A Novel Double-Grating-Based Horizontal Geophone
Zong Ze Xing Hongyan (College of Electronic and information Engineering, Nanjing University of Information Science and Technology, Nanjing, Jiangsu 210044, China)
A new type of short-period horizontal geophone is developed for solving the low sensitivity for detecting ground motion caused by S-wave, which mainly constructed with a vibration limit pendulum and a double-grating diffraction system. The working principle of this geophone is introduced. Its feasibility is proved by analyzing the differential vibration equation and frequency characteristic of mechanical model using a new simulation function of artificial ground motion and theoretical studying the diffraction field after superposed gratings. The light intensity on receiving screen is derived by applying Fresnel diffraction theory. The horizontal velocity of ground motion is calculated through putting forward an easy way to determine the relation between fringe frequency and ground motion speed, and record the time when pendulum passes through the equilibrium position based on the symmetric indicative grating. The dynamic range and sensitivity of geophone system that uses 1800 grooves/mm gratings and 4096 fringe subdivision technique is around 144 dB and 136 pm, respectively. Compared with other kinds of geophone, the corresponding parameters of this newly system is improved at least 10%, showing its application potential in exploration engineering.