Ground Penetrating Radar (GPR) is an electromagnetic technique in which electromagnetic waves are emitted into the ground surface which reflects from structures/targets with different dielectrical properties. GPR is similar to seismic reflection with an electromagnetic pulse being used instead of a pulse of seismic energy. GPR typically operates in the frequency range of 10 to 1000 MHz. A typical system consists of two antennas, spaced a fixed distance apart, with a data recorder connected to the antennas with communication cabling (Fiber-optic or copper). GPR is an extremely attractive geophysical method as it has the highest resolution of available geophysical techniques for imaging the subsurface.
Method of Operation
A transmitter antenna is used to transmit a high energy pulse into the subsurface with reflections measured by the receiver antenna. While pulses are continuously transmitted, the whole system is moved continuously, or in fixed station spacing, while readings are being taken. This allow for the construction of a pseudo cross section of reflections as a function of depth and chainage.
Choice of antenna depends on the required resolution, depth-to-target and site conditions. A 1000 MHz antenna gives cm resolution for imaging targets in the 0-0.5 meter depth range, but will not give penetration beyond those depths. A low frequency antenna (100 MHz) will have improved penetration, but with the lower resolution cannot image small and shallow targets. Penetration is to a large extent determined by the conductivity of the subsurface. The more conductive the subsurface, the lesser penetration is achieved. In general GPR does not perform well in areas where a combination of any two of water, salt or clay is present.
GPR allows for data interpretation on-site as data can be processed relatively quickly. An assessment of the effectiveness of the technique for the particular application can also be made during data acquisition, allowing effective Data Quality Control during acquisition. GPR is a rapid method, and large distances can be covered in a single day. Systems can be vehicle mounted, or towed behind a vehicle.
GPR is commonly used in the following applications:
- Delineation of subsurface pipes and services
- Mapping of leaks
- Mapping of subsurface cavities and undermining
- Mapping of buried objects such as drums and tanks (both metallic and non-metallic)
- Mapping depth to water table
- Mapping depth to bedrock
- Road and pavement investigations
- Underground mining: Roof Integrity Investigations
- Underground mining: Mapping of geological structures
- Concrete integrity investigations
- Mapping of rebar and post tension cables
- Mapping of shallow voids