Low-frequency scattering from submerged bubble clouds

Abstract

Preliminary results are presented from a recent experiment carried out at the NUWC Seneca Lake test facility and designed to investigate the scattering properties of bubble clouds produced in fresh water in the absence of boundaries and under known propagation conditions.The test range consisted of both conventional and parametric sources, conventional receivers, and a transient bubbler that was submerged to 91 m. The cloud possessed an elongated teardrop shape (lengthm 1.4 m, diameters0.5 m) with a void fraction of 0.25% and a mean bubble radius of 1.5 mm, which corresponds to a single-bubble resonance frequency of 6.6 kHz at depth. Backscatter target strengths were measured for frequencies ranging from 300 Hz to 14 kHz. These measurements revealed high target strengths (up to + I dB) and distinctive peaks in the spectrum at 450 Hz, and 1.3, 3.0, 6.5, and 11 kHz. The low-frequency results are consistent with calculations (based on the theory of collective bubble oscillations) of the resonance frequency and target strength of a comparably sized, spherical bubble cloud. Certain features of the high-frequency backscatter data are consistent with single-bubble resonance scattering. These preliminary results are consistent with the notion that, at low frequencies, the scattering characteristics of a bubble cloud are determined, not by the bubble sizes and number density, but rather by the free-gas volume fraction and the length scales of the cloud proper.

The Freely-oscillating Bubble

“…Roy et al [226] eliminated such boundary effects when they investigated the low-frequency (< 2 kHz) scattering of incident sound from roughly cylindrical bubble clouds injected at 91.4-m depth into Seneca Lake, New York (130 m of fresh water). Both conventional and parametric sources were used to generate the sound.…”

On bubble clouds produced by breaking waves: An event analysis of ocean acoustic measurements

“…Subsequent field measurements at a depth of 20 cm [Lamarre and Melville, 1992] were consistent with the laboratory results. Finally, several theories have been proposed to explain measurements in the 20-Hz to 2-kHz range that show enhanced acoustic scattering from the sea surface under sea state conditions characterized by wave breaking (compare, for example, Henyey [1991], McDonald [1991], and Roy et al [1992]).…”

The effect of wind-generated bubbles on sea-surface backscattering at 940 Hz

“…Alternative lowfrequency backscattering mechanisms are Bragg scattering at bubble plumes 6,12,13,36 and coupled oscillations between bubbles in bubble clouds. 14-16 Gauss et al 13 succeeded in obtaining a good match with the CST data, and Roy et al 16 compare theoretical predictions to measurements acquired in Lake Seneca. It is, however, difficult to assess the relative importance of each mechanism individually, as each approach uses data-derived tuning parameters.…”

“…(1), (3), (16), (19)- (23). The value of p is determined by the value that yields the minimum value of the objective function.…”

“…6,10-13 Another proposed mechanism for low frequency scattering from bubble clouds involves coupled oscillations between bubbles. [14][15][16] Our hypothesis is that scattering from individual resonant bubbles may contribute significantly to the total surface backscattering strength, at or around 1 kHz. This is motivated by the observation of bubbles with larger than 1 mm radius generated by breaking waves in lab exeperiments [17][18][19][20][21] and at sea.…”