Analysis of a feedback-controlled seismometer

The use of feedback as applied to a long-period seismometer is investigated analytically and experimentally. This technique is advanced as a means of constructing a compact, portable, long-period seismograph for unattended operation at remote sites where the problem of long-term drift of the elastic suspension becomes a serious limitation and where sufficient signal power for telemetry is required. The technique as applied here provides not only the stability and sensitivity required for normal seismic recording but also a means for both measuring the earth tide and shaping instrument response. The output power is sufficient to drive standard analog tape recorders and digital encoders. Instruments in which this concept is used have been constructed and successfully operated. The primary sources of instrument noise are analyzed in terms of the ground motion amplitude which would produce an output signal equivalent to that of each noise source. Thermal energy in the damping circuit is the dominant noise source in the long-period range (greater than 6 sec). Noise associated with the capacitor-plate transducer and output amplifier is dominant at shorter periods. The minimum detectable signal is of the order of 1-mμ, peak-to-peak ground motion for periods shorter than 20 sec.