Monitoring snow avalanches in Northwestern Italian Alps using an infrasound array

Risk assessment of snow avalanches is mostly related to weather conditions and snow cover. However, a robust risk validation requires avalanche activity data, in order to compare predictions to actual events. For this purpose in December 2009 we installed a temporary 4-element, small aperture (150 m), infrasound array in the northwestern Italian Alps. The array was installed south of Mt. Rosa, at an elevation of 2000 m a.s.l. in the valley of Gressoney, Italy, where natural avalanches are expected and avalanche control by explosives is regularly performed. A multi-channel correlation analysis is carried out on the continuous infrasound data set recorded by the array as a function of apparent velocity, back-azimuth and frequency of recorded infrasound. This allowed detectionof infrasonic signals propagating across the array from the background noise. During the 5-month-long experiment, 343 infrasonic events have been detected and characterized. These include sharp infrasonic transients (99 events) produced by explosions during avalanche control as well as longer lasting signals (244 events) possibly caused by avalanches. Although only few of these events were validated by direct avalanche observation, obtained results are promising and encouraging application of infrasound for long-term avalanche observation on wide areas, as the peak in avalanche activity in winter 2009–2010 was observed shortly after the infrasound events peaked as well.