A passive signal is one that is naturally occurring around a conductor, or in this case an underground utility. Some examples of passive signals include the following:
- Current flowing along an electric supply cable.
- Earth return current from power systems that use metal pipes or cable sheaths as a convenient conductor.
- Radio frequency currents from very low frequency (VLF) radio transmissions that have penetrated the ground and flow along a buried utility.
A passive sweep is performed to search for inaccessible, abandoned or unknown utilities using only a receiver. To perform a passive sweep, a survey grid is traversed in “power” mode, with the receiver blade in line with the direction of movement and at right angles to any utilities that may be crossed. When the receiver indicates the presence of a utility, it is pinpointed, traced and marked. The sweep is then continued until all detected utilities have been marked and the entire grid has been traversed in both directions. After completing the sweep, the entire process is repeated in “radio” mode to search for utilities that radiate VLF radio signals.
Passive signals enable utilities to be located, but not identified, because the same signal may appear on multiple utilities within the grid. To solve this problem, an active signal must be applied to each individual utility line.
An active signal is one that is intentionally generated by a transmitter. In this mode, the signal can be applied directly to the utility via direct connection or induction. This enables utilities to be identified, traced and their depth determined with a receiver.
Direct connection involves plugging a connection cable into a transmitter output socket and connecting directly to the target line. This can be accomplished with connection leads or with a transmitter clamp. Connection leads are generally used to apply a signal to metallic conduits, site lighting structures and metallic pipes. This is the preferred method for locating secondary electric, water and gas.
Many electric, telephone and cable lines are housed within plastic conduits or buried into the ground without protection. In addition, directly connecting to these lines is usually too risky or forbidden. In such instances, a transmitter clamp is used to apply a signal to the cable without interrupting service to the line. The clamp is easy to apply, but the signal may not travel as far as it does with connection leads, and works best if the target line is grounded at each end .This is the method of choice for locating primary electric, telephone and cable lines.
If an active signal cannot be applied to a line because it is inaccessible, an induction sweep must be performed. The transmitter contains an antenna, that when placed on the ground directly on top of a utility line, can induce a signal into it. The advantage of using induction is that a signal can be applied without access to the line and it is very quick and easy to use. The disadvantages are that induction efficiency is poor on deep targets, it is only useful at depths down to 6 feet and the signal can induce into lines other than the target. In addition, signal strength is often lost in the surrounding soil, the signal is shielded by reinforced concrete and a signal will not apply to a well-insulated line unless it is effectively grounded at each end. Despite its shortcomings, an induction sweep can sometimes successfully locate unknown or abandoned utilities when GPR results are inconclusive.
An active signal cannot be applied to non-conductive (non-metallic) utility lines. To combat this, a detectable duct rod or self contained transmitting sonde must be inserted into the line via a manhole, handhole, cleanout or catch basin. The disadvantages of this method are that some non-metallic utility lines do not have access points or might be obstructed by detritus. Nonetheless, this is the best method for locating fiber optics, future use lines, sanitary sewer and storm sewer.
To see how cable and pipe locator technology is used to perform an underground utility locating survey, please visit the underground utility locating services or underground utility locating projects page.