Searching for Utilities Requires Understanding of Proper Locating

Understanding the method and best practices will ensure each job goes smoothly

Searching for Utilities Requires Understanding of Proper Locating

Jeremy Marginson, project manager/safety officer for Ultra Engineering, uses the company’s portable ground-penetrating radar system to quickly scan for known and unknown utilities in an area, providing engineering firms a more accurate picture of the area to plan from.

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It’s easy to pick out small, common mistakes in utility locating — like swinging the locator wildly from side to side — but consistent results require an in-depth knowledge of the locating method.

Locators and equipment-specific recommendations can vary wildly, but there are a few rules of thumb that any good locating technician should know.

“It’s never going to be the same for every locate. The locator or contractor really has to open their mind up,” says Jake Jeffords, director of marketing and global accounts for Vermeer. “There are going to be variables on every job, and you’ve got to be open to using that variable, changing your frequencies to what’s best for your particular locate.”


Understanding frequency is essential. Often, workers will assume that a frequency they have had success with in the past will work again. That’s not really the case, however.

Jeffords likens it to AM and FM radio. AM radio works over longer distances, particularly when the receiver has a straight shot to the transmitter. But if something blocks the signal, it’s not strong enough to overcome that barrier. FM, on the other hand, is strong enough to reach anything in its radius — but only over short distances.

“I like to start on a midrange frequency, and then go either way,” Jeffords says. “If I know I’ve got a lot of lines underground (I’m seeing a lot of marks on the ground from 811), then maybe I would lean more toward the low frequency so I get a good, narrow, long-distance run.

“If I know I’ve got a water main with a rubber gasket and I want to push over it, then I’m going to use that higher frequency,” he says, “because it’s going to give me more power over a shorter distance.”


Once you’ve determined the best frequency for the transmitter, narrowing down the true position of the utility requires an understanding of peak and null frequency. This is a setting on the receiver that refers to the ups and downs of a radio wave.

“It goes in a mountain and a valley: You want to use the two valleys to get your general area, but the peak of the mountain is where the actual line is supposed to be,” Jeffords says.

Operators tend to fall back on using the null setting because it is simpler. Essentially, the peak setting will give you a locate on the strongest part of the signal. There’s less room for error, which makes it trickier to get a bead on, but it produces more accurate results because you’re narrowing down the signal to the area underground that is transmitting the strongest signal — the utility itself.

“When you get down to putting paint on the ground, potholing or finding the bore, put it in peak because it gives you the peak signal that you’re looking for,” Jeffords says. “It gives you a more accurate locate. You can be off 3, 4, 5 inches either way otherwise, because peak and null are not the same.”


All of this also relies on how you’re sending out your signal. There are three methods: conductive, inductive and coupling.

Conductive is the preferred method. Simply put, a conductive signal is applied directly to the utility through lead wires. Inductive is when a signal is sent into the utility from above ground without an actual physical connection. As you can imagine, this is less reliable and produces a weaker signal. It’s also possible in some cases to apply a clamp, or coupler, around a utility line. While technically considered a type of inductive locating, it’s usually referred to as coupling.

There is also passive locating, which is attempting to locate off the natural frequency of the utility.

“Your pipes and cables act like antennas, radiating the signal from power or phone lines, and in a lot of cases you can go out without the transmitter and detect the presence of that utility,” says Don Dillon, a sales manager for locating products with Subsite Electronics. “Passive is not a means of identification; it’s a means of avoidance. All it’s saying is, ‘I believe there’s a pipe or cable down there, and I should exercise caution.’”


When locators run into problems that can’t be fixed by tailoring the frequency, there are a few tricks.

A common practice is “dotting out” utilities, or essentially taking a problem area and attempting to overlay the locator’s results with a utility map or visual surface observations.

Move in a 360-degree circle around the problem area, marking each locator hit with a dot. Then try to line up each dot with what you know is supposed to be in the ground. This can help clarify the locate when the signal is bleeding into other utilities.

Also, take time to analyze the site before breaking out any equipment. A lot of problems can be solved or prevented by just understanding the site and taking cues from what can be seen of the utilities above ground.

“When you walk onto a job site, you need to really survey the area,” Jeffords says. “Don’t think so much underground, but look at the resources you have above ground. I can usually decipher what I need to do to start my locate, what the best method is, to find the line.”

This can be anything from power lines and boxes, water meters, telecom boxes, transformers, gas mains and more.

“For me, the biggest thing when my locators go out is I want them to take a look at the site and try to get a general understanding of what’s going on there. Almost every structure is going to have multiple utilities going to it,” says Jeremy Marginson, project and safety manager with Ultra Engineering. “They really need to navigate the site and figure out what’s going on there, make sure everything makes sense.”


For locating-specific subcontractors, Marginson recommends taking the customer on a tour of the site after the locate to explain any concerns and make sure they understand exactly what’s going on under the ground — including what isn’t known.

“Once everything is marked on the ground, then we’ll do a job walk. We’ll take the time to do that and explain everything, any concerns: ‘That there might possibly be something here, we weren’t able to locate it, I just want to bring this to your attention, but using all our means of locating, we aren’t sure,’” he says. “Making sure they’re aware of it so they can take extra caution.”

They also provide customers with photos and a written explanation of the site and what was located. A simple subsurface utility map then accompanies all this.

“It’s not going to be survey grade, just a drawing on a satellite-image map,” Marginson says. “The basic vicinity of where it’s at so they have that to look at as well.

“From my perspective as a locator and contractor, there’s always a misconception that when you call 811, contractors will take that as ‘OK, we’re good to go,’” Marginson says. “It’s the responsibility of the contractor to get an accurate locate, to keep the structure safe, the people safe, and I think there’s a big misconception on that.”

Services like the federally designated call line 811 routes callers through local call centers, which typically notify the utilities that may be affected. Some regions have nonprofit groups that handle utility marking, but in most areas, the people doing 811 marks are municipal workers who don’t necessarily specialize in utility locating.

As such, they can’t always be relied on for an accurate locates, often only indicating the general vicinity of a utility. Besides potential inaccuracy, it also doesn’t necessarily give all the information you would want in order to safely excavate or drill.

“Safe digging is super important for us, and the locator is first step of verification,” Jeffords says. “It’s really important to follow all the steps, not just the basics — call 811 before you dig, have your locate done — but also we really preach and try to train our contractors to use that locator to verify the paint that is already on the ground. In that respect, to not just do their 811 diligence, but to make sure, to double-check. It’s their responsibility, to be a good citizen and a good contractor.”



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