Observation of nonlinear scattering of acoustical waves at a subsurface bubble layer

Abstract

To observe nonlinear acoustic effects in a subsurface bubble layer, two echo sounders working at different frequencies were used. The experiments were performed in the shallow area of the southern part of the Baltic Sea aboard the research vessel ‘‘Oceania.’’ Two echo sounders were run at 30 and 37 kHz; each had a power of about 0.3 kW. The depth of the echo sounders was about 15 m and the intensity of the acoustic signal in the subsurface bubble layer was enough to observe nonlinear effects. The sum and difference frequency components in the spectra of the scattered signal were determined. A theory of such a nonlinear scattering, based on summarizing the signals scattered by separate bubbles, was developed. It was shown that the sum frequency component was due to a noncoherent reverberation in the bubble layer and the difference frequency component was coherent. This component was generated in the process of primary wave propagation and can be received after sea surface reflection. Experimental and theoretical results have shown that the nonlinear scattering can be used to determine bubble density at sea. [Work was supported by ONR.]