Principles of Ground Penetrating Radar
GPR is a flexible, non-intrusive and non-destructive technique for the investigation and characterisation of the sub-surface. For more information about the wide and varied applications of GPR, visit our dedicated page about Ground Penetrating Radar surveys offered by Sandberg.
How Ground Penetrating Radar Works.
Ground Penetrating Radar utilises an antenna (comprising a transmitter and receiver a small fixed distance apart) to send electromagnetic waves into the subsurface. The antenna is moved over the surface of the medium to be inspected. The transmitter sends a diverging beam of energy pulses into the subsurface and the receiver collects the energy reflected from interfaces between materials of differing electrical properties. The reflected energy is recorded as a “pattern” on radargrams which are displayed in real time. The radargrams constitute the raw Ground Penetrating Radar data. They are displayed in real time on the control unit and basic interpretation can be conducted on site. Usually however, the radargrams are processed and analysed off-site using specialist software.
Depending on the application, antennas with different central frequencies are used to suit conditions and survey requirements. In general, low frequency systems are more penetrating but data resolution is lower; high frequency systems have limited penetration but offer a much higher resolution. By scanning the same area using two or more different frequencies, the information collected in the GPR data can be maximised.
|Antennae Frequency||Maximum Penetration Depth||Examples of Potential Use|
|1500 MHz||0.5 m||Rebar mapping and concrete evaluation.|
|900 MHz||1 m||Pipe and void detection or assessing concrete thickness.|
|400 MHz||4 m||Utility surveys, pavement evaluation, storage tank detection and assessing structural integrity.|
|270 MHz||6m||Utility surveys, geology and archaeology.|
The above table shows the maximum penetration that the different antenna frequencies are capable of achieving. In practice the limiting factor is is generally dictated by ground conditions. GPR works best in sandy soil. It does not work well in heavy clay soils where depth penetration is severely compromised.
GPR Data Analysis
Sandberg uses GSSI Radan software for off-site processing and analysis. Data processing can filter out unwanted noise and interference, apply corrections and enhancements, calculate depth and more. If the data has been collected on regular orthogonal grids, adjacent scans can be combined to produce 3D models if required.
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