Electrical Resistivity measurements are made by placing four electrodes in contact with the soil or rock (ASTM D6431-99).   A current is caused to flow in the earth between one pair of electrodes while the voltage across the other pair of electrodes is measured.  The depth of measurements is related to the electrode spacing.  The resistivity measurement represents the apparent resistivity averaged over a volume of earth determined by the soil, rock, and pore fluid resistivity, along with the electrode geometry and spacing.  Resistivity measurements include profiling with fixed electrode spacing, sounding (1D) by increasing electrode spacing at a fixed location and 2D and 3D imaging using multi-electrode arrays (sometimes known as ERI or MER) to depths of several hundred feet.  Capacitively-coupled measurements can be made for detailed imaging of the shallow (<30 feet) subsurface.

Applications

• Determine the depth and thickness of geologic strata
• Detect lateral changes and locate anomalous geologic conditions
• Measure soil resistivity for estimating metal corrosion rates and designing grounding grids
• Map saltwater intrusion and contaminant plumes
• Locate buried wastes (e.g. locate landfill boundaries)
• Azimuthal measurements to determine fracture orientation
• Create a detailed 3D resistivity model
• Monitor changes in groundwater or contaminants

Advantages

• Depth range of up to 200 feet
• Various electrode configurations are available for different applications

Limitations

• Length of electrode array is typically 5 times the depth of exploration. Finding sufficient accessible space can sometimes be a problem.
• Susceptible to interference from nearby grounded metal fences, buried pipes, cables, etc.
• Dry, highly resistive surface materials will limit the amount of electrical current injected in the ground and will reduce the signal to noise ratio