Portable 30-Day Unattended System for Recording Earth Strain in a 66-dB Range

Earth strains of approximately 1 × 10-9 to 1 × 10-5 in the frequency range 0-0.1 Hz are recorded on magnetic tape with a 30-day unattended recording capability. Strains over a horizontal 6-meter interval are detected by a dual variable-capacitance transducer attached to a quartz-tube translating member. Differential earth displacements sensed by capacitor plates are converted to variations in oscillator pulsewidths. The pulsewidths are averaged and amplified before being transmitted by cable from a tunnel or a buried-trench installation to a nearby recording facility. A voltagecontrol circuit maintains the output signal within a dynamic range of 30 dB for an input-signal range of 66 dB. This is accomplished by an offset-biasing technique using a precision level detector and a digital-to-analog converter. The number and polarity of the offset increments are recorded on a separate track of the tape recorder for restoration of the input signal. Temperature measurements are resolvable to within 0.001°C by a voltage-control circuit. Calibration of the strainmeter is accomplished with a temperature-compensated electromagnetic calibrator mounted at the fixed end of the quartz tubing. Repositioning of the capacitor plates and calibration of the transducer are controlled by a fine-adjust mechanism. Use of a stepping motor and motion reducer permits positioning of the capacitor plates over a range of 12 × 10-6 meters in steps of 5 × 10-9 meters.