One of the most versatile underground locating tools available is ground penetrating radar, or GPR. GPR uses high-frequency radar waves to create a detailed image of what lies under the earth. GPR is particularly good at helping to locate non-metallic subsurface utilities such as plastic, concrete and clay pipes, which conventional electronic locating equipment sometimes has a tough time with. For instance, we have been having great success in locating small-diameter plastic propane and natural gas pipes - very difficult to see with standard locating techniques - with GPR.

Ground-penetrating radar is also great for ledge profiling, and our instrument can even tell us how far down bedrock is: data which could save a contractor thousands of dollars in blasting costs. GPR is also very effective at locating underground storage tanks ... or anything else you might need to find without tearing up your whole property.

GPR is also very handy for finding small, discrete objects that don't show up well against the background using other locating methods. In the picture at left, Glen is searching for a concrete cover to an electrical vault underneath a parking lot. Once the cover's exact position was found, the contractor was able to minimize damage to the pavement by cutting only where he needed to in order to access the vault.

The scan below, of an area at the Verso Paper mill in Jay, Maine, shows five three-foot water pipes, at depths ranging from four to thirteen feet. Our ground penetrating radar unit can detect objects buried up to twenty feet beneath the surface.

Technical Notes

Ground-penetrating radar works better in some soil types than in others: GPR works best in gravelly or sandy soil, and less well in heavy clay materials. The efficiency of GPR is strongly affected by the target soil's conductivity (the soil's ability to carry a charge) and dielectric permittivity (the soil's capacity to hold a charge).

Since different soils have different electrical characteristics, the effectiveness and scan depth of GPR is highly dependent on the particular target site. Clay soils, along with subsurface brackish or salt water, greatly attenuate (absorb) the radar signal used by the GPR unit to "paint" a picture of the subterranean environment. So, in certain soils, the GPR's scan depth is limited. Especially salty or clayey soils can block the signal completely, although we don't often encounter that.

Assuming the soil composition of the target site is reasonably conducive to GPR operation, the other main factor influencing GPR's maximum scan depth and resolution is the type of antenna used. Generally, the lower the GPR antenna frequency, the deeper the GPR can scan, but the lower the resolution of the scan – so only larger objects will be detected. Conversely, a higher-frequency antenna will give a shallower scan depth, but at a much higher scan resolution, which allows smaller subsurface features to be detected. Naturally, we have a full range of antennas for our GPR units to suit any terrain and target. Our higher-frequency antennas are best for finding small buried objects, while our lower-frequency antennas are excellent for finding larger subterranean features, and ledge profiling.

The best conditions for ground-penetrating radar are sandy or gravelly soils, such as silica sands, fresh water, ice, and most solid rocks of low conductivity. Under such conditions, we can use a relatively shallow-scanning, high-frequency antenna, which will pick out the smallest detectable underground features while still providing all the scan depth we need. Using a lower-frequency antenna in compatible soils lets us find larger objects several dozen feet below the surface.