Cable & Pipe Locator Technology

Because many electric, telephone and cable lines are housed within plastic conduits or buried directly into the ground without protection, connecting to them is usually impossible, too risky, or forbidden. In such instances, a transmitter clamp is plugged into the transmitter and wrapped around the target line to discharge a signal onto it without any interruption of service. We have 2 inch, 4 inch, and 8.5 inch transmitter clamps, enabling us to induce lines of various sizes up to 8. 5 inches in diameter. Although this method is generally successful, the signal may not travel as far as it does with connection leads and only works if the target line is grounded at each end . This is the method of choice for locating primary electric, telephone, and cable lines.

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 a current flowing along an electric supply cable, earth return current from power systems that use metal pipes or cable sheaths as a convenient conductor, and 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 using only a receiver to search for high voltage electrical or telecommunication lines or inaccessible, abandoned, or unknown utilities. 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.

When a line is non-metallic or non-conductive and it cannot be found using GPR technology, a detectable fiberglass duct rod must be snaked through it. 

Afterwards, a signal is applied to the copper wire inside the duct rod using the direct connection method described above, and the location and depth of the line is traced out using a hand-held receiver. This is the best method for locating fiber optics, empty conduits, future use lines, drains, sanitary sewer, and storm sewer. We have three different sized DCD Manufacturing duct rods, which helps us snake through conduits and pipes of various depths, diameters, distances, and bends:

1. 3/16" Diameter - Used to locate shallow, small diameter lines running a short distance with tight bends.

2. 5/16" Diameter - Used to locate shallow or deep small, medium, and large diameter lines running various distances with various types of bends.

3. 7/16" Diameter - Used to locate deep, large diameter lines running a long distance with minimal to no bends.

As noted above, radio signals can sometime "bleed" onto non-target lines. This is a common occurrence when using detectable duct rods in highly congested industrial and municipal settings or when the target line is deeper than 8 feet. To combat this issue a sonde, which is French for "sounding line," can be connected to the tip of the duct rod and snaked inside the target line. The sonde emits a radio signal that can be picked up with a hand-held receiver. The position and depth of the sonde is then located in increments of 10 feet along the entire path of the line, providing one with the location and depth of the line. This method is normally only used as a last resort to duct rodding, and is primarily used for finding deep industrial and municipal storm and sanitary sewer lines.

If an active signal cannot be applied to a line because it is inaccessible, an induction sweep can be performed before giving GPR a try. The transmitter houses an antenna that when placed on the ground directly on top of a line can induce a signal onto it. The advantages of using induction are that a signal can be applied without access to the line and it is very quick and easy to do. 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 "bleed" onto non-target lines. In addition, signal strength is often lost in the surrounding soil, the signal can be 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 be used to locate unknown or abandoned utilities. However, we rarely ever use this method and do not believe it is all that useful. The advantages of using induction are that a signal can be applied without access to the line and it is very quick and easy to do. 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 "bleed" onto non-target lines. In addition, signal strength is often lost in the surrounding soil, the signal can be 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 be used to locate unknown or abandoned utilities. However, we rarely ever use this method and do not believe it is all that useful.