A higher-order tangent linear parabolic-equation solution of three-dimensional sound propagation

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): EL251-EL257, doi:10.1121/1.4813852.A higher-order square-root operator splitting algorithm is employed to derive a tangent linear solution for the three-dimensional parabolic wave equation due to small variations of the sound speed in the medium. The solution shown in this paper unifies other solutions obtained from less accurate approximations. Examples of three-dimensional acoustic ducts are presented to demonstrate the accuracy of the solution. Future work on the applications of associated adjoint models for acoustic inversions is proposed and discussed.This work was sponsored by the Office of Naval Research under Grant No. N00014-13-1- 0026

3D acoustic propagation through an estuarine salt wedge at low-to-mid-frequencies: Modeling and measurement

Author Posting. © Acoustical Society of America, 2019. 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 146(3),(2019): 1888-1902, doi:10.1121/1.5125258.The estuarine salt wedge presents a dynamic and highly refractive waveguide, the acoustic propagation characteristics of which are controlled by the water column sound speed gradient and boundary interactions. Acoustically, the salt wedge consists of two isospeed layers separated by a thin, three-dimensional (3D), high-gradient layer. The behavior of a broadband (500–2000 Hz) acoustic field under the influence of an estuarine salt wedge in the Columbia River estuary is explored using two 3D acoustic propagation models: 3D rays and 3D parabolic equation. These model results are compared to data collected during the field experiment. Results demonstrate that the dominant physical mechanism controlling acoustic propagation in this waveguide shifts from 3D bottom scatter in a non-refractive waveguide (before the entrance of the salt wedge) to 3D acoustic refraction with minimal bottom interaction in a refractive waveguide (when the salt wedge occupies the acoustic transect). Vertical and horizontal refraction in the water column and out-of-plane scattering by the bottom are clearly evident at specific narrowband frequencies; however, these mechanisms contribute to, but do not account for, the total observed broadband transmission loss.Environmental input to the acoustic models included high resolution bathymetric survey data provided by Guy Gelfenbaum (USGS), and modeled temperature and salinity profiles of the water column provided by Antonio Baptista, Charles Seaton, and Paul Turner at CMOP. The authors thank Derek Olson (NPS) for invaluable assistance with running the 3DPE model on NPS HPC resources. This work was supported by the Office of Naval Research.2020-03-3

Three-dimensional ambient noise modeling in a submarine canyon

Author Posting. © Acoustical Society of America, 2019. 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 146(3), (2019): 1956-1967, doi:10.1121/1.5125589.A quasi-analytical three-dimensional (3D) normal mode model for longitudinally invariant environments can be used to compute vertical noise coherence in idealized ocean environments. An examination of the cross modal amplitudes in the modal decomposition of the noise cross-spectral density shows that the computation can be simplified, without loss of fidelity, by modifying the vertical and horizontal mode sums to exclude non-identical mode numbers. In the special case of a Gaussian canyon, the across-canyon variation of the vertical wave number associated with each mode allows a set of horizontally trapped modes to be generated. Full 3D and Nx2D parabolic equation sound propagation models can also be used to calculate vertical noise coherence and horizontal directionality. Intercomparison of these models in idealized and realistic canyon environments highlights the focusing effect of the bathymetry on the noise field. The absolute vertical noise coherence increases, while the zero-crossings of the real component of the coherence are displaced in frequency when out-of-plane propagation is accounted for.The authors wish to acknowledge Arthur Newhall for his technical support. This work was supported by the Office of Naval Research, Ocean Acoustic Code 322OA, under Grant Nos. N00014-15-1-2629 and N00014-17-1-2692, and by the Natural Sciences and Engineering Research Council of Canada's Research Chair, and Discovery Grant program.2020-03-3

Three-dimensional global scale underwater sound modeling: The T-phase wave propagation of a southern Mid-Atlantic ridge earthquake

