Techniques
Ground resistivity 2D/3D imaging
An effective tool for surveys in site investigations, foundation engineering, aggregate prospecting and environmental surveys
The principle
The basic principle behind resistivity imaging is that because different geologic materials have different electrical properties, layers in the subsurface can be identified on the basis of these properties. The effective parameter is resistivity (Ohmm) or its inverse, conductivity (S/m). Resistivity depends mainly on water and clay mineral content of materials.
The method is based on measurements of the potential field stimulated by a direct current or very low frequency alternating current in the ground by means of metal electrodes. An electrical current is injected into the earth through a pair of current electrodes, and the potential difference is measured between a pair of potential electrodes.
Instruments
New advances in resistivity imaging technology include systems that acquire large amounts of resistivity data using a computer operated measuring system. After a series of electrodes are spread out in an array (usually Wenner, dipole-dipole or pole-pole), the computer automatically collects the data from many different electrode separations. Wider electrode spacing results in deeper penetration. The maximum penetration depth is about 60 meters e.g. with ABEM Lund imaging system.
The final report includes subsurface true resistivity models and geological interpretations from them. Results are usually plotted as true resistivity contour maps. The interpretation results can also be transferred into CAD design programs.
The pseudosection is constructed from the measured data. The true resistivity is calculated from measured data using resistivity inversion. The measurements can be made in 2D or 3D, depending on the problem.
Figure. Principle of 2D resistivity imaging and measured apparent resistivity pseudosection.
Applications
Resistivity imaging is a non-invasive technique that can be used in variety of applications. Resistivity imaging is efficient especially in conductive environments e.g. in clay areas where GPR or other EM methods are unsuitable.
In geotechnical investigations and foundation surveys, resistivity imaging can be used e.g. to determine depth of bedrock or overburden, thickness of clay and silt layers, or landfill thickness; to map dam leakage or dam structure; and to locate sinkholes, cavities, and fracture zones.
Groundwater and watercourse investigations include determining groundwater depth, mapping marine and river bottom, mapping of salt water intrusions, and characterising of subsurface hydrogeology.
In environmental surveys, resisitivity imaging is efficient in mapping conductive contaminant plumes or heavy metals soil contamination. The method can also be used to delineate disposal area.
Resistivity imaging has also been applied in sand and gravel aggregate prospecting, in mapping of archaeological sites, and in railway and highway tunnel investigations.