1AGU Spring ‘98

Improved Low Frequency Performance of

a Geophone

S32A-19

2AGU Spring ‘98

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

3AGU Spring ‘98

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 for his advice and assistance.

4AGU Spring ‘98

Poster Map

Title Introduction

Conventional Geophone

Capacitive Geophone

Conclusions

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Objective

• Develop an Affordable, Robust Broadband Seismometer with Resolution Comparable to the Earth’s Seismic Noise

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

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Seismometer Information Flow

AccelerationInput Mechanical

System

RelativeMotion Electrical

System

OutputVoltage

7AGU Spring ‘98

Cross-sectionSchematic

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

Magnet

Cylinder

Coil

GeophoneHousing

LeafSpring

8AGU Spring ‘98

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]

9AGU Spring ‘98

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]

10AGU Spring ‘98

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]

11AGU Spring ‘98

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

12AGU Spring ‘98

Conventional Geophone Resolution

• Poorer Resolution at Low Frequency caused by Reduced Sensitivity

• Resolution worse than Fundamental Limit

Resolution gHz[ ] =

Noise VHz[ ]

Sensitivity Vg[ ]

Reference: Barzilai et al., “Technique for Measurement of the Noise of a Sensor in the Presence of Large Background Signals,” Rev. Sci. Instrum., Accepted for Publication

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

13AGU Spring ‘98

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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Photos

Guralp CMG-40T and Capacitive Geophone

Capacitive Geophone

15AGU Spring ‘98

Capacitive HardwareAdditionalHousing

MovingElectrode

FixedElectrodes

Insulation

33.37 mm39.37 mm

y

CircuitModel

a = Balanced Gap ≈ 250µm

A = Area = 3.4 ×10-4 m2

CNOMINAL = 12.1pF

C =εε0 A

a − y

C =εε0 A

a + y

16AGU Spring ‘98

Electrical System Overview

VBR is a Sine Wave at the Same Frequency as VSIN with Amplitude Modulated by y. The Lock-In AmplifierDemodulates the Signal to Produce an Output that isProportional to the Displacement of the Proof Mass.

y

+VSIN

x11

-VSIN

Lock-InAmplifier

VOUTVBR

Reference

InputVIN

17AGU Spring ‘98

Preamplifier Circuit

fSIN = 100kHz

VSIN = 3.8Vpk

fc = 12πRC

≈ 600Hz

10 M

1 k

10 k

ModulatedSignal

VSIN

VBR

-VSIN

VINCA

CB TLC 2274

18AGU Spring ‘98

Preamplifier Circuit Sensitivity

VIN = VSIN − −VSIN[ ]( )1

sCA1

sCA+ 1

sCB

− VSIN

1+10kΩ1kΩ

VIN = VSINsCB − sCA

sCB + sCA

11( )

VIN = VSIN

1a −y − 1

a+y1

a −y + 1a+y

11( )

VIN = VSINy

a11( ) = 1.7 ×105 V

m

y m[ ]

Exact Solution: Voltage Amplitude is a Linear Function of Displacement

19AGU Spring ‘98

Demodulator

• Demodulator Output is Directly Proportional to the Amplitude of the Input at the Reference Frequency.

• In the Frequency Domain, Low Frequency Amplitude Variations Appear as Signals Near the Reference Frequency. The Demodulator Shifts These Signals Back to Low Frequency.

20AGU Spring ‘98

Electrical Sensitivity

Demodulator Sensitivity

VOUT =10V

5 10−3( )V1

2VIN pk = 1414 VIN pk

VSIN

SRS 510Lock-In

Amplifier

Reference

Input

Output

VIN

VOUT = 1414( )1.7 ×105 V

m

y m[ ] = 2.3 108( ) V

m

y m[ ]

21AGU Spring ‘98

Total Sensitivity

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

100

101

102

103

104

105

106

107

10-2 10-1 100 101 102

Geo

phon

e S

ensi

tivity

[V/g

]

Frequency [Hz]

Capacitive

Conventional

22AGU Spring ‘98

Preamplifier Circuitry Noise

• Noise is Constant Amplitude vs. Frequency

• Lock-In Amplification Produces .18 mV/√Hz Output Voltage NSD at Low Frequencies

Noise Near the Reference Frequency is Mapped to Low Frequencies by Demodulator

10-7

10-6

99.9 100 100.1

Pre

amp

Out

put N

SD

[V/√

Hz]

Frequency [kHz]

ReferenceFrequency

23AGU Spring ‘98

Capacitive Geophone Resolution

Resolution gHz[ ] =

Noise VHz[ ]

Sensitivity Vg[ ]

• At Low Frequencies, Resolution is Limited by Thermomechanical Noise, not Circuitry

• Better Resolution at Low Frequencies as a Result of Constant Sensitivity

10-11

10-10

10-9

10-8

10-7

10-2 10-1 100 101 102

Res

olut

ion

[g/√

Hz]

Frequency [Hz]

Thermomechanical Limit

Circuitry Limit

24AGU Spring ‘98

Clip Level

• Demodulator Output Range ±10V

• Corresponds to .040 µm Displacement

10-6

10-5

10-4

10-3

10-2

10-2 10-1 100 101 102

Clip

Lev

el [g

]

Frequency [Hz]

25AGU Spring ‘98

Resolution Comparison

USGS LNM:Seismic Noise At Quietest Sites On Earth

10-11

10-10

10-9

10-8

10-7

10-6

10-2 10-1 100 101 102

Res

olut

ion

[g/√

Hz]

Frequency [Hz]

Thermomechanical Limit

Circuitry Limit

ConventionalGeophone

USGS LowNoise Model

CMG-40T

26AGU Spring ‘98

Resolution Comparison

• Low Frequency Resolution of a Geophone is Improved by using Capacitive Detection

• Circuitry Noise does not Limit Capacitive Geophone at Low Frequency

• Limits on Resolution of a Capacitive Geophone are Better than the Resolution of a Guralp CMG-40T Broadband Seismometer

27AGU Spring ‘98

Performance ComparisonConventional

Geophone

100 ng/√Hz

90 mg

120 dB

$50

Resolution

Clip Level

DynamicRange

Estimated Cost

CapacitiveGeophone

.1 ng/√Hz

5 µg

90 dB

$500

GuralpCMG-40T

.5 ng/√Hz

1 mg

130 dB

$10,000

28AGU Spring ‘98

Conclusions

• The Low Frequency Resolution of a Geophone can be Improved by Adding Capacitive Detection

• Capacitive Detection Does Not Improve High Frequency Resolution

• Thermomechanical Noise Sets a Resolution Limit ≈ 0.1 ng/√Hz on all Geophone Based Seismometers

29AGU Spring ‘98

Future Work

• Experimentally Validate the Predicted Resolution of a Capacitve Geophone

• Reduce Size by Integrating Electronics onto a Single Printed Circuit Board

• Operate as a Closed Loop Sensor to Increase the Dynamic Range and Tune the Frequency Response