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  • 1
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    Modal analysis of the range evolution of broadband wavefields in the North Pacific Ocean : low mode numbers (2012)
    Acoustical Society of America
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © Acoustical Society of America, 2012. This article is posted here by permission of Acoustical Society of America for personal use, not for redistribution. The definitive version was published in Journal of the Acoustical Society of America 131 (2012): 4409-4427, doi:10.1121/1.4707431.
    Description: The results of mode-processing measurements of broadband acoustic wavefields made in the fall of 2004 as part of the Long-Range Ocean Acoustic Propagation Experiment (LOAPEX) in the eastern North Pacific Ocean are reported here. Transient wavefields in the 50–90 Hz band that were recorded on a 1400 -m long 40 element vertical array centered near the sound channel axis are analyzed. This array was designed to resolve low-order modes. The wavefields were excited by a ship-suspended source at seven ranges, between approximately 50 and 3200 km, from the receiving array. The range evolution of broadband modal arrival patterns corresponding to fixed mode numbers (“modal group arrivals”) is analyzed with an emphasis on the second (variance) and third (skewness) moments. A theory of modal group time spreads is described, emphasizing complexities associated with energy scattering among low-order modes. The temporal structure of measured modal group arrivals is compared to theoretical predictions and numerical simulations. Theory, simulations, and observations generally agree. In cases where disagreement is observed, the reasons for the disagreement are discussed in terms of the underlying physical processes and data limitations.
    Description: This work was supported by the Office of Naval Research, Code 322, Grant Nos. N00014-08-1-0195, N00014-06-1-0245, and N0014-11-1-0194.
    Keywords: Acoustic field ; Underwater acoustic propagation
    Repository Name: Woods Hole Open Access Server
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  • 2
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    A test of basin-scale acoustic thermometry using a large-aperture vertical array at 3250-km range in the eastern North Pacific Ocean (2014)
    Acoustical Society of America
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © Acoustical Society of America, 1999. This article is posted here by permission of Acoustical Society of America for personal use, not for redistribution. The definitive version was published in Journal of the Acoustical Society of America 105 (1999): 3185, doi:10.1121/1.424649.
    Description: Broadband acoustic signals were transmitted during November 1994 from a 75-Hz source suspended near the depth of the sound-channel axis to a 700-m long vertical receiving array approximately 3250 km distant in the eastern North Pacific Ocean. The early part of the arrival pattern consists of raylike wave fronts that are resolvable, identifiable, and stable. The later part of the arrival pattern does not contain identifiable raylike arrivals, due to scattering from internal-wave-induced sound-speed fluctuations. The observed ray travel times differ from ray predictions based on the sound-speed field constructed using nearly concurrent temperature and salinity measurements by more than a priori variability estimates, suggesting that the equation used to compute sound speed requires refinement. The range-averaged oceansound speed can be determined with an uncertainty of about 0.05 m/s from the observed ray travel times together with the time at which the near-axial acoustic reception ends, used as a surrogate for the group delay of adiabatic mode 1. The change in temperature over six days can be estimated with an uncertainty of about 0.006 °C. The sensitivity of the travel times to ocean variability is concentrated near the ocean surface and at the corresponding conjugate depths, because all of the resolved ray arrivals have upper turning depths within a few hundred meters of the surface.
    Description: This work was supported largely by the Strategic Environmental Research and Development Program through Defense Advanced Research Projects Agency ~DARPA! Grant No. MDA972-93- 1-0003. Additional support was provided at SIO by the Office of Naval Research ~ONR! through Grant No. N00014- 97-1-0258. J. Colosi wishes to acknowledge support from an ONR Young Investigator Award, from the J. Lamar Worzel Assistant Scientist Fund, and from the Penzance Endowed Fund in support of scientific staff at WHOI.
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  • 3
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    Deep seafloor arrivals in long range ocean acoustic propagation (2013)
    Acoustical Society of America
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © Acoustical Society of America, 2013. This article is posted here by permission of Acoustical Society of America for personal use, not for redistribution. The definitive version was published in Journal of the Acoustical Society of America 134 (2013): 3307-3317, doi:10.1121/1.4818845.
    Description: Ocean bottom seismometer observations at 5000 m depth during the long-range ocean acoustic propagation experiment in the North Pacific in 2004 show robust, coherent, late arrivals that are not readily explained by ocean acoustic propagation models. These “deep seafloor” arrivals are the largest amplitude arrivals on the vertical particle velocity channel for ranges from 500 to 3200 km. The travel times for six (of 16 observed) deep seafloor arrivals correspond to the sea surface reflection of an out-of-plane diffraction from a seamount that protrudes to about 4100 m depth and is about 18 km from the receivers. This out-of-plane bottom-diffracted surface-reflected energy is observed on the deep vertical line array about 35 dB below the peak amplitude arrivals and was previously misinterpreted as in-plane bottom-reflected surface-reflected energy. The structure of these arrivals from 500 to 3200 km range is remarkably robust. The bottom-diffracted surface-reflected mechanism provides a means for acoustic signals and noise from distant sources to appear with significant strength on the deep seafloor.
