| This Thesis analyzes the refocusing properties of time reversed acoustic waves propagating in the ocean. Time reversal is based on the time invariance of the wave equation. In a time reversal experiment a signal is emitted from a localized source, is recorded by an array of detectors, and is then time reversed and sent back into the medium. Time reversion means that what is recorded last is sent back first and vice versa. The striking property of time reversal is that the backpropagated signal refocuses much more tightly at the location of the original source term when propagation occurs in heterogeneous media rather than in homogeneous media. This phenomenon has many applications in medicine, nondestructive testing, imaging, wireless communication and underwater acoustics.;This thesis provides theoretical explanations for experimental results obtained in the Mediterranean sea by W. Kuperman and al. from 1996 to 2000. The aim of these experiments was to demonstrate the advantages of time reversal in underwater communication. One of the most relevant aspects of these experiments was the different behavior of the refocused signal for high and low frequencies. Whereas low frequency refocusing is stable in time, high frequency refocusing is not. After recalling generic properties of time reversal, wave propagation in the ocean, and the model of radiative transfer, the main contributions of this thesis is a theoretical and numerical analysis of time reversal refocusing in both the high frequency (3.5KHz) and low frequency (445Hz) regimes considered in the experiments. We have obtained a remarkable agreement between numerical simulations and theoretical predictions for these behaviors. This theory qualitatively explains the experimental results obtained by the experiments. |
