1. An Affordable Broadband Seismometer : The Capacitive Geophone
Aaron Barzilai1, Tom VanZandt2,
Tom Pike2, Steve Manion2,
Tom Kenny1
1Dept. of Mechanical Engineering
Stanford University
2Center for Space Microelectronics Technology
Jet Propulsion Laboratory

2. An Affordable Broadband Seismometer : The Capacitive Geophone
Aaron Barzilai4, Tom VanZandt2,
Tom Pike3, Steve Manion2,
Tom Kenny1
1Dept. of Mechanical Engineering
Stanford University
2SiWave, Inc.
3Imperial College
4CapitalOne, Inc.

3. A Conventional Geophone:
OYO Geospace 4.5 Hz GS-11D
Schematic
Cross-section

4. Mechanical Sensitivity
Acceleration Causes Relative Motion Between the Coil and the Housing
Constant Sensitivity Below the Resonant Frequency

5. Electrical Sensitivity
Inductively Measure Motion of the Coil Relative to the Magnetic Field
Output Voltage Proportional to the Proof Mass Velocity

6. Total Sensitivity
At Low Frequency, Measurement of Proof Mass Velocity Reduces Sensitivity
At High Frequency, Mechanical System Reduces Sensitivity

7. Conventional Geophone Resolution
Poor Resolution at Low Frequency Exacerbated by Reduced Sensitivity
Resolution not at Thermomechanical Limit
Barzilai et al., “Technique for Measurement of the Noise of a Sensor in the Presence of Large Background Signals,” Rev. Sci. Instrum., July 1998, Vol. 69, Num. 7, pp

8. An Improved Seismometer: A Capacitive Geophone
Use a Commercial, Off The Shelf Geophone as the Mechanical System
Improve Low Frequency Sensitivity by Capacitively Measuring Proof Mass Displacement with only Simple, External Modifications

9. Capacitive Hardware

10. Capacitive Hardware

11. Electrical System Overview
VCEN is a Square Wave at the Same Frequency as VSQ with Amplitude Modulated by y.
VCEN is Demodulated to Produce an Output that is Proportional to the Displacement of the Proof Mass.

12. Acceleration Sensitivity
Constant Sensitivity At Low Frequency since Output is Proportional to Proof Mass Displacement
Bandwidth from sec to 50 Hz Defined by Controller

13. Velocity Sensitivity Velocity Output Produced by Integrator Circuit
Bandwidth from sec to 50 Hz Defined by Controller and Integrator

14. M 7.3 Near Australia 11/26/99 Recorded in Ground Floor Lab in the Durand Building at Stanford.

17. Conclusions
The Low Frequency Resolution of a Geophone can be Improved by Adding Capacitive Detection
Real Instrument Packaging needed to reach full potential performance of this approach.
A Capacitive Geophone is an affordable seismometer suitable for broadband use

18. Acknowledgements
This work was supported by the Center for Space Microelectronics Technology, Jet Propulsion Laboratory, California Institute of Technology, and is sponsored by the NASA Office of Space Access and Technology. We also acknowledge the NSF Career Award(ECS ), the Charles Lee Powell Foundation, and the Terman Fellowship.
Special thanks to Marcos Alvarez at PASSCAL and Larry Cochrane of the PSN for their advice and assistance.

19. Full Circuit Diagram Part I

20. Full Circuit Diagram Part II

21. Block Diagram High Frequency Low Frequency High Pass Integrate
Thus far, we have measured the sensitivity and resolution of our capacitive geophone with integral feedback. The story could be done, expect that now the frequency range is limited at the high end by the resonance of the mechanical system. Since conventional geophones are typically used at frequencies well above their resonance, we have added a lead controller to extend the bandwidth.