Interferometer as seismometer displacement transducer

(in Croatian)

Abstract

 Displacements of the pendulum of S-13 Teledyne Geotech seismometer were measured using a small Michelson interferometer. The oscillations of the pendulum were simultaneously recorded by the PDAS-100 digital seismograph. These records were integrated and compared with those made by the interferometer. The pendulum was simulated by sine (0.5, 1 and 2 Hz) and random signals. Michelson interferometer was simplified by using the cube beam splitter with mirror film deposited onto one of its faces. The source of radiation was thermally stabilised collimated laser diode (LD) driven with constant current. Wavelength of LD was 670 nm. Fringes were counted with 16-bit presentable up/down CMOS counter. Counter state was sampled by Atari ST 1040 computer with sampling frequency equal to 33.4 Hz. Resolution of the transducer was 335 nm. The transducer would be able to record oscillations of the pendulum caused by an earthquake with magnitudes between 1.1 and 5.2 at epicentral distance of 0.1°, and could therefore find the application in a displacement strong-motion seismograph.