    Description: The OBS/Hs used in the LOAPEX field program were provided by Scripps Institution of Oceanography under the U.S. National Ocean Bottom Seismic Instrumentation Pool (SIO-OBSIP, http:// www.obsip.org). The OBS/H deployments themselves were co-funded through direct funding to SIO-OBSIP by the National Science Foundation and by Woods Hole Oceanographic Institution under a grant from the WHOI Deep Ocean Exploration Institute. The LOAPEX source deployments and the moored DVLA receiver deployments were funded by the Office of Naval Research under Award Nos. N00014-03-1-0181 and N00014-03-1-0182. The data reduction and analysis in this paper were funded by the Office of Naval Research under Award Nos. N00014-06-1-0222 and N00014-10-1-0510. Additional post-cruise analysis support was provided to RAS through the Edward W. and Betty J. Scripps Chair for Excellence in Oceanography.
    Keywords: Acoustic arrays ; Acoustic noise ; Acoustic signal processing ; Acoustic wave reflection ; Acoustic wave velocity ; Long-range order ; Ocean waves ; Seafloor phenomena ; Seismometers ; Surface acoustic waves ; Surface energy ; Underwater acoustic propagation
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  • 4
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    Estimating the horizontal and vertical direction-of-arrival of water-borne seismic signals in the northern Philippine Sea (2014)
    Acoustical Society of America
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © Acoustical Society of America, 2013. This article is posted here by permission of Acoustical Society of America for personal use, not for redistribution. The definitive version was published in Journal of the Acoustical Society of America 134 (2013): 3282, doi:10.1121/1.4818843.
    Description: Conventional and adaptive plane-wave beamforming with simultaneous recordings by large-aperture horizontal and vertical line arrays during the 2009 Philippine Sea Engineering Test (PhilSea09) reveal the rate of occurrence and the two-dimensional arrival structure of seismic phases that couple into the deep ocean. A ship-deployed, controlled acoustic source was used to evaluate performance of the horizontal array for a range of beamformer adaptiveness levels. Ninety T-phases from unique azimuths were recorded between Yeardays 107 to 119. T-phase azimuth and S-minus-P-phase time-of-arrival range estimates were validated using United States Geological Survey seismic monitoring network data. Analysis of phases from a seismic event that occurred on Yearday 112 near the east coast of Taiwan approximately 450 km from the arrays revealed a 22° clockwise evolution of T-phase azimuth over 90 s. Two hypotheses to explain such evolution—body wave excitation of multiple sources or in-water scattering—are presented based on T-phase origin sites at the intersection of azimuthal great circle paths and ridge/coastal bathymetry. Propagation timing between the source, scattering region, and array position suggests the mechanism behind the evolution involved scattering of the T-phase from the Ryukyu Ridge and a T-phase formation/scattering location estimation error of approximately 3.2 km.
    Description: This research is supported by the Office of Naval Research, both the Applied Research Laboratory program and Code 322(OA).
    Repository Name: Woods Hole Open Access Server
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  • 5
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    Deep water towed array measurements at close range (2014)
    Acoustical Society of America
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © Acoustical Society of America, 2013. This article is posted here by permission of Acoustical Society of America for personal use, not for redistribution. The definitive version was published in Journal of the Acoustical Society of America 134 (2013): 3230, doi:10.1121/1.4818869.
    Description: During the North Pacific Acoustic Laboratory Philippine Sea 2009 experiment, towed array receptions were made from a towed source as the two ships transited from a separation of several Convergence Zones through a Closest Point of Approach at 3 km. A combination of narrowband tones and broadband pulses were transmitted covering the frequency band 79–535 Hz. The received energy arrives from two general paths—direct path and bottom bounce. Bearing-time records of the narrowband arrivals at times show a 35° spread in the angle of arrival of the bottom bounce energy. Doppler processing of the tones shows significant frequency spread of the bottom bounce energy. Two-dimensional modeling using measured bathymetry, a geoacoustic parameterization based upon the geological record, and measured sound-speed field was performed. Inclusion of the effects of seafloor roughness and surface waves shows that in-plane scattering from rough interfaces can explain much of the observed spread in the arrivals. Evidence of out-of-plane scattering does exist, however, at short ranges. The amount of out-of-plane scattering is best observed in the broadband impulse-beam response analysis, which in-plane surface roughness modeling cannot explain.
    Description: This work was supported by the Office of Naval Research, Ocean Acoustics, under contract N00014-11-M-0170.
