Comparisons of measured and predicted acoustic fluctuations for a 3250-km propagation experiment in the eastern North Pacific Ocean

During the Acoustic Engineering Test (AET) of the Acoustic Thermometry of O cean Climate (ATOC) program, acoustic signals were transmitted from a broad band source with 75-Hz center frequency to a 700-m-long vertical array of 2 0 hydrophones at a distance of 3252 km receptions occurred over a period of -six days. Each received pulse showed early identifiable timefronts, follow ed by about 2 s of highly variable energy. For the identifiable timefronts, observations of travel-time variance, average pulse shape, and the probabi lity density function (PDF) of intensity are presented, and calculations of internal-wave contributions to those fluctuations are compared to the obse rvations. Individual timefronts have rms travel time fluctuations of 11 to 19 ms, with time scales of less than 2 h. The pulse time spreads are betwee n 0 and 5.3 ms rms, which suggest that internal-wave-induced travel-time bi ases are of the same magnitude. The PDFs of intensity for individual ray ar rivals are compared to log-normal and exponential distributions. The observ ed PDFs are closer to the log-normal distribution, and variances of log int ensity are between (3.1 dB)(2) (with a scintillation index of 0.74) for lat e-arriving timefronts and (2.0 dB)(2) (with a scintillation index of 0.2) f or the earliest timefronts. Fluctuations of the pulse termination time of t he transmissions are observed to be 22 ms rms. The intensity PDF of noniden tified peaks in the pulse crescendo are closer to a log-normal distribution than an exponential distribution, but a Kolmogorov-Smimov test rejects bot h distributions. The variance of the nonidentified peaks is (3.5 dB)(2) lan d the-scintillation index is 0.92. As a group, the observations suggest tha t the propagation is on the border of the unsaturated and partially saturat ed regimes. After improving the specification of the. ray weighting functio n, predictions of travel-time variance using the Garrett-Munk (GM) internal -wave spectrum at one-half the reference energy are in good agreement with the observations, and the one-half GM energy level compares well with XBT d ata taken along the transmission path. Predictions of pulse spread and wave propagation regime are in strong disagreement with the observations. Pulse time spread estimates are nearly two orders of magnitude too large, and La mbda-Phi methods for predicting the wave propagation regime predict full sa turation. (C) 1999 Acoustical Society of America. [S0001-4966(99)04606-8].