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  • 1
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    Studies of horizontal refraction and scattering of low-frequency acoustic signals using a modal approach in signal processing of NPAL data
    Acoustical Society of America (ASA) ; 2003
    In:  The Journal of the Acoustical Society of America Vol. 113, No. 4_Supplement ( 2003-04-01), p. 2333-2333
    Details
    In: The Journal of the Acoustical Society of America, Acoustical Society of America (ASA), Vol. 113, No. 4_Supplement ( 2003-04-01), p. 2333-2333
    Abstract: In our previous paper [J. Acoust. Soc. Am. 112, 2232], we obtained a time dependence of the horizontal refraction angle (HRA) of acoustic signals propagating over a range of about 4000 km in the ocean. This dependence was computed by processing of acoustic signal s recorded during the North Pacific Acoustic Laboratory (NPAL) experiment using a ray-type approach. In the present paper, we consider the results obtained in signal processing of the same data using a modal approach. In this approach, the acoustic field is represented as a sum of local acoustic modes with amplitudes depending on a frequency and arrival angle. We obtained a time dependence of HRA for a time interval of about a year. Time evolution of HRA exhibits long-period variations which could be associated with seasonal trends in the sound speed profiles. The results are consistent with those obtained by the ray approach. Different horizontal angles within arrivals were impossible to resolve due to sound scattering by internal waves. A theoretical estimate of the angular width of the acoustic signals in a horizontal plane was obtained. It appears to be consistent with the observed variance of HRA data. [Work supported by ONR.] a)J. A. Colosi, B. D. Cornuelle, B. D. Dushaw, M. A. Dzieciuch, B. M. Howe, J. A. Mercer, R. C. Spindel, and P. F. Worcester.
    Type of Medium: Online Resource
    ISSN: 0001-4966 , 1520-8524
    URL: Article
    DOI: 10.1121/1.4780853
    RVK:
    EQ 1000
    Language: English
    Publisher: Acoustical Society of America (ASA)
    Publication Date: 2003
    detail.hit.zdb_id: 1461063-2
    detail.hit.zdb_id: 219231-7
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  • 2
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    Horizontal coherence of tracked arrivals in the North Pacific Acoustic Laboratory98 (NPAL98)
    Acoustical Society of America (ASA) ; 2006
    In:  The Journal of the Acoustical Society of America Vol. 120, No. 5_Supplement ( 2006-11-01), p. 3022-3022
    Details
    In: The Journal of the Acoustical Society of America, Acoustical Society of America (ASA), Vol. 120, No. 5_Supplement ( 2006-11-01), p. 3022-3022
    Abstract: The NPAL98 experiment offered a unique opportunity to study horizontal coherence because of the deployment of a billboard array. The billboard consisted of five vertical line arrays with a 3600-m horizontal aperture listening to a 75-Hz acoustic signal 3500 km away. Original estimates of horizontal coherence were complicated by the bottom-interacting sound field, which made it difficult to track individual arrivals. Thus, previous studies of the horizontal coherence in this experiment were limited to the arrival pattern in aggregate. By using a turning-point filter and a rake correlator, it is now possible to track individual arrivals during the NPAL98 experiment. We can thus measure the coherence of individual early non-bottom-interacting arrivals. Results are compared to recent calculated predictions using both Monte Carlo and path-integral methods for the same environment. [Work supported by ONR.]
    Type of Medium: Online Resource
    ISSN: 0001-4966 , 1520-8524
    URL: Article
    DOI: 10.1121/1.4787104
    RVK:
    EQ 1000
    Language: English
    Publisher: Acoustical Society of America (ASA)
    Publication Date: 2006
    detail.hit.zdb_id: 1461063-2
    detail.hit.zdb_id: 219231-7
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  • 3
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    Observing horizontal wave fronts from the NPAL billboard array data
    Acoustical Society of America (ASA) ; 2001
    In:  The Journal of the Acoustical Society of America Vol. 109, No. 5_Supplement ( 2001-05-01), p. 2385-2385
    Details
    In: The Journal of the Acoustical Society of America, Acoustical Society of America (ASA), Vol. 109, No. 5_Supplement ( 2001-05-01), p. 2385-2385
    Abstract: One of the main objectives of the NPAL experiment is to investigate the horizontal refraction and coherence of the acoustic wave fronts at long range. Given time series of acoustic arrival times and angles of resolved ray arrival arrivals, a detailed look at the acoustic wave fronts is possible. First and second order statistics (density functions and coherences) of the wave fronts are investigated. The wave fronts are shown to vary with time, frequency, depth and across the horizontal aperture.
