Even though underground scanning has been around for decades, we still meet people who aren’t familiar with it. Some are understandably skeptical about its accuracy. They worry that they might pay for an underground scan only to hit an unexpected utility line or buried object once excavation begins.
In this blog post, we’ll explain the technology behind underground scanning and discuss the accuracy of the tools we use at Enhanced Scanning.
The “Big Two” in Underground Scanning
Two technologies dominate the underground scanning industry: ground penetrating radar (GPR) and electromagnetic (EM) locating. Depending on the site conditions and what we’re trying to locate, we may use one technology or the other. However, on many projects, we use both together to provide the most complete picture possible.
Both GPR and EM locating are remarkably effective tools, but neither is infallible. Understanding their accuracy begins with understanding how each technology works.
How GPR “Sees” Underground
Ground penetrating radar sends high-frequency electromagnetic pulses into the ground and measures how those signals bounce back. Every time a pulse encounters a boundary between materials with different electrical properties—such as soil and pipe, soil and rock, or soil and a void—a portion of the signal reflects back to the surface. A skilled technician interprets these reflections and converts them into a map of what lies below.
Under favorable conditions, GPR can locate utilities and other buried objects within approximately one inch of their actual position. It can also estimate depth with similar precision. One of GPR’s greatest advantages is its ability to detect non-metallic objects, such as PVC pipes, concrete structures, and underground storage tanks, which traditional locating equipment often cannot identify.
However, GPR’s accuracy depends heavily on site conditions. Soil composition has a significant impact on how well radar signals travel. Clay-rich or highly conductive soils absorb radar energy quickly, reducing the scanning depth and making the returned signals more difficult to interpret. Rocky ground or soil filled with debris can scatter the radar waves, creating misleading reflections. Moisture also plays a major role. Very wet soil can significantly reduce the depth that GPR can penetrate. In ideal conditions, such as dry, sandy soil, GPR may provide clear readings at depths of 10 feet or more. In dense clay, however, its effective range may be limited to just two or three feet.
How EM Locating Works
Electromagnetic locating works very differently. Rather than sending signals into the ground, it detects electromagnetic fields generated by conductive utilities. A technician uses a receiver to detect the field and determine both the utility’s location and approximate depth based on the strength and orientation of the signal.
EM locating is extremely accurate within its area of specialization. When a metallic pipe or energized cable carries a strong, clean signal, the technology can often pinpoint its location within about an inch. It is fast, reliable, and requires less interpretation than GPR.
Its limitations are just as specific. EM locating only works on conductive materials. Plastic water lines, empty conduits, and abandoned utilities without a continuous conductive path are essentially invisible to EM equipment. In areas where multiple utilities are buried close together, signals can also “bleed” from one line to another, making it more difficult for a technician to distinguish the exact location of each individual utility.
Why the Two Are Often Combined
Because each technology has strengths that compensate for the other’s limitations, professional locating companies like Enhanced Scanning frequently use GPR and EM locating together. EM locating excels at finding metallic and energized utilities, while GPR helps identify non-metallic pipes, underground voids, buried structures, and other objects that have no electrical signature. Comparing the results from both technologies also increases confidence in the findings. When a utility is detected by both GPR and EM locating, the likelihood of an accurate identification is significantly higher.
Even when underground scanning results inspire a high degree of confidence, caution should always be exercised during excavation. No locating technology can guarantee the detection of every buried object under every set of conditions. For that reason, we recommend maintaining at least one inch of clearance from our markings whenever penetrating the ground. Taking this extra precaution helps reduce the risk of damaging underground utilities and contributes to a safer excavation process.
What can Enhanced Scanning help you locate underground? Contact us to schedule service with one of our highly trained underground scanning experts.