Infrasound signals in the band 0.02 to 4 Hz are sensed in the presence of
ambient noise generated chiefly by wind as integrated pressure variations,
which induce detectable changes in the optical path length, along optic
fibers, typically extending 100 m. to 1000 m. and more, arrayed at
arbitrary geometries. Two fibers connected as a Michelson, Mach-Zehnder or
equivalent interferometer where (i) one fiber is coupled to atmosphere
while (ii) the other is not for being hermetically sealed in a tube,
permit common mode rejection of noise from (i) temperature changes and
(ii) strain, including ground vibration. Because the optic fiber
infrasound sensors are longer than the distance over which wind-induced
pressure changes are coherent, the effects of wind noise on the sensing of
infrasound is reduced, and signal-to-noise ratio is increased over a wide
bandwidth.
