Project Description-

• Various 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-entries, and atmospheric nuclear tests. After the approval of the Comprehensive Test-Ban Treaty (CTBT) in 1996, the United Nations International Monitoring System established a network of more than 300 sites to conduct seismological, hydroacoustic, radionuclide, and infrasound monitoring. This advanced sensor array is part of that network.

• Where other infrasound sensors consist of microbarometers or microphones that measure atmospheric pressure changes, the new optical fiber infrasound sensors (OFIS) consist of a compliant tube wrapped helically with optical fibers. Pressure fluctuations produced by infrasonic soundwaves cause strain in the tubular diaphragm, which induces changes in the optical path length, thereby creating an interferometer whose change in path length measures time-dependent fluctuations in atmospheric pressure.

• The 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 OFIS arrays are currently in operation at this site, 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. The ultimate goal is to create an array of OFIS sensors that can detect directional signals and transmit data in real time for analysis.

Principal Investigators-
Mark Zumberge, University of California, San Diego
Jon Berger, University of California, San Diego

Participating Institutions-
Scripps Institution of Oceanography
Laboratory for Atmospheric Acoustics

Online Information-
International Monitoring System
Laboratory for Atmospheric Acoustics

Funded by the
United Nations International
Monitoring System