Time evolution of a nonequilibrium polariton condensate has been studied in the framework of a microscopic approach. It has been shown that due to polariton-polariton scattering a significant condensate depletion takes place in a comparatively short time inter-

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Bubbles in a subsurface ocean layer play a significant role in underwater acoustics and oceanography. Many works have been devoted to different methods of bubble density measurement. Among them, the nonlinear methods of bubble sizing based on combination acoustic scattering have a very high selectivity. These methods were utilized in several types of bubble counters developed in the Institute of Applied Physics for the last decade. In the devices the difference and the sum frequency waves of the incident ones generated by a bubble in the working volume are detected. Another approach to the problem based on the parametric generation of subharmonic of the incident wave frequency was also tested. The results of the measurements of bubble size distribution and density in the subsurface ocean layer and in the wakes of ships are given in the paper. Simulteneous measurements by different linear and nonlinear methods were done to make a comparison between them. The results of linear acoustic echosounding of a subsurface bubble cloud from an underwater research platform are also presented. Possibilities and advantages of linear and nonlinear methods of spectroscopy of structure and movements in a subsurface layer are discussed. [Work supported by RFBR (Grants 94-05-16755, 95-05-15632).]

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