Map of the United States showing a typical jet stream. The wind speeds are shown in knots. The “threshold” wind speed for CAT (Ref. 2 ) (and the wind shear that it generates) is 110 knots. The regions shown in brown and yellow are the Appalachian Mountains and the state of Virginia, respectively. The triangle indicates the location of the array.
Photograph of the infrasonic microphone and windscreen.
Satellite image of the three-microphone field array. Long-dashed line: drainage pipes from the microphone stations to a ditch surrounding the array. Short-dashed line: underground cable from array to control room.
Coherence spectra of received signals during the turbulence event of 9/08/06. The frequency (0–15 Hz) is shown on the y-axis and the time (6-h period) on the x-axis. The coherence, evaluated over 30-s intervals, is color coded, ranging from 1 (dark red) to 0 (dark blue). The figure represents 720 individual coherence plots.
(Color online) Acoustic slowness vector projections.
(a) Pilot reports (PIREPS) on the morning of 9/08/06 at 0700 EDT. The yellow lines show the directions from the PIREP locations to the array. (b) Normalized slowness map of signals received at the array on 9/08/06 at 0700 EDT. The triangle indicates the array location. Each remaining symbol represents a datum point evaluated over a 30-s interval. The solid lines mark the same directions as shown in (a). The dashed lines indicate the uncertainty in azimuth based on an error of one sample count (0.002 s) in the time delay measurement. The solid circle represents a slowness of 1 (propagation horizontal to the ground). The inner dashed circle represents a slowness of 0.66, corresponding to an elevation angle of 41°. The outer dashed circle represents a slowness of 1.5, indicating wind aided or seismic signals. The symbols inside the solid circle indicate propagation of signals arriving from high elevations. (c) Power spectral density of signals received from 0700 to 0800 on 9/08/06. The symbols represent 12 sets of data spaced in 5-min intervals. The background noise spectrum was taken during a time when emissions from turbulence were absent. The solid line represents the best fit to Eq. (2) . (d) Pilot reports have disappeared at 0800 on 9/08/06. (e) Normalized slowness map of signals received at the array on 9/08/06 at 0800 EDT. Signals continue to arrive at the array even after the disappearance of PIREPS. The absence of a PIREP does not necessarily indicate the absence of a turbulence event.
(a) Pilot reports (PIREPS) on the morning of 8/25/06 at 0700 EDT. (b) Normalized slowness map of signals received at the array on 8/25/06 at 0700 EDT. (c) Power spectral density of signals received from 0700 to 0725 (region I) and from 0735 to 0800 (region II). The magnitude of the power spectral density fell over 40 dB within a ten-minute interval.
(a) Coherence spectra (0.2–5 Hz) of received signals attributed to a microbarom. See caption of Fig. 4 for details. (b) Power spectral density of the microbarom signals. Each symbol represents a measured spectrum taken in 5-min intervals. The peak at 0.2 Hz and high coherence from 0.2–1 Hz are characteristic of a microbarom.
(Color online) (a) Normalized slowness map of signals received from Space Shuttle STS-115 launch noise. (b) Sound pressure level of signals received at the array from Space Shuttle STS-115 launch noise.
D matrix for the microphone field array.
Summary of significant infrasonic signals detected at the array.
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