ACOUSTICAL MEASUREMENTS OF MICROBUBBLES WITHIN SHIP WAKES

High-frequency sonar measurements of target strength due to microbubbl es were obtained within the wakes of three oceanographic vessels. 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, , as a function of depth and time was calculated from the echo intensi ty of six upward-looking, conical beam sonars (28-400 kHz). Four 100-k Hz steerable sidescans allowed measurement of wake locations, widths, and persistence. In the near-surface core of the wake M, 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 cons tant at all frequencies within the top 5-6 m of the wake, suggesting a roughly homogeneous vertical bubble distribution. However, difference s in the volumetric backscatter at different acoustical frequencies su ggest a higher relative concentration of larger bubbles (greater than or similar to 100 mu m) 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.