Ground Penetrating Radar (GPR)
The Earth’s magnetic field interacts with rock, or other subsurface materials, which causes variations in the Earth’s magnetic field received at the Earth’s surface. The magnetic susceptibility of the geology has a large influence on the induced magnetic field (i.e. a high-susceptibility body will produce a stronger induced field than a low susceptibility body).
The Ground Magnetics (Ground-mag) method measures the physical parameter described as the Total Magnetic Intensity (TMI) and this is measured in units of nano-Tesla (nT) using a magnetometer. Steel and other ferrous metals in the vicinity of a magnetometer can also distort the recorded data (so metal objects should be removed from clothing when recording). The strength of the TMI field is dependent on location (geographically) and varies with distance from the Earth’s centre (elevation) and with time. These factors are removed during the data processing phase. Reduction to the pole (RTP) is a transformation of the observed data with simulates the magnetic field distribution which could be observed if the inducing field were vertical (i.e. at the geomagnetic pole). The RTP transformation places magnetic highs directly over their causative bodies, and simplifies qualitative interpretation of data from moderate to low geomagnetic latitudes. The instabilities in the RTP process begin to become significant for geomagnetic latitudes <20 degrees, and generates strongest artefacts in the direction of magnetic declination (Rajagopalan, 2003). Further processing steps can also be performed on the data i.e. First Vertical Derivative (1VD), Analytical Signal (Ansig), to enhance the techniques visualisation of certain features. The processed magnetic maps (TMI, RTP) can be further modelled in 2D, by modelling the data response along a profile line with a geologic body of an inferred magnetic susceptibility. The model can be further refined with known drilling results and other available geophysical data. An example of a ground-mag contour map can be viewed in Figure 1.
Figure 1. An example image showing a GPR data cross-section with preliminary void/cavity and geological layer interpretation.
Ground-mag is a useful geophysical technology, especially in mineral exploration programmes. It is logistically straightforward to deploy, and provides detailed mapping of targeted zones very rapidly. Ground-mag is generally acquired on foot, but magnetic data can also be collected in an airborne (aero-mag) or marine environment (marine-mag).
Limitations of the ground-mag technique include:
- The magnetic method only responds to variations in the magnetic properties of the Earth
- Highly magnetic geologic or modern materials may obscure subtle features of interest
Ground-mag surveying could be used together with a complimentary geophysical technology (e.g. aero-mag, Gravity), conventional mapping, drilling, and indeed any other available source of information.
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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