Snow Attenuation of Infrasound Signals and Wind Noise

Infrasound monitoring of geophysical and human-sourced activity, such as explosions, eruptions, earthquakes, and avalanches, may be impacted by burial of sensors in a snowpack. We studied the frequency-dependent attenuation of sound, vertical sound speed, and wind noise reduction in a natural snowpack using earthquakes as a natural source of infrasound. Our experiment used a 2 m vertical distribution of sensors deployed during the 2021-2022 winter in the Sawtooth mountains of Idaho. We recorded 38 earthquakes and their primary infrasound propagating vertically upward through the snow. For these events, average attenuation appeared negligible between 0 and 8 Hz, rose to 0.1 dB/cm at similar to 12 Hz, and remained at this level for higher frequencies. Using timing lags across the vertical profile the speed of infrasound in the snow was measured at 160 +/- 51 m/s, significantly subsonic and indicative of pore-space waves. Finally, we quantified the significant reduction in the amount of background noise measured for snow-buried compared with free-air sensors. The 24 dB of reduction of noise observed for high wind speed conditions was evident with only 20 cm of snow burial and demonstrates the powerful capability of snowpack as a noise reduction filter. This study provides a clearer understanding of how snow burial filters infrasound and minimizes noise levels and has implications for monitoring of both natural and human-made hazards.