Ground Penetrating Radar, also called GPR, is a non-destructive geophysical method capable of detecting most underground utilities, including concrete, fiberglass, transite, and plastic pipes that may not be detectable with conventional utility locating methods. In fact, GPR can be thought of as a subsurface imaging tool, offering solutions for many underground problems like subsurface voids, underground storage tanks, and more.
How Does it Work?
The Ground Penetrating Radar (GPR) equipment is mounted on a cart that is pushed along the surface of the ground that is being investigated. It transmits pulses of ultra high frequency radio waves down into the ground through a transducer or antenna. The transmitted energy is reflected back from various buried objects or distinct contacts between different dielectric properties of subsurface materials. An onboard antenna then receives the reflected radio waves, which are stored and interpreted in the digital control unit. This data makes up the final image, that is then reviewed and interpreted by the experts on the Master Locators team.
The end result is a constant profile or record of both metallic and non-metallic subsurface features without probing, digging or drilling. This allows you to evaluate the location and approximate depth of buried objects, as well as investigate the presence and continuity of natural subsurface conditions and features.
The readable depth is a function of the earth’s conductivity in the area tested, which is largely based on the materials that make up the area being investigated. The lower the conductivity, the deeper the depth of penetration. GPR waves can reach depths of up to 100 feet in low conductivity materials such as dry sand and granite. Clay, shale, and other high conductivity material may attenuate or absorb the radar signals, greatly decreasing the depth of penetration to 3 feet or less. The depth readability of GPR is also dependent on the ground saturation as it is less effective when there is a high water table or when it has recently rained.
The type of GPR antenna will also determine the depth of penetration. Antennas with low frequencies from 25 to 200 MHZ obtain subsurface reflections from deeper depths, but have lower resolutions than higher frequency antennas. In addition to underground utilities, these low frequency antennas are used for investigating the geology of a site, such as for sinkholes or fractures, and also to locate large deep buried objects, such as underground storage tanks and septic tanks.
What can ground penetrating radar find?
GPR can locate underground utilities, underground voids, and other buried objects and geotechnical or environmental features.
How deep can GPR detect?
The depth that GPR can reach depends on the soil type, antenna frequency and size of the target. In areas that have a clay soil, the depth is only up to a few feet but in areas with sandy soils, GPR can penetrate much deeper.
Is GPR easily interpreted?
How easily GPR can be interpreted depends on what kinds of objects or features are being investigated, and what the soil or surface area conditions are like. GPR profiles may be easily interpreted, or may require someone with years of training and experience to interpret them.