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| Installation of a Tiltmeter |
The figure shows borehole tiltmeters being installed at a depth
of about 200m near the Hayward fault, San Francisco Bay, California,
as part of a cooperative experiment with M. Gladwin of the University
of Queensland, Australia. A commercial drill rig is used to drill
and case 8" holes and to core the bottom of these holes until a
section of about 10 feet of unfractured rock is obtained. The tiltmeter
is then installed in a bath of expansive grout within the cored
section of the hole. After the grout has set, the instrument will
detect tilting of the rock using calibrations based on tides in
the solid earth.
The tiltmeters are highly sensitive instruments with precisions
of less than 1 part per billion at short periods (ie less than 1
inch in 16,000 miles). These instruments are used to measure ground
tilt near active faults and volcanoes.
Tilt is detected using either 1) differential capacitance tranducers
that detect the position of a pendulum in deep boreholeinstruments,
2) differential capacitance transducers that measure the height
above two pools of mercury separated by about 10m and connected
by a tube (mercury-liquid level tiltmeter), and 3) a resistance
bridge that locates the position of a bubble under a concave quartz
lens (shallow borehole tilemeter). Two mercury tube tiltmeters were
installed at the Presidio in San Francisco and at Berkeley in the
early 1970's. Two deep borehole tiltmeters were installed along
the Hayward fault in the San Francisco Bay area in 1992.
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