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A specialized method of surveying a borehole, utilized when a magnetic
compass cannot be used because of local magnetism. The instrument used is
essentially a rigid tube, up to 30 ft (9.2 m) long, which is lowered into
a borehole. The tube fits the borehole closely and contains a fine wire
under tension. The difference between the arc of the tube, when bent at a
crook in the borehole, and the chord of the wire is indicated by a stylus
marking, which can be measured. The dip is read by etch tubes, and a
directional orientation taken at the surface is carried down the hole by
precise alignment of the tube and rods as they are lowered into the
borehole. Long
Source:
Dictionary of Mining, Mineral, and Related Terms
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