Performance and experimental evidence of GPR in density estimates of snowpack

Ground probing radar (GPR) and microwave devices can be successfully used to analyse the structure and the density of the snowpack. The performances of standard radar systems for snowpack characterisation are analysed here; the main aim was to assess the reliability of the method for estimating snow density, snowpack thickness and the depth resolution in terms of capability of detecting thin layers. The main relationships between the electrical permittivity and the density of dry-snow are discussed; these relationships are applied to estimate the vertical density profiles inferred from GPR surveys performed at several test sites in the Italian Alps. The data are calibrated and compared with the results obtained from direct density and layer thickness measurements. 1. Foreword The estimation of snow density and the water equivalent is a challenging task in hill-slope investigations. Snow water equivalent determines the amount of snow-melt discharge which is used in runoff modelling, and hence, to design drainage systems with on economic impact, if the snow-melt-water is to be used for power generation. As melted snow water carries pollution from snow dumps, snow density and water equivalent estimation are also related to the contaminant transport modelling. Finally, snow density has a direct effect on the mechanical strength of the snowpack and is, therefore, a parameter that should be considered in forecasting avalanches. The measurements of density are also relevant to validate the data obtained using remote sensing systems. Ground-based surveys, using portable ground-based equipment (TDR - Time domain reflectometry or GPR - Ground Probing Radar), can be useful to validate the information of remote sensing (InSAR) imaging of large areas. The mechanical properties of the snowpack are usually inferred by using conventional approaches to estimate the density, the cohesion and the free (liquid) water content; for instance static penetrometers are adopted to estimate the consistency through the measurement of the depth to which a standard needle penetrates into the snowpack. The measurement of vertical density profiles assumes great importance as spatial changes and time-varying of density along vertical profiles must be considered to estimate the avalanche risk (Conway and Abrahamson, 1984). Furthermore, different degrees of snow metamorphism can be recognised through density estimation. Combined high (using TDR) and low-frequency permittivity measurements are usually performed for continuous snow wetness and snow density determination (Stacheder et al., 2005);