Principles of Ground Penetrating Radar

The radargrams are saved electronically. 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.

Radargrams showing how the anomaly in the animation above would appear in a GPR radargram. The left radargram shows the raw data; the right radargram is after surface correction and migration has been applied. Note: depth and longitudinal scales are displayed.

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.

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.

Horizontal depth slice from 3D model showing rebar distribution in concrete beam. Vertical slice shows tendon below rebar layer.