Infrasound from the eruptions of Mt. Etna was detected in the Netherlands at the Deelen Infrasound Array (DIA) during the summer of 2001. Zonal cross winds lead to a deviation in the observed back azimuth. An average azimuthal deviation of 2.6 deg is explained by westward stratospheric winds along the infrasonic trajectories through the atmosphere. The signals have signal-to-noise power ratios lower than 0.6. The detectability of Mt. Etna's infrasound is tested on sub-arrays of the 13 elements of DIA. Doing so, sparse arrays used in the International Monitoring System (IMS) are mimicked. Both small aperture sub-arrays of 800 m and large aperture sub-arrays of 1400 m are applied. The results are evaluated on the basis of the number of detections made. The number of detections strongly increases with the number of elements. Small aperture sub-arrays perform significantly better than large aperture sub-arrays. With a factor of two reduction in the aperture, two instruments less can be used to obtain the same number of detections. Larger aperture arrays perform less well because of the loss of spatial coherence of the infrasound. The average accuracy of the detections slightly increased, when the number of elements is increased. The result of the use of more elements is that lower signal-to-noise power ratio events could be detected. The trade-off between more detections and the larger average deviation in back azimuth and apparent sound speed causes the slight increase accuracy. The results were comparable to the detections obtained at IMS array IS26 in Germany. It is concluded that successfully applying infrasound as monitoring technique strongly depends on the array configuration. The frequency versus spatial coherence of signals will play a decisive role in detecting low signal-to-noise ratio events.

Number of detections versus the number of elements for large aperture sub- arrays of DIA, circles, and IS26, square at five elements. The number of detections for small aperture sub-arrays are represented by the triangles.

Reference:
The detectability of infrasound in the Netherlands from the Italian volcano Mt. Etna L.G. Evers and H.W. Haak, Journal of Atmospheric and Solar-Terrestrial Physics, vol 67 no 3, 259-268, 2005. doi:10.1016/j.jastp.2004.09.002