Calculating the velocity of a fast‐moving snow avalanche using an infrasound array

Abstract

On 19 January 2012, a large D3 avalanche (approximately 103 t) was recorded with an infrasound array ideally situated for observing the avalanche velocity. The avalanche crossed Highway 21 in Central Idaho during the largest avalanche cycle in the 15 years of recorded history and deposited approximately 8 m of snow on the roadway. Possible source locations along the avalanche path were estimated at 0.5 s intervals and were used to calculate the avalanche velocity during the 64 s event. Approximately 10 s prior to the main avalanche signal, a small infrasound signal originated from the direction of the start zone. We infer this to be the initial snow pack failure, a precursory signal to the impending avalanche. The avalanche accelerated to a maximum velocity of 35.9 ± 7.6m s−1 within 30 s before impacting the highway. We present a new technique to obtain high spatial and temporal resolution velocity estimates not previously demonstrated with infrasound for avalanches and other mass wasting events.

Infrasound array criteria for automatic detection and front velocity estimation of snow avalanches: towards a real-time early-warning system

“…The lack of data between 700 and 800 m distance corresponds to a blind area of the Doppler radar field of view, while the drop of velocity at 1150 m distance corresponds to the avalanche moving outside the radar field of view. Recorded velocity for the 23 December 2012 event is in the range commonly reported for snow avalanches (see Havens et al, 2014, for a review).…”

“…Seismic measurements are widely used both for monitoring and research on snow avalanches in many countries worldwide (e.g., Schaerer and Salway, 1980;Kishimura and Izumi, 1997;Leprette et al, 1998, Surinach et al, 2000van Herwijnen and Schweizer, 2011). Seismic observations provide time of occurrence of snow avalanches regardless of the visibility conditions.…”

“…The use of infrasound for avalanche monitoring has been increasing rapidly in the last decades, with significant improvements also on avalanche dynamics research (Bedard, 1989;Chritin et al, 1996;Adam et al, 1998;Comey and Mendenhall, 2004;Scott et al, 2007;Ulivieri et al, 2011;Kogelnig et al, 2011;Havens et al, 2014;Thüring et al, 2015). After the initial works with single infrasound sensors (e.g., Bedard, 1989), the use of infrasound arrays has improved significantly the signal-to-noise ratio (e.g., Scott et al 2007;Ulivieri et al, 2011;Havens et al, 2014), thus resulting in a larger efficiency of infrasound in detecting snow avalanches even at larger (few km) distances.…”

“…After the initial works with single infrasound sensors (e.g., Bedard, 1989), the use of infrasound arrays has improved significantly the signal-to-noise ratio (e.g., Scott et al 2007;Ulivieri et al, 2011;Havens et al, 2014), thus resulting in a larger efficiency of infrasound in detecting snow avalanches even at larger (few km) distances. Array processing techniques showed that back azimuth and apparent velocity of infrasound generated by snow avalanches nicely trace the downhill moving front at a source-to-receiver distance of 2 km (Ulivieri et al, 2011) and can be used to evaluate avalanche front velocity (Havens et al, 2014). Recently, a network of three infrasound arrays deployed in three different valleys in Valle d'Aosta, Italy, allowed Ulivieri et al, 2012) to detect and locate a size-3 avalanche at a sourceto-receiver distance of ∼ 20 km.…”

“…Ulivieri et al (2011) tracked the motion of an avalanche front at a source-to-receiver distance of 2 km with infrasonic back azimuth and compared it to video imagery showing that infrasonic back azimuth matches the migration of the avalanche front and could be used to derive infrasound velocity. More recently Havens et al (2014) evaluated instantaneous front velocity of a snow avalanche by applying Fisher statistics of infrasound array observations along the section of the avalanche path. Here we present a procedure to derive automatically the avalanche front velocity directly from infrasound array observations based on the conversion of infrasound back azimuth into position of the moving source of infrasound along a given avalanche path.…”