Very-small-aperture 3-D infrasonic array for volcanic jet observation at Stromboli Volcano

SUMMARY This study tested a very-small-aperture 3-D (VSA-3D) infrasonic array. A 3-D array is ideal for resolving the back elevation angle (BEL), which has become important in the analysis of volcanic jet noise or geological flows on steep mountain slopes. Although a VSA infrasonic array, with an aperture as small as a few tens of metres, has recently been shown to have a sufficient resolution of the backazimuth (BAZ) of incident signals, its BEL resolution is considered to be poor. We performed a four-element 3-D array experiment with a 20-m aperture and 2-m height at the summit of Stromboli Volcano. We analysed the direction of arrival (DOA) with the MUSIC algorithm as a function of frequency and conducted a cluster analysis for the estimated DOA–frequency functions of eruption signals. As a result, individual infrasonic signals were successfully related to eruptive vents. We also calculated the standard deviation (STD) of the DOAs in each cluster. Of the observed BAZ-STDs and BEL-STDs, 80 per cent were