Speed and attenuation of acoustic waves in snow: Laboratory experiments and modeling with Biot's theory

Achille Capelli,Jagdish C Kapil,Ingrid Reiweger,Dani Or,Jürg Schweizer

doi:10.1016/j.coldregions.2016.01.004

Abstract

Monitoring acoustic emissions (AE) prior to imminent failure is considered a promising technique for assessing snow slope instability. Gaps in elastic wave propagation characteristics in snow hinder quantitative interpretation of AE signals. Our study focuses on characterizing the propagation of acoustic reference signals in the ultrasonic range across cylindrical snow samples with varying density (240–450kgm−3). We deduced the acoustic attenuation coefficient within snow by performing experiments with different column lengths to eliminate possible influences of the snow-sensor coupling. The attenuation coefficient was measured for the entire burst signal and for single frequency components in the range of 8 to 35kHz. The acoustic wave propagation speed, calculated from the travel time of the acoustic signal, varied between 300ms−1 and 950ms−1, depending on the density and hardness of snow. From the sound speed we also estimated the Young's modulus of our snow samples; the values of the modulus ranged from 30 to 340MPa for densities between 240 and 450kgm−3. In addition, we modeled the sound propagation for our experimental setup using Biot's model for wave propagation in a porous medium. The model results were in good agreement with our experimental results and suggest that our acoustic signals consisted of Biot's slow and fast waves. Our results can be used to improve the identification and localization of acoustic emission sources within snow in view of assessing snow slope instability.

Highlights

Snow avalanches present a significant hazard for human activity and infrastructure in snow-covered mountainous regions world-wide, yet, to date, the precise time and location of a single avalanche event remain unpredictable (e.g., McClung and Schaerer, 2006; Schweizer, 2008)
The source pencil lead fracture (PLF) signal, measured before propagation in snow by the sensor S1, had a very sharp rising to the maximum amplitude and a fast decrease, while the amplitude of the signal recorded with the sensor S2 exhibited a more gradual increase and decrease (Fig. 5a, c)
The values of the Young's modulus derived from the speed measurements were in good agreement with the data published by Mellor (1975); they were lower but correlated with the results obtained from micro-computed tomography images (Köchle and Schneebeli, 2014)

Summary

Introduction

Snow avalanches present a significant hazard for human activity and infrastructure in snow-covered mountainous regions world-wide, yet, to date, the precise time and location of a single avalanche event remain unpredictable (e.g., McClung and Schaerer, 2006; Schweizer, 2008). When piezoelectric sensors in direct contact to the snow were used, the obtained speed was higher than the sound speed in air and increased with increasing density (solid symbols, Reiweger et al, 2015; Smith, 1965; Takei and Maeno, 2004; Yamada et al, 1974). On the other hand, the speed was measured with an impedance tube (Buser, 1986; Ishida, 1965; Marco et al, 1998) or with microphones not in direct contact with the snow's ice skeleton (Gudra and Najwer, 2011; Iwase et al, 2001; Lee and Rogers, 1985; Oura, 1953), the measured speed was lower than the speed in air and decreased with increasing density (open symbols, Fig. 1)

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