An Affordable Broadband Seismometer : The Capacitive Geophone Aaron Barzilai 1 , Tom VanZandt 2 ,

1Aaron Barzilai

An Affordable Broadband Seismometer : The Capacitive Geophone

Aaron Barzilai1, Tom VanZandt2,

Tom Pike2, Steve Manion2,

Tom Kenny1

1Dept. of Mechanical EngineeringStanford University

2Center for Space Microelectronics TechnologyJet Propulsion Laboratory

2Aaron Barzilai

An Affordable Broadband Seismometer : The Capacitive Geophone

Aaron Barzilai4, Tom VanZandt2,

Tom Pike3, Steve Manion2,

Tom Kenny1

1Dept. of Mechanical EngineeringStanford University

2SiWave, Inc.3Imperial College

4CapitalOne, Inc.

3Aaron Barzilai

Cross-sectionSchematic

A Conventional Geophone:OYO Geospace 4.5 Hz GS-11D

MagnetCylinder

CoilGeophoneHousingLeafSpring

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Mechanical Sensitivity

• Acceleration Causes Relative Motion Between the Coil and the Housing

• Constant Sensitivity Below the Resonant Frequency

10-6

10-5

10-4

10-3

10-2

10-2 10-1 100 101 102

Mechanical Sensitivity [m/(m/s

2)]

Frequency [Hz]

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Electrical Sensitivity

• Inductively Measure Motion of the Coil Relative to the Magnetic Field

• Output Voltage Proportional to the Proof Mass Velocity

102

103

104

105

106

107

10-2 10-1 100 101 102

Electrical Sensitivity [V/m]

Frequency [Hz]

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Total Sensitivity

• At Low Frequency, Measurement of Proof Mass Velocity Reduces Sensitivity

• At High Frequency, Mechanical System Reduces Sensitivity

100

101

102

103

104

10-2 10-1 100 101 102

Geophone Sensitivity [V/g]

Frequency [Hz]

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Conventional Geophone Resolution

• Poor Resolution at Low Frequency Exacerbated by Reduced Sensitivity

• Resolution not at Thermomechanical Limit

Resolution gHz[ ] =

Noise VHz[ ]

SensitivityVg[ ]

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. 2767-2772

10-10

10-9

10-8

10-7

10-6

10-5

10-4

10-2 10-1 100 101 102

Resolution [g/

√]Hz

[ ]Frequency Hz

Thermomechanical Limit

Predicted Circuitry Limit

Measured Circuitry Limit

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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

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Capacitive Hardware

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Capacitive HardwareAdditionalHousingMovingElectrodeFixedElectrodes

Insulation33.02 mm43.18 mm

yCircuitModel

C =εε0Aa−y

C=εε0Aa+ y

a= Balanced Gap≈250μm

A=Area=6.0 ×10-4m2

CNOMINAL≈21pF

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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.

y+VSQx12

-VSQDemodulatorVOUTReferenceInput

VCEN

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Acceleration Sensitivity

• Constant Sensitivity At Low Frequency since Output is Proportional to Proof Mass Displacement

• Bandwidth from 83 sec to 50 Hz Defined by Controller

100

101

102

10-2 100 102

Sensitivity [V/g]

Frequency [Hz]

3dB pts

63 V/g

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Velocity Sensitivity

• Velocity Output Produced by Integrator Circuit

• Bandwidth from 45 sec to 50 Hz Defined by Controller and Integrator

100

101

10-2 100 102

Sensitivity [V/m/s]

Frequency [Hz]

3dB pts

6.3 V/m/s

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M 7.3 Near Australia 11/26/99 Recorded in Ground Floor Lab

in the Durand Building at Stanford.

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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

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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-9502046), 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.

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Full Circuit Diagram Part I

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Full Circuit Diagram Part II

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Block Diagram1

s2 + 23s+ 645

1

37428

1

.026

˙ ̇ X

1

300s +1

X

VIF˙ ̇ X

1

2⋅5.0 ×103(4.7×10−3s+1)47×103(5.0 ×10−4s+1)

˙ ̇ X

VACCELERATION

High Frequency

Low Frequency

1.0 ×106˙ ̇ X IN

1

37428

1

.026

VIF

V

33s

33s +1

High Pass10

(10s +1)

Integrate

VVELOCITY