Fill variation can be mapped effectively and quickly using various geophysical technologies. Fill typically consists of materials with a differing electrical signature to the host geological material. These electrical contrasts can be resolved laterally and with depth using combinations of geophysical techniques. Common methods used to achieve these objectives are GPR, EM and Magnetics.
- Ground Penetrating Radar (GPR) – Fill generally comprises of a different source material to the surrounding host geology and local ground conditions. The fill zone tends to act like a porous material and/or contains a high moisture content, making this (in general) an electrical conductive zone. These fill regions can effectively be imaged using GPR. The boundary between the fill and natural ground often provides a strong dielectric contrast that can be interpreted using GPR’ this makes it possible to map the depth and extent of fill zone.
- Electromagnetic (EM) methods target large contrasts in subsurface electrical conductivity associated with uncontrolled fill. The bulk fill region tends to act as a porous material and/or contains a high moisture content, making this (in general) an electrical conductive zone in comparison to the surrounding host geological material. This technique is generally used to map the extent of the fill laterally. EM is a very effective geophysical tool for this application and fast to deploy and to acquire data.
- Electrical Resistivity Imaging (ERI) also targets conductivity variations throughout the uncontrolled-fill region. The fill region will generally appear as a more conductive zone in comparison to the generally more resistive host geological material. ERI is a preferred methodology for showing the depth to base of fill presenting the lateral electrical variation along the profile. With a series of parallel or orthogonal profile lines it is possible to interpret the volume of the fill-region.
Tailings dam integrity and groundwater contamination are two examples where geophysics can be an effective tool to map zones of higher conductivity generally associated with contamination. Contaminants seeping into subsurface soils can alter the electrical properties of the ground. Geophysical methods such as Electromagnetics (EM) and Electrical Resistivity Imaging (ERI) are preferred techniques utilised for these types of environmental applications.
- Electromagnetic (EM) methods, in particular frequency domain EM (FDEM), are quick and effective at monitoring contamination plumes or regions of tailings seepage. Depending on depth from surface, extent of the contamination and type of contaminant being mapped, closely-spaced parallel FDEM profile can laterally constrain electrically high conductive zones often associated with contamination. Through repeat surveys over calendar time it is possible to observe and monitor the extension, contraction or migration of plume/seepage zones.
- Electrical Resistivity Imaging (ERI) also targets regions of conductivity variation associated with contamination plumes or tailings seepage. ERI is a preferred methodology for showing the depth extent of a contamination plume/tailings seepage region along a profile line. By arranging ERI cross-sectional profiles in a parallel orientation, it is possible to periodically map over calendar-time any observable migration of contamination zones between profile lines. It is also possible to monitor any changes in dip/strike direction of the migrating contamination.
- Electromagnetic (EM) methods are excellent pre-screening tools for locating buried electrical services and steel pipes. Buried metal objects will generally have a large electrical contrast in comparison to the host geological material. EM methods can map the lateral extent of the buried object across the site, and the depth to the object can often be determined using the complimentary GPR technique.
- Magnetic methods target metallic or ferrous objects sometimes found in areas of uncontrolled fill. It can detect magnetic objects such as drums or tanks that may otherwise be too deep for GPR or EM methods to detect. EM methods may not necessarily pick up highly magnetic objects, so the Magnetic technique acts as a great pre-screening and complementary tool to EM methods; especially if the investigation site is known to contain possible buried magnetic objects. Depths to interpreted objects can often be determined using the GPR technique.
- Ground Penetrating Radar (GPR) is a quick and effective technique for interpreting the depth to buried objects such as underground services, underground storage tanks (UST) and other large buried man-made objects. Generally, the targets chosen for further investigation with GPR are often located prior using EM and Magnetic methods. These objects often have a large electrical contrast in comparison to the host material, which provide good targets to produce a strong GPR reflection.
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