The Theory of Buried Cable and Pipe Location

Any kind of excavation project will eventually put construction workers in close proximity to an underground utility line (including gas, electricity, telecommunications, and water). So, they need to know the exact location of all the buried utilities before they start digging. As a safeguard against utility strikes, they will use an underground pipe locator to find their presence and location. Whenever an alternating current (AC) travels along a cable, it generates an electromagnetic field. The oscillation of the current between the positive and the negative also creates a frequency that’s measured in Hertz (Hz). This field can be detected by a pipe locator.

Locating Buried Utility Lines

Information is an important ingredient in any kind of project because it allows you to plan ahead, but the challenge of getting information about buried utility lines is that nothing is visible. Drawings, plans, and other pieces of information should always be gathered, but they may inaccurate or incomplete. An appropriate technique should always be used to get the information you need about the location of underground utility lines, and there are a number of methods you can use. Some of them include but may not be limited to:

Ground-Penetrating Radar (GPR) — In some cases, this technique has certain advantages over other methods because it can “see” both metallic and non-metallic objects, but there's one caveat. GPR products tend to be bigger and more expensive. They also need special training to get the most out of the equipment. Surveying with this kind of equipment can take longer than with other methods, which will put it beyond the scope of many operators.
Sonic Surveying — Sending sound or ultra-sonic waves into the ground or along a utility line is a way of tracing plastic water pipes, but it’s not good for locating other buried services.
Dowsing — It’s one of the oldest techniques, but it doesn’t use any kind of technology. While the hazel twig and all of its variants continue to be used with different degrees of success, it’s a relatively obscure art form that isn’t used as much. And while its information may not be as accurate, its ease of handling is one of the features that can be claimed with any amount of certainty.
Electromagnetic Location — This method of pipe location has become almost universal, but its main drawback is its inability to locate non-metallic lines (such as plastic pipes). However, utility lines that were built with tracer wire aren’t affected by this shortcoming, and the technology does have its advantages.

Electromagnetic location has many advantages when it comes to getting information about underground utility lines that aren’t available from other techniques (or even a combination of them).

The Theory of Electromagnetic Location

The technology behind an underground pipe locator has three basic building blocks:

A signal generator or transmitter used to apply a signal to a buried line.
A small self-contained transmitter that’s suitable for inserting drains or ducts.
A handheld receiver that’s used to locate the signals from the transmitters or signals that are occurring “naturally” on buried lines.

The current flowing along a conductor generates a magnetic field, which forms a cylindrical shape along the conductor and is called “the signal.” This field is produced by the flowing current and not by the voltage. While it’s possible to insulate against the electrical current, you can’t insulate against a magnetic field. And its shape won’t be changed by cable insulation or by the presence of different soil types. For a DC current, you will need to use an instrument that can measure magnitude and polarity. And because of the difficulty in measuring a static magnetic field, it’s not a feasible technology for a pipe finder.

Active and Passive Signals

There are two types of signals that come from buried conductors:

Passive Signals — These signals are “naturally” present in many conductors without any action needed by the user. This can include power cables that carry currents as part of their normal operation, but there are some less obvious examples. The earth is full of power system return currents that can flow along convenient paths of lower resistance, which can be provided by metal pipes and cable sheaths. There can also be frequency currents that come from long-wave radio transmissions, which can penetrate the ground and flow along buried pipes and cables (whether it’s electrically live or dead).
Active Signals — These signals can come from deliberate action by the user to connect or induce a known AC signal from a signal transmitter onto a target line. Active signals will not only allow buried lines to be located but will also allow you to positively identify and trace them in congested areas (which is typically found in below-street services in a city). The fact that the signal source is under your control allows you to do more precise work (such as depth measurement and signal strength comparisons). The choice of frequency can also be made to meet the need of a particular job, especially when a multifrequency signal transmitter is being used.

For passive location, you only need the receiving instrument. It’s a simple operation, because it allows the digging crew to find buried utility lines while giving them protection from accidental contact with a live cable pipeline (which can lead to a disruption or loss from a severed line). Active location uses two components of a pipe finder: a transmitter and a receiver. This process is essential if target lines need to be positively identified and traced.

