Frequency‐dependent influence of the sea bottom on the near‐surface sound field in shallow water

An undulating bottom in shallow water has a significant effect on sound propagation. An acoustic propagation experiment was carried out in the East China Sea in 2020. Measurements along two separate propagation tracks with flat and undulating bottoms were obtained. Abnormal transmission losses (TLs) were observed along the track with the undulating bottom. By using the parabolic equation model RAM and ray theory, these abnormal TLs and the distribution of the sound field energy were analyzed. Numerical simulations indicate that under the shallow water condition with a negative thermocline and for a high frequency (1000 Hz), the incidence and reflection angles of sound rays on the sea bottom are changed due to the undulating sea bottom. The larger the inclination angle of the undulating bottom, the greater the grazing angle changes. These angles changes lead to different sound propagation paths for the undulating bottom and the flat bottom, resulting in the difference of TLs at a certain distance and depth. The undulating bottom will cause energy convergence in the mixed layer when the source and receiver locate above the thermocline.

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ИССЛЕДОВАНИЯ АКУСТИЧЕСКИХ ХАРАКТЕРИСТИК ВЕРХНЕГО СЛОЯ МОРЯ МЕТОДОМ МНОГОЧАСТОТНОГО АКУСТИЧЕСКОГО ЗОНДИРОВАНИЯ

Podvodnye issledovaniia i robototehnika ◽

10.37102/24094609.2020.31.1.006 ◽

2020 ◽

Author(s):

V.A. Bulanov ◽

I.V. Korskov ◽

A.V. Storozhenko ◽

S.N. Sosedko

Keyword(s):

High Frequency ◽

Wave Motion ◽

High Spatial Resolution ◽

Wind Waves ◽

Acoustic Parameter ◽

Sea Surface ◽

Near Surface ◽

Acoustical Properties ◽

Acoustic Spectroscopy ◽

Sea Bottom

Описано применение акустического зондирования для исследования акустических характеристик верхнего слоя моря с использованием широкополосных остронаправленных инвертированных излучателей,устанавливаемых на дно. В основу метода положен принцип регистрации обратного рассеяния и отраженияот поверхности моря акустических импульсов с различной частотой, позволяющий одновременно измерятьрассеяние и поглощение звука и нелинейный акустический параметр морской воды. Многочастотное зондирование позволяет реализовать акустическую спектроскопию пузырьков в приповерхностных слоях моря,проводить оценку газосодержания и получать данные о спектре поверхностного волнения при различных состояниях моря вплоть до штормовых. Применение остронаправленных высокочастотных пучков ультразвукапозволяет разделить информацию о планктоне и пузырьках и определить с высоким пространственным разрешением структуру пузырьковых облаков, образующихся при обрушении ветровых волн, и структуру планктонных сообществ. Участие планктона в волновом движении в толще морской воды позволяет определитьпараметры внутренних волн спектр и распределение по амплитудам в различное время.This paper represents the application of acoustic probingfor the investigation of acoustical properties of the upperlayer of the sea using broadband narrow-beam invertedtransducers that are mounted on the sea bottom. Thismethod is based on the principle of the recording of thebackscattering and reflections of acoustic pulses of differentfrequencies from the sea surface. That simultaneouslyallows measuring scattering and absorption of the soundand non-linear acoustic parameter of seawater. Multifrequencyprobing allows performing acoustic spectroscopy ofbubbles in the near-surface layer of the sea, estimating gascontent, and obtaining data on the spectrum of the surfacewaves in various states of the sea up to a storm. Utilizationof the high-frequency narrow ultrasound beams allows us toseparate the information about plankton and bubbles and todetermine the structure of bubble clouds, created during thebreaking of wind waves, along with the structure of planktoncommunities with high spatial resolution. The participationof plankton in the wave motion in the seawater columnallows determining parameters of internal waves, such asspectrum and distribution of amplitudes at different times.

