Ocean acoustic tomography (OAT) is a powerful approach for observation of mesoscale ocean fluctuations. A precise estimation of travel time is essential for measuring ocean current using OAT. Generally, a reciprocal propagation sound waveforms that are transmitted at the same time are similar mutually, and it is considered that their correlation is high. However, we found that reciprocal sound waveforms sometimes become different in the continuous estimation of travel time difference. In this paper, we present the estimation results of the correlation between the reciprocal transmissions. The correlation coefficient of signals depends on the signal-to-noise (SN) ratio of the signals. We propose an estimation method for the correlation of the reciprocal signals without the effect of SN ratio. Using the method, we estimate the variation of the correlation of reciprocal propagated sound transmissions for a distance of 500 km. A low correlation between the reciprocal propagated sound waveforms occurs at a certain rate. These phenomena seem to occur at a short periodical time.
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