April 2017 Vol. 72 No. 4
Features
EM, GPR: Complementary Damage Prevention Technologies
by Daniel Bigman, Ph.D.
Damage prevention plays a vital role in underground construction. Identifying existing buried services prior to installing new ones is critical to maintaining site safety and limiting project costs. There are various technologies that assist in detecting and marking buried facilities, but sometimes there is confusion regarding the benefits and limitations of each. While electro-magnetic (EM) locators are popular and have been used in the underground construction industry for a long time, ground penetrating radar (GPR) continues to increase in popularity.
EM locators use the principles of electrical circuits to locate pipes and utilities. By placing a current through a pipe, utility or tracer wire with a transmitter, the current can be tracked using a receiver. Working on a different set of principles, GPR produces an electromagnetic wave that travels through the subsurface and reflects any changes in the physical properties of the material it is traveling through. For example, if the wave is traveling through soil and then encounters a water-filled pipe, some or all of the energy will be reflected off of the pipe and return to the surface. GPR records the two-way travel time and amplitude of the reflected energy.
This article will compare these two technologies to help clarify some of the confusion.
How fast can they locate pipes and utilities?
EM locators allow a technician to rapidly identify buried services, and since the instrument is generally small enough to carry in one hand, marking is also a relatively fast process. GPR often takes more time. These locators are usually pushed across the ground surface in three- or four-wheeled carts, and GPR data can be more difficult to interpret in real-time compared to EM. GPS technology is speeding up data collection times for GPR, and some systems now come with accessories to streamline paint marking.
What can they locate?
One of the major benefits of GPR is that it can locate pipes and utilities of any material, as long as the pipe contrasts in its physical properties with the material it is embedded in. This means that a fiberglass, ceramic or PVC pipe can produce a GPR wave reflection if it is embedded in soil, concrete, gravel or other materials. One drawback of EM locators is that the utility must be able to hold a current for the locator to detect it. If a gas pipe made of PVC has a tracer wire buried with it, than an EM locator can still identify the wire. However, if the tracer wire was broken for some reason, the EM device could not identify it, but the GPR still could.
Can they distinguish between services?
Only EM can consistently distinguish between services. The ability to induce an electric current in a specific utility allows the EM locator to solely identify that service on many occasions, which is a huge advantage. One circumstance where this becomes difficult for the EM locator is in highly congested situations where numerous conductive features are buried near each other. In these cases, the signal can “bleed” from one service to another and identification can become confusing. GPR cannot generally distinguish between services. While a technician with proper training can identify different reflection events coming from services made of different materials and can, in some cases, interpret what the materials are, it can be difficult to confidently identify a specific service from GPR data. One advantage of GPR is that despite its inability to identify which service it is locating, it can delineate separate services without “bleeding” the signal.
How accurate is each instrument?
This question is one of the most important in this discussion. Getting accurate locations of buried pipes and utilities is critical for maintaining overall site safety and project success. Damaging a gas pipe can cause an explosion harming many people; damaging an electric line can harm a single technician. While both techniques are accurate, EM locators often give a range of spatial accuracy, and “peak” response can be interpreted as not directly over the utility being located. GPR provides incredible resolution in spatial accuracy. The way a GPR wave responds to pipes and utilities makes it easy for a technician to pinpoint exact locations of buried services.
How much does each cost?
EM locators are priced at a few thousand dollars with high-end models going for around $5,000. GPR has traditionally been more than this − much more − but prices have decreased over the past decade. Several GPR manufacturers have created base models in the $10,000 range, offering very basic functionality with little to no control over data. For the most part, they are “press and go.” Higher-end models with more functionality can cost $20,000 or more. Some now come with multi-frequency antenna, which produce multiple GPR signals that image the subsurface at different depths.
In summary, these two locating techniques vary in performance across several characteristics. Some debates have revolved around “which is better,” but this discussion takes the position that these are complimentary techniques rather than competitive ones. Where EM locators fall short, such as in accuracy and limits on what they can locate, GPR can enhance the likelihood of project success. Where GPR falls short, such as difficulty to determine which service it is identifying and slow survey speeds, EM locators shine. These techniques are two tools in the same toolkit, for use as appropriate. Doing so will help protect people and infrastructure, which should be the highest priority on any project.
About the author:
Daniel Bigman, Ph.D., is the founder of LearnGPR.com and trains engineers, locators and construction professionals from around the world on ground penetrating radar. He can be reached at bigmangeophysical.com or (706) 201-3378.
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