    Type of Medium: Online Resource
    ISSN: 0001-4966 , 1520-8524
    URL: Article
    DOI: 10.1121/1.4744409
    RVK:
    EQ 1000
    Language: English
    Publisher: Acoustical Society of America (ASA)
    Publication Date: 2001
    detail.hit.zdb_id: 1461063-2
    detail.hit.zdb_id: 219231-7
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  • 4
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    Modeling the acoustic receptions at the NPAL array from the Kauai source
    Acoustical Society of America (ASA) ; 2003
    In:  The Journal of the Acoustical Society of America Vol. 113, No. 4_Supplement ( 2003-04-01), p. 2279-2279
    Details
    In: The Journal of the Acoustical Society of America, Acoustical Society of America (ASA), Vol. 113, No. 4_Supplement ( 2003-04-01), p. 2279-2279
    Abstract: Acoustic transmissions from a 75-Hz source near Kauai to a vertical line array near California were recorded as part of the North Pacific Acoustic Laboratory (NPAL) experiment. Extensive environmental measurements were also performed as part of the experiment and were intended to ensure correspondence between numerical simulations and the data. Despite the availability of this information, the process of identifying the recorded arrivals with predictions has not been a simple one. Since the source is near the seafloor at about 800 m depth, and the depth at the receiver is approximately 1800 m, acoustic interaction with the bathymetry has been explored as a possible complication. Ray simulations that allow for specular reflection from the bottom indicate that fully-refracted and bottom-interacting paths can reach the receiver range (about 3900 km) at similar travel times. The simultaneous presence of both kinds of acoustic energy could contribute to the identification difficulties. A series of parabolic-equation simulations have been performed for different geoacoustic parameters in an attempt to correspond more closely to the data. The sensitivity of the predictions to the method used to extract and interpolate the sound speeds has also been investigated. [Work supported by ONR.] a)J. A. Colosi, B. D. Cornuelle, B. D. Dushaw, M. A. Dzieciuch, B. M. Howe, J. A. Mercer, R. C. Spindel, and P. F. Worcester.
    Type of Medium: Online Resource
    ISSN: 0001-4966 , 1520-8524
    URL: Article
    DOI: 10.1121/1.4780566
    RVK:
    EQ 1000
    Language: English
    Publisher: Acoustical Society of America (ASA)
    Publication Date: 2003
    detail.hit.zdb_id: 1461063-2
    detail.hit.zdb_id: 219231-7
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  • 5
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    Noise field statistics and coherence on the NPAL array
    Acoustical Society of America (ASA) ; 2001
    In:  The Journal of the Acoustical Society of America Vol. 109, No. 5_Supplement ( 2001-05-01), p. 2386-2386
    Details
    In: The Journal of the Acoustical Society of America, Acoustical Society of America (ASA), Vol. 109, No. 5_Supplement ( 2001-05-01), p. 2386-2386
    Abstract: While the NPAL array was primarily deployed to examine the spatial coherence of the Hawaii source, it is also a rich data set for ambient noise studies. Shipping noise, earthquakes and biologics all have been identified in the NPAL data. Moreover, ambient noise coherence is the primary issue in maximizing the SNR output of a sonar system. The first and second order statistics of data from the NPAL ‘‘noise only’’ segments have been analyzed with the following results: (i) There is a wide spread in the peak levels, most likely due to the proximity to shipping lanes. The maximum peak level in the recording band is 117 dB. (ii) Full broadband coherences tend to be low because of the presence of many ships. (iii) If one examines frequency bands of 1–2 Hz, then lines of individual ships can be identified and associated and they are very coherent across NPAL aperture. (iv) Vertical beamforming indicates relatively highly directional spectra at low grazing angles and ‘‘noise notch’’ for the spectra at higher frequencies. Horizontal beamforming has been difficult to implement due to element positioning errors and the large array transit time.
