Simultaneous acoustic and microwave backscattering from the sea surface

Simultaneous and coincident measurements of acoustic and microwave backscatter from the air/sea interface were obtained during Phase II of the SAXON-FPN experiment in December 1992 and again in March 1993. The acoustic and microwave grazing angles were both set to 17 degrees, and the wavelengths were matched, being set to 2.14, 3.00, and 5.66 cm, corresponding to, respectively, acoustic frequencies of 26.5, 50, and 70 kHz and microwave frequencies of 5.3, 10, and 14 GHz. Backscattering cross sections normalized by ensonified area for the acoustic (sigma(0)(a)) and microwave (sigma(0)(m)) returns were determined, and their dependence on wind speed was investigated. The acoustic scattering strength is defined as 10 log(10)(sigma(0)(a)) and the microwave scattering strength is defined as 10 log(10)(sigma(0)(m))-10 log(10)(4 pi). The results of these experiments show that the two scattering strengths are comparable at wind speeds below about 3 m/s but that the acoustic scattering strength increases much faster than the microwave Scattering strength with increasing wind speed until reaching saturation. If these wind-speed dependencies are represented by a power law, U-n, then n is 5-6 for sigma(0)(m) and 2-4 for sigma(0)(m) for wind speeds between 2 and 7 m/s. This difference is ascribed to the effect of bubbles on the acoustic backscatter. The more rapid increase of sigma(0)(a) compared to sigma(0)(m) implies that for our 17 degrees grazing angle acoustic scattering from the surface is negligible at all but the lowest wind speeds. Therefore a simple model is used for bubble scattering to fit the acoustic data as a function of wind speed for all three acoustic frequencies. The bubble densities required to fit the data agree well with previous measurements of near-surface bubble distributions. The model predicts an overshoot of the acoustic scattering strength (above the saturation level) at moderate wind speeds which is clearly seen in the data at 26.5 and 70 kHz. Finally, a composite surface scattering model is utilized for the pure surface scattering component along with the bubble model to predict the wind-speed dependence of the acoustic scattering strength at a 45 degrees grazing angle and compare the results with earlier measurements. (C) 1997 Acoustical Society of America.