Snow Avalanche Detection and Source Constraints Made Using a Networked Array of Infrasound Sensors

• Published in: Journal of geophysical research. Earth surface 2021-03, Vol.126 (3), p.n/a
• Main Authors: Johnson, J. B., Anderson, J. F., Marshall, H. P., Havens, S., Watson, L. M.
• Format: Article
• Language: English
• Subjects: Acoustics; array analysis; Avalanches; Cliffs; Debris flow; Echoes; Explosions; Gravity; Highways; Infrasound; Landslides; Microphones; Mudflows; Sensor arrays; Snow; snow avalanche; Snow avalanches; Snowstorms; Sound intensity
• ISSN: 2169-9003; 2169-9011
• DOI: 10.1029/2020JF005741

We studied a triggered snow avalanche (∼60 s in duration and with ∼1,100 m run‐out) using a network of infrasound arrays and time‐synced video, with the objective of understanding the relationship between infrasound generation and flow dynamics. Using standard array processing techniques, we compared the infrasound source back azimuths with the avalanche flow path identified by frame‐differenced, geo‐referenced video. Results show that infrasound records begin with direct arrivals followed by echoes from the avalanche‐triggering explosions and these decay within 35 s of the detonations. Subsequent infrasound, which lasts 20–30 s, could then be attributed exclusively to the avalanche. These infrasound detections, and their triangulated source locations, progress downhill over time and the most intense infrasound appears to originate from a steep, mid‐path cliff band, where the avalanche reached speeds in excess of 30 m/s and accelerations of more than 5 m/s2. The recorded infrasound was compared to two candidate source models extracted from video: total flow motion and advancing flow motion. Advancing source locations were compared to acoustic intensity time series using a nonnegative least squares inversion to solve for, and to quantify, time‐varying infrasound source intensity. We observed that certain portions of the flow, most notably the early stages and the end stages (when the powder cloud was expanding and settling) were infrasonically quiet. Plain Language Summary Rapid gravity‐driven flows, such as mud flows, debris flows, and snow avalanches, produce intense infrasound that may be recorded at distances of many kilometers. Infrasound is low‐frequency sounds that are inaudible, but travel long distances efficiently and can be recorded using specialized microphones. This study uses eight microphones distributed around a snow avalanche path to map how the avalanche advances and to quantify which part of the snow avalanche produces the majority of the infrasound. We use a video record of the featured avalanche to confirm findings and determine that much of the sound is created as the avalanche accelerates over a steep cliff band. Lessons learned here can be used to better monitor snow avalanche activity elsewhere including highways that are threatened by avalanches following snowstorms. Key Points Infrasound source intensity for a distributed moving source is calculated from a network of arrays Video and infrasound observations are jointly used to tri