GPR Scanning Technology

How Does GRP Work

Ground Penetrating Radar (GPR) is a well-established method of monitoring civil structures such as bridges, tunnels, and buildings. GPR is also used as part of a nondestructive detection method to identify underlying features or buried services. It is well known to be a reliable method of identifying the location and depth of underground pipes of different materials and sizes.

GPR operates by transmitting electromagnetic waves (in the range of 10 ~ 10 00 Hz) into the probed material and receiving the reflected pulses as they encounter discontinuities. The discontinuity could be a boundary or interface between materials with different dielectrics or it could be a subsurface object such as a debone or delamination. The amplitudes of the received echoes and the corresponding arrival times can then be used to determine the nature and location of the discontinuity.

With the advancement in GPR technology, especially the increase in frequency of commercially available GPR antennae and better data processing software, GPR can now be used for subsurface condition assessment in materials consisting of thin layers, such as FRP composites. Careful analysis of GPR waveforms can potentially help detect subsurface debonds between the wearing surface and the underlying FRP bridge deck, and delaminations within the flanges of the FRP deck.

Compared to other non-destructive techniques such as infrared thermography, ultrasonic or microwave, GPR offers more penetrating power and so can detect concrete defects or deteriorations at greater depths. Results have shown that the lower frequency GPR antenna (1 GHz) cannot detect shallow defects such as debonding in FRP wrapped members, but a higher frequency antenna (2 GHz) can detect those defects. On the other hand, a ground coupled 1.5 GHz antenna was found to offer higher penetration capability, which is crucial for testing FRP bridge decks. GPR is an excellent tool for detecting water-filled debondings.

The main components that make up the GPR include a waveform generator, a single transducer comprised of an emitting and receiving antenna, a signal processor, and a data storage/display unit. Various approaches have been used for structural applications, such as frequency modulation, synthetic pulse-radar, and pulse systems. GPR data can be acquired and presented in different modes. A measurement in a single position will produce a time series that can be presented as a curve or colour coded. The vertical axis is a time axis and it is usually pointing downwards because larger times correspond to larger depths.

If the antenna is moved along a horizontal line, GPR measurements can be recorded at close intervals. The recorded time series can be plotted side by side with the time axis pointing downwards. The horizontal axis is then corresponding to the line along which the antenna was moved. This display format is called a radargram.

If data acquisition is carried out over an area, for example by measuring along many parallel lines, a data cube can be built and then slices corresponding to a certain Twt can be cut out and displayed. This display format is called a time-slice. The two axes of the time slice are corresponding to the two horizontal axes of the acquisition area. It is also possible to cut vertical slices from the data cube or to display all values that meet certain criteria, for example values beyond a defined threshold.