The effect of the balloon on balloon-borne infrasound measurements

Abstract

Deploying acoustic sensors on free-flying, long-living balloons helps to reach the areas not accessible with the traditional ground-based sensors, reduce flow noise, and improve characterization of various infrasound sources. In particular, instrumented balloons can potentially increase the infrasonic detection range and the early warning lead time for natural hazards, such as tornadoes and avalanches. When assessing the capabilities of balloon-borne infrasonic sensors and interpreting the measurements, it is important to recognize that the balloon inevitably distorts both signals and the ambient infrasound field by scattering the incoming sound. Measurement distortions due to a nearby compact scatterer prove to be rather different from the well-understood effect of a rigid boundary on ground-based sensors. Using the recently developed theory of sound scattering by thin, prestressed elastic shells [O. A. Godin, J. Acoust. Soc. Am. 154, 3223–3236 (2023)], this paper quantifies the effects of hot-air and helium balloons on acoustic pressure and oscillatory velocity. It is found that balloon-borne vector sensors are more susceptible to the distortions than pressure sensors, leading to large differences between the apparent and true source bearing and directionality. Possible approaches to compensate for the distortions and retrieve the free-field acoustic quantities will be discussed.