Author Posting. © Acoustical Society of America, 2019. 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 146(3), (2019): 2124-2135, doi:10.1121/1.5126010.A three-dimensional (3D) geodesic Cartesian parabolic equation model is utilized to study the propagation of low-frequency underwater sound (5 to 20 Hz), the so-called T-phase wave, triggered by a Southern Mid-Atlantic Ridge earthquake. The sound from the earthquake was recorded at 1050 km from the epicenter by the deep water hydrophones of the Comprehensive Nuclear-Test-Ban Treaty Organization network near Ascension Island. A few hours later and at 8655 km from the epicenter, the hydrophones of the Shallow Water 2006 experiment in the U.S. East coast also registered the sound. Recorded field data showed discrepancies between expected and measured arrival angles indicating the likely occurrence of horizontal sound reflection in the long waveguide journey. Numerical modeling of this T-phase wave propagation across the Atlantic Ocean with realistic physical oceanographic inputs was performed, and the results showed the importance of 3D effects induced by the Mid-Atlantic Ridge and Atlantic Islands. Future research directions, including localization of T-phase wave generation/excitation locations, are also discussed.The authors acknowledge David Bradley and Stephen Nichols for the access and support analyzing CTBTO data. The first author acknowledges the Postdoctoral Scholar Program at the Woods Hole Oceanographic Institution, with funding provided by the University of Haifa. The funding supports for the second author were from the Office of Naval Research, USA, under Grant No. N00014-17-1-2692.2020-03-3

Approximate formulas and physical interpretations for horizontal acoustic modes in a shelf-slope front model

Author Posting. © Acoustical Society of America, 2016. 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 140 (2016): EL20, doi:10.1121/1.4954881.The structure and behavior of horizontal acoustic modes for a three-dimensional idealized model of a shelf-slope front are examined analytically. The Wentzel–Kramers–Brillouin–Jeffreys (WKBJ) method is used to obtain convenient simple expressions and to provide physical insight into the structure and behavior of horizontal modes as trapped, leaky, or transition types. Validity regions for WKBJ expressions in terms of slope and frontal parameters are found, and outside the regions the asymptotic formulas for large order and large argument Hankel functions are used. These combined approximations have very good accuracy as shown by comparisons with numerical solutions for modal shapes and horizontal wavenumbers.This work has been supported by the Office of Naval Research through grants to Rensselaer Polytechnic Institute and to Woods Hole Oceanographic Institution

Estimating the parameter sensitivity of acoustic mode quantities for an idealized shelf-slope front

Author Posting. © Acoustical Society of America, 2018. 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 143 (2018): 706-715, doi:10.1121/1.5022776.The acoustic modes of an idealized three-dimensional model for a curved shelf-slope ocean front [Lin and Lynch, J. Acoust. Soc. Am. 131, EL1–EL7 (2012)] is examined analytically and numerically. The goal is to quantify the influence of environmental and acoustic parameters on acoustic field metrics. This goal is achieved by using conserved quantities of the model, including the dispersion relation and a conservation of mode number. Analytic expressions for the horizontal wave numbers can be extracted by asymptotic approximations and perturbations, leading to accurate and convenient approximations for their parameter dependence. These equations provide the dependence on model parameter changes of both the real horizontal wavenumbers, leading to modal phase speeds and other metrics, and the imaginary parts, leading to modal attenuation coefficients. Further approximations for small parameter changes of these equations characterize the parameter sensitivities and produce assessments of environmental and acoustic influences.This work was supported by the Office of Naval Research under grants to Rensselaer Polytechnic Institute (Grant No. N00014-14-1-0372, which is a Special Research Award in Ocean Acoustics for the first author's Ph.D. degree, and also Grant No. N00014-17-1-2370), and to Woods Hole Oceanographic Institution (Grant No. N00014-11-1-0701), which is a Multidisciplinary University Research Initiative

