Ground Penetrating Radar (GPR) – An Electromagnetic Investigation Method
Ground penetrating radar (GPR) is an electromagnetic investigation method. It is the technique that employs radio waves to map structures and features buried in the ground. It provides a profile of subsurface features, which can be used to determine the location of buried objects.
How does it work?
Ground penetrating radar operates by transmitting pulses of high frequency radio waves, in the range of 10MHz to 2.6 GHz, down into the ground through a transducer (also called an antenna). As they encounter discontinuities the pulses are reflected back to be received by the antenna. These reflections can be caused by any objects that have sufficient contrast from the surrounding soil such as rocks, tree roots, or underground utilities. The amplitudes of the return signal and the corresponding arrival times allows the operator to determine the size and location of these objects. The data is displayed on a control unit as images based on the amplitudes of the reflections and their travel times. Radar is sensitive to changes in material composition. Detecting these changes requires movement. GPR systems are designed to locate stationary targets, so one must move the radar to detect them.
What can GPR find?
GPR can find anything that has contrast within the surrounding soil. A PVC pipe and the air inside of it, for example, has a different composition than the dirt that surrounds it. Similarly, voids and backfilled excavations will also have contrast from the surrounding soil.
Here are some objects that GPR can help you find:
Utility Service: Clay Pipes, Plastic/PVC, Concrete Pipe, Transite Pipe, Metal Pipe, missing Valves, Water Boxes, Abandoned Lines, Conduit, Water/Wastewater, Gas, Power, CATV, Telecom Wire, Fiber Optic, Septic Tanks, Voids, Manholes etc.
Structural Analysis: Reinforcing, Voids, Slab/Wall Thickness, Asphalt Layer Thickness.
Law Enforcement: Weapons, Drugs, Cash.
Environmental: Contaminant Underground Storage Tanks, Landfill Limits, Buried Drums.
Humanitarian: UXO, Graves.
How Accurate is it?
Generally, GPR will reveal the horizontal positioning of targets in their exact locations. However, there are several factors that affect GPR’s ability to locate certain objects and to obtain accurate depths. Moisture content in the soil is the biggest variable that impacts the effectiveness of GPR by absorbing the signal and limiting depth penetration. Depth accuracy can also vary but can often be estimated to within 90% accuracy.
The radar signal velocity depends on the composition of the material being scanned and the breadth of the target. The signal will travel slower through ground that are more difficult to penetrate. The distance between the antenna and its target is calculated based on the amount of time that it takes for the radar signal to be reflected off the target and back to the system. Similarly, the moisture content of the material affects the velocity of the signal as well.
Though it is usually not possible to know the exact velocity that the GPR signal travels through the material, it is usually possible to estimate this within 90% accuracy.
What are its advantages and disadvantages?
The main advantages of the method are:
- GPR is non-destructive
- It is fast (hundreds of measurements per second),
- GPR can be used in non-contact mode
- It can detect non-metallic materials
- It provides superior resolution and reliability
- GPR is very cost effective
The main disadvantages are:
- GPR requires significant training because a GPR display is subject to interpretation.
- Results can be affected by the moisture content and specific materials comprising the ground. GPR cannot tell the composition of a target. It can only tell if there is a contrast between the target and the surrounding area. Water reflects the signals differently from materials in the ground, which can mask the presence of utility lines.
- The cumbersome nature of GPR and the fact that it must be dragged or pushed along the ground surface makes it difficult to collect data in areas with obstructions.