    Type of Medium: Online Resource
    ISSN: 0001-4966 , 1520-8524
    URL: Article
    DOI: 10.1121/1.4744417
    RVK:
    EQ 1000
    Language: English
    Publisher: Acoustical Society of America (ASA)
    Publication Date: 2001
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  • 6
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    On ‘‘ray-like’’ arrivals in the deep-ocean shadow zone of megameter range acoustic transmissions
    Acoustical Society of America (ASA) ; 2001
    In:  The Journal of the Acoustical Society of America Vol. 109, No. 5_Supplement ( 2001-05-01), p. 2385-2386
    Details
    In: The Journal of the Acoustical Society of America, Acoustical Society of America (ASA), Vol. 109, No. 5_Supplement ( 2001-05-01), p. 2385-2386
    Abstract: Receptions of long-range acoustic transmissions by deep hydrophone arrays in the Pacific and Atlantic often have ‘‘ray-like’’ arrivals that occur in the shadow zone of the predicted time front. These ‘‘ray-like’’ arrivals can frequently be identified with the cusps of the predicted time front, but the receivers are up to 750 m below the depth of the cusps. Preliminary calculations show that the observed acoustic energy is not accounted for by errors in the sound speed, leakage of acoustic energy below the cusps as predicted by the full wave equation, or scattering due to internal waves. Data obtained during experiments in the Atlantic and Pacific will be reviewed. Experiments that have been conducted with receivers of vertical line arrays have not had receivers deep enough to observe this phenomena. The effect is seen when bottom-mounted or midwater acoustic sources are used. These data present a number of problems: If the ray paths are wandering all over the water column, why are predictions of ray travel times usually accurate? How does the energy loss associated with these data increase the attenuation of very long-range acoustic transmissions? Without knowing the forward problem, how can these data be used to determine oceanographic information?
    Type of Medium: Online Resource
    ISSN: 0001-4966 , 1520-8524
    URL: Article
    DOI: 10.1121/1.4744412
    RVK:
    EQ 1000
    Language: English
    Publisher: Acoustical Society of America (ASA)
    Publication Date: 2001
    detail.hit.zdb_id: 1461063-2
    detail.hit.zdb_id: 219231-7
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  • 7
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    The North Pacific Acoustic Laboratory (NPAL) experiment.
    Acoustical Society of America (ASA) ; 2001
    In:  The Journal of the Acoustical Society of America Vol. 109, No. 5_Supplement ( 2001-05-01), p. 2384-2384
    Details
    In: The Journal of the Acoustical Society of America, Acoustical Society of America (ASA), Vol. 109, No. 5_Supplement ( 2001-05-01), p. 2384-2384
    Abstract: The North Pacific Acoustic Laboratory program augmented the existing ATOC acoustic network with a sparse, two-dimensional receiving array installed west of Sur Ridge, California, from July 1998 through June 1999, to receive transmissions from the 75-Hz ATOC source north of Kauai. The NPAL array consisted of four 20-element vertical arrays, each with a 700-m aperture, and one 40-element vertical array with a 1400-m aperture. The arrays were deployed transverse to the 3900-km acoustic path from the Kauai source and had a total horizontal aperture of 3600 m. Data collected with the billboard array and the U.S. Navy SOSUS receivers are being used (i) to study the temporal, vertical, and horizontal coherence of long-range, low-frequency resolved rays and modes, (ii) to study 3-D propagation effects, (iii) to examine directional ambient noise properties, and (iv) to improve basin-scale ocean nowcasts via assimilation of acoustic data and other data types into models. In addition to acoustic data, environmental data along the path from the Kauai source to the billboard array were acquired by two oceanographic sub-surface moorings and by two XBT/CTD/ADCP transects along the path. The experiment will be described and some preliminary results presented.
    Type of Medium: Online Resource
    ISSN: 0001-4966 , 1520-8524
    URL: Article
    DOI: 10.1121/1.4744406
    RVK:
    EQ 1000
    Language: English
    Publisher: Acoustical Society of America (ASA)
    Publication Date: 2001
    detail.hit.zdb_id: 1461063-2
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  • 8
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    The terminal problem: Model-data comparisons of basin scale transmissions including effects of shallow bathymetry near source and receiver
    Acoustical Society of America (ASA) ; 2001
    In:  The Journal of the Acoustical Society of America Vol. 109, No. 5_Supplement ( 2001-05-01), p. 2385-2385
    Details
    In: The Journal of the Acoustical Society of America, Acoustical Society of America (ASA), Vol. 109, No. 5_Supplement ( 2001-05-01), p. 2385-2385
    Abstract: Acoustic transmissions on basin scale ranges are being used to determine depth-dependent temperature variability. With travel time being the primary observable, stationary sources and nearly stationary receivers are experimental requirements. This has led to the use of bottom-mounted sources and receivers to reduce travel time variability. The NPAL (North Pacific Acoustics Laboratory) experiment has transmitted broadband acoustic pulses from two bottom-mounted sources near the sound channel axis. Recordings have been taken on the NPAL billboard array, a linear series of five vertical line arrays moored in 1800 m of water near Monterey, CA. Additional recordings have been taken from the SOSUS system throughout the Pacific basin. The effects of the near source and near receiver scattering are examined. In particular, near source scattering leads to excess high-angle energy entering deep water with a travel time delay of nearly 1 s due to the low group speeds of high-angle rays/modes in shallow water. We also compare the energetics of the arriving rays that have bounced once on the rising seafloor near the NPAL receivers. Comparisons of models and data for bottom interacting acoustics lead us to the perennial issue of geoacoustic parameters.
