An Optical Approach for Sensing Seismic Vibrations

This experimental study demonstrates a novel optical method which is used as a readout system for a vertical seismometer that is, based on the moiré technique. Our purpose was to build an optical seismometer whose performance is similar to seismic sensors. The oscillation system of the sensor is a spring-suspended mass which its position is monitored by moiré technique. The maximum displacement is limited by mechanical issues to a few millimeters. We used two similar overlaid grids at a small angle that, one of them is fixed to the frame of the sensor and the other one is attached to the suspended mass. Moiré pattern is illuminated with a laser diode. The laser beam passes through the moiré pattern and a narrow slit and hits on a light detector. Due to moving the oscillatory mass and the fringes movements, the light intensity on the detector varies and is recorded as voltage. A digital signal processor samples the output voltage and produces a record of the seismometer mass displacement. The response of the optical seismometer was validated through comparison of recorded waveforms with those obtained by CMG-6TD seismometer. Comparisons with conventional seismometer show that, in terms of both noise and signal fidelity, the optical approach is quite viable. Our seismometer was found to be compatible with the reference seismometer.