A direct digitizing seismograph

Abstract

A seismograph is described in which capacitance transducers on either side of the inertial reactor determine the frequencies of two Clapp oscillators. The oscillators are heterodyned and the resulting frequency, varying between 10 kc and 100 kc, is detected by a frequency or period counter and an analog output. The latter drives a self-balancing potentiometer through a cathode follower and an adjustable twin-T filter. It also drives a circuit with a very long time constant which keeps the boom centered through a coil and magnet transducer. The electronic noise corresponds to a boom displacement of less than 1.25 mμ. The sensitivity is about 400 cps per micron. The dynamic range is about 100 db at constant resolution. The digital output is temporarily in the form of printed or punched tape. A special magnetic tape recorder is being built.