Backscattered Acoustic Energy by Monobubbles Experimental Approach and Statistical Study of the Attenuation

Abstract

Problem statement: As the number of air bubbles in the sea is very high, they are so many acoustic diffusers who make illegible the recordings the purpose of which is to quantify the alive bodies. The signals backscattered by air bubbles constitute a parasite in offshore recordings and must be eliminated. It is planned to finalize techniques allowing the localization and the identification of a signal backscattered by an air bubble. Once this type of signal was localized and identified on an offshore recording, it is easy to eliminate it. From then, we could have recordings where the only diffusers would be alive bodies like the zooplankton. Approach: We began a work of characterization of signals of bubbles to discriminate between them and those backscattered by alive diffusers. We realized in laboratory a bench test then we finalized an original method of production of air bubbles with known size in a liquid medium. Five types of monobubbles were generated in a water column by a technique using a peristaltic pump. This technique allowed obtaining a continuous water flow carrying same-sized air bubble. The bubbles radii were calculated from the measure of rise limit speed. The acoustic responses of these bubbles (to a frequent wide bandwidth ultrasonic wave) were studied by statistical methods in order to determine the variation of the energy backscattered by a calibrated bubble according to its depth. Results: Besides the production technique of calibrated bulles that was finalized, we established that the variation of backscattered energy according to depth can be explained by simple exponential models which permitted to estimate the constant of absorption. Conclusion: The coming step will be to correct the signal of the effect of the absorption of energy by the middle, then to elaborate a protocol of localization of the signals of bubbles on recordings where multiple diffusers appear. The results had to be refined and adapted for in-situ applications.