The Journal of the Acoustical Society of America · 1994 · 91 citations · 0 references
Natural SciencesOceanographyPhysicsAcousticsOcean AcousticsUnderwater AcousticsAcoustic MeasurementAcoustic SensingSonar SystemsUltrasonicsUltrasound PhysicsWake Field AnalysisAcoustic Backscatter MeasurementAcoustic PropertiesBubble DistributionBubble Size DistributionExperimental Sea TrialsScattering Cross SectionsSea TrialsVolumetric Scattering
During three sea trials, two self‑contained high‑frequency acoustic instruments at 25 m depth measured wake properties, computing backscatter cross‑section per unit volume from six upward‑looking conical beam sonars (28–400 kHz) and using four 100‑kHz steerable sidescans to map wake location, width, and persistence. High‑frequency sonar measurements revealed that within ship wakes the volumetric backscatter cross‑section peaks at ~0.3 m⁻¹ in the near‑surface core, remains roughly constant across frequencies in the top 5–6 m indicating a homogeneous vertical bubble distribution, but higher backscatter at higher frequencies suggests a larger bubble concentration in the wake center; wakes spread to widths up to 66 m, depths 7–12 m, and persist as strong acoustic scatterers for ~7.5 min, with bubble dissolution via gas diffusion proposed as the decay mechanism.
High-frequency sonar measurements of target strength due to microbubbles 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, Mv, 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 Mv 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.