Tracing Non-Metallic Pipes

The Theory of Buried Cable and Pipe Location

Magnetic location for an underground pipe locator is based on the detection of electrical current flowing along a conductive metal line. But with plastic or concrete pipes, ducts, and drains, there’s no way to detect and trace them based on current flow. You will have to use a tracer wire, which you can either insert into or lay along the line so a signal can be applied for active location. You can also use a signal transmitter (known as a “sonde”) into the line with a receiver that has been tuned to that particular frequency, so you can determine its position from above the ground.

Detection and Location of Signal Sources

A coil that’s suspended in a particular space produces an AC signal that’s proportional to any alternating field or signal passing through it, which is a process known as the “induction principle.” The aerials of radio detection instruments are built in this fashion, but how they perform will depend on their orientation relative to the buried line.

The Twin Aerial Antenna and Depth Estimation

The detection of a single buried conductor in an open field is simple and can be done with a basic aerial instrument, but buried pipes and cables are often close to other conductors and can even be located under overhead power lines or other sources of electromagnetic interference. Using a single aerial instrument in these situations can lead to confusing results, or the interference can make it ineffective. Radio detection technology is based on the twin-aerial system, which uses two horizontal aerials that are spaced 16 in. (400mm) apart to find the same signal.

Current Measurement (CM)

Current measurement is a function that’s available on all precision locators that use radio detection, and it can be used to sort out any difficulties in location (especially in congested areas). Current measurement on a pipe finder is only accurate if it’s reading a “clean” signal from a single conductor and will depend on depth accuracy. That’s why this feature is only available in the active modes and should only be used when the site has been checked for signal distortions.

Current Direction (CD)

Electromagnetic locators can only find AC signals, but an alternating current doesn’t have a direction because of the backward-and-forward movement along the conductor. Even so, many transmitters used with locators use arrows to indicate the direction of current flow. These arrows actually show the direction of the current flow at one point in time (typically a few thousandths of a second later). But in reality, the current may be moving in the opposite direction. Despite this discrepancy, the arrows are still useful because they can indicate the behavior of the current being applied. Whether you need a pipe laser or some other product used for locating underground utilities, be sure to look at what we have at Engineer Supply.

Frequently Asked Questions

How do I locate underground pipes?

The Theory of Buried Cable and Pipe Location

You can find an underground pipe by performing the following steps:

Mark any potential pipelines and make a note about the location of faucets or any other outlets.
Press your probe rod into the ground along the lines you have marked.
Use a shovel to dig along the assumed pipeline. If you didn’t find a pipe with your probe rod, use a shovel again to dig small holes.
Use an underground pipe locator to find out where the pipe might be located.

Whether you’re looking for a pipe laser or some other underground location device, be sure to browse through the broad selection we have at Engineer Supply.

How do I find a stormwater pipe underground?

There are a couple of different ways to find underground stormwater pipes:

Through the Manhole — The easiest and fastest way to find the location of stormwater pipes is by opening the manhole lid, which can be an access point to either a stormwater or sewer pipe. Once you open the lid, you’ll be able to measure the size of the pipe and invert level, which will become known as the Quality A location.
With an Underground Pipe Locator — Non-metallic stormwater pipes can’t be found with a pipe finder. A flexible fiberglass rodder is the most common tool used to find these kinds of pipes, which can either have a traceable wire that’s fitted inside or a traceable sonde that’s fitted onto one end. The sonde transmits a frequency that can be tracked, but you need to have a receiver to determine its location.

To completely trace the pipe, there needs to be enough of the rodder or sonde to feed through the pipe and estimate the scope.

How does an underground pipe locator work?

A pipe locator has two parts: a transmitter and a receiver. The transmitter is used to send a signal though the pipe or cable, while the receiver is used to locate the signal moving through the line. This technique can be used to find any of the following:

Electric lines.
Cable TV lines.
Telephone lines.
Water or gas pipelines.

If you want to find a pipe finder or some other device used for utility location, be sure to look at what we have at Engineer Supply.

What does a pipe laser do?

A pipe laser is a type of construction laser that can be used to determine the grade for pipelines, so it’s used as a type of electronic level. It sends out a high-powered beam of light at a predetermined angle, so an appropriate grade can be established.

Where can I buy a quality pipe laser?

If you’re looking for a pipe finder or some other tool used for pipe layout and location work, you can find what you need at Engineer Supply. We have a variety of products from some of the best manufacturers in the industry. So if you need a pipe laser that you can use on your next project, be sure to look at what we have in stock.