Etna animation
The top frame shows the likelihood of coherent signal as a function of slowness. The slowness vector resolves back azimuth and apparent sound speed belonging to the event. Fisher values (signal likelihood) are contoured upto a value of 3 in orange. Red colours indicate Fisher values higher than 3. The white circle corresponds to a sound speed of 340 m/s.
The middle frame displays the best beam for the 13 operational elements of DIA. The best beam belongs to the event characteristics, back azimuth and apparent sound speed, of the event with the largest Fisher value. This is the infrasound around 1000 seconds in the lower frame.
The lower frame gives the maximum fisher value (signal likelihood) as function of time. The time axis zero time is 2001, July 29 06h30m20.82s GMT.


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A continuous source of infrasound is present to the southeast of DIA, or the lower right panel in the slowness contour plot. Voilent eruptions of Mt. Etna are the cause of these coherent infrasound signals. The most coherent waveforms travelled over DIA, around 785 seconds adding up to Fisher values of 10. Around 1000 seconds another source of infrasound can be identified. The energy first appears to the west of DIA, around 270 degrees. The source gradually travels to the south. It appears to be a plane flying at close range from DIA. Microbarom activity can be seen in the upper left panel, or to the northwest of DIA. An unidentified source is present to the east of DIA.

March 2002
Läslo Evers