Mv. Trevorrow et al., ACOUSTICAL MEASUREMENTS OF MICROBUBBLES WITHIN SHIP WAKES, The Journal of the Acoustical Society of America, 95(4), 1994, pp. 1922-1930
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.