The job required locating shallow PVC gas lines, recalls Matt Crosby, environmental scientist and business development rep at Master Locators in Philadelphia. Since the lines were nonmetallic, ground-penetrating radar (GPR) was used. But the Master Locators crew was still having difficulty seeing much of what was below the surface, especially with the moist clay soil hindering the GPR’s penetration ability. The 250 megahertz antenna on the unit was switched out for a 500 megahertz antenna. Suddenly the view belowground was much better.

“Little changes like that can definitely help depending on the soils you’re working in,” Crosby says.

It’s one example of how an operator can overcome the challenges of a certain soil type and still achieve an accurate locate. And those conditions can greatly vary within a certain area, so it helps to have a variety of go-to tricks.

“We service a broad area, so it’s different on every job,” Crosby says of the common soil types Master Locators encounters.

SAND

In sandy soil, GPR is the most effective tool. Its high-frequency radio waves are able to easily penetrate the surface to provide a clear picture belowground. The opposite is true of electromagnetic locating equipment. Dry soil conditions can make it difficult to form a solid circuit, preventing the transmitter from properly inducing a signal onto a utility line. But that’s why you don’t rely on any single locating method or tool.

“You don’t go to a site, see that it’s sandy, and decide to use one tool over another,” Crosby says. “You’re going to use everything every time and hope that it will help you best see what’s underground.”

In the case of sandy soil, Crosby says it’s trial and error with electromagnetic locating tools.

“You just have to play around with it to see what works best,” he says. “If you’re locating with one frequency and not getting that good, solid signal, you might want to try a different frequency. Or move your grounding device because perhaps the circuit is jumping on to some other things and the frequencies aren’t focusing in the right matter.”

Then GPS can be used to confirm findings and fill in the gaps, especially since it will be most effective in the dry, sandy conditions.

CLAY

More clay-like soils will produce better results for electromagnetic tools. But GPR units can struggle.

“Any time the soil gets really clayey — or dense with some water like a shallow water table or a recent rain — those waves are going to have a lot more difficulty penetrating through the soil and you won’t be able to see as well,” Crosby says.

Overcoming that obstacle can be as simple as waiting. Crosby says if the GPR unit is deployed in the morning and not working very well, put it away and try again later in the day when soil conditions may be drier.

“There have definitely been times where we’re running the GPR in the morning and not really getting good results right off the bat,” he says. “We just put it away, continue with something else, come back a little later to see if the soil has dried up some, try again, and a lot of times get much better results.”

Changing out the antenna on a GPR unit — as Master Locators did on the job with the PVC gas lines Crosby described — can also help in clay soils, especially when you just want to improve visibility and aren’t necessarily worried about penetrating as deeply into the ground. Crosby says on the PVC gas lines job, the crew went in knowing it was looking for lines that were shallow.

“A typical GPR unit runs at 250 megahertz and that has pretty good depth penetration. But when you’re in clay soils where you can’t see as deep, you might want to change to a higher-frequency antenna like 500 megahertz,” Crosby says. “You won’t be able to see as deep, but if you’re already not seeing as deep because of the clay, that’s probably the better option because it will give you better visibility.”

ROCK

For electromagnetic tools, the same methods employed in sand and clay should be used in rocky soil conditions — experiment with frequencies and the location of the grounding device.

“There’s nothing in particular that’s noteworthy,” Crosby says.

But with GPR, he says it’s possible to confuse utility lines with large rocks and other objects that may be present in such conditions.

“With GPR, what it is looking for are anomalies in the surface,” Crosby says. “So in rocky soil conditions you would want to confirm that those anomalies are in fact a utility line and not, say, a large cobble or tree stump.”

Operators should make several passes in the area surrounding the anomaly to look for any further evidence that it may be a utility line.

“If nothing comes up, you can probably rule it out as being just a large stone or something similar,” Crosby says.

DO YOUR HOMEWORK

No matter the challenges of certain soil conditions, a good locate ultimately starts with doing the proper amount of preparation beforehand.

“It’s not about just showing up with one piece of equipment and doing the locate,” Crosby says. “It’s about investigating the area and solving a problem with a variety of methods. The more information you have about the utilities in an area beforehand, the better.”

And that can include researching the area’s soil conditions, so you know going in what equipment and methods have the best chance of producing an accurate locate.

“Whatever you can get,” Crosby says. “Talk to the personnel on the site, that guy who has been there for 35 years and has a little bit of knowledge about what the soil looks like based on past excavations. That all comes into play and helps on a good locate. For us it starts from the moment someone calls for a quote. We try to get as much information as possible to help us determine the best way to tackle that job.”