GPR is the acronym for Ground Penetrating Radar that is also known as Georadar or Ground Penetration Radar or Ground Probing Radar.
GPR is a compact device that scans the subsurface in a non-destructive way. It can penetrate the surface from a few cm down to tens or hundreds of meters, based on soil conditions and antenna characteristics.
GPR emits electromagnetic (EM) waves to the subsurface and calculates the time these waves need to travel through the various subsurface material and get back to the GPR receiver. This two-way travel-time, along with a few other parameters such as the dielectric, provide the user an estimate of the target depth.
GPR is a very useful tool in a variety of applications. Most popular uses of GPR are the non-destructive testing and mapping of objects inside the concrete, such as rebars, pipes, ducts, cables etc. and infrastructure assessment (bridge decks, roads), utility location, archaeology, forensics, environmental studies (mapping contaminants), shallow geology and geophysics, mine exploration and safety, transportation (pavement thickness and density, ballast fouling), agriculture, military (UXO), sedimentology, glaciology, quarrying, space exploration.
Dielectric, also known as relative permittivity, is a unitless value that determines how fast EM waves travel in a specific medium. Its values range from 1 to 81, with lower values being better for GPR. Air, which is the best medium for radar waves has a value of 1 and seawater has a value of 81. Metal objects have infinite dielectric.
GPR can detect metallic and non-metallic targets below the surface. However, certain limitations exist, like the depth to target limitation. The rule of thumb is that GPR can detect almost any target if it is at least one inch (2.54cm) in diameter and is buried at one foot (0.30m) or shallower. For example, it may be impossible for GPR to locate a 3-inches (7.6cm) plastic pipe down to 6 (1.82m) feet depth. Metallic targets are doing better with this rule.