Acoustic tomography assessment of the acoustic characteristics of bubble clouds

In the seawater environment, interactions of the rotation of ship propellers with the wind tend to produce masses of localized bubbles. These bubble clouds cause acoustical interference in the acquisition of sonar data during marine surveys and marine exploration. For example, pronounced bubble-attenuation of pressure levels results in acoustic signals received by sonar equipment being below predicted values. In addition, a strong backscattering signal may be detected due to the impedance difference between liquid water and intra-bubble air. These effects distort underwater sonar measurement data. If the acoustic characteristics of a bubble cloud in the seawater environment can be known in advance, more precise measurement data could be obtained through data processing. Thus, the aim of this study was to assess the acoustic characteristics of experimenter-produced bubbles. Acoustic tomography techniques were used to obtain data descriptive of the acoustic characteristics and distribution of bubble clouds. We developed six sets of buoy systems equipped with multiple projectors and hydrophones for acoustic tomography. The buoy systems were installed in a hexagonal arrangement in seawater. A transmitter emitted sequential sound signals into the water in response to radiofrequency-transmitted commands from a control device located on land. Each acoustic signal was recorded by multiple hydrophones. Applying repetitive optimization techniques to the tomography data, it was possible to analyze acoustic characteristics such as transmission loss of signals transmitted through bubble clouds, magnitude of backscattering associated with bubble clouds, and bubble distributions. The acoustic effects and distribution characteristics of bubbles documented in this experiment will be used as foundational data for subsequent research.