Digging and construction can mean big problems for underground utilities. That's where utility locators come in — it's their job to find out exactly where mains and lines are located, so they aren't accidentally damaged. This might seem simple to bystanders, especially when all you see are the spraypainted lines or planted flags when the locator's finished. In reality, it's a complicated task that involves some pretty sophisticated equipment and specialized skills.
In the past, there was just the 811 system. 811 is the US's call-before-you-dig number, which involves utility companies coming out to mark the approximate location of buried structures with paint or small flags. This can give a general idea of where a utility is located but isn't really helpful when you need to be more exact.
Two modern tools that today's utility locator has at their disposal include ground penetrating radar and electromagnetic locating. How do these work, and what's the difference? Is one better than the other?
Ground penetrating radar (GPR) is pretty much just what it says on the label: radar that's capable of penetrating the ground. It's just like aerial radar, only waves are sent into the ground. These reflect off of objects and structures down there and return a picture of what's under the surface.
These aren't "pictures" the way most people think of pictures, however. They appear as peaks on a graph. It's up to the technician to interpret them and determine what's a significant finding, and what's an anomaly.
This technology is best for certain kinds of buried structures. It relies on utilities reflecting radar waves differently from the soil around them. Therefore, it's best for materials that contrast sharply with soil, like hollow pipes.
On the other hand, electromagnetic locating (EM) uses electromagnetic waves to test for conductivity. This is similar to the way that ground penetrating radar works, in that the technician is sending waves into the ground and looking for objects that show discrepancies with the background of the soil. Instead of relying on objects reflecting waves back to create an image, it looks at the differences in conductivity and resistivity.
This technology is best for utilities made either with conductive materials or ones that are marked with conductive tracing wires. It can't find things with a conductivity similar to soil, since there won't be a difference in readings for the technician to spot.
Each system has its own strengths and weaknesses. Depending on the situation, one may clearly be a better choice than the other, but both have their place.
Ground penetrating radar technology is still evolving, but, at the moment, electromagnetic tracers are faster.
To find something with electromagnetic waves, it has to be conductive. That's an issue when you're looking for structures that aren't made of conductive material. If their conductivity is too similar to the surrounding soil, they're effectively invisible.
This isn't an issue for ground penetrating radar. It can see anything that contrasts with the soil around it, no matter what it's made of.
As of this writing, electromagnetic locators are less expensive than ground penetrating radar. This is almost certainly going to change in the future, as radar technology advances and becomes more commonplace.
Electromagnetic locating is quicker, but ground penetrating radar can provide location data that's accurate to within a few inches horizontally and vertically.
Metal is more conductive than soil. EM can spot metallic objects (or those marked with metallic wire for tracing purposes) quickly, easily, and cheaply. You probably don't need to use GPR in order to find a metal pipe.
GPR can spot pretty much anything -- there aren't too many objects that reflect radar waves exactly as the soil does. If you're trying to locate utilities that are nonmetallic and not marked with an EM tracer wire, then GPR is your best bet.
It's best not to rely on a single piece of tech if you want truly accurate results. If you can spot a utility on both GPR and EM, then you can be pretty certain that it's there.
Both of these methods are non-destructive, so you can use them with confidence that you're not harming any underground structures. As long as a greater level of confidence is worth some additional time and expense, it's worth it to locate buried utilities using both GPR and EM. This may not always be the situation, in which case it's up to the technician to choose the method that best suits the utility in question, soil conditions, and other factors that can influence their readings.
Whether you're trying to find utilities buried under soil or located under a thick slab of concrete, ground penetrating radar or electromagnetic locating can help you. Neither is really better than the other, and both rely on their user having a lot of knowledge and experience in interpreting their results. These methods can give you a crystal-clear picture of exactly where crucial infrastructure is located, so you know exactly where to dig (or where to avoid).
POSTED: September 14, 2022
TAGS: Utility Construction