Subsurface Utility Engineering (SUE) is a branch of engineering that manages the risks of utility coordination. The mapping of underground utilities is an essential part of identifying potential conflicts and communicating those conflicts to concerned parties. Electromagnetic (EM) location and ground penetrating radar (GPR) are currently the most common non-evasive methods of locating underground utilities. GPR provides a numbers of advantages over EM location, especially in specific SUE applications.
GPR uses a single unit that’s both a transmitter and receiver. It transmits high-frequency EM signals in the microwave range, typically between 10 megahertz (MHz) and 2.6 gigahertz (GHz). Signals in this frequency range are the most effective in detecting changes in the soil, such as those caused by utility lines. These signals reflect off objects in the ground and are displayed on the operator’s screen, with disturbances generally appearing as hyperbolic patterns.
The operator must then interpret these patterns to identify the ones that indicate utility lines. An experienced GPR operator can determine the depth and position of utility lines, which are then marked on the surface. This information will then be incorporated into a three-dimensional map. Operator skill is a key factor in determining the effectiveness of GPR, even though it doesn’t have the limitation on materials that EM location does.
GPR requires significantly more training than EM location because a GPR display is much more subject to interpretation. For example, GPR results can be affected by factors such as the moisture content and specific materials comprising the soil. Water reflects the signals differently from soil materials, which can mask the presence of utility lines. Post-processing software can improve the technician’s interpretation, although this step requires additional time.
EM location uses a transponder that transmits a very low-voltage alternating current (AC) into an electrically conductive probe, typically a steel pipe. This current creates an EM field around underground utility lines, which is detected by a wand-like receiver. These lines can then be located and traced onto a map for later reference.
The specific methods of EM location include conductive location, inductive location and passive location. Conductive location requires the operator to connect the transmitter directly to a utility line, allowing it to create the EM field outside the line’s conductive material. Inductive locating involves applying the signal to the utility line, typically by placing the transmitter into the ground over the utility line. Passive location detects EM signals that the line already produces. so it doesn’t require a transponder to detect utility lines for electrical power and communication.
The primary benefit of GPR over EM location is that GPR provides better imaging for utility lines, regardless of their composition. GPR primarily detects ground disturbances rather than the lines themselves, so it doesn’t matter if the lines are metallic or nonmetallic. EM location, on the other hand, detects the EM signals produced by a flow of electrical current. This method therefore requires the utility line to be made of an electrically conductive material when using passive location. EM location can use a conductive drain rod to locate pipes made of nonconductive material in some cases, although a drain rod is frequently impractical or at least inconvenient.
The frequency of the signal emitted by GPR greatly affects its performance with respect to the size and depth of the object. For example, a higher frequency signal provides better resolution but penetrates the ground less deeply than a low-frequency signal. This property means that a high-frequency signal is better for detecting small, shallow objects, while a low-frequency signal is better for detecting large, deep objects. Older GPR systems only use signals with one frequency, so their effectiveness varies greatly depending on the size and depth of the utility lines. However, more advanced models use multiple frequencies to maximize their effectiveness for a particular application.
These factors have resulted in an increase in the practice of combining GPR and EM. SUE companies often perform a sweep with an EM locator first to detect the utilities made with conductive materials. They can then follow up with GPR to detect all the other utilities. This combination of technologies allows engineers to discriminate between conductive and non-conductive utilities while improving overall mapping performance.
GPR measures differences in the density of materials, which makes it useful for detecting a variety of other subsurface objects besides utility lines, including large rocks, tanks and void spaces. It’s therefore useful in many industrial activities that require information on subsurface conditions. For example, GPR is also used in bridge and road construction, which require knowledge of soil density. It also has applications in building inspections to determine if foundations are sound. Rail networks frequently use GPR to monitor the density of ballast used to balance the loads in rail cars.
Other industrial sectors that use GPR include law enforcement, which often requires non-invasive testing in forensic investigations. The study of archaeological sites also benefits from GPR’s ability to examine subsurface structures without disturbing them. The operator’s skill is particularly important for these applications, both for the high degree of accuracy required and use of GPR in conjunction with other techniques. The number of GPR applications is therefore quite large, although the precision needed is highly dependent upon the application.
The most common use of GPR technology is likely to remain in SUE, where GPR’s superior resolution and reliability will make it the preferred choice over EM location. GPR innovations will include producing GPR signals that can be accurately interpreted by multi-skilled operators rather than GPR specialists.
Underground Services, Inc., commonly known as SoftDig, is one of the oldest, full-service professional Subsurface Utility Engineering (SUE) companies in the United States. We provide a complete range of SUE services throughout the United States, including the innovative SoftDig non-destructive air/vacuum excavation system for locating underground utilities.
For more information about the standards that regulate our work, a portfolio of our previous clients or any other question, you may have, get in touch with us at (610) 738-8762 or use our online estimate request form.