A. Ansari¹ , S. Esmaeili¹

¹International Institute of Earthquake Engineering and Seismology (IIEES), Tehran, Iran

Abstract:

We have built a new three component optical seismometer. The oscillation systems consist of a spring-suspended mass whose position is monitored by the moiré technique. We use a pair of similar gratings with one of them is fixed to the suspended mass and another fixed to the seismometer frame. The gratings are installed close to each other with no physical contact; the planes of the gratings are parallel to each other and their lines make a small angle with one another. Thus, a moiré pattern is formed. A narrow beam of a laser diode passes through the moiré patterns and the beam power is detected by a light detector. When a typical impulse is applied, the second grating is displaced with respect to the first grating, and as a result the moiré fringes are moved with a magnification of more than ten times. So, the detector output is a time series of the mass displacement. The response of the optical seismometer was validated through the comparison of recorded waveforms with those obtained by a CMG-6TD seismometer. Comparisons show that, in terms of both noise and signal fidelity, the optical approach is quite reliable.