Resistivity

Resistivity techniques provide valuable information for hydrologic and environmental studies. It is a flexible method that is used to map both the lateral changes in geology and changes in geology with depth. Resistivity provides detailed profiles of shallow buried landfills and stream channels. Resistivity can probe 1000 feet into the earth, delineating sandy aquifers from clayey aquicludes. Unlike exploratory drilling, resistivity requires no special permits and avoids the risk of cross contaminating aquifers. Resistivity also:

  • Defines rock and soil types
  • Maps buried sand channels
  • Delineates contamination plumes
  • Maps stratigraphic layering
  • Determines soil corrosiveness
  • Delineates faults and slide planes

Grounding Grids and IEEE Standard 81-1983

Wenner ArrayResistivity techniques are used to analyze soils for electrical grounding grids. The size of a grounding grid is in part dependent on the electrical resistivity of the soil. Soils that are electrically resistive require a larger grid than soils that are electrically conductive. The IEEE standard 81-1983 was developed to provide a standard for measuring earth resistivity for grounding grids.

J R Associates follows the IEEE Four-Point method employing equally spaced electrodes. In geophysics this is called a Wenner Array. Four electrodes are placed in a straight line at intervals of "a" and hammered a short distance into the ground. A measured electrical current is injected into the outer electrodes and a voltage is measured between the inner electrodes. The average resistivity to a depth equal to the "a" spacing is calculated from the voltage, current and "a" spacing. A profile of resistivity versus depth can be made by making incremental increases in the "a" spacing and repeating the measurements. The resistivity profile can then be used by electrical engineers to design a grounding grid.

At J R Associates we stand ready to apply our geophysical expertise to your particular requirements. Please contact us to discuss how our combination of techniques, knowledge and experience can provide you with a window on what's underground.

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J R Associates, 1886 Emory Street, San Jose, CA  95126

Ph: (408) 293-7390 Email: jra@Greatgeophysics.com