A laser ultrasound system to non-invasively measure compression waves in granular ice mixes

Accurate knowledge of snow mechanical properties, including Young's modulus, shear modulus, Poisson's ratio, and density, is critical to many areas of snow science and to snow-related engi- neering problems. To facilitate the assessment of these properties, an innovative non-contacting laser ultrasound system (LUS) has been developed. This system acquires ultrasound waveform data at frequencies ranging from tens to hundreds of kHz in a controlled cold-lab environment. Two different LUS devices were compared in this study to determine which recorded more ro- bust ultrasound in granular ice mix samples. We validated the ultrasound observations with poro-elastic traveltime modeling based on physical and empirical constitutive relationships, com- parison to and replication of previous studies, and the use of other accredited snow property measurement systems, i.e., the SnowMicroPen. For ice mixes, we determined that the PSV-400 Scanning Vibrometer (Polytec GmbH) produces higher quality ultrasonic wavefield observations (i.e. has a better signal-to-noise ratio) than the VibroFlex Fiber Vibrometer (Polytec GmbH) in the lab conditions tested here. Using the PSV-400, we then demonstrate the utility of this new LUS to study the relationship between snow compression-wave speed and density during snow compaction experiments.