Quantifying the contribution of ship noise to the underwater sound field

Author Posting. © Acoustical Society of America, 2020. 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 148(6), (2020): 3863-3872, https://doi.org/10.1121/10.0002922.The ambient sound field in the ocean can be decomposed into a linear combination of two independent fields attributable to wind-generated wave action at the surface and noise radiated by ships. The vertical coherence (the cross-spectrum normalized by the power spectra) and normalized directionality of wind-generated noise in the ocean are stationary in time, do not vary with source strength and spectral characteristics, and depend primarily on the local sound speed and the geoacoustic properties which define the propagation environment. The contribution to the noise coherence due to passing vessels depends on the range between the source and receiver, the propagation environment, and the effective bandwidth of the characteristic source spectrum. Using noise coherence models for both types of the sources, an inversion scheme is developed for the relative and absolute contribution of frequency dependent ship noise to the total sound field. A month-long continuous ambient sound recording collected on a pair of vertically aligned hydrophones near Alvin Canyon at the New England shelf break is decomposed into time-dependent ship noise and wind-driven noise power spectra. The processing technique can be used to quantify the impact of human activity on the sound field above the natural dynamic background noise, or to eliminate ship noise from a passive acoustic monitoring data set.The work was funded by Office of Naval Research, Code 32 (Grant No. N00014-17-1-2692 for Y.T. Lin), and the Canada Research Chair program and the Natural Science and Engineering Research Council Discovery program. N. S. would like to thank Transatlantic Ocean System Science and Technology (TOSST) for his graduate fellowship.2021-06-2

Effects of front width on acoustic ducting by a continuous curved front over a sloping bottom

Author Posting. © Acoustical Society of America, 2019. 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 146(3), (2019): 1923-1933, doi:10.1121/1.5125426.The behavior of sound near an ocean front in a region with wedge bathymetry is examined. The front is parameterized as a zone of variation with inshore and offshore boundaries parallel to a straight coastline. The importance of frontal width and frontal sound speed on the ducting of acoustic energy is examined. Previous analytical studies of sound propagation and parameter sensitivity in an idealized wedge environment use an unphysical but convenient single interface front representation, which is here replaced by a continuous sound speed profile. The continuous profile selected is convenient for analytical investigation, but encourages the use of asymptotic approximation methods which are also described. The analytical solution method is outlined, and numerical results are produced with an emphasis on comparing to the single interface front. These comparisons are made to highlight the strengths and weaknesses of the idealized model for capturing the horizontal ducting effects.The authors would like to thank Dr. T. F. Duda for his comments on this work. This work was supported by the Office of Naval Research under grants to Rensselaer Polytechnic Institute (Grant No. N00014-14-1-0372, which is a Special Research Award in Ocean Acoustics for the Ph.D. degree of B.J.D., and also Grant No. N00014-17-1-2370), and to Woods Hole Oceanographic Institution (Grant No. N00014-11-1-0701 which is a Multidisciplinary University Research Initiative, and Grant No. N00014-18-1-2172 which is a Postdoctoral Fellowship for B.J.D.).2020-03-3

Acoustic mode radiation from the termination of a truncated nonlinear internal gravity wave duct in a shallow ocean area

Author Posting. © Acoustical Society of America, 2009. 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 126 (2009): 1752-1765, doi:10.1121/1.3203268.Horizontal ducting of sound between short-wavelength nonlinear internal gravity waves in coastal environments has been reported in many theoretical and experimental studies. Important consequences arising at the open end of an internal wave duct (the termination) are examined in this paper with three-dimensional normal mode theory and parabolic approximation modeling. For an acoustic source located in such a duct and sufficiently far from the termination, some of the propagating sound may exit the duct by penetrating the waves at high grazing angles, but a fair amount of the sound energy is still trapped in the duct and propagates toward the termination. Analysis here shows that the across-duct sound energy distribution at the termination is unique for each acoustic vertical mode, and as a result the sound radiating from the termination of the duct forms horizontal beams that are different for each mode. In addition to narrowband analysis, a broadband simulation is made for water depths of order 80 m and propagation distances of 24 km. Situations occur with one or more modes absent in the radiated field and with mode multipath in the impulse response. These are both consistent with field observations.This work was supported under ONR Grant No. N00014-05-1-0482 and the ONR postdoctoral fellowship award, Grant No. N00014-08-1-0204

Erratum : “Acoustic mode radiation from the termination of a truncated nonlinear internal gravity wave duct in a shallow ocean area” [J. Acoust. Soc. Am. 126, 1752–1765 (2009)]

Author Posting. © Acoustical Society of America, 2010. 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 128 (2010): 947, doi:10.1121/1.3436545