The term “infrasound” refers to atmospheric pressure fluctuations that proceed faster than weather but more slowly than can be heard by people. Infrasonic pressure waves are caused by a number of natural sources including bolides, lightning, earthquakes, volcanoes, and ocean waves, and by some man-made sources like supersonic aircraft, missile launches and explosions.
Generally the band of interest for infrasound is from 0.01 Hz to 10 Hz. Atmospheric pressure waves at these frequencies can travel intercontinental distances. Currently, most infrasound detection is done with microbarometers. However wind turbulence is a major source of noise as is cost.
Where is it best used? Defense and surveillance. Monitoring of both large-scale but distant signals (i.e. from rocket and missile launches, to artillery and nuclear test detection) as well smaller, local signals (i.e. foot traffic, boundary and barrier control) or even tornado modeling and tracking.
How does it work? The OFIS is a spatially distributed pressure sensor, which averages pressure along a line by means of optical fiber sensing of strain in a long tubular diaphragm. The sensitivity to pressure changes has proven to be superior to other techniques in the band from 1 Hz to 10 Hz. The sensor provides a continuous signal representing the instantaneous spatial average of pressure along the line described by the sensor.
How does it compare? The optical nature of the OFIS allows for significant cost savings and performance improvements over microbarometers. Add in the fact that the OFIS is isolated from surface conditions and its signal-to-noise ratio is unparallelled.
Quad Geometrics can manufacture and install an OFIS to a customer’s specifications, with lengths of up to many hundreds of meters. The complete system consists of: sensor tube(s), an interferometer, photo detectors, an optical fringe signal processing system and remote power and communications.
Contact Quad for sample results or more details.
Walker, K.T., and Hedlin, M.A.H., 2010, A review of wind noise reduction methodologies, in: Infrasound Monitoring for Atmospheric Studies, ed. by A. Le Pichon, E. Blanc, and A. Hauchecorne, Springer, p. 141-182, doi:10.1007/978-1-4020-9508-5.
Zumberge, M. A., Berger, J., Hedlin, M. A. H., Husmann, E., Nooner, S., Hilt, R., and Widmer-Schnidrig, R. (2003). “An optical fiber infrasound sensor: A new lower limit on atmospheric pressure noise between 1 and 10 Hz”, Journal of the Acoustical Society of America 113, 2474–2479.
Walker, K.T., Zumberge, M.A., Hedlin, M.A.H., and Shearer, P., 2008, Methods for determining infrasound phase velocity direction with an array of line sensors, J. Acoust Soc. Am., 124, 2090-2099.
DeWolf, S., Walker, K. T., Zumberge, M. A., and Denis, S., 2013, Efficacy of spatial averaging of infrasonic pressure in varying wind speeds, J. Acoust. Soc. Am., 133, 3739-3750, doi:10.1121/1.4803891.