Electrical Resistivity Survey

Geotechnical & Structural Monitoring Division   Electrical Resistivity Survey   The purpose of electrical survey is to determine the subsurface resistivity distribution.  The method is based on an artificial electric field in the earth by introducing a current into ground surface through metal electrodes, standard array configuration, a voltage is measured across two electrodes (Figure 1).   Form these measurements, the true reisstivity of the subsurface can be estimated.  Electrical resistivity of soils and rocks correlated with other soil/rock properties which are of interest to the geologist, hydrogeologist and geotechnical engineer.  Several geologic parameters affect earth resistivity including:   ž         Clay content ž         Groundwater conductivity ž         Soil or formation porosity ž         Degree of water saturation   Wider electrode spacing results into larger penetration.  Intelligent nodes are connected to the electrodes that allow each to be in either standby, current or potential measuring modes.  These nodes are controlled by an automatic multi-electrode switching system that steps through a programme.   Utilizing multiple surface electrodes and a switching system that automatically selects electrode arrangements, surveys can be carried out rapidly and accurately.  Figure 2 shows the typical setup of a 2-D survey with a number of electrodes along a straight line attached to a multi-core cable.  Normally a constant spacing between adjacent electrodes is used.