Geology Reference
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movement (i.e., observing a moving conductor in a magnetic field). In the
geothermal case, chemical reactions induced by mineralized waters may add to
any voltage caused by movement of water. Another possible use of spontaneous
potential surveys is mapping the concentration gradients of chemically active
leachate. A spontaneous potential survey might be sensitive to water in mine
works that are reacting with their surroundings. Spontaneous potential data may
be interpreted from contour maps of voltages or by more quantitative model
calculations employing geometrical shapes similar to those used in magnetic
and gravity studies. The fundamentals of near-surface spontaneous potential
applications can be found in Corwin (1990).
Active electromagnetic surveying employs a primary field induced by an
electrical current passing through a coil, which induces three-dimensional
currents through underground conductors according to the physical laws of
electromagnetic induction. This underground current induces a secondary
electromagnetic field which then distorts the primary field; the resulting final
field is sensed by a receiving coil. The field detected by the receiving coil varies
in intensity, phase, and direction from the primary field, which reveals
information about subsurface electrical conductivity. Electromagnetic methods
have an advantage over DC resistivity because they do not require inserting
electrodes into the ground. Low-flying, small (maximum dimension 3 to 6 feet)
unmanned aircraft have been employed to conduct some surveys to provide
access to polluted, dangerous, or inaccessible areas. Such airborne surveys have
disadvantages, including small separation between the source and receiver coils
and a higher noise level caused by the relatively high velocity movement of the
coils through the Earth's magnetic field. The practical background and the
theoretical basis for electromagnetic methods are presented in McNeill (1990).
In contrast to active electromagnetic surveying discussed above, passive
electromagnetic surveying employs Earth's natural electromagnetic fields to
provide the variations in the electric field. Electric fields generated by distant
lightning flashes are the source used in the audio-frequency magnetic field
technique. The passive very-low radio frequency method relies upon the 15 to
25 kilohertz electric field from distant, powerful radio transmitters used to
communicate with submarines. These passive techniques may be useful for
regional studies, but they do not have the resolution to find underground mine
works.
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