OFIS schematic. The OFIS is a Michelson interferometer that measures the volumetric change of a sealed compliant tube imparted by a passing pressure wave. The double wrapped fiber (blue) deforms less than the single wrapped fiber (red), thereby providing a fringe signal when the two fibers are recombined on the output side of the fiber splitter.
(Color online) Camp Elliott experiment design. (a) The 18 -m diameter, 32-inlet reference pipe array (gray) was collocated with an OFIS (black), as well as two reference sensors at the center of the pipe array: a single-inlet MB2005 microbarometer and a sealed MB2005 (not shown). (b) Inlets were buried beneath gravel during the summer of 2011, which improved the WNR.
PFO Layout. Circular clusters indicate the locations of some of the circular spatial averaging rosettes of the IS57 infrasonic array. The IS57 rosette filters have two diameters. The high-frequency (H) and low-frequency (L) filters have diameters of 18 and 70 m, respectively. The nine 30 m OFIS segments start near IS57 L2 element and run to the south-southeast. The 30, 60, and 180 m OFISs used in the 2010 data set were the first, the second, and third, and fourth through ninth 30 m segments, respectively, starting from the north-northeast (i.e., closest to L2).
WNR summary for Camp Elliott experiment. (a) The median curves from the winter 2011 experiment with exposed rosette inlets. (b) Median curves for the same experiment, but with gravel-covered inlets in the summer of 2011. The OFIS-predicted and Pipe-predicted curves are predictions made using the Inlet curves and are discussed in Sec. VI B .
Median spectra for 1 to 2 and 5 to 6 m/s wind speeds. (a) The 270 m OFIS spectra are compared with the rosette spectra for the 2009 experiment. (b) The spectra for the 30 and 270 m OFIS lengths are compared with the rosette spectra for the 2010 experiment.
Wind noise reduction (WNR) as a function of OFIS length with respect to the 18 m rosettes of IS57. The columns represent different frequency bands [(a) and (c) 0.1 to 1 Hz, and (b) and (d) 1 to 5 Hz] and rows for the two wind speeds [(a) and (b) 3–4 m/s, and (c) and (d) 5–6 m/s]. Because we are plotting noise reduction, a positive value in dB means the integrated noise spectrum for the OFIS is lower than that for the IS57 array element. The circles and error bars correspond to the observations, while the solid curves represent the predicted WNR shown in Eqs. (5) and (6) .
WNR as a function of OFIS length with respect to the 70 m rosettes of IS57 for different frequency bands and wind speeds. As in Fig. 6 , the LF, HF, and two wind speed subplots are similarly laid out, and the data and their uncertainties are shown as filled circles and the solid curves are the predicted WNR.
Signal-to-noise ratio comparison for rocket-generated infrasound recorded during moderate winds at PFO. (a) 8 August 2009 Delta II Vandenberg rocket launch generated infrasound in the 1 to 5 Hz band. (b) 18 August 2009 Atlas V rocket launch generated infrasound in the 0.5 to 2 Hz band. Yellow and red shading are the noise and signal windows, respectively. The lowermost plot in each column shows the frequency-time windows of significant infrasonic energy, with color indicating the signal back azimuth obtained by array processing using the IS57 array.
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