We present the stability estimation results of the reciprocal acoustic transmission data collected during the tomography experiment performed by Japan Marine Science and Technology Center (JAMSTEC) in 1999 in the Central Equatorial Pacific. Travel-time perturbations due to ocean currents are correspondingly one to two orders of magnitude smaller than travel-time signals due to sound-speed perturbations. It is important to estimate the stability of signals of reciprocal transmission. The standard deviation of the phase of the ray signal is very stable within 130 s. Between reciprocal transmissions, the overall structures of the signal are similar, however, the fine structures are different. The standard deviation of the effective phase is a useful index for determining whether or not the ray exists. The received signal is judged as an arrived ray when the standard deviation of the effective phase of 13 consecutive shots is less than 1 rad. The combination of the amplitude and phase information is effective for observing the ocean structure change.
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