A NEW VERTICAL SEISMOMETER BASED ON THE MOIRE TECHNIQUE

This paper describes a new optical seismic sensor for geophysical applications. Following the work done Institute Advance studies in Basic sciences in Zanjan, we have built a new vertical short period seismometer based on the Moiré technique. This seismometer consisting of a spring-suspended mass whose position is monitored using the moiré technique. A pair of gratings in relative displacement, magnifies the amplitude of the displacement of the oscillator inside the seismometer. The intensity of a light beam passing through the moving moiré fringes changes with time, and a detector converts these power fluctuations to a voltage signal. This voltage signal gives the time-series of the ground motion. We derive the theoretical response of the moiré readout system. The natural frequency of our sensor is 8 Hz and the damping factor is 0.65. Also, a 12 bit A/D device is used to digit the analog responses of optical seismometer. We compare the actual response of the moiré seismometer and a conventional seismometer (6TD Guralp, 0.1 Hz). Comparisons with conventional seismometers show that, in terms of both noise and signal fidelity, the optical approach is quite viable. Also, our sensor has some additional advantages.