Reduce Self-Noise of Velocity Broadband Seismometer in Low Frequency Using Internal Position Output

This paper proposes a method for enhancing seismometer detection capabilities in the low-frequency range (on the order of mHz) by utilizing the seismometer’s Position output(PosOut) replacing the traditional broadband velocity output(VelOut). Theoretical analysis of Equivalent Input Noise(EIN) and noise transfer functions for two output configurations (VelOut and PosOut) reveals that while forward-path noise remains unaffected by the output change, the integrator and amplifier driver noises are significantly influenced. As a result, PosOut exhibits lower EIN at low frequencies but higher noise at higher frequencies. This is primarily due to the integrator shifting from the feedback path to the forward path, along with changes in the closed-loop system’s frequency response. Based on these findings, several noise optimization strategies are proposed. To validate the theoretical predictions, simulations based on the parameters of a commercial seismometer were conducted, confirming the noise shaping behavior under both output configurations. Experimental validation using four 3T seismometer further supports the theory, showing that the EIN of the PosOut is lower than that of the VelOut below 0.02 Hz, with PSD reductions of 7.5 dB to 10 dB at 1 mHz. The results indicate that the PosOut provides a significant advantage in detecting weak low-frequency signals, and offers a promising approach for both future seismic sensor designs and signal processing applications focused on low-noise, low-frequency detection.