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Seismo-acoustic analysis of the infrasound events on 2010, February 03

The propagation of infrasound from the Vliehors to FDG through the 06h UTC model at 53N,5E. The lower frame shows the raytrajectories for energy leaving the source towards FDG. The frame to right gives the effective sound speed (Ceff) and sound speed (Ct). The top frame shows the traveltime for rays impinging on the earth's surface.
LG Evers

The Seismology Division of the Koninklijk Nederlands Meteorologisch Instituut received reports of vibrations in Friesland on 2010, February 03. In order to evaluate such accounts, a seismometer network and infrasound arrays are being operated. An infrasound array on Texel (TEX) and borehole seismometer near Firdgum (FDG) appeared to have measured impulsive waveforms on that specific day. The analysis of the signals pointed to an atmospheric source near Vlieland. A military exercise was confirmed by the Ministry of Defense; a total of 12 MK-82 500 pound bombs had been thrown from F-16 fighters on the Vliehors (Vlieland).
Infrasound waves of the explosions were efficiently ducted from the Vliehors towards Friesland by a strong wind between 5 and 10 km altitude. This eastward jet stream refracted the infrasound back to earth's surface after which it was experienced as vibrations. Such wind conditions are not unique but appeared 5% of the time in a year of atmospheric specifications.
The modeling of the infrasound amplitudes for a specific source under actual wind and temperature conditions might be of help in forecasting sound pressure levels at a variety of source-receiver distances.

Bibliografische gegevens

LG Evers. Seismo-acoustic analysis of the infrasound events on 2010, February 03
KNMI number: TR-314, Year: 2010, Pages: 31

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