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FIG. 9.14.1 Electromagnetic survey. (Reprinted from U.S. Environmental Protection Agency, 1993, Subsurface characterization and monitoring techniques, a desk reference guide, U.S. EPA/625/R-93/003a [May] U.S. EPA.)

tow line in the subsurface electrical properties to a given depth. Then, they can use the data to delineate hydrogeological anomalies or map inorganic plumes. Sounding measurements, on the other hand, are made at increasing depths so that engineers can map vertical changes in electrical properties. Engineers use data from sounding measurements to determine the depth, thickness, and type of soil or rock layer at the site. The data from ER surveys can be interpreted with the use of computer models or master curves to create geoelectric sections (Orellana and Mooney 1966). These sections illustrate changes in the vertical and lateral resistivity conditions at the site.

The ER surveys are useful for identifying shallow contaminated groundwater bodies where (1) a significant contrast exists in water quality; (2) the water table is less than 40 feet deep; (3) the geology of the water table aquifer is relatively homogeneous; and (4) local interferences, such as buried pipelines, power lines, or metal fences, are not present.

The advantages of the ER methods are that they are well established and their equipment is inexpensive, mobile, and easy to operate and provides relatively rapid areal coverage. In addition, the ER methods are superior to the EM methods for detecting thin resistive layers. The disadvantage, however, is that continuous profiling is not possible, and the requirement for ground contact can cause problems in resistive material and generally makes the ER surveys slower to use than the EM surveys. Furthermore, use of the ER methods is limited in wet weather and on paved areas, and the methods are less sensitive to conductive pollutants than the EM methods.

Current Source

Current Meter

Current Source

Current Meter

Current Flow Through Earth

Current . Voltage

Current Flow Through Earth

Current . Voltage

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