Geology Reference
In-Depth Information
methods in which air, water, or drilling mud (often containing high-density
barite) is circulated down the center of the drill pipe and back up the outside of
the drill pipe to help move the cuttings to the top of the hole. If solid pieces of
rock rather than cuttings are required for analysis, the investigator must resort to
core drilling, which is usually more expensive than simple rotary drilling.
REMOTE SENSING
One of the principal uses of remote sensing in coal-related activities is to
detect linear features, such as faults and fracture zones, on scales from miles to
tens of miles. Delineation and evaluation of such regional linear features is
necessary as part of the permit application process for a new impoundment.
Remote sensing provides observations of the Earth's surface and shallow (less
than 3 feet) subsurface to complement information from mapping and
geophysical methods. Because the measurements are commonly made from a
moving airborne platform, a large area can be examined quickly and cost-
effectively.
For remote sensing to be useful in evaluating subsidence-related problems,
spatial resolution on the order of 3 feet or less is required. Measurements from
low-flying aircraft, or even from unmanned aircraft, can provide such high-
resolution measurements and photography. For additional information on
remote sensing applications, see NRC (1995) and Watson and Knepper (1994).
The oldest form of remote sensing is aerial photography. Historic black-
and-white aerial photos, often dating from 1930s Works Projects
Administration files, are sometimes available for comparison with modern
photography. Modern scanning and digital registration techniques can be used
to enter these old photos into a geographic information system format. Historic
photos may indicate shafts and other facilities that have been abandoned and
covered but not properly plugged, and are also useful for analysis of drainage
patterns, detection of geologic conditions, such as landslides and debris flows,
and signs of vegetative change, including stress from pollution or from shallow
voids.
Multispectral scanning samples the field at several different parts of the
electromagnetic spectrum (including outside the wavelength range visible to
humans) and digitizes the data for later analysis. The spectral bands sampled are
selected to provide the most sensitivity to features that can characterize
problems of interest. Individual portions of the spectrum are analyzed with
digital signal processing techniques and the results are compared with
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