Ground-based exploding balloon noise propagating over a grass-covered field

Abstract

With ground-based, impulsive acoustic sources, the initial blast wave can be followed by additional noise that is difficult to interpret. As outlined by Embelton [J. Acoust. Soc. Am. 100 (1996)], a porous ground can lead to creeping, ground, and surface waves. In a recent test on a grass-covered field, the blast noise from exploding balloons had evidence of a secondary arrival that in some cases was larger than the blast wavefront. The balloons were filled with oxy-acetylene gas and placed on the ground or in holes. The balloons were ignited, and the sound from the resulting explosions were measured at distances of 100, 130, and 160 m. At each of these stations, microphones were placed at four heights: 0.01, 1.2, 2.4, and 3.6 m. For every explosion, the blast wave amplitude increases with height, while the secondary arrival amplitude decreases with height. This variation in height can help identify the type of wave responsible for the secondary arrival. For example,the amplitude of surface waves propagating over grass exponentially decay with height. This study will help distinguish different types of acoustic signals produced by ground-based explosions, such as the Volcano Hazards Workshop in 2018.