HR: 1330h
AN: S52C-0144    [PDF]
TI: Experiments With an Optical Seismometer
AU: * Zumberge, M
EM: mzumberge@ucsd.edu
AF: IGPP, UCSD, 9500 Gilman Drive, La Jolla, CA 92093-0225 United States
AU: Berger, J
EM: jberger@ucsd.edu
AF: IGPP, UCSD, 9500 Gilman Drive, La Jolla, CA 92093-0225 United States
AU: Wielandt, E
EM: Erhard.Wielandt@geophys.uni-stuttgart.de
AF: Stuttgart University, 44 Richard Wagner St., Stuttgart, D-7000 Germany
AB: Modern seismometers rely on electronic displacement transducers to sense the motion of an inertial mass suspended by a spring. The more sophisticated systems use electrostatic or electromagnetic force-feedback on the inertial mass to ameliorate the shortcomings of the spring and the displacement transducer. Recent advances in optical fiber technology and digital signal processing offer an alternative to the modern observatory seismometer. We have developed an optical fringe resolver to replace the electronic displacement transducer, which promises to lead to a greatly improved seismometer. The use of optical fiber interferometry rather than traditional electronic displacement transducers affords significant advantages, including: $\bullet$ A linear, high-resolution displacement detector - the proposed optical sensor includes the functionality of a digitizer providing about a 30-bit digital output; \\ $\bullet$ Absolute displacement measurement referenced to the wavelength of light; \\ $\bullet$ Bandwidth sufficient to resolve the USGS Low Noise Model from DC to > 15 Hz; \\ $\bullet$ Dynamic range sufficient to record the largest teleseisms and most regional and local earthquakes; \\ $\bullet$ Minimum electronics in package - only optical fiber connection to the seismometer, minimizing heat from electronics in the sensor package and noise pickup from connecting electrical cables; \\ $\bullet$ Smaller package - our design will be applicable to both vault and borehole installations and should be relatively easy to manufacture. Our first test of this concept was to apply it to a standard STS-1 seismometer. For this experiment, we added interferometric components to the seismometer frame and a retroreflector to the seismometer's mass. We removed the feedback electronics and recorded the STS-1 mass displacement with our new interferometric system. Simultaneously, we recorded the output of a standard STS-1 set up on the same pier. The results, which include observations of large teleseisms and microseisms, indicate that the new technique is promising. In our second experiment, we measured the inherent noise floor of the optical fringe resolver. In a 100 Hz bandwidth, the RMS noise was approximately 5 x 10-12 m, sufficient to resolve the USGS ground noise model up to at least 15 Hz with the STS-1 suspension.
DE: 7294 Instruments and techniques
SC: Seismology [S]
MN: 2003 Fall Meeting