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
Resistivity 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.
J
R Associates,
Ph: