Topographically Scattered Infrasound Waves Obeserved on Microbarometer Arrays in the Lower Stratosphere

When an acoustic wave strikes a topographic feature, some of its energy is scattered. Sensors on the ground cannot capture these scattered signals when they propagate at high angles. We report observations of upwardly-scattered acoustic waves prior to refraction back to the ground, intercepting them with a set of balloon-borne infrasound microbarometers in the lower stratosphere over northern Sweden. We show that these scattered waves generate a coda whose presence can be related to topography beneath balloons and low-altitude acoustic ducts. The inclination of the coda signals changes systematically with time, as expected from waves arriving from scatterers successively closer to receivers. The codas are present when a temperature inversion channels infrasound from a set of ground chemical explosions along the ground, but are absent following the inversion's dissipation. Since scattering partitions energy away from the main arrival, these observations imply a mechanism of amplitude loss that had previously been inaccessible to measurement. As such, these data and results allow for a better comprehension of interactions between atmospheric infrasound propagation and the solid earth. Plain Language Summary Sound is a useful tool in monitoring phenomena of both natural and manmade origins. Sound below the limits of human hearing, known as infrasound, can travel very long distances, making it particularly worthwhile for study. Infrasound is typically recorded at ground level. However, vantage points higher in the atmosphere will record sound that cannot be observed at the ground. As a result, experiments where infrasound is recorded on balloons have become more popular. In this study, explosions are recorded at several balloon-based microphones. In some cases, the sound of the explosion at the microphones is followed by additional pulses of sound. We show that the timing of these pulses and the angle they approach the balloon from can be explained by sound bouncing off of the ground. Further, these pulses only occur in the morning, which can be explained by the differences in the atmosphere during the morning and afternoon.

Automated summary

When an acoustic wave encounters a topographic feature, it scatters and loses some of its energy. In this study, we observed the scattered acoustic waves that travel upward before refracting back to the ground. These waves were intercepted by balloon-borne infrasound microbarometers in the lower stratosphere over northern Sweden. The presence of the scattered waves generated a coda, which can be linked to the topography beneath the balloons and low-altitude acoustic ducts. The inclination of the coda signals changed over time, indicating waves arriving from scatterers closer to the receivers. These observations provide insights into the mechanism of amplitude loss due to scattering, which was previously difficult to measure. They contribute to a better understanding of the interaction between atmospheric infrasound propagation and the solid earth.