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A modern method for acoustic underwater channel characterization is the passive tomography concept. The technique relies on the use of opportunity sources thus avoiding the use of dedicated active sources. As a consequence, one of the main difficulties of the concept is due to the lack of any information about the emitted signals. In this paper, we will propose a time-frequency method in the case of linear frequency modulated signals. For an ideal acoustic channel, the received signal is composed of a sum of shifted chirps that have the same chirp rate and start frequency as the emitted signal. Hence, using the fractional Fourier transform (FRFT), it is possible to express the received signal as a sum of sinusoids. Their frequencies are directly related to the time delays of the arrivals. In order to distinguish the closed arrivals, we apply a high resolution spectral estimation method, providing their time delays. Furthermore, we obtain some information related to their amplitudes by comparing the pseudo and the real spectra. These results can further be used in order to estimate the envelope of the impulse response of the acoustic channel.