Geology Reference
In-Depth Information
statistical or physical models or to information extracted from specimen-based
laboratory measurements.
For regional analysis (tens to hundreds of square miles), the 100-foot
resolution satellite-based thematic mapper is a useful tool. It employs six
different bands in the reflected spectrum, along with a 400-foot resolution
thermal channel. Because of the spatial resolution limitations, the thematic
mapper is not useful for the detailed analysis needed for localized problems.
Regional studies can also benefit from archival radar and photographic data
from national surveys, available from the U.S. Geological Survey's Earth
Resources Observation System Data Center. The IKONOS high-resolution
satellite, which offers 3-foot resolution, also is useful for mapping and surveying.
For localized studies, aircraft systems that use additional spectral channels
and better spatial resolution can be employed. Reflectance data have been used
to distinguish among geologic units, to detect geologic structures, and to trace
linear fractures. They have also been used to indirectly infer lithologic and soil
information in vegetated areas based on empirical relations between vegetation
and geological conditions. Thermal infrared data can be used to detect
exothermic reactions, such as underground coal fires, and to find hydrological
features such as springs (NRC, 2000a).
GEOPHYSICAL METHODS
Noninvasive active geophysical methods used to search for coal mine
voids employ artificial electrical, electromagnetic, or mechanical energy to
examine the shallow subsurface of the Earth (Sharma, 1997; Ward, 1990). In
contrast, passive geophysical techniques measure some natural physical
parameters of the Earth, such as minute variations in the Earth's gravitational
field (see Dobecki and Romig's 1985 review).
Physical parameters measured directly during shallow geophysical surveys
include: electrical, thermal and stress fields, gravitational and magnetic fields,
electrical conductivity, elastic (i.e. seismic) properties, transparency to and
polarizability of electromagnetic waves, and natural gamma radiation. These
measurements can then be used to infer the permeability, porosity, chemical
constitution, stratigraphy, geologic structure, and various other properties of a
volume of material near the Earth's surface, including the presence of voids.
Geophysical methods can be used to guide exploratory drilling programs, but
they cannot be expected to eliminate confirmation drilling.
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