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2005
Colosi, JA, Baggeroer AB, Cornuelle BD, Dzieciuch MA, Munk WH, Worcester PF, Dushaw BD, Howe BM, Mercer JA, Spindel RC, Birdsall TG, Metzger K, Forbes AMG.  2005.  Analysis of multipath acoustic, field variability and coherence in the finale of broadband basin-scale transmissions in the North Pacific Ocean. Journal of the Acoustical Society of America. 117:1538-1564.   10.1121/1.1854615   AbstractWebsite

The statistics of low-frequency, long-range acoustic transmissions in the North Pacific Ocean are presented. Broadband signals at center frequencies of 28, 75, and 84 Hz are analyzed at propagation ranges of 3252 to 5171 km, and transmissions were received on 700 and 1400 in long vertical receiver arrays with 35 in hydrophone spacing. In the analysis we focus on the energetic "finale" region of the broadband time front arrival pattern, where a multipath interference pattern exists. A Fourier analysis of 1 s regions in the finale provide narrowband data for examination as well. Two-dimensional (depth and time) phase unwrapping is employed to study separately the complex field phase and intensity. Because data sampling occured in 20 or 40 min intervals followed by long gaps, the acoustic fields are analyzed. in terms of these 20 and 40 min and multiday observation times. An analysis of phase, intensity, and complex envelope variability as a function of depth and time is presented in terms of mean fields, variances, probability density functions (PDFs), covariance, spectra, and coherence. Observations are compared to a random multipath model of frequency and vertical wave number spectra for phase and log intensity, and the observations are compared to a broadband multipath model of scintillation index and coherence. 2005 Acoustical Society of America.

Voronovich, AG, Ostashev VE, Colosi JA, Cornuelle BD, Dushaw BD, Dzieciuch MA, Howe BM, Mercer JA, Munk WH, Spindel RC, Worcester PF, The NPAL Group.  2005.  Horizontal refraction of acoustic signals retrieved from the North Pacific Acoustic Laboratory billboard array data. Journal of the Acoustical Society of America. 117:1527-1537.   10.1121/1.1854435   AbstractWebsite

In 1998-1999, a comprehensive low-frequency long-range sound propagation experiment was carried out by the North Pacific, Acoustic Laboratory (NPAL). In this paper, the data recorded during the experiment by a, billboard acoustic array were used to compute the horizontal refraction of the arriving acoustic signals using both ray- and mode-based approaches. The results obtained by these two approaches are consistent. The acoustic signals exhibited weak (if any) regular horizontal refraction throughut most of the experiment. However, it increased up to 0.4 deg (the sound rays were bent towards the south) at the beginning and the end of the experiment. These increases occurred during midspring to midsummer time and seemed to reflect seasonal trends in the horizontal gradients of the sound speed. The measured standard deviation of the horizontal refraction angles was about 0.37 deg, which is close to an estimate of this standard deviation calculated using 3D modal theory of low-frequency sound propagation through internal gravity waves. (c) 2005 Acoustical Society of America.

Andrew, RK, Howe BM, Mercer JA, Group NPAL, Cornuelle B, Colosi J.  2005.  Transverse horizontal spatial coherence of deep arrivals at megameter ranges. Journal of the Acoustical Society of America. 117:1511-1526.   10.1121/1.1854851   AbstractWebsite

Predictions of transverse horizontal spatial coherence from path integral theory are compared with measurements for two ranges between 2000 and 3000 km. The measurements derive from a low-frequency (75 Hz) bottom-mounted source at depth 810 m near Kauai that transmitted m-sequence signals over several years to two bottom-mounted horizontal line arrays in the North Pacific. In this paper we consider the early arriving portion of the deep acoustic field at these arrays. Horizontal coherence length estimates, on the order of 400 m, show good agreement with lengths calculated from theory. These lengths correspond to about 1 degrees in horizontal arrival angle variability using a simple, extended, spatially incoherent source model, Estimates of scintillation index, log-amplitude variance, and decibel intensity variance indicate that the fields were partially saturated. There was no significant seasonal variability in these measures. The scintillation index predictions agree quite well with the dataset estimates; nevertheless, the scattering regime predictions (fully saturated) vary from the regime classification (partially saturated) inferred from observation. This contradictory result suggests that a fuller characterization of scattering regime metrics may be required. (c) 2005 Acoustical Society of America.

