Multi-frequency Acoustic Measurements of Near-Surface Ari Bubbles in Lake Biwa

Abstract

Using an instrument with six vertical sonars (29 to 397 kHz) in Lake Biwa, Japan during August and September of 1993, measurements were made of acoustic backscattering due to microscopic air bubbles created by wave breaking processes. Due to the limited acoustic frequency range, these methods were focused on bubble sizes from 8 to 120um radius. From measurements of volume scattering strength at six frequencies, depth profiles of bubble size spectral densities, acoustic attenuation, and void fraction were extracted. In a typical example with 9m⋅sec^-1 winds, the volume scattering strengths reached peaks near 1.0m^-1 for the 51 to 397 kHz sonars at 0.45 m depth, with peak bubble spectral densities of 1.4×10⁶m^-3 (re 1μm spectral bins) near bubble radii of 16um. At depths from 0.4 to 1.7m, bubble size spectra decreased exponentially with increasing radius with e-folding scale of 10μm. Additionally, bubble concentrations were found to decrease exponentially with depth at all sizes, with averaged e-folding depth scale of 0.26 ±0.05 m. These freshwater bubble size spectral densities contained much smaller quantities of larger bubbles (>80μm radius) than oceanic spectra found in the literature. Finally, time-averaged bubble penetration depths were found to increase roughly parabolically with wind speed above a threshold of 4m⋅sec^-1, to typical maximum penetrations of 1.8 to 2.2 m under winds of 10 m⋅sec^-1.