Study On The Acoustic Inversion Of Bubble Size Distribution

High-speed winds can lead to breaking waves, then a bubbly surface layer, which is inherently different from a calm ocean's surface, is formed. Gas bubbles in sea water not only scatter but also absorb the acoustic energy. The bubbles are one of most efficient sound scatters and the source of ocean ambient noise. The bubbles have a strong effect on the sound transmission. Meanwhile, Bubble size distributions in the surface and subsurface layers of the oceans are of considerable importance in physical oceanography. Bubbles play a crucial role in several important air-sea interactive processes. It is important to study the bubble size distribution in the ocean since it contains critical information for understanding sound propagation and ambient noise in the ocean.Both optical and acoustical methods have been used to determine bubble distributions functions at various locations in the ocean. In the thesis, we mainly study how to inverse the bubble size distribution by using acoustical methods. First, the theory of sound scattering and attenuation by air bubbles in sea water will be presented, then we discuss resonant acoustical approximation theory which can be used to inverse the bubble size information, but there are considerable errors in bubble estimation occurs in the larger radius end of the spectrum and in high void fraction. In the thesis, sound speed correction and Andreeva-Weston(AW) model correction are introduced to improve acoustic resonant approximation method. The improved methods are more accurate than resonant approximation method by numerical simulation.The improved resonant approximation method is used to inverse the bubble size distribution by using laboratory measurements. The inversion results of bubble populations and size spectra are summarized and analyzed by using AW model correcting method. And in electrolysis experiments, bubble radius are measured by using optical method, the acoustical inversion results are in good agreement with optical measurements. The bubble size distribution are also inversed from the sea experimental data, which were performed in August and October 2010. At last, the relation between bubble size distribution and wind speed is received based on the sea experimental data, and the wind speed inversed by applying acoustic method are in agreement with measurements of anemometer.