Geology Reference
In-Depth Information
Radiometric Techniques
Radiometric methods measure emissions from radioactive isotopes. The
techniques can be used to sense the presence of radioactive contaminants or to
explore for radioactive ores. Specific isotopes that occur within 3 to 6 feet of
the Earth's surface can be identified by spectral gamma methods, which may
also be useful for locating radioactive hazards, such as natural radon gas
sources. Nielson et al. (1990) review natural gamma and radon emanation
methods.
Methods that measure anomalously high radon concentrations show
promise for identifying abandoned underground mine workings. Research using
this method was pioneered by geologists looking for buried geologic features
such as faults (Fleischer and Mogro-Campero, 1979; Heirendt, 1988), and the
method has since been adapted to locate abandoned coal mines (Misquitta,
1989). Radon is thought to be concentrated in the voids left by mining, then
released to the surface by way of subsidence fractures that result from mine
collapse. The method is based on the concept that the alpha particle emission
that occurs during the radioactive decay of radon will leave impressions on high-
density plastic detectors and that the number of impresssions on the detectors
can be directly correlated with standardized radon concentrations (
Figure 5.3
).
Anomalously high radon concentrations may correlate with the portion of the
profile above the mine voids. Further refinement and development of this
method could lead to a cost-effective, noninvasive screening method for
detecting abandoned underground mines.
Measurement of soil gases or gases that emanate from the ground is
commonly used to detect buried wastes or containers. For example,
photoionization detectors are hand-held instruments that detect gasoline
derivatives or additives from underground, leaking storage tanks. Natural gas
utilities and transmission companies also use similar instruments (“sniffers”) to
detect gas leaks. The technical literature is sparse regarding the innovative use
of gas monitoring to detect old mine workings in coal seams.
In the case of underground coal mines, the cracks induced by mine
collapse and subsequent subsidence create fractures that can allow for
accumulated gases to escape from mine workings to the overlying soil or
ground surface. Methane gas is often associated with coal seams, and thus,
portable gas detectors could be useful as a screening tool to identify where mine
voids with accumulated gas are connected to the surface by natural or
subsidence-induced fractures. Detection of anomalous gas concentrations may
not alone be indicative of mine voids, but this method could prove useful as a
screening tool for a drilling or geophysical investigation of a specific area. The
method could reduce costs while helping to focus more
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