a self-contained transmitter that generates an AC signal that creates an electromagnetic field around a buried conductor. These signals are picked up by a handheld receiver.
By determining the proper method, a private utility locator can use this equipment to detect both passive and active signals. Passive signals include those emitted by power cables, telephone lines, power-system return currents, and radio-frequency currents from long-wave radio transmissions that penetrate the ground and flow along buried pipes and cables. Locating a line using passive signals requires only a receiver. Passive signals can’t identify a specific line, however, if multiple lines are present. That’s where the ability to detect active signals produced by the transmitter pays off. It allows utility locators to identify lines more precisely in terms of depth and signal strength. By varying the transmitter frequencies, private utility locators can use this type of electromagnetic locator to positively identify and trace a single line in a congested area, such as below-street services in a city.