Design of Environmental Remediation Systems, Cathodic Protection, Landfill Liner Evaluations, or Grounding Systems requires, as part of its input, the value of or distribution of the earth’s resistivity. For grounding systems, as an example, the area of the earth covered by the grounding electrode and the earth resistivity are the dominant considerations for design. A number of geophysical methods have been developed to provide an in situ measurement of bulk earth resistivity. Geophysical measurements are preferred to measurements made on samples because they assess a larger (bulk) volume of the soil mass and represent in situ (undisturbed) determinations.

Direct Current (DC) resistivity measurements are made by injecting electrical current into the ground via a pair of metal electrodes driven into the soil and then measuring the resulting voltages set up on an independent pair of similar electrodes. Expanding the geometry of the four-electrode set results in deepening the depth-of-investigation. By matching the observed data of instrument reading versus electrode spacing with the theoretical results predicted by a layered earth model, a best-fitting model of the subsurface is found. Such a procedure is termed a vertical electrical sounding (VES) and is used to determine the vertical distribution of resistivity (layer resistivity versus depth).

An alternative way of conducting DC resistivity surveys is to fix the electrode spacing geometry and traverse a site. This method is termed electrical profiling. Its key objective is to locate lateral changes in resistivity, much like a fixed coil spacing electromagnetic survey. Such lateral changes could be caused by changes in lithology as in a sand channel within a clay matrix, fluid conductivity as in a chloride plume or by changing metal content as in an ore zone or a metal reinforced foundation.

EMC uses the Advanced Geosciences, Inc. Sting R1 Electrical Resistivity system and either the Wenner or Schulmberger Arrays to measure resistivity soundings. Raw sounding data are analyzed using a state-of-the-art computer program, Interpex’s RESIX, to transform the raw data into the required resistivity versus depth distribution.