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Closed, Illegal and Abandoned (CIA) Disposal Sites Electrical and Electromagnetic Methods |
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Geological materials have different electrical properties. The variations in these properties are useful geophysical parameters for characterizing earthen materials. Subsurface variations in electrical conductivity (or its inverse, resistivity) typically correlate with variations in water content, fluid conductivity, porosity, permeability, and the presence of metal. These variations may be used to locate subsurface features whose electrical properties contrast with the surrounding earth. For example, decaying solid waste and metal have a higher electrical conductivity than most soil and therefore produce anomalous readings in measured conductivity readings. The methods used to measure the properties of geologic materials can be divided into two types: methods using applied currents and those using naturally occurring currents. Those methods that used applied currents include electrical resistivity, induced polarization, and electromagnetic surveying. Methods that use naturally occurring current flow include telluric surveying, magnetotelluric surveying, and the self-potential method. Two of the more commercially utilized techniques are the resistivity method and the electromagnetic (EM) method. Resistivity can provide better vertical resolution and is generally less sensitive to interfering noise such as fences, buildings and overhead power lines. EM requires no direct contact with the ground surface, so the data can be acquired more quickly than with resistivity. For more specific information on all the previously mentioned methods, visit the Colorado School of Mines Web site. More in-depth information: More on electromagnetic surveys: Example of an electromagnetic subsurface image:
More on electrical resistivity: |
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Last updated: April 18, 2008 Closed, Illegal, and Abandoned (CIA) Disposal Sites, http://www.ciwmb.ca.gov/LEACentral/CIA/ John Macanas, jmacanas@ciwmb.ca.gov (916) 341-6722 |
