Geophysics: Electrical Resistivity Profiling
Electrical Resistivity (Electrical Resistivity - Fall-of-Potential and Dipole/Dipole)
Electrical resistivity surveys frequently use a digital resistivity instrument system. Either a dipole-dipole array with programmable electrodes at an appropriate spacing for the detection of the desired target is laid out on the ground surface for profiling, or a Wenner four-point array is used for resistivity soundings.
The objective of an electrical resistivity survey is to map the subsurface distribution of apparent electrical resistivity by means of injection of DC current in the ground and by measuring the voltage produced at points on the surface for the purposes of geologic, geohydrologic, void detection, and/or buried target detection evaluations.
Electrical resistivity values are digitally recorded and processed using specialized software to build a two- or three-dimensional model of a site which can be used to evaluate existing void spaces, depth of conductive materials, and approximate volumes of conductive objects within the subsurface soils. Topographic elevations are used to make any needed topographic corrections to the collected resistivity data. Depth sounding involves digital computer controlled current electrodes and potential electrodes expanding out from a common midpoint. As the electrode pairs are expanded farther apart, deeper measurements of apparent resistivity are recorded. The system is capable of producing resistivity profiles over 500 feet long with a depth of evaluation of up to about 100 feet below the ground surface.
Electrical Resistivity - Fall-of-Potential Ground Testing
The ground impedance Fall of Potential (FOP) test method for measuring ground resistance is typically performed at existing installations of electrical grounding equipment and subsurface earth grounding grids. Examples include electrical substations, power station power blocks, and electrical transmission switching yards.
FOP testing is conducted as per any project specification(s) and in general accordance with 2011 International Electrical Testing Association Maintenance Testing Standards (NETA MTS 2011) using a three point FOP method and a calibrated digital earth resistance meter at varying distances from the existing grounding grid. We observe whether the existing ground grid cable is electrically connected to the existing infrastructure or not, and our tests are conducted with our ground test wire attached to the existing ground grid cable and/or grounding rod.
FOP test results include reporting the test’s electrode separation distance, recorded resistance in ohms at each distance measured, and a graph of the recorded data.
Electrical Resistivity – Wenner Array
Electrical resistivity surveys are typically performed to evaluate corrosion potential of soils, grounding potentials, presence of clay layers, extent and depth of landfills, and to resolve resistivity layering useful in interpreting possible geology, presence of voids, and depth to groundwater.
Soil or material resistivity testing of field soil resistance (R) is measured and recorded in the field in order to calculate the apparent resistivity (Pa), in ohm centimeters and/or ohm-ft in general accordance with ASTM G57. A Wenner equally spaced electrode array is used, with electrode spacings based on project specifications, experience, and/or site conditions, or on a combination of all three considerations.
A calibration test is performed at the beginning and end of the field survey using an electrical calibration harness of known resistances supplied to Ninyo & Moore by the instrument manufacturer that contain three very high quality test resistors of differing resistance values. The calibration harness is traceable through our supplier to the United States of America National Institute of Standards and Technology (NIST).
Data are reduced in accordance with ASTM G57, and processed in accordance with IEEE Std. 81-2012 Annex B, or other optional modeling procedures to resolve the resistivity structure of the subsurface. The test is relatively non-invasive and the equipment is hand portable, so that environmentally sensitive locations lacking vehicle access can be surveyed.