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

Abstract

During the Acoustic Engineering Test ͑AET͒ of the Acoustic Thermometry of Ocean Climate ͑ATOC͒ program, acoustic signals were transmitted from a broadband source with 75-Hz center frequency to a 700-m-long vertical array of 20 hydrophones at a distance of 3252 km; receptions occurred over a period of six days. Each received pulse showed early identifiable timefronts, followed by about 2 s of highly variable energy. For the identifiable timefronts, observations of travel-time variance, average pulse shape, and the probability density function ͑PDF͒ of intensity are presented, and calculations of internal-wave contributions to those fluctuations are compared to the observations. 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 between 0 and 5.3 ms rms, which suggest that internal-wave-induced travel-time biases are of the same magnitude. The PDFs of intensity for individual ray arrivals are compared to log-normal and exponential distributions. The observed PDFs are closer to the log-normal distribution, and variances of log intensity are between (3.1 dB) 2 ͑with a scintillation index of 0.74͒ for late-arriving timefronts and (2.0 dB) 2 ͑with a scintillation index of 0.2͒ for the earliest timefronts. Fluctuations of the pulse termination time of the transmissions are observed to be 22 ms rms. The intensity PDF of nonidentified peaks in the pulse crescendo are closer to a log-normal distribution than an exponential distribution, but a Kolmogorov-Smirnov test rejects both distributions. The variance of the nonidentified peaks is (3.5 dB) 2 and the scintillation index is 0.92. As a group, the observations suggest that the propagation is on the border of the unsaturated and partially saturated regimes. After improving the specification of the ray weighting function, predictions of travel-time variance using the Garrett-Munk ͑GM͒ 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 data 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 ⌳-⌽ methods for predicting the wave propagation regime predict full saturation.

A test of basin-scale acoustic thermometry using a large-aperture vertical array at 3250-km range in the eastern North Pacific Ocean

“…11 because the predictions would have even shorter travel times, making the measured minus predicted travel times more positive. Colosi et al ͑1999͒ find that the observed travel-time variances for the experiment described here are consistent with predictions assuming that the internal-wave field has one-half of the standard GarrettMunk energy level. Using this energy level would reduce the biases in the simulations from Ϫ10 to Ϫ5 ms. Further, the observed pulse spreads are only 0-5 ms ͑Colosi et al, 1999͒.…”

Effects of internal waves on low frequency, long range, acoustic propagation in the deep ocean

“…Second, for both regimes it has been shown that the edges of the shadow zones of the wavefront are significantly extended in depth, and in time [19,20][26]This extension of the shadow zone shows that the effect of scattering in long range low frequency ocean acoustic propagation is to introduce a significant bias into the wavefront intensity pattern; that is to say, the acoustic fluctuations cannot be considered a zero mean effect superimposed upon an otherwise deterministic wavefront pattern. Finally, in spite of the large fluctuations in the wavefront finale the time stability of the phase is surprisingly large and close to that of the wavefront region [22,79,30]. Observed coherence times are between 5 and 15 minutes.…”

“…These observations provide significant quantification of space-time scales of ocean sound to Gaussian random noise [22,23]. Second, for both regimes it has been shown that the edges of the shadow zones of the wavefront are significantly extended in depth, and in time [19,20][26]This extension of the shadow zone shows that the effect of scattering in long range low frequency ocean acoustic propagation is to introduce a significant bias into the wavefront intensity pattern; that is to say, the acoustic fluctuations cannot be considered a zero mean effect superimposed upon an otherwise deterministic wavefront pattern.…”

“…In the SLICE89 experiment it was discovered that acoustic fluctuations were much stronger than previously predicted, especially for acoustic energy which traveled within a few hundred meters vertically from the sound-channel axis [25]. The Acoustic Thermometry of Ocean Climate (ATOC) Acoustic Engineering Test (AET) [22,23,83], were qualitatively similar to SLICE89 and the AET signals also showed surprising vertical and temporal coherence for the early ray-like arrivals which were far in excess of the then currently predicted values of [44]. Furthermore, the AET showed pulse time spreading to be lower than predictions [22], and intensity fluctuations were slightly larger than predicted by weak fluctuation theory [71,23,22].…”

“…The long-range (3250km) transmission data has been analyzed and results are described in a series of papers [22,23,83]. For the 87 km transmission data (addressed in this thesis), the arrival pattern of these transmissions consists of two time resolved and identifiable wavefronts, one with an initially downwards ray angle and two lower turning points (LTP) and one UTP, to be referred to as ID -3, and the other arrival with an initially upwards ray angle and two LTP's and two UTP's, to be referred to as ID +4.…”