Technologies for Locating Mine Workings - Coal Waste Impoundments: Risks, Responses, and Alternatives - page 76

The geophysical methods used and how they are applied vary according to
a project's objectives, resolution requirements, budget, and geological situation.
For example, seismic methods are sensitive to the mechanical properties of
earth materials but not to the chemical makeup of these materials and the fluids
they contain. In contrast, electrical methods are sensitive both to fluids and to
magnetic or electrically conductive materials. Usually, multiple geophysical
methods offer better answers than any individual method. The difficulty (and
sometimes the success) of geophysical surveys is affected by topography. For
example, it is easier to collect, process, and interpret geophysical data in the
agricultural fields of Illinois than in the steep valleys of West Virginia.
In addition to topography, vegetation and cultural features—such as
buildings, roads with traffic, and fences—can be barriers to geophysical
surveys. Furthermore, because of the need to obtain regional geophysical
background information, the area needed for a geophysical survey may greatly
exceed that owned or leased by a mining company, in which case rights of
ingress and egress for geophysical measurements may be a serious issue.
Electrical Resistivity and Electromagnetic Methods
Resistivity techniques sense the electrical properties of the material
through which a current passes. Electrically conductive contaminants can be
tracked using resistivity methods. For example, resistivity would be expected to
be more effective for finding mine workings full of polluted water than for
detecting mine workings full of air.
Under some conditions, these methods can be used to find geological faults
and buried valleys—but usually not with the precision of seismic reflection
techniques. Resistivity surveys are usually cheaper than seismic surveys.
Multichannel electrical cables similar to seismic cables have recently been
developed to increase the flexibility and the rate of resistivity data collection.
Electrical and electromagnetic survey data interpretation often involves
mathematical inversion, producing a model that fits the data (e.g., Ellis and
Oldenburg, 1994; Zohdy, 1989). Electromagnetic methods have partially
replaced resistivity surveys because equivalent information is obtained faster
and without inserting electrodes into the ground.
Electromagnetic methods include active methods in which an
electromagnetic signal is induced in the ground by human activity, and passive
methods in which natural variations in the electromagnetic field of the Earth are
analyzed ( Appendix E ). The induced polarization method is