Near-surface geophysical techniques are now widely applied to a broad spectrum of subsurface problems, and new applications continue to arise. The non-invasive geophysical tools used to examine near-surface (e.g., <100m depth) Earth materials employ electrical, electromagnetic, or mechanical energy sources, along with passive techniques that measure the physical parameters of the earth. Compelling factors, such as the need for potable groundwater, as well as construction and exploration, has led to the advancement of near-surface geophysical techniques and accompanying software over the last two decades.
Physical parameters which are directly measured during shallow geophysical surveys include elastic properties, gravitational and magnetic fields, electrical conductivity, and transparency to and reflection of electromagnetic waves. This data is then used to infer the stratigraphy, geological structure, permeability, porosity, and various other properties of the near-surface. Potential applications of near-surface geophysics include projects which are aimed at:
Mitigating existing geotechnical and environmental problems, such as subsurface pollutants (DNAPLs - dense non-aqueous phase liquids) and to guide shallow drilling programs;
Exploring and optimize the production of resources such as coal, and mineral deposits;
Inputting into design parameters for civil engineering and construction projects;
Enhancing basic geological and hydrogeological knowledge.
Geophysical methods chosen will vary according to a project’s objectives, resolution requirements, available budget, and most importantly, the prevailing geological conditions. For example, microgravity surveys sometimes are used in shallow geophysical exploration, particularly where a large density contrast might be present such as abandoned mine workings. In contrast, high-resolution magnetic surveys can be useful in searches for buried metal objects such as steel drums, or in mapping faults and locating magnetic bodies. Ground penetrating radar (GPR) can be useful in non-conductive ground to image at a range of resolutions, depending on antenna frequency. In contrast, in clayey ground where GPR may be less effective, by applying small electric currents across arrays of ground electrodes, electrical resistivity imaging (ERI) can be highly useful in imaging faults, stratigraphy and voids. Education concerning all of these methods needs improvement along with the smooth transfer of technology from its developers to users and potential beneficiaries.