Technologies for Locating Mine Workings - Coal Waste Impoundments: Risks, Responses, and Alternatives - page 83

Ground-Penetrating Radar
In many places, ground-penetrating radar has become the method of choice
for exploring the upper several feet of the Earth's subsurface (e.g., Daniels,
1996; Davis and Annan, 1989). Ground-penetrating radar employs a source of
microwave radiation that is radiated into the Earth via an antenna at a known
time. From the time measured for the wave to echo back to a receiving antenna
at the surface, one can calculate the depth to various layers in the Earth, once an
accurate wave-propagation velocity has been measured. Velocity determination
is a critical factor and can be done by measurements on samples, by one-way
travel time in a borehole, or by fitting a least-squares hyperbola to an observed-
travel time versus horizontal-distance curve (Tillard and Dubois, 1995). In
geometrical concept, ground-penetrating radar and seismic reflection are
similar. Ground-penetrating radar data can be displayed in a format identical to
that used for seismic sections. However, the Earth environments in which the
two techniques perform optimally tend to be mutually exclusive. Ground-
penetrating radar works best in dry media in the absence of clays or other
electrically conducting earth materials because electromagnetic radiation cannot
penetrate into electrical conductors (e.g., moist clay). In contrast, seismic waves
transmit well through moist clays but are rapidly attenuated in dry sand. Ground-
penetrating radar imaging works at depths of 30 to 60 feet under favorable
conditions, but may fail at depths of less than a 3 feet if clays or other
conductive materials are present near the Earth's surface.
Nuclear Magnetic Resonance
Magnetic resonance imaging was developed for medical diagnosis as an
outgrowth of physicists' nuclear magnetic resonance experiments (Knight et al.,
1999). Proton nuclear magnetic resonance, which responds to the state of
hydrogen nuclei in the ground, is of interest to geoscientists. Proton nuclear
magnetic resonance might be used to detect water-filled cavities because of its
sensitivity to hydrogen nuclei. However, owing to the immature state of
research in this field, referring to the geophysical use of nuclear magnetic
resonance as "imaging" may be premature ( Appendix E ).