Focusing of sound pulses using the time reversal technique on 100-km paths in a deep sea

Numerical and analytical studies are performed on how unstable fluctuations of the parameters of the medium in a deep sea affect the focusing of sound pulses using the time reversal method. The simplest situation, when point sources and receivers are used for emission and reception, is considered. Pulse propagation in the direct and backward directions is numerically simulated by the parabolic equation method. Calculations are performed for sound signals with frequencies of several tens of hertz. It is shown that, in the presence of sound velocity fluctuations caused by random internal waves, noticeable attenuation of the field amplitude at the center of the focal spot can be observed beginning from distances of 200 to 400 km. As the central frequency of the pulsed signal increases, the effect of nonstationarity of the perturbation on the focusing is amplified. This phenomenon is explained qualitatively and quantitatively in the geometrical optics approximation.