Cable & Pipe Locator Technology

Cable and pipe locator technology, which is composed of both a transmitter and hand-held receiver, is our primary method for locating underground utilities. We currently use the RD7000+, which is manufactured by SPX Corp. of Raymond, ME. This model is widely recognized as the industry standard because it maintains the highest locate performance, contains multiple locate modes, and is relatively easy to use.

RD7000 Transmitter

Cable and Pipe Locator RD7000+ Transmitter

RD7000 Receiver

Cable and Pipe Locator RD7000+ Receiver

Basic Principles of Locating Utilities With Cable & Pipe Locator Technology

Cable and pipe locator technology makes use of the scientific fact that conductive wires and pipes emitting radio signals--either passive or active--can be detected with a hand-held receiver. To understand how this process works, please note the orthogonal grid shown below.
Cable & Pipe Locating Passive Line Search

Sweeps are made within the orthogonal grid while holding the receiver vertical and in line with the direction of movement. When the receiver indicates the presence of a utility, the user stops walking the grid to pinpoint and mark its location and depth using the three-step process shown below:

1) The receiver is moved left and right over the line to search for the signal.
Cable & Pipe Locating Line Search

2) The receiver is held near the ground with the blade vertical, and it is rotated around until the position of maximum response is found.
Cable & Pipe Locating Receiver Rotation

The receiver is moved slowly from side to side to find the exact position of the peak response, and once this position is found and the receiver blade is directly on top of the target line and at a right angle to it, the ground is marked.
Cable & Pipe Locating Line Pinpointing

Once the line is marked out its entire distance, the grid search is resumed to search for other potential lines running through it.

The Six Cable & Pipe Locating Techniques

Cable and pipe locating technology can be harnessed to its full potential by using the following six techniques in conjunction with the single line pinpointing process described above:

1. Direct Connection

In the first two methods, direct connection and direct induction, the transmitter discharges a radio signal  onto the target line (with a frequency of 8 KHz, 33 KHz, 65 KHz, etc.), and the line can then be located and traced out using a hand-held receiver set to the same frequency. The Direct Connection method is performed by plugging a direct connection lead into the transmitter output socket and connecting directly to the target line with an alligator clamp or, if the line is too large for it, a magnet.

The clamp can be placed onto a shut off valve.
Cable & Pipe Locator Direct Connection of a Site Light

The clamp can also be applied to a lighting structure.

Cable & Pipe Locator Direct Connection of Conduit

If there is an accessible wall outlet connecting to the target line, a plug connector can be used to apply a signal to the line. 

Cable & Pipe Locator Plug Connector

If a signal needs to be applied to a live cable, the live cable connector can be used to do so safely.

Cable & Pipe Locator Live Cable Connector

Direct connection is 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.

2. Direct Induction

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.
Cable & Pipe Locator Direct Induction

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.

3. Passive Radio & Power

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. 

Cable & Pipe Locator Passive Search

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.

4. Detectable Duct Rodding

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.
Cable & Pipe Locator Duct Rodding

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.

5. Sonding

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. 
Cable & Pipe Locator Sonding

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.

6. Passive Induction

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.

Cable & Pipe Locator Passive Induction

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.

Advantages of Cable and Pipe Locating Technology

  • Determines the location and depth of almost all types of underground utilities 
  • The equipment is portable and easy to handle.
  • One can confirm the identity of a target line, using direct connection and direct induction.
  • The basics are easy to teach and learn.
  • The equipment works in almost all soil conditions.
  • If drains, sewers, or other non-metallic ducts or pipes are accessible, they can be located by duct rodding or sonding.
  • The component parts of the technology are low cost enough to be purchased by small contractors or issued on a large scale by regional or national organisations.

Limitations of Cable and Pipe Locating Technology

  • Cannot locate non-metallic or non-conductive utilities unless there is access to snake them or a tracer wire is wrapped around them.
  • The target signal can often "bleed" onto non-target lines
  • Multiple lines laid next to one another cannot be resolved into multiple targets.
  • Does not work on sites where there are too many utilities.
  • In some industrial setting, especially power plants and sewer treatment facilities, there is too much background signal interference to produce a reliable active signal.
  • The basics are easy to learn, but the more advanced techniques take years to master.
  • Cannot be used in inclement weather