Topographic Effects of Infrasound Wave Propagation

Abstract:

Infrasound radiation, which is routinely recorded for volcano dynamics research and volcano monitoring, is directly correlated with physical processes occurring at a volcanic vent. Past study at Volcán Villarrica, Chile, for instance, has been used to determine volcanic activity levels, including the depth to a fluctuating lava lake. Villarrica has an open vent and active lava lake, which is an excellent source of powerful and continuous infrasound, making it ideal for studies of the topographic effects on propagation. One of the research goals of our January 2020 expedition to Villarrica was to use the reliable and continuous infrasound radiation from Villarrica to determine the influence of intervening topography on the amplitude and spectral content of infrasound data recorded far from the vent. To address this question, we installed a 17-sensor, 2-km linear array perpendicular to the source-receiver propagation path and ~13 km distant. The 17 sites represent varying degrees of topographic interference, from hills located between the sensors and the volcanic vent. To assess the effects of diffraction over these hills, we utilized the Fresnel number and relate it to the spectral variations across the array of sensors. The data show a correlation between infrasound character and topographic obstruction. This study explores the impacts of topography and discusses how it may influence the placement of future installations for volcano monitoring applications.