SOUTHWEST RESEARCH INSTITUTE

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	SwRI uses multichannel, multiple-electrode resistivity systems with efficient software to rapidly and accurately conduct electrical resistivity imaging surveys.

Direct current (DC) resistivity measurements are made by inducing a current into the earth between a pair of electrodes and measuring the potential of the induced current at a second pair of electrodes. The resistivity measurement (expressed in ohm-m) represents the apparent resistivity over a volume of the earth and is a function of:

• Soil and rock type

• Porosity and permeability

• Pore fluid chemistry

• Electrode geometry and spacing

Using the electrical resistivity imaging (ERI) technique, Southwest Research Institute (SwRI) combines many individual resistivity measurements along a linear electrode array to produce a two-dimensional (2D) resistivity cross section of the subsurface environment. Measurements from various electrode separations and positions along the array provide information at various lateral and vertical locations beneath the array.

Using the "roll-along" survey technique, the length of a resistivity transect can be substantial (kilometers). Scientists have conducted resistivity surveys that have generated continuous 2D cross sections with lengths on the order of 1,500 m.

Direct Current Resistivity Applications

Direct current resistivity applications include:

• Delineating the depth, thickness, and lateral extent of geologic strata (sands and gravels associated with paleo-stream channels)

• Locating geologic and structural anomalies (karst features and faults)

• Locating buried wastes (landfill boundaries)

• Mapping saltwater intrusion and contaminant plumes

Paleo-stream channel deposits serve as shallow aquifers for water supply development and as a source of sand and gravel. This electrical resistivity imaging profile illustrates the depth, extent, and thickness of electrically resistive paleo-stream channel deposits beneath a portion of a floodplain.

Shallow karst features in limestone environments can affect building and foundation designs. This electrical resistivity imaging profile illustrates the probable depth and extent of electrically resistive karst features, such as caves and solution cavities, beneath a retail development site.

Resistivity data are typically collected at ground surface. The data are a response of the earth to an active or passive signal. Data are interpreted, generally using an inversion method, to determine the properties of the earth that generate such a response. Typically, one-dimensional (1D) soundings using a Schlumberger array are used to create a 1D (layered earth) image of the subsurface, and a dipole-dipole array is used to generate a 2D profile or image of the subsurface.

Advantages of Direct Current Resistivity

• Good vertical resolution

• Depth range of as much as 300 feet, depending on the electrode array used and the total cable length

• Various electrode configurations or arrays are available for different applications

Limitations of Direct Current Resistivity

• Requires intrusive ground contact

• Susceptible to interference from nearby metal fences, buried pipes, and utilities

For more information about our direct current resistivity capabilities, or how you can contract with SwRI, please contact Ronald T. Green, Ph.D., at rgreen@swri.org or (210) 522-5305, or James Prikryl at jprikryl@swri.org or (210) 522-5667.

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Southwest Research Institute^® (SwRI^®), headquartered in San Antonio, Texas, is a multidisciplinary, independent, nonprofit, applied engineering and physical sciences research and development organization with 11 technical divisions.

August 18, 2011