Electrical resistivity methods involve the measurement of the apparent resistivity of soils and rock as a function of depth or position. The resistivity of soils is a complicated function of porosity, permeability, ionic content of the pore fluids, and clay mineralization. The most common electrical methods used in hydro-geologic and environmental investigations are vertical electrical soundings (resistivity soundings) and resistivity profiling. During resistivity surveys, current is injected into the earth through a pair of current electrodes, and the potential difference is measured between a pair of potential electrodes. The current and potential electrodes are generally arranged in a linear array. The apparent resistivity is the bulk average resistivity of all soils and rock influencing the flow of current. It is calculated by dividing the measured potential difference by the input current, and multiplying by a geometric factor (specific to the array being used and electrode spacing). With resistivity soundings, the distance between the current electrodes or the distance between the current and potential dipoles is expanded in a regular manner between readings, thus yielding information of the electrical properties of soils from deeper and deeper depths.
In 2D resistivity profiling, the electrode spacing is fixed, and measurements are taken at successive intervals along a profile. Data is generally presented as profiles or contour maps and interpreted qualitatively.
2D electrical resistivity profiling
For complex 3D subsurface conditions, 3D electrical resistivity tomography instead of 2D resistivity profiling should be performed.
Depth slices of 3D electrical resistivity tomography
For water-occupied areas such as lakes, rivers or oceans, marine resistivity can be used for sub bottom electrical resistivity imaging.
Enviroprobe also provides resistivity tomography services utilizing ground and downhole electrodes. The typical applications include underground void locating, bedrock fracture/fault locating and underground water monitoring, etc.
Resistivity tomography utilizing both ground and downhole electrodes