An Affordable Broadband Seismometer: The Capacitive...

1SSA‘99 Aaron Barzilai

An Affordable Broadband

Seismometer:The Capacitive

GeophonePoster F1

http://micromachine.stanford.edu/smssl/projects/Geophones/SSA99PosterFinal.pdf

2SSA‘99 Aaron Barzilai

Aaron Barzilai1, Tom VanZandt2,

Tom Pike2, Steve Manion2,

Tom Kenny1

1Dept. of Mechanical EngineeringStanford University

2Center for Space Microelectronics TechnologyJet Propulsion Laboratory

Contact: barzilai@leland.stanford.edu

3SSA‘99 Aaron Barzilai

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.

4SSA‘99 Aaron Barzilai

Objective

• Develop an Affordable, Robust Broadband Seismometer

• Enhance the Ability of Seismometer Arrays to Detect Low Frequency Signals

5SSA‘99 Aaron Barzilai

Seismometer Information Flow

AccelerationInput Mechanical

System

RelativeMotion Electrical

System

OutputVoltage

6SSA‘99 Aaron Barzilai

Cross-sectionSchematic

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

Magnet

Cylinder

Coil

GeophoneHousing

LeafSpring

7SSA‘99 Aaron Barzilai

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

Mec

hani

cal S

ensi

tivity

[m/(

m/s

2)]

Frequency [Hz]

8SSA‘99 Aaron Barzilai

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

Ele

ctric

al S

ensi

tivity

[V/m

]

Frequency [Hz]

9SSA‘99 Aaron Barzilai

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

Geo

phon

e S

ensi

tivity

[V/g

]

Frequency [Hz]

10SSA‘99 Aaron Barzilai

Circuitry NoiseOutput Voltage

Noise Spectral Density

Geo100Ω

10kΩ

VO

Typical Circuit

100x Ampifier10-7

10-6

10-5

10-4

10-2 10-1 100 101 102

Out

put V

olta

ge N

SD

[V/√

Hz]

Frequency [Hz]

Peak CausedBy Resonance

11SSA‘99 Aaron Barzilai

Conventional Geophone Resolution

• Poor Resolution at Low Frequency Exacerbated by Reduced Sensitivity

• Resolution not at Thermomechanical Limit

Resolution gHz[ ] =

Noise VHz[ ]

Sensitivity Vg[ ]

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

Res

olut

ion

[g/√

Hz]

Frequency [Hz]

Thermomechanical Limit

PredictedCircuitry Limit

MeasuredCircuitry Limit

12SSA‘99 Aaron Barzilai

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

13SSA‘99 Aaron Barzilai

Capacitive HardwareAdditionalHousing

MovingElectrode

FixedElectrodes

Insulation

33.02 mm43.18 mm

y

CircuitModel

C =εε0 A

a − y

C =εε0 A

a + y

a = Balanced Gap ≈ 250µm

A = Area = 6.0 ×10-4 m2

CNOMINAL ≈ 21pF

14SSA‘99 Aaron Barzilai

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

+VSQ

x12

-VSQ

DemodulatorVOUT

Reference

Input

VCEN

15SSA‘99 Aaron Barzilai

Full Circuit Diagram Part I

16SSA‘99 Aaron Barzilai

Full Circuit Diagram Part II

17SSA‘99 Aaron Barzilai

Block Diagram1

s2 + 23s + 645

1

37428

1

.026

˙ X

1

300s +1

X

VIF˙ X

12

⋅5.0 ×103(4.7 ×10−3s + 1)

47 ×103(5.0 ×10−4 s +1)

˙ X

VACCELERATION

High Frequency

Low Frequency

1.0 × 106

˙ X IN

1

37428

1

.026

VIF

V

33s

33s +1

High Pass

10

(10s + 1)

Integrate

VVELOCITY

18SSA‘99 Aaron Barzilai

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

Sen

sitiv

ity [V

/g]

Frequency [Hz]

3dB pts

63 V/g

19SSA‘99 Aaron Barzilai

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

Sen

sitiv

ity [V

/m/s

]

Frequency [Hz]

3dB pts

6.3 V/m/s

20SSA‘99 Aaron Barzilai

Resolution Comparison

USGS LNM:Seismic Noise At Quietest Sites On Earth

Capacitive Geophone Measurement Limited byAmbientSeismic Noise

10-10

10-9

10-8

10-7

10-6

10-2 10-1 100 101 102

Res

olut

ion

[g/√

Hz]

Frequency [Hz]

ConventionalGeophone

USGS LowNoise Model

CMG-40T

CapacitiveGeophoneUpper Limit

Capacitive GeophoneThermomechanical Limit

21SSA‘99 Aaron Barzilai

Conclusions• Need to collaborate with a vault

operator to take data at a quieter site• The Low Frequency Resolution of a

Geophone can be Improved by Adding Capacitive Detection

• A Capacitive Geophone is an affordable seismometer suitable for broadband use