Electrical Resistivity Tomography

ERT surveys are conducted by introducing a DC electrical current into the ground through two electrodes and measuring the voltages on the surface. Resistivity (the inverse of conductivity) is a bulk physical property that describes how difficult it is to pass an electrical current through a material. Sub-surface materials will have a broad range of resistivity responses depending on many factors such as saturation, ion concentration, faulting, jointing, etc. Measurements provide an indication of direction and amount of current flow in the sub-surface which can be processed into an image of sub-surface resistivity.

To collect ERT data, a linear array of electrodes is laid out on the area of interest and connected to a resistivity transmitter/receiver and electrode control box. Various electrode spacings and arrangements can be used to best meet the objective of the survey and to accommodate the local geology. An EM survey is generally completed first in order to locate regions for optimal ERT line placement. Resistivity methods are used to map lateral sub-surface features such that a map in a cross-sectional format is possible. Specialized interpretation of the data then defines the vertical extent of inorganic contamination. Combining EM and ERT data provides an estimate for the volume of contamination at a site.