Snow cover properties from impulsive noise propagation measurements

The amplitude and waveform shape of atmospheric acoustic pulses, propagating horizontally over a seasonal snow cover, are profoundly changed by the air forced into the snow pores as the pulses move over the surface, When compared with waveforms propagated above other ground surfaces, this interaction greatly reduces the pulse amplitude and elongates the waveform. A comparison between waveforms from blank pistol shots and those calculated for a rigid porous media model for the snow and ground can be used to determine the snow cover properties, By varying the source and receiver positions during the experimental measurements, the spatial variations in snow properties near the edge of a forest were sampled at the site of the 1995 Norwegian winter blast tests, Just inside the forest, the snow became very shallow because of the warming effect of the trees which absorb and reradiate solar energy. This shallow area was detected by an inversion procedure that automatically matches observed and theoretical waveforms, Solar action and heating also produced snow of higher flow resistivity at the forest edge than was found inside the shaded forest, These acoustically derived measurements were in agreement with direct depth measurements and snow pit observations, This waveform inversion procedure is able to accurately determine snow cover conditions, even in the highly variable region at the edge of the forest, Measurements a few days later in an open field, covered with slush instead of dry snow, showed very little pulse attenuation or waveform changes, because rain and melt water had completely filled the snow pores. (C) 1998 Institute of Noise Control Engineering, [S0736-2501(98)00105-2].