Utility Detection Technology: How to Use Ground Penetrating Radar (GPR)

X-ray vision is something that is usually associated with superheroes. Lacking the ability to see through pavement, contractors have to rely on the next best thing: advanced utility detection technology. A construction project, especially in an urban area, is a potential minefield of utilities and objects that can be ruptured, dislodged, or broken by excavator activity: electrical power lines, communication wires, sewers, gas pipelines, etc. Even though reasonably accurate records of a site’s underground utilities may be available, no contractor should rely on what could be outdated written records. Contractors must know before they break ground that unrecorded utilities are absent. Though a utility may have been decommissioned and abandoned in place, contractors can’t risk damaging expensive utilities or endangering their work force. Even abandoned and decommissioned utilities can prove dangerous. For example, an old abandoned natural gas pipeline could have retained significant quantities of flammable methane that could explode upon contact with an excavator bucket. The construction industry relies on subsurface detection technologies of various types and applications.

X-ray vision is something that is usually associated with superheroes. Lacking the ability to see through pavement, contractors have to rely on the next best thing: advanced utility detection technology. A construction project, especially in an urban area, is a potential minefield of utilities and objects that can be ruptured, dislodged, or broken by excavator activity: electrical power lines, communication wires, sewers, gas pipelines, etc. Even though reasonably accurate records of a site’s underground utilities may be available, no contractor should rely on what could be outdated written records. Contractors must know before they break ground that unrecorded utilities are absent. Though a utility may have been decommissioned and abandoned in place, contractors can’t risk damaging expensive utilities or endangering their work force. Even abandoned and decommissioned utilities can prove dangerous. For example, an old abandoned natural gas pipeline could have retained significant quantities of flammable methane that could explode upon contact with an excavator bucket. The construction industry relies on subsurface detection technologies of various types and applications. [text_ad] Magnetic detection can be used to find ferrous metals of all kinds used in the construction of conduits, pipe lines, power cables carrying an electric current, and communication lines transmitting a signal. Non-ferrous materials must be detected by other means. One of the oddities of magnetic detection occurs because the strongest signals come from the ends of the object being detected. This is because the ends are where the lines of magnetic force tend to concentrate. As a result, a vertically oriented object (even a small steel drum) can often be easier to find than a horizontally oriented cast-iron water main hundreds of feet long. This same effect occurs at the joints of this same water line where sequentially laid metal pipe segments join to form the pipeline. The magnetic detection display can show a series of aligned peak signals where these joints are located, providing information on the pipe’s alignment and depth by connecting the dots. Utility Detection Methods and Technologies There are several categories of utility detection technologies: magnetic, ground penetrating radar (GPR), passive sound detection, and location data recorders. Though usually made of bundled copper wire, thin electrical power lines must usually be carrying a current to be detected magnetically. If a cable or line has been abandoned or is disconnected, operating the magnetic detector in direct mode can be used to induce a magnetic field. This is done by attaching a low voltage power source via clamps to the ends of the cable or pipeline. The result is a stronger signal running directly along the target alignment that allows for detection at greater depths and longer distances. Similarly, the transmitter operating in broadcast induction mode—after it is placed over the target line and activated—will also induce a signal that can be traced by the receiver. As the magnetic detection unit operator walks over a buried line, the receiver generates an audio signal indicating when the operator is directly above the buried utility. When not above a utility, the receiver’s magnetic field sensors balance out the earth’s background magnetic field and produce a low frequency (about 40 Hz) audio signal indicating the absence of a utility line. The frequency of the audio signal increases as the operator approaches the utility until it hits its peak and then declines as the operator moves and passes over the line. Marking the surface locations of these peak signals will provide a visual indication of the utility’s location and alignment. Similarly, damaged electrical cables can be located by measuring the resultant voltage differentials in the soil. Utilizing an electromagnetic antenna that broadcasted radio waves tuned to a frequency that can penetrate soils (including rock, concrete, ice, and other common natural and man-made materials), GPR provides a broader detection technology than the more narrowly focused magnetic detection technique. Though it has capabilities that make it suitable for obtaining geotechnical information as well as locating utilities, a GPR unit often requires more time to process data than simpler magnetic detection techniques and it tends to cost more. However, its abilities make it useful for outside of urban areas where natural conditions are dominant and utility locations are sparser than in cities. [text_ad use_post='27747'] GPR is functionally not much different than the more common aerial radar. By sending out pulses of high frequency radio waves, it can determine underground conditions from the radio waves that bounce back from solid objects such as buried utilities. These radio echoes bounce back to the surface where they are detected by the receiving antennae. The rest of the radio energy penetrates deeper into the soil until it encounters other objects and bounces signals back to the surface. The echoes delineate underground interfaces and surfaces such as changes in soil moisture and clay content, fractures and voids, cement chunks and solid pavement, gravel bedding, and man-made utilities. Increasing the transmitted radio frequency also increases the resolution of the radar reflections. Though its primary focus is on delineating regional hydro-stratigraphy, sensitive GPR units can also reveal the locations and depths of utilities.