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Akito ARAYA (Earthquake Reseach Inst., Univ. of Tokyo, Japan). Broadband observation …. requires low-frequency sensitivity Advantages of Laser Interferometers: ・ high resolution owing to short wavelength of light

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Laser-interferometer application to broadband observations

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Laser interferometer application to broadband observations l.jpg

Akito ARAYA

(Earthquake Reseach Inst., Univ. of Tokyo, Japan)

Broadband observation …. requires low-frequency sensitivity

Advantages of Laser Interferometers:

・high resolution owing to short wavelength of light

・low drift using frequency-stabilized laser

・in-situ calibration with reference to wavelength of light

・free from EM noise or heating based on optical sensing

・operation at high-temperature (deep underground)

Laser-interferometer application to broadband observations


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Features of the laser-interferometric seismometer

・Self calibration (with reference to the laser wavelength)

Principle

Block diagram


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Features of the laser-interferometric seismometer

・Laser-diode as a light source (l=850nm, 5mW)

・Long-period pendulum (f0=7s with a 10-cm mass)

Laser diode as a light source

Prototype laser seismometer


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Features of the laser-interferometric seismometer

・Wideband feedback

(UGF=1kHz,

phase delay < 0.2deg.

below 100Hz)

Frequency response


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Two calibration method were applied

and both results were agreed well.


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・Accuracy of the self calibration

----- ~1% as compared with an STS-2 seismometer

・Self-noise level

----- well below the Low Noise Model (50mHz ~ 100Hz)

estimated from two identical laser seismometers

Comparison with an STS-2 seismometer

Self-noise estimation

(measured at Black Forest Obs.)


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Current problems

---- thermal and

barometric response


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Optical-fiber-linked version of a laser seismometer (under development)

Accelerometer and an air-tight case

No electronic components are installed in the accelerometer

Accelerometer (vertical)


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Optical-fiber-linked borehole tiltmeter


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A pendulum-type tiltmeter with laser interferometers


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Test observation in an 80-m-deep borehole at Nokogiriyama observatory


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Comparison with water-tube tiltmeters

Earth tides (10-7~10-6rad)

Seiche (~10-8rad) occurred in Uraga Channel (entrance of Tokyo Bay)


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Earthquake observation

Turkey earthquake (17 Aug. 1999)

Initial motion -- (~10-8rad)

Maximum amplitude -- (~10-5rad)

Rapid response than that of water-tube tiltmeters.


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Laser strainmeter as a broadband seismometer

・Iodine-stabilized Nd:YAG laser (l =532nm, dn/n=2×10-13)

・100-m baseline, 1000-m underground (in Kamioka Mine)


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Location of Kamioka Mine --- about 200km northwest of Tokyo


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Map in the mine (-1000m level)

Super KAMIOKANDE

(neutrino detector)

L-shaped 100-m tunnel for

laser strainmeters and

a gravitational-wave detector


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Schematic diagram


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Iodine-stabilized Nd:YAG laser


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Twin bellows

Optics in vacuum


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Background

noise level


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Comparison with

CMG seismometers


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Excited free oscillation

Tokachi earthquake (M=8.0)


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Earth tides compared

with predicted one


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Summany

Laser interferometer … promising method for

・opening up a new window (deep borehole, ocean floor, etc.)

optical-fiber-linked borehole seismometer, tiltmeter, and strainmeter

・improving sensitivity

long-baseline strainmeter with a highly-stabilized laser


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