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Title: Military hydrology : Report 10, Assessment and field examples of continuous wave electromagnetic surveying for ground water
Authors: Butler, Dwain K.
Keywords: Military hydrology
Magnetic instruments
Publisher: Environmental Laboratory (U.S.)
Geotechnical Laboratory (U.S.)
Series/Report no.: Miscellaneous Paper;EL-79-6 Report 10
Abstract: Abstract: This report reviews the concept of a particular type of electromagnetic (EM) geophysical method applied to ground-water exploration and assessment. The method utilizes a transmitter with a small loop antenna, which is excited with a continuous wave (CW) EM signal. The magnetic field generated by the transmitter couples inductively with subsurface geological materials and generates eddy currents. The secondary magnetic fields generated by the eddy currents couple inductively with a receiver with a small loop antenna. The specific system type considered in this report operates under the conditions which allow a "small induction number approximation" to be made. This approximation allows the receiver response to be calibrated directly in terms of apparent ground conductivity. By operating the system at multiple transmitter and receiver coil spacings and multiple transmitter frequencies, it is possible to conduct EM soundings. A specific CW EM system was field tested at White Sands, N. Mex., and Fort Carson, Colo., at locations where previous geophysical surveys were conducted using seismic and electrical resistivity methods. For the White Sands EM surveys, a critical evaluation of data interpretation procedures is presented. Specifically, the EM interpretations are compared to models deduced from the previous geophysical work at the locations. At Fort Carson, an EM sounding was conducted at 20-m intervals along a 700-m profile line. Also, an EM sounding was conducted on an outcrop of Dakota Sandstone. The results of an EM sounding can be interpreted, in principle, to yield a model of the vertical variation in electrical conductivity in the subsurface. In some cases the conductivity model can be interpreted in terms of a hydrogeological model, although supplementary geological data or complementary geophysical data are usually required. Analysis of the White Sands EM survey results indicates the multilayer response calculations, for predicting the low induction number performance of the device, should be used with caution because the response functions overpredict the proportion of the response from deeper layers. For three of the five cases, two-layer equivalent model interpretations of the EM data are useful and correlate satisfactorily with resistivity models. The most useful interpretation procedure is to assign measured apparent conductivities to rule-of-thumb depths of investigation and use the model only as a qualitative indicator of the conductivity versus depth variation. At Fort Carson, the conductivities measured on the outcrop agreed with values deduced for the sandstone in the subsurface from a resistivity sounding. The EM data along the Fort Carson profile line agreed well with a geological model for the site deduced from available geological information, site reconnaissance, and previous geophysical surveys. The EM device evaluated in this study is lightweight and easy to operate in the field; surveys proceed rapidly. These comments apply in general to CW EM systems. The device considered in this report could not "stand alone" nor replace electric resistivity in a complementary methods approach for groundwater detection and assessment. However, the device can be useful as a rapid survey technique to supplement resistivity surveys in a complementary methods approach. The CW EM methods are not limited to the small number of conductivity determinations (coil spacing-frequency combinations) of the device considered here. Currently available CW EM systems with greater capability and applicability, however, are larger and more cumbersome to use, and the data interpretation is not as straightforward.
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