The View Underground: Ground Penetrating Radar
GPR can help keep you out of trouble, but it won't do so by itself. Use it to confirm the information you already know or suspect. Anything beyond that may be serendipity.
Like potholing, only smarter. You’ve got the list of underground utilities with a drawing of their planned or located paths that may cross the designed path of your trenchless project.
Your Geotech team took a shovel and dug some test holes - potholes - to spot-check the information, but…what if you strongly suspect there are things down there that aren’t on the drawings from One Call – some centuries-old obstruction, like a forgotten cemetery that potholing could miss?
The simple answer is to take a comprehensive look underground using ground penetrating radar, a technology that’s been around since the 1980s that gives you a comprehensive idea of what you’re drilling into. Used early, it can help you avoid costly delays, errors and omissions that can mean the difference between a newsworthy nightmare and a profitable project. (Also read "Why A Geotechnical Report Means Success for Your Trenchless Project.")
GPR: The Basic Principles
One of the first questions people ask about ground penetrating radar (GPR) is, “Is it acoustic, or is it really radar?” The short answer is, it’s radar, not a sound-powered, acoustic substitute. It uses a microwave transmitter to send a signal into the ground.
When the signal encounters a change in the subsurface makeup, it bounces back to the surface and the GPR’s receiver. The GPR then records the underground anomaly and creates a visual file for review. GPR has some constraints. Electrical conductivity in the ground tends to attenuate the depth at which it’s effective.
If, for example, your project’s design path is crisscrossed with electric power or communications cables, that limits how much and how deep your view is. There are tradeoffs available, though. Your geotechnical team can compensate for this with changes in the GPR transmitter frequency and power. A higher frequency means better image resolution, but shallower ground penetration.
Using Ground Penetrating Radar
Using GPR effectively is a specialized geotechnical skill. Anyone can drag a GPR unit over the ground and some image will appear. Maybe these images will answer your needs in a simple situation, but when your project’s path is fraught with multiple -- perhaps hundreds -- of utility crossings, littered with suspicions about unknowns and plain old “unknowns,” you have two options.
You can learn all there is to know about GPR, do the work in-house and hope you get it right, or you can retain the services of a firm that specializes in the subject to build your geotechnical report.
Like using any geotech service related to trenchless construction, using a dedicated GPR contractor or a geo-contractor with GPR skills depends on the same cost/benefit factors that lead you to bid or pass on a project. Project complexity, better project completion times, increased profit potential that results from avoiding subterranean conflicts and sidestepping avoidable public relations problems can weigh heavily in favor of contracting for GPR use.
Likewise, projects like pipe bursting (which sometimes requires pipe penetrating radar) and relining of sewers probably doesn’t merit the inclusion of GPR. A healthy dose of common sense applies: consider GPR for new projects no matter how sketchy or comprehensive the available information on the design path is.
WYSIAWYG: What You See Isn’t Always What You Get
Like any radar image, GPR gives you the location and distance of a subsurface anomaly from the transmitter head. It’s not a bird’s eye view. Rather, it shows a profile view with the ground surface at the top and the anomalies – and their depth and horizontal location.
An existing pipeline may appear as nothing more than a hump or other trace underground that indicates the presence of something. It may be a facility that shows up accurately in the One Call data or it may be a pipe or cable that’s several feet away from the position found in the data. Or it may be something that was never entered into the system.
It is, however, a potential conflict with your project that you have to avoid, meaning additional investigation, whether with a shovel or a backhoe, is needed. (Read more in "The Science of Getting it Right: Locating Underground Utilities.")
Whatever the result, GPR gives you a starting point. Like any radar system, it doesn’t always deliver a finished, defined image, but it does give you the edges, like a coastline to be investigated directly rather than by electronic means.
The Bottom Line of GPR
Don’t expect GPR to give you the moon, stars and sun on a platter. Proper pre-planning, having an idea of what your target will show up as and foreknowledge make life easier. It can help keep you out of trouble, but it may add to your design team’s woes with the project design path, or change the method and manner of your digging altogether.
While GPR will show you some things of which you weren’t aware, it’s best to use it to confirm the information you already know or suspect. Anything beyond that is serendipity. Either way, proper underground planning is recommended.