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  • 6
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    Weakly dispersive modal pulse propagation in the North Pacific Ocean (2013)
    Acoustical Society of America
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © Acoustical Society of America, 2013. This article is posted here by permission of Acoustical Society of America or personal use, not for redistribution. The definitive version was published in Journal of the Acoustical Society of America 134 (2013): 3386, doi:10.1121/1.4820882.
    Description: The propagation of weakly dispersive modal pulses is investigated using data collected during the 2004 long-range ocean acoustic propagation experiment (LOAPEX). Weakly dispersive modal pulses are characterized by weak dispersion- and scattering-induced pulse broadening; such modal pulses experience minimal propagation-induced distortion and are thus well suited to communications applications. In the LOAPEX environment modes 1, 2, and 3 are approximately weakly dispersive. Using LOAPEX observations it is shown that, by extracting the energy carried by a weakly dispersive modal pulse, a transmitted communications signal can be recovered without performing channel equalization at ranges as long as 500 km; at that range a majority of mode 1 receptions have bit error rates (BERs) less than 10%, and 6.5% of mode 1 receptions have no errors. BERs are estimated for low order modes and compared with measurements of signal-to-noise ratio (SNR) and modal pulse spread. Generally, it is observed that larger modal pulse spread and lower SNR result in larger BERs.
    Description: This work was supported by the Office of Naval Research, Code 322, Grant Nos. N00014-06-1-0245, N00014-08-1-0195, and N00014-11-1-0194.
    Repository Name: Woods Hole Open Access Server
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  • 7
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    Bottom interacting sound at 50 km range in a deep ocean environment (2012)
    Acoustical Society of America
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © Acoustical Society of America, 2012. This article is posted here by permission of Acoustical Society of America for personal use, not for redistribution. The definitive version was published in Journal of the Acoustical Society of America 132 (2012): 2224-2231, doi:10.1121/1.4747617.
    Description: Data collected during the 2004 Long-range Ocean Acoustic Propagation Experiment provide absolute intensities and travel times of acoustic pulses at ranges varying from 50 to 3200 km. In this paper a subset of these data is analyzed, focusing on the effects of seafloor reflections at the shortest transmission range of approximately 50 km. At this range bottom-reflected (BR) and surface-reflected, bottom-reflected energy interferes with refracted arrivals. For a finite vertical receiving array spanning the sound channel axis, a high mode number energy in the BR arrivals aliases into low mode numbers because of the vertical spacing between hydrophones. Therefore, knowledge of the BR paths is necessary to fully understand even low mode number processes. Acoustic modeling using the parabolic equation method shows that inclusion of range-dependent bathymetry is necessary to get an acceptable model-data fit. The bottom is modeled as a fluid layer without rigidity, without three dimensional effects, and without scattering from wavelength-scale features. Nonetheless, a good model-data fit is obtained for sub-bottom properties estimated from the data.
    Description: This work was supported by the Office of Naval Research, Code 322, Grant Nos. N00014- 10-1-0987, N00014-11-1-0194, and N00014-10-1-0510.
    Keywords: Acoustic wave reflection ; Acoustic wave scattering ; Acoustic wave transmission ; Bathymetry ; Parabolic equations ; Uunderwater acoustic propagation
    Repository Name: Woods Hole Open Access Server
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  • 8
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    The North Pacific Acoustic Laboratory deep-water acoustic propagation experiments in the Philippine Sea (2014)
    Acoustical Society of America
    Details
    Publication Date: 2022-05-26
    Description: Author Posting. © Acoustical Society of America, 2013. This article is posted here by permission of Acoustical Society of America for personal use, not for redistribution. The definitive version was published in Journal of the Acoustical Society of America 134 (2013): 3359, doi:10.1121/1.4818887.
    Description: A series of experiments conducted in the Philippine Sea during 2009–2011 investigated deep-water acoustic propagation and ambient noise in this oceanographically and geologically complex region: (i) the 2009 North Pacific Acoustic Laboratory (NPAL) Pilot Study/Engineering Test, (ii) the 2010–2011 NPAL Philippine Sea Experiment, and (iii) the Ocean Bottom Seismometer Augmentation of the 2010–2011 NPAL Philippine Sea Experiment. The experimental goals included (a) understanding the impacts of fronts, eddies, and internal tides on acoustic propagation, (b) determining whether acoustic methods, together with other measurements and ocean modeling, can yield estimates of the time-evolving ocean state useful for making improved acoustic predictions, (c) improving our understanding of the physics of scattering by internal waves and spice, (d) characterizing the depth dependence and temporal variability of ambient noise, and (e) understanding the relationship between the acoustic field in the water column and the seismic field in the seafloor. In these experiments, moored and ship-suspended low-frequency acoustic sources transmitted to a newly developed distributed vertical line array receiver capable of spanning the water column in the deep ocean. The acoustic transmissions and ambient noise were also recorded by a towed hydrophone array, by acoustic Seagliders, and by ocean bottom seismometers.
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