Design and Construction of a Seismometer Based on the Moiré Technique: Detailed Theoretical Analysis, Experimental Apparatus, and Primary Results

We have built a vertical seismometer that works based on the moiré technique. One grating is attached to the suspended mass and one to the frame of the instrument. The two gratings are in relative motion, and the resulting moiré pattern magnifies this motion. The intensity of a light beam passing through the moving moiré fringes changes with time, and a light detecting system converts this power fluctuation into a voltage signal. From this voltage signal the time series of ground motion is obtained. We investigated the performance of the instrument theoretically, as well as experimentally. Our instrument can measure the amplitude of oscillation through moiré magnification. It is free from EM environmental noise, has adjustable sensitivity and dynamic range, and has simple and straightforward design. Furthermore, we introduce a novel way of data acquisition from moving moiré patterns.