A digital low-frequency geophone based on 4th-order sigma-delta modulator and single-coil velocity feedback

• Published in: Sensors and actuators. A. Physical. 2020-09, Vol.312, p.112074, Article 112074
• Main Authors: Zhang, Xiaopeng, Wei, Xueyong, Wang, Tongdong, Li, Xin, Xiao, Weiguo, Wu, Zutang
• Format: Article
• Language: English
• Subjects: Acceleration; Balancing; Bandwidths; Digital readout interface; Feedback control systems; Frequency response; Geophone; Integrators; Interface stability; Moving coil; Programmable logic controllers; Seismic engineering; Seismic exploration; Seismic stability; Seismic waves; Sigma-Delta modulator; Velocity feedback; Waveform analysis
• ISSN: 0924-4247; 1873-3069
• DOI: 10.1016/j.sna.2020.112074

[Display omitted] •A digital low-frequency geophone for deep Earth exploration was designed and characterized.•PID controlled force balancing ΣΔM demonstrates the band extension and digital readout interface.•A conventional geophone can be directly inserted to the interface for industry popularization.•The resulting prototype achieves a low-frequency response down to 0.16 Hz and an instrumental noise of 4.3 ng/√Hz. Widely deployed conventional moving-coil geophones lack the responses needed at low frequencies for detecting long-period seismic waves, and rely heavily on the A/D conversion chips for data acquisition. This work reports on a digital low-frequency geophone based on the PID force balancing technique and a single loop 4th-order sigma-delta modulator. The system is studied by Simulink modeling and is designed to satisfy demanding bandwidth and stability requirements. In addition, the hardware implementation adopts a readout interface for a geophone that consists of two parts: the analog part picks off the coil voltage while driving a control balancing force; and the digital part performs a real-time time DF integrators filtering in an FPGA for high-order noise shaping. The experimental results confirm the design concept of the proposed digital geophone. The balancing loop extends the low-frequency response of a commercial 4.5 Hz geophone down to 0.16 Hz. Within a 200 Hz bandwidth, the sigma-delta modulator achieves an equivalent acceleration noise floor of 4.3 ng/√Hz @ 1.0 Hz and shapes the in-band quantization noise to a level below other noises in the system. This investigation shows its potential to detect low-frequency seismic waves for efficient and high precision deep Earth exploration and hazard predictions.