High-frequency surface backscattering: A 24-h time series of data showing transition between Bragg and resonant bubble scattering

Abstract

Measurements of acoustic backscattering from the sea surface made continually over a 24-h period are discussed. The frequency was 30 kHz and grazing angle coverage was 15–25 deg. The experiment was conducted in 25 m of water near the Dry Tortugus collection of islands off south Florida. During the experiment, the wind speed varied from less than 1 to 6 m/s in three major cycles. Directional wave spectra, and air and sea temperature were also continually measured. For wind speeds ≥ 2.5 m/s, the data are consistent with scattering from resonant-sized bubbles (in this case, ∼100 μm radius) residing near the surface. These data were inverted for bubble concentration and results compared with other field measurements of subsurface bubble concentration. For wind speeds <2.5 m/s, the data are consistent with scattering from wind-generated centimetric waves, or Bragg scattering. The 24-h time series of surface backscattering demonstrates the transition between centimetric Bragg, and resonant bubble scattering modalities in high-frequency surface scattering. [Work supported by ONR Codes 322OM and 3210A.]