Design of Portable Strainmeter System
Special rept. 15 Aug-30 Dec 1969
GEOTECH GARLAND TX
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Six portable strainmeter systems were designed and built for deployment in mine tunnels and shallow trenches to measure strains induced by high yield underground events at epicentral distances as short as 30 km. The system is designed to detect earth strains of 5 times 10 to the minus 10 power or smaller over a horizontal interval of 6 meters in the period range 10 seconds to dc, and to record the signals on magnetic tape. Strains are detected by a variable-capacitance transducer attached to a quartz-tube translating member. Output signals are maintained within a dynamic range of 30 dB for an input- signal range of 66 dB by an offset biasing technique which uses a precision voltage-level detector and a digital-to-analog converter. Temperature measurements are resolvable to within 0.001 degree Centigrade by the same offset biasing technique. The strainmeter is calibrated with a temperature-compensated electromagnetic calibrator mounted at the fixed end of the quartz tubing. The strain detector is calibrated and the capacitor plates positioned over a range of 12 times 10 to the minus 6 power meters in steps of 5 times 10 to the minus 9 power meters by use of a stepping motor and motion reducer.
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