Ground Penetrating Radar:
Development of new technologies in today’s world seems ever more so to move at light speed in many disciplines — utility location technology is no exception.
It was just a few short years ago that the use of GPR technology for locating utilities was relegated to a few high skilled specialists sporting systems of considerable cost and complexity to simply locate problematic utilities such as those composed of plastic, concrete, and transite to name a few.
Electromagnetic (EM) locators — for years the only non-invasive tool — are unmatched in their ability to trace CATV, power or other utilities that behave as conductors when subjected to electromagnetic fields whether from ambient sources or those induced intentionally. However, GPR is often the only non-destructive solution for the suite of utilities that simply cannot be traced with an EM locator.
Just how did GPR become widely accepted in the utility market? The answer is simple: GPR systems can be purchased for as little as $10,000 and have user interfaces that mimic the simplicity of an EM locator. In addition, advances in interpretive software have made the job of deciphering the information even easier. This is a far cry from the days when GPR cost at least $30,000 and required several days of training and experience to operate. It is safe to say that every day of the week utility locate professionals — whether they represent a municipality with miles of plastic water lines, utility locate contractors, subsurface utility engineering companies or contractors that install underground utilities — use GPR to augment their EM investigations. Pushing a GPR system across a street or right of way is as standard a practice for many as “swinging” an EM locator.
An additional bonus of using GPR beyond detecting non-metallic objects is the ability to scan large areas and detect many unknown utilities at one time without prior knowledge of their locations or directly connecting to them. Once a potential utility is detected with the GPR, an effective way to integrate a cable locator is to place the transmitter on the ground over the target to see if a signal can be induced. This allows confirmation that the target is a conductor and it may also be traced to areas not accessible or through soils that are not favorable to GPR.
Is GPR an effective locating tool in all areas? Unfortunately, the answer to this question is no. GPR has its limitations like any tool and does not work equally well in all soils. GPR works best in soils that are non-conductive. Soils in the United States range from very favorable for GPR to unfeasible. In general, clay soils inhibit the performance of the GPR signal. Sandy or gravely soils are generally ideally suited to the method.
Many GPR operators understand that GPR is most effective when used as part of a holistic approach to a particular project. Wiley Rickerson with Central Florida Locating (CFL) specializes in the location of underground utilities throughout the Southeastern United States. According to Rickerson, the use of GPR is integral to avoiding conflicts in difficult areas, especially along proposed directional drilling locations. For example, on a recent jobsite the directional driller contacted Sunshine one-call, which came onsite with its cable/pipe locator and marked out all the utilities it found. CFL was contacted by the directional drilling contractor to ensure that all the marks by one-call were accurate and also look for other possible utilities. By combining data collected with a GPR system and an EM locator, Rickerson was able to verify all of one-call’s marks, as well as find a completely unknown utility that was located within the bore path. By potholing down to the utility, Rickerson was able to confirm the GPR information and uncover an 8-in. gas main 5 ft down. It is unknown just how much money and lives were saved on this project but it is safe to say everyone was pleased. Rickerson commented on this project by saying, “This is a day-in, day-out example of how our GPR equipment is an invaluable tool in our ability to confidently locate utilities.”
Consistent with the rapid advances in GPR hardware, many software packages are available to aid in interpretation. Transferring GPR target information from the GPR data set to the map in the past involved the manual transfer of selected targets, which takes a considerable amount of time. Now with GPS Marker software, the utility professional can use the GPR to locate utilities in the field and simultaneously generate a GPS data point to download to your utility maps later. The locations are automatically geo-referenced with the GPS system you have connected to your GPR equipment.
The need for better locating tools manifested in both hardware and software is more apparent every year due to the increasing complexity of the underground environment. The exciting news is that as the underground world is increasing in complexity GPR is rapidly moving in the direction of simplicity. More and more you will find stories like Rickerson with Central Florida Locating in which GPR was able to provide the final piece to the underground puzzle.
Matthew J. Wolf is president of Mala GeoScience USA Inc., which is headquartered in Charleston, S.C.