A Low-Frequency Response Extension Method With Low Noise Floor, High Sensitivity, and High-Temperature Stability for Geophone

In the fields of microseismic monitoring and deep earth exploration, low-frequency signal detection faces problems, such as weak vibration signals, large ambient temperature changes, and complex ambient noise. A novel comprehensive low-frequency response extension method for a moving coil geophone is proposed to implement a high-sensitivity low-frequency vibration sensor with low noise floor for measuring seismic wave velocity. Based on exploring the principle of increasing the damping ratio of geophone to expand the linear variation range of acceleration with frequency, a model in which both damping and gain vary with temperature is established by introducing the temperature-dependent coil resistance parameter. Developed on the typical negative impedance conversion method, a damping adjustment circuit with temperature compensation, gain adjustment, and differential noise suppression is designed comprehensively; thereby, a new scheme of low-frequency response extension based on the force balance principle is proposed. To further suppress the inherent 1/f noise of the operational amplifier in the low-frequency band, an acceleration-velocity conversion scheme based on a dual operational amplifier narrowband filter is designed to flatten the velocity–frequency curve while suppressing low-frequency noise. Through standard vibration table and field comparative tests, the proposed comprehensive low-frequency extension method not only extends the natural frequency of the conventional moving coil detector from 5 to 0.25 Hz but also has low noise, high gain [sensitivity of 420 V/(m $\cdot $ s−1)], and high-temperature stability.