What is apparent resistivity?
Apparent resistivity (Dutch: schijnbare weerstand) is, put simply, a kind of average electrical resistivity of the ground. The value is made up of the resistivities of all soil layers within the range of the measuring array. It is expressed in ohm-metres (Ωm).
How is it measured?
In a geoelectrical (resistivity) survey, an electric current is passed into the ground through two electrodes, and the voltage is measured between two other electrodes. The apparent resistivity is calculated from the current, the voltage and the electrode spacing. Well-known arrays are the Wenner and Schlumberger arrays.
The wider the electrode spacing, the deeper the measurement reaches. Measuring with increasing spacing (vertical electrical sounding) or along a line gives a picture of the ground’s layering. It is called “apparent” because the value is not the true resistivity of a single layer but a blend of several layers. The actual layering is then derived using inversion models.
Resistivity depends mainly on water content, groundwater salinity and soil type: clay usually conducts better (lower resistivity) than dry sand, and salty groundwater conducts far better than fresh water.
Application
- Groundwater studies: locating the boundary between fresh and saline groundwater and finding aquifers.
- Ground conditions: detecting clay or peat layers, old channels or contamination, alongside boreholes and cone penetration tests.
- Earthing and cathodic protection: soil resistivity determines the design of earthing systems.
- Archaeology: locating buried foundations and ditches.
A resistivity survey is an indirect method. Always have the results interpreted by a specialist and check them against boreholes or probes.
Related Terms
- Ground investigation
- Cone penetration test
- Groundwater level
- Cathodic protection
- Seepage
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