Acoustical measurements of microbubbles within ship wakes

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Abstract
High‐frequency sonarmeasurements of target strength due to microbubbles were obtained within the wakes of three oceanographicvessels. Two self‐contained, high‐frequency acoustics instruments suspended at 25‐m depth were used to measure the wake acoustic properties during three separate sea trials. The backscatter cross section per unit volume, M v , as a function of depth and time was calculated from the echo intensity of six upward‐looking, conical beam sonars (28–400 kHz). Four 100‐kHz steerable sidescans allowed measurement of wake locations, widths, and persistence. In the near‐surface core of the wake M v reached peak values of approximately 0.3 m−1 for the 120‐ and 200‐kHz sonars. The volumetric scattering cross sections were observed to be roughly constant at all frequencies within the top 5–6 m of the wake, suggesting a roughly homogeneous vertical bubble distribution. However, differences in the volumetric backscatter at different acoustical frequencies suggest a higher relative concentration of larger bubbles (≳100 μm) in the center of the wake. The ship wakes were observed to spread to typical widths of up to 66 m (ship speed 10 kn) and to depths of 7–12 m and to persist as strong acoustic scatterers for approximately 7.5 min. Gas diffusion causing bubble dissolution is suggested as the mechanism for decay of the wake bubble clouds.

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