A Theoretical Model for the Self-Noise of a Velocity-Broadband Seismometer

A theoretical model of the seismometer self-noise can be used to predict the self-noise. Its inputs are mechanical and electrical parameters of the seismometer. In this article, we studied the theoretical model of the self-noise of the velocity-broadband seismometer, using the CS60 seismometer as an example. Velocity-broadband seismometer is a type of feedback seismometer. The previous theoretical model of the self-noise of the feedback seismometer only involved noise sources in the forward path of the feedback system. Our model involved not only noise sources in the forward path, but also noise sources in the feedback path and external to the feedback loop.We introduced noise sources in the feedback system of the seismometer and the method of calculating their levels with mechanical and electrical parameters, developed expressions for referring all noise sources to the input terminal of the feedback system, and finally established the model. We compared the CS60 seismometer's predicted self-noise calculated using this model with the measured self-noise, and we found good agreement between them.The contribution of each noise source to the total noise was studied, and it was found that the dominant noise source in CS60 is different over different frequency bands. Over the frequency band below 0.0095 Hz, the noise of the integrator (in the feedback path) is the dominant noise source. Over the frequency band from 0.0095 to 0.21 Hz, suspension noise is the dominant noise source. Over the frequency band from 0.21 to 39 Hz, the noise of the differential driver (external to the feedback loop) is the dominant noise source. Over the frequency band above 39 Hz, the noise of the preamplifier (in the forward path) is the dominant noise source. In addition, some viewpoints about the low-noise design of seismometers were proposed.