An Employee-Owned Company®

LP Sensors3/24/04 1

SensorsDeveloped in the US for

Low Frequency Seismology

W. E. FarrellScience Applications International Corp

An Employee-Owned Company®

LP Sensors3/24/04 2

Topics

• Summary• Motivation

– Nuclear test detection– Physics and geophysics

• Geotech KS series• Other Geotech sensors• Academic projects

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Summary

• The principal driver for 1/2 century of development of LP sensors has been nuclear test monitoring– The heyday lasted a long time, 1958-2000– Currently here seems to be little interest within DoD or DoE

for R&D in LP sensors

• Much innovation came from University programs– Academics partly rode on the shirt tails of the test detection

funding, but were focused on fundamental problems– NSF and private foundation money was key to these

programs • AFOSR was a supporter in the early years

– Sensor R&D in universities tapered off through the 70s• Perception that available technology good enough?

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Motivation - Nuclear Test Detection

• The problem of nuclear test detection has generated the most money for R&D in long period sensors– Berkner Report 1959

• Mb:Ms discriminant requires good SNR in the .1-.03Hz range

– Lower frequency data required for estimating shallow earth structure, but ultra low frequencies were not a concern

– Borehole packaging deemed critical

• DARPA & Vela Uniform– See Farrell, 1985, Sensors, Systems & Arrays

• AFTAC– Sponsored sensor R&D related to its monitoring mission

• DOE

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Motivation - Physics and Geophysics

• Fundamental research in physics & geophysics has been an equally important driver of progress– Dicke

• Theories of gravity

• Lock-in amplifiers, capacitor pickoff & electrostatic feedback

– IDA • Systematic observation of free oscillations for splitting and Q(r)

• Inherited and applied Dicke approach

– Superconducting gravity meter• “Zero” drift would lead to data in the tectonic frequency band

• A few in service as seismometers

– Strain meters

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Geotech KS series

Instrument Years Description Deployment

KS36000 1973-2000 3 component, capacitive sensing, magnetic force feedback, borehole package & remote leveling

Seismic Research Observatory (SRO, 13 stations)

KS44000 1978-1982 3 component, capacitive sensing, electrostatic feedback

Experimental system, not deployed

KS54000 1980-present

3 component, capacitive sensing, magnetic force feedback, borehole package & remote leveling

Global Seismic Network (GSN, many stations)

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Other Geotech sensors

Instrument Years Description Deployment

S-11, S-12

1960-1975?

Vertical sensor packaged in sealed container

High-gain, long-period (HGLP) array, upgraded to ASRO, 5 stations

Strain-Inertial

Quartz-bar strain sensor and conventional inertial seismometer

Not deployed

11550 1960- Open-loop EM sensor with 5 kg mass, 20 second period in a tetrahedral configuration

Alaska long-period array (ALPA)

SL210, SL220

1968-1985 Open-loop EM sensor with 20 second period for surface vaults

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Academic projects

Instrument Years Description Deployment

Block Moore 1965-1975 Quartz cantilever, Dicke feedback (capacitive sensing, electrostatic forcing). High Q, 1 Hz resonance (electronic refrigeration)

Diax failed to commercialize. Never deployed

LaCoste-Romberg gravity meter

1968-1995 Ca 1950’s vintage underwater gravity meter with Dicke feedback

International Deployment of Accelerometers (IDA) 20 stations

Superconducting gravity meter

1968-present

Levitated Nb sphere Commercialized by GWR. 20 in operation

Strain meters 1968-present

1 km Michelson interferometet, volumetric

Not conducive to global network