1999
Colosi, JA, Scheer EK, Flatte SM, Cornuelle BD, Dzieciuch MA, Munk WH, Worcester PF, Howe BM, Mercer JA, Spindel RC, Metzger K, Birdsall TG, Baggeroer AB.  1999.  Comparisons of measured and predicted acoustic fluctuations for a 3250-km propagation experiment in the eastern North Pacific Ocean. Journal of the Acoustical Society of America. 105:3202-3218.   10.1121/1.424650   AbstractWebsite

During the Acoustic Engineering Test (AET) of the Acoustic Thermometry of Ocean Climate (ATOC) 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 (PDF) 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 0.74) for late-arriving timefronts and (2.0 dB)(2) (with a scintillation index of 0.2) 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-Smimov test rejects both distributions. The variance of the nonidentified peaks is (3.5 dB)(2) land 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 (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 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 Lambda-Phi methods for predicting the wave propagation regime predict full saturation. (C) 1999 Acoustical Society of America. [S0001-4966(99)04606-8].

Colosi, JA, Grp A.  1999.  A review of recent results on ocean acoustic wave propagation in random media: Basin scales. Ieee Journal of Oceanic Engineering. 24:138-155.   10.1109/48.757267   AbstractWebsite

Measurements of basin-scale acoustic transmissions made during the last four years by the Acoustic Thermometry of Ocean Climate (ATOC) program have allowed for the study of acoustic fluctuations of low-frequency pulse propagation at ranges of 1000 to 5000 km, Analysis of data from the ATOC Acoustic Engineering Test conducted in November 1994 has revealed new and unexpected results for the physics of ocean acoustic wave propagation in random media, In particular, use of traditional Lambda, Phi methods (using the Garrett-Munk (GM) internal wave model) to identify the wave propagation regime for early identifiable wavefronts predict the saturated regime, whereas observations of intensity probability density functions, intensity variance, and pulse time spread and wander suggest that the propagation is more likely near the border between the unsaturated and partially saturated regimes. Calculations of the diffraction parameter Lambda are very sensitive to the broad-band nature of the transmitted pulse, with CW calculations differing from a simplistic broad-band calculation by 10(3)! A simple model of pulse propagation using the Born approximation shows that CW and broad-band cases are sensitive to a random medium very differently and a theoretical description of broad-band effects for pulse propagation through a random media remains a fundamental unsolved problem in ocean acoustics. The observations show that, at 75-Hz center frequency, acoustic normal mode propagation is strongly nonadiabatic due to random media effects caused by internal waves. Simulations at a lower frequency of 28 Hz suggest that the first few modes might be treated adiabatically even in a random ocean. This raises the possibility of using modal techniques for ocean acoustic tomography, thereby increasing the vertical resolution of thermometry. Finally, the observation of unsaturated or partially saturated propagation for 75-Hz broad-band transmissions, like those of ATOC, suggests that ray-based tomography will be robust at basin-scales. This opens up the possibility of ray-based internal wave tomography using the observables of travel time variance, and vertical and temporal coherence, Using geometrical optics and the GM internal wave spectrum, internal wave tomography for an assortment of parameters of the GM model can be formulated in terms of a mixed linear/nonlinear inverse, This is a significant improvement upon a Monte Carlo approach presented in this paper which is used to infer average internal wave energies as a function of depth for the SLICE89 experiment. However, this Monte Carlo approach demonstrated, for the SLICE89 experiment, that the GM model failed to render a consistent inverse for acoustic energy which sampled the upper 100 m of the ocean, Until a new theory for the forward problem is advanced, internal wave tomography utilizing the signal from strong mode coupling can only be carried out using time-consuming Monte Carlo methods.