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Research Projects

Optical Fiber Infrasound Sensor (OFIS)

OFIS design.
OFIS design.

Spectrograms for a forced signal (1-50 Hz sweeps in 50 seconds)
Spectrograms for a forced signal (1-50 Hz sweeps in 50 seconds) at Piñon Flat Observatory. We compared two OFIS sensors to two B&K microphones along with wind speed data. The OFIS sensors have a lower infrasound noise limit than the microphones in windy conditions and operate better in the audible (> 20 Hz) range.

A variety of atmospheric and anthropogenic phenomena create infrasonic soundwaves (0-20 Hz) that can travel thousands of kilometers through the atmosphere. Example sources include meteors, tornadoes, wave swells, earthquakes, rocket and spacecraft launches/re-entry, and atmospheric nuclear tests. Most infrasound sensors consist of microbarometers or microphones that measure atmospheric pressure changes. We have developed a new sensor consisting of a compliant tube wrapped helically with optical fibers. Pressure fluctuations cause strain in the tubular diaphragm that induce changes in the optical path length, thereby creating an interferometer whose change in path length measures time dependent fluctuations in atmospheric pressure.

Our optical fiber infrasound sensor (OFIS) averages pressure over the length of the tube (~ 89 m) rather than as a point sensor. Integrating over the OFIS's length reduces small-scale atmospheric noise due to air turbulence. Current results indicate that the OFIS records pressure fluctuations with less noise than mechanically filtered sensors and sets a new low noise limit in the band from 1-to-10 Hz, depending on local wind conditions.

Three OFISs are currently in operation at Piñon Flat Observatory with more to be added. A number of instrumental issues are currently under study, including variation in the instrument calibration with time and its dependence on temperature, stability of the interferometer, and methods to mitigate environmental problems. We ultimately aim to create an array of OFIS sensors that can detect directional signals and transmit data in real time for analysis.

For more information about OFIS, including actual recorded events, visit postdoctoral researcher Kris Walker's OFIS page.

Media

  • Download a Quicktime movie of the Optical Fiber Infrasound Sensor [OFIS] being fabricated. The file is copyrighted by Dr. Mark Zumberge of UCSD.
    [10MB 360x240 ]
  • Download an MP3 soundfile of OFIS recording of a helicopter sped up 50 times. The file is copyrighted by Drs Matt Dzieciuch & Mark Zumberge of UCSD.
    [heli.mp3 572 KB ]

People

Publications

  • Zumberge, M.A., Berger, J., Hedlin, M.A.H., Hilt, R., Nooner, S., and Widmer-Schnidrig, R., An optical fiber infrasound sensor: a new lower limit on atmospheric pressure noise between 1 Hz and 10 Hz, Journal of the Acoustical Society of America, 113(5), pp. 2474-2479, 2003.

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