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Precise measurement of the local geomagnetic field can be used
to map the distribution of magnetic minerals, mainly magnetite,
in the rocks beneath the surface of the Earth. Magnetic minerals
often can be used to identify rock type, or at least to narrow
the field of possible rock types. Abrupt lateral changes in the
volume of magnetic minerals, whichtypically indicate lateral
changes in rock type, produce local distortions of the geomagnetic
field that reflect the three-dimensional geometry of the rock
contact. Magnetic techniques are widely applied to mineral exploration
problems, to crustal structure and plate tectonic studies, to
location and delineation of concealed faults, to tracing of outcropping
rock bodies under sedimentary cover, and to location of buried
manmade objects such as abandoned well casings, unexploded ordinance,
containers and other debris in forgotten toxic waste sites. Magnetic
techniques also are used in hydrocarbon exploration and as a
guide to geologic mapping in areas of poor exposure. Recently,
high-resolution magnetic surveys have been effective in delineating
faults and other structures within young water-bearing sedimentary
deposits, thus providing detailed information crucial to the
understanding of ground water systems.
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