The Experiments And Studies On Broadband Electric Feedback Seismometer Based On KS-1 Pendulum

Seismometer is an important instrument which can acquire earthquake observation data and this data is the basis of earthquake prediction. It is one kind of sensor which can transform ground mechanical movement into electric signal. Seismometer's main performance indicators include: bandwidth, dynamic range and resolution and so on. The quality of these indicators directly related to the achieved accuracy of earthquake observation data. The actual needs of earthquake observation point out the direction of seismometer's researching, which is: broadband, large dynamic range, high-resolution development.The study of long-period surface wave, earth's free oscillations and slow earthquake need good ultra-long cycle of seismic records. Ultra-long cycle seismometer is important instrument which can acquire good earthquake observation data. This paper elaborates the current situation of seismometer development as well as the tendency, and analyze its working principle systematically. By creating system function block of seismometer, and ratiocinate its transfer function, it proved: the introduction of electric feedback can expand the seismometer's frequency band and dynamic range, and also can reduce nonlinear distortion. By analyze the system function, introduces how to design the parameters of the seismometer.The self-noise level is one the most important parameters of the seismometer. So, how to lower down the self-noise level is a big issue in the seismometer designing. This paper introduces how the two major noise source– Brownian noise and Electronic noise(1/f noise)affect seismometer's self-noise level and transfer function. We analyzed noise of station base recorded by three different kinds of seismometers and study how the self-noise affects the data recorded.Based on study of brown thermal noise affects on the seismometer performance, the longer of the pendulum's natural cycle, the bigger of the pendulum's weight, the greater of the quality factor, and the smaller of the mechanical pendulum brown thermal noise. Erhard Wielandt suggested the conditions to lower the thermal noise: MTQ> 1kgs, the bigger value of MTQ product, the lower of brown thermal noise. We chose KS-1 pendulum to carry out this study based on the existing experimental conditions.This paper designed the broadband feedback plan and carried it out based on KS-1 medium-and long-cycle seismometer. We retrofitting the referential capacitance displacement transducer on KS-1 Medium-and long-cycle seismometer, and also change the electromagnetic transducer coil to feedback coil. We design the circuit to achieve transducer, feedback, filtering and so on. We also change the open-loop seismometer to close-loop seismometer and lower long-period band and expand the seismometer bandwidth. We tested the example machine, and get some results. Compare the Noise power spectrum of station base by different seismometer, we proved that: the bigger value of MTQ, the lower noise of seismometer.