    Type of Medium: Online Resource
    ISSN: 0001-4966 , 1520-8524
    URL: Article
    DOI: 10.1121/1.4744411
    RVK:
    EQ 1000
    Language: English
    Publisher: Acoustical Society of America (ASA)
    Publication Date: 2001
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  • 9
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    Analysis of mode coherence and intensity at megameter ranges
    Acoustical Society of America (ASA) ; 2001
    In:  The Journal of the Acoustical Society of America Vol. 109, No. 5_Supplement ( 2001-05-01), p. 2385-2385
    Details
    In: The Journal of the Acoustical Society of America, Acoustical Society of America (ASA), Vol. 109, No. 5_Supplement ( 2001-05-01), p. 2385-2385
    Abstract: The low-order modes constitute some of the most energetic arrivals at long ranges. Understanding fluctuations of these mode arrivals is crucial to their use as observables in matched field processing and tomography. Both simulated and experimental data indicate that at megameter ranges, the low modes have complex arrival patterns due to internal-wave-induced coupling. Analysis of broadband receptions at 3515 km from the ATOC experiment has shown that mode coherence times are on the order of 6 minutes and that centroid statistics provide useful measures of arrival time trends over the course of several months [Wage et al., IEEE Sensor Array and Multichannel Signal Processing Workshop Proceedings, pp. 102–106, 2000] . The North Pacific Acoustic Laboratory (NPAL) experiment presents an opportunity for further research on broadband mode arrivals at megameter ranges. This study examines temporal coherence, intensity variations, and other mode statistics using data from the 40-element NPAL vertical line array. Experimental results are compared with PE simulations of propagation through internal waves of varying strengths, and the impact of the up-slope propagation near the receivers on the mode statistics is discussed.
    Type of Medium: Online Resource
    ISSN: 0001-4966 , 1520-8524
    URL: Article
    DOI: 10.1121/1.4744408
    RVK:
    EQ 1000
    Language: English
    Publisher: Acoustical Society of America (ASA)
    Publication Date: 2001
    detail.hit.zdb_id: 1461063-2
    detail.hit.zdb_id: 219231-7
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  • 10
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    The effect of bottom interaction on transmissions from the North Pacific Acoustic Laboratory Kauai source
    Acoustical Society of America (ASA) ; 2005
    In:  The Journal of the Acoustical Society of America Vol. 117, No. 3 ( 2005-03-01), p. 1624-1634
    Details
    In: The Journal of the Acoustical Society of America, Acoustical Society of America (ASA), Vol. 117, No. 3 ( 2005-03-01), p. 1624-1634
    Abstract: Acoustic signals transmitted from a 75-Hz broadband source near Kauai as part of the North Pacific Acoustic Laboratory (NPAL) experiment were recorded on an array of receivers near California at a range of 3890 km, and on a vertical line array at a range of 3336 km in the Gulf of Alaska. Because the source is approximately 2 m above the seafloor, and the bottom depth at the receivers near California is approximately 1800 m, acoustic interaction with the bathymetry complicates the identification of the recorded arrivals with those present in numerical simulations of the experiment. Ray methods were used to categorize acoustic energy according to interactions with the sea bottom and surface and to examine the significance of seafloor geometry. A modal decomposition was also used to examine the role of range-dependent bathymetry and to associate the effects on the acoustic field with seafloor features at specific ranges. Parabolic-equation simulations were performed in order to investigate the sensitivity of the received signal to geoacoustic parameters; shear excitations within the seafloor were modeled using a complex-density, equivalent-fluid technique. Incorporation of bottom interaction into models of the propagation enables an identification between experimental and simulated arrivals.
    Type of Medium: Online Resource
    ISSN: 0001-4966 , 1520-8524
    URL: Article
    DOI: 10.1121/1.1854491
    RVK:
    EQ 1000
    Language: English
    Publisher: Acoustical Society of America (ASA)
    Publication Date: 2005
    detail.hit.zdb_id: 1461063-2
    detail.hit.zdb_id: 219231-7
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