Buried beneath your feet is a complex system of subsurface utilities, silently supplying power and connectivity to your home, neighborhood, office, farm and every city in between. Detecting these invisible networks of water, sewer, gas, broadband and electricity is an essential operation for new construction and rehabilitation professionals. Careful digging and vacuum excavation can provide verifiable, see-with-your-eyes results, but there is a different way to detect underground utilities without moving even a single clump of dirt or debris.
Electromagnetic-based locating systems (EM) and ground penetrating radar (GPR) are two technologies that allow utility contractors, locating firms, municipalities and subsurface utility engineering (SUE) companies to identify and record underground “facilities” using evasive methods. The first step before any digging project is to contact your state’s one-call office and provide it with the location of the proposed area to be excavated. A one-call contractor will visit the site and often mark the location of the existing utilities using these locating technologies. The locators come in all shapes and designs (pull or push carts, hand-held units and even big models that can be strapped to a mower, utility vehicle or truck).
While most contractors rely on these one-call services to find and mark utilities, it never hurts to have your own utility locating equipment (if you can afford it), confirming the marks on the ground are actually underground facilities and discovering if any utilities have been missed. Choosing between these different systems and technologies will depend on many things, but most importantly ground conditions and the type of utility.
“The first and most important question that has to be asked is what is your geography?” says Matthew Wolf, president of MALA Geoscience (a global leader in GPR technology). “Where are you? Secondly, what sort of depth are you looking at? If you’re in a particular region, what’s the average depth of your facilities? Also, what is the size and type of your facilities — how big are they? Are you looking for a 2-in., air-filled gas line at 10 ft or a plastic water main 8 in. in diameter and down about 4 ft? You have to dissect what the customer’s actual targets are.”
How Does It Work?
The choice between GPR- and EM-based locating systems will largely have to do with soil conditions, cost and the construction material of the utility. The bigger market today is for EM technology, which boasts it can detect any continuous metal utility, including: iron, steel and copper water lines; gas lines; tracer wire by plastic pipe; telephone and TV cables; copper and aluminum wire; and power lines whether energized or not. EM is also much cheaper, noting that it only works mostly on metal-constructed utilities.
A unit’s transmitter emits electromagnetic waves, which creates a magnetic field. If a metal pipe or a cable is laid within the magnetic field, induced current (signal current) is produced and flows through the underground metal pipe or cable according to the principle of electromagnetic induction. Then, a receiver picks up the magnetic field generated by the subsurface metal pipe or cable. The location and depth of the subsurface pipe or cable is located by the angle of the magnetic force concentrically generated by the metal pipe and the strength of the magnetic field. Buyers will have the choice of using single- or multi-frequency antennas to accomplish this.
“Single-frequency electromagnetic locators have been around for decades,” explains Matt Manning, products manager for electronics at McLaughlin (home to the Verifier line of EM locators). “Single-frequency systems work well on lines and pipes in non-congested easements, but putting multi-frequency into the ground has the tendency to light up everything underground and may produce a distorted signal. In other words, you cannot distinguish whether it is a power, gas or communications line. The other limitation is that single-frequency locators cannot determine the depth of the line and limit the user as there isn’t one frequency that is good for locating all utilities in every soil condition.”
Lines and pipes are constructed of different materials (copper, aluminum, iron, steel, plastic, etc.), so a higher or lower frequency may do the best job of locating them. Multi-frequency systems (some offer up to five frequencies or antennas) allow you to tune the frequency you are putting into the ground to the type of line or pipe you are trying to locate. The lower the frequency, the better it will stay on the line you are trying to locate, thus making the job of distinguishing a gas from a water line easier. Single-frequency locators start around $900, and multi-frequency units can cost up to $5,500.
Both use two methods of EM locating — active and passive. Active locating is applying a specific frequency on the utility and involves searching for a specific line using either the direct connection or inductive method. The locator is either attached directly to the line or a frequency is selected and induced into the ground and re-radiated by the utility.
“Passive locating uses the frequency that the utility creates or re-radiates,” says Manning. “Passive locating is a method used by contractors to check the area for unknown lines, but this method does not allow the operator to distinguish between the types of lines. Passive is good for sweeping or a final check of an area, while active is better for locating the targeted utility.”
Ground Conditions and GPR Location
Ground conditions will always affect the locator signal. It’s a big reason why buyers use multi-frequency locators. The best soil conditions for EM locators are moist clay soil, and the worst soil conditions for EM locators are dry sand or rocky soil. That’s almost the exact opposite for GPR locators, which work best in dry sand and alluvial gravel.
“In general, we say clay is bad for GPR, but not all clays are created equal,” says Wolf. “For example, in the Piedmont area of the Southeast, some of the clays that may look pretty insidious are actually decent for GPR. They are not as good as sand, but not zero reception, which we get in some areas in the Southwest with some of the clays there.”
GPR is pulse radar system, emitting a totally different frequency than EM units. It puts a wave pulse into the ground and receives reflected energy back. Whereas electromagnetic technology basically induces a signal on a utility and gets a secondary electromagnetic field back, GPR actually reflects energy off a utility, which is why GPR locators can detect plastic and non-conductive material. That’s one of the key differences between technologies.
GPR isn’t used to find buried electrical or metal gas lines; that’s EM locator work. GPR is used to find what EM locators can’t — like a plastic water line in sandy soil conditions with no tracer on it. GPR can also blindly scan a whole area much better than the EM locator and has no need to know where a particular utility is because it’s not inducing a signal. You just scan and get reflective energy off facilities. So a lot of times people will scan with GPR on large projects, and then take an EM locator and put the induction source over specific utility targets. Many GPR operators understand that GPR is most effective when used as part of a holistic approach to a particular project. GPR locators come with multiple antennas just like EM locators, but they also cost about twice as much (starting around $10,000).
“The average frequency range is between 250 and 500 megahertz,” explains Wolf. “That is the most widely used and most versatile and successful range of antennas for most GPR situations. Then you get some extremes, so there’s not a one-size fits all, but in the 250-500 range you’ve got 85 percentile of locating with radar. Most people get by having an antenna with one frequency.”
GPR- and EM-based locators are different technologies and different manufacturers specialize in each method. Companies like McLaughlin, Underground Imaging Technologies and Schonstedt Instrument Co. make EM locators. Manufacturers of GPR locators include experts like MALA Geoscience, US Radar and Sensors and Software. Both systems function and cost differently and are often used to detect different types of subsurface utilities. Before and after purchase, training will be essential, but the right unit can make learning a whole lot easier.
“You want to make sure that the unit is simple to use and doesn’t require you to send your operator to a day-long class to learn how to run it,” says Manning. “In a number of cases your operator may not use the unit from week to week, and it should be simple enough for them to pick it up and recall how to use it without much instruction. Durability is also important. Contractors take equipment in and out of a truck on a daily basis and unfortunately the locator may not be handled with absolute care. Weather can affect your locator too, so before you make your purchase ask if the unit has limitations when being used in various weather conditions, such as rain. Lastly, make sure the instrument features a multi-year warranty.”
Keith Gribbins is managing editor for Compact Equipment, based in Peninsula, Ohio.