Electrical Resistivity Tomography (ERT)

Resistivity, like density, is an intrinsic property of a material. The variations in subsurface materials can be measured using resistivity method. In the resistivity method, electric current will be applied across arrays of ground-coupled electrodes. Then potential differences are measured using a voltmeter. The potential differences can then be converted to resistivity of the subsurface materials. Resistivity is mostly a function of lithology, porosity, clay content, permeability, fluid saturation and fluid salinity. A 2-D or 3-D resistivity profile can be transformed into a realistic 2-D geologic/hydrologic model of the subsurface.

Usually, accuracy with geophysical methods can be increased if external constraints (i.e. borehole control) are available. Resistivity method can be a reconnaissance tool to optimally locate exploratory boreholes for geotechnical investigations. It is also a cos-effective means of establishing control between boreholes.

Significant applications:

  • Locate voids, abandoned mines and tunnels
  • Map variable depth to bedrock
  • Mapping faults, fractures, weathered zones
  • Mapping clay layers and lenses
  • Mapping sand and gravels
  • Mapping lithologic contacts
  • Differentiate/map rock units
  • Variable depth to water table
  • Variations in porosity
  • Variations in salinity
  • Mapping contaminants
  • Landslide investigation

The figure below displays an electrical resistivity data with the overarching goal of imaging the subsurface to a depth of approximately 120 ft. the specific objectives of this case study included mapping variations in the elevation of top of rock, mapping variations in soil thickness, mapping  variations in rock quality, mapping subsurface voids, characterizing existing sinkholes, and provide insight into groundwater flow patterns.

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