Ground Penetrating Radar (GPR)
Single-channel GPR utilises a single transmitter (Tx) and receiver (Rx) pairing for GPR data acquisition.
Traditionally the acquired GPR data is presented in standard 2D sections (radargrams – see Figure 1), or if multiple GPR profile lines are orientated parallel to each other or in a grid formation, depth-slices of the gridded amplitude reflection data can be presented. Depth-slices are useful for observing if a feature is present across multiple GPR profiles.
General GPR Limitations
- GPR requires a dielectric contrast between the sub-surface target and the surrounding host material in order to observe a reflection at the material interface boundary;
- Strong electrically conductive soils (e.g. clays) or saline groundwater (e.g. salt water) can have strong attenuating effects on the GPR waves limiting (or completely ending) signal penetration.
Figure 1. An example image showing a GPR data cross-section with preliminary void/cavity and geological layer interpretation.
An advantage of Multichannel 3D GPR over conventional single channel GPR (2D GPR) is the area of investigation coverage. A swath of multiple GPR transmitter (Tx) and receiver (Rx) antennas are utilised in Multichannel 3D GPR investigations in comparison to standard GPR investigation which utilise a single channel Tx and Rx antenna. Figure 1 provides an example of potential GPR ray-paths between multiple transmitters and receivers for an example Multichannel 3D GPR set-up.
Figure 1. An example image illustrating multiple radar ray path combinations for a Multichannel 3D GPR set-up (Image source. Mala GPR, www.malagpr.com.au).
The 3D GPR data results produced from using Multichannel 3D GPR data are of a higher lateral resolution in comparison to reconstructing 3D GPR from multiple parallel 2D GPR profile data (see 3D GPR data comparison for the same depth interval in Figure 2). Note: The main limitation of GPR is that the depth of investigation is limited in the presence of electrically conductive materials (e.g. clay, saline groundwater).
Figure 2. An example image showing a comparison of a 3D GPR dataset created from a Multichannel 3D GPR data and multiple parallel 2D GPR data for the same depth interval (Image source. Mala GPR, www.malagpr.com.au).
The GPR method can be utilised to display subtle changes in the sub-surface of the ground or material of interest. Changes in material dielectric contrast can be caused by many factors including material deformation, such as cracking or contamination seepage. 3D regions of interest in the subsurface highlighted using the GPR method can be monitored over calendar time (4D monitoring) to determine if these regions are propagating, diminishing or showing no signs of change. This information can be particular useful to civil engineers or environmental scientists in reducing the risk of their ground, or material health monitoring assessments. An example illustrating the net volumetric changes in a GPR dataset monitoring a dense non-aqueous phase liquid (DNAPL) is shown Figure 1.
Figure 1. An example image illustrating the net volumetric changes of DNAPL seepage in the subsurface (image source: Birken, R. and Versteeg, R. (2000). “Use of four-dimensional ground penetrating radar and advanced visualization methods to determine subsurface fluid migration”. Journal of Applied Geophysics, 43 (2000), 215-226.)
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