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magnetic earth materials. Such surveys are used to detect variation in the
magnetite content of rocks and unconsolidated materials, so they can detect
changes in some types of igneous rocks and other geologic structures. They are
used also at contaminated sites to measure the perturbation of the Earth's
magnetic field caused by buried ferrous metal objects such as steel drums, the
ferrous metal waste in landfills, and iron pipes (e.g., Roberts et al., 1990a).
Microgravity surveys measure minuscule changes in the gravitational field
of the Earth using gravity-meters with a sensitivity of 1 microGal (where 1 Gal
is a gravitational acceleration of 1 cm/sec/sec). Readings are made along a
profile line or on a grid with typical spacing of 1 to 100 meters (3 to 330 feet).
The sensitivity of microgravity measurements is one part per billion in
comparison to the Earth's gravitational field. A map of gravity anomalies
reflects the lateral density contrasts detectable after removing all known effects
that can cause changes in gravity, such as, tide, instrument drift, elevation and
latitudinal variation, and terrain.
For near-surface geophysical exploration, microgravity surveys sometimes
are used where a high contrast in density occurs between bedrock and overlying
alluvium, or between air and rock in a mine. Minute gravity anomalies can be
caused by artificial features such as trenches, tunnels, disposal containers, and
incipient subsidence problems (e.g., Roberts et al., 1990b; Yule et al., 1998), as
well as by geologic features such as cavities, faults, folds, dipping layers, and
lateral intralayer heterogeneity.
While microgravity methods could be applicable in finding shallow air-
filled mine workings, they would not be the first choice for finding water-filled
mine workings because the density contrast between the missing coal and the
water that replaced it is too small. Figure 5.2 shows the calculated gravity
anomaly at the Earth's surface above an air-filled 20-foot-diameter horizontal,
cylindrical mine entry.
Near-Surface Seismic Methods
Seismic research has met with limited success when conducted to detect
cavities resulting from abandoned subsurface coal mines (Fisher, 1971;
Hasbrouck and Padget, 1982), salt-solution mining (Cook, 1965), lava-flow
tunnels (Watkins et al., 1967), and natural caverns (Rechtien and Stewart,
1975). Most researchers using seismic techniques for cavity detection cite three
phenomena as evidence of a cavity: free oscillations or resonance of the cavity
walls, anomalous amplitude attenuations, and delay of arrival time (Cook, 1965;
Fisher, 1971; Godson and Watkins, 1968; Robinson and Coruh,
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