5pAO8. High-frequency surface backscattering: A 24-h time series of data showing transition between Bragg and resonant bubble scattering.

Session: Friday Afternoon, December 6

Time: 4:00

Author: Peter H. Dahl
Location: Appl. Phys. Lab., Univ. of Washington, Seattle, WA 98195


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 (greater than or equal to) 2.5 m/s, the data are consistent with scattering from resonant-sized bubbles (in this case, ~100 (mu)m 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.]

ASA 132nd meeting - Hawaii, December 1996