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→Core-based orientation techniques
==Core-based orientation techniques==
==Core-based orientation techniques==
[[file:core-orientation_fig3.png|thumb|300px|{{figure number|3}}Paleomagnetic plug sample.]]
Core-based techniques rely at least in part on data derived from the core for orientation. If bedding planes and fractures can be discerned in the core, it may be possible to use dipmeter logs or data from borehole televiewers to determine the orientation of the core (see “[[Dipmeters]]” and “[[Borehole imaging devices]]”). Another approach to core-based orientation involves analysis of naturally existing paleomagnetic signals in rock to determine their relationship to present-day geographic north.
Core-based techniques rely at least in part on data derived from the core for orientation. If bedding planes and fractures can be discerned in the core, it may be possible to use dipmeter logs or data from borehole televiewers to determine the orientation of the core (see “[[Dipmeters]]” and “[[Borehole imaging devices]]”). Another approach to core-based orientation involves analysis of naturally existing paleomagnetic signals in rock to determine their relationship to present-day geographic north.
===Paleomagnetic core orientation===
===Paleomagnetic core orientation===
The paleomagnetic technique is based on the fact that nearly all rocks, including quartzite, chert, and chalk, contain at least trace amounts of magnetic minerals such as magnetite and hematite. These minerals act as miniature compasses and lock-in the earth's ambient magnetic field. A primary signal is imprinted near the time of deposition; over time, one or more secondary signals may be overlain on the primary signal. In specially equipped laboratories, the signals in rock samples can be separated, interpreted, and used to orient core to present-day geographic north.
The paleomagnetic technique is based on the fact that nearly all rocks, including quartzite, chert, and chalk, contain at least trace amounts of magnetic minerals such as magnetite and hematite. These minerals act as miniature compasses and lock-in the earth's ambient magnetic field. A primary signal is imprinted near the time of deposition; over time, one or more secondary signals may be overlain on the primary signal. In specially equipped laboratories, the signals in rock samples can be separated, interpreted, and used to orient core to present-day geographic north.