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A Green’s Function for Acoustic Problems in Pekeris Waveguide Using a Rigorous Image Source Method

Applied Sciences ◽

10.3390/app11062722 ◽

2021 ◽

Vol 11 (6) ◽

pp. 2722

Author(s):

Zhiwen Qian ◽

Dejiang Shang ◽

Yuan Hu ◽

Xinyang Xu ◽

Haihan Zhao ◽

...

Keyword(s):

Green’S Function ◽

Shallow Water ◽

Green's Function ◽

Plane Waves ◽

Spherical Wave ◽

Sound Field ◽

Efficient Computation ◽

Image Source ◽

Source Method ◽

Pekeris Waveguide

The Green’s function (GF) directly eases the efficient computation for acoustic radiation problems in shallow water with the use of the Helmholtz integral equation. The difficulty in solving the GF in shallow water lies in the need to consider the boundary effects. In this paper, a rigorous theoretical model of interactions between the spherical wave and the liquid boundary is established by Fourier transform. The accurate and adaptive GF for the acoustic problems in the Pekeris waveguide with lossy seabed is derived, which is based on the image source method (ISM) and wave acoustics. First, the spherical wave is decomposed into plane waves in different incident angles. Second, each plane wave is multiplied by the corresponding reflection coefficient to obtain the reflected sound field, and the field is superposed to obtain the reflected sound field of the spherical wave. Then, the sound field of all image sources and the physical source are summed to obtain the GF in the Pekeris waveguide. The results computed by this method are compared with the standard wavenumber integration method, which verifies the accuracy of the GF for the near- and far-field acoustic problems. The influence of seabed attenuation on modal interference patterns is analyzed.

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Near-surface frequency-dependent nonlinear damping ratio observation of ground motions using SMART1

Soil Dynamics and Earthquake Engineering ◽

10.1016/j.soildyn.2021.106798 ◽

2021 ◽

Vol 147 ◽

pp. 106798

Author(s):

Chun-Hsiang Kuo ◽

Jyun-Yan Huang ◽

Che-Min Lin ◽

Chun-Te Chen ◽

Kuo-Liang Wen

Keyword(s):

Ground Motions ◽

Damping Ratio ◽

Nonlinear Damping ◽

Frequency Dependent ◽

Near Surface

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A Prediction Method for Acoustic Intensity Vector Field of Elastic Structure in Shallow Water Waveguide

Shock and Vibration ◽

10.1155/2020/5389719 ◽

2020 ◽

Vol 2020 ◽

pp. 1-23

Author(s):

Wenbo Wang ◽

Desen Yang ◽

Jie Shi

Keyword(s):

Spatial Structure ◽

Shallow Water ◽

Transfer Functions ◽

Source Parameters ◽

Prediction Method ◽

Sound Field ◽

Elastic Structure ◽

Superposition Method ◽

Acoustic Intensity ◽

Sound Energy

Compared with scalar sound field, vector sound field explained the spatial structure of sound field better since it not only presents the sound energy distribution but also describes the sound energy flow characteristics. Particularly, with more complicated interaction among different wavefronts, the vector sound field characteristics of an elastic structure in a shallow water waveguide are worthy of studying. However, there is no reliable prediction method for the vector sound field of an elastic structure with a high efficiency in a shallow water waveguide. To solve the problem, transfer functions in the waveguide have been modified with some approximations to apply for the vector sound field prediction of elastic structures in shallow water waveguides. The method is based on the combined wave superposition method (CWSM), which has been proved to be efficient for predicting scalar sound field. The rationality of the approximations is validated with simulations. Characteristics of the complex acoustic intensity, especially the vertical components are observed. The results show that, with constructive and destructive interferences in the depth direction, there could be quantities of crests and vortices in the spatial structure of time-dependent complex intensity, which manifest a unique dynamic characteristic of sound energy. With more complicated interactions among the wavefronts, a structure source could not be equivalent to a point source in most instances. The vector sound field characteristics of the two sources could be entirely different, even though the scalar sound field characteristics are similar. Meanwhile, source types, source parameters, ocean environment parameters, and geo parameters may have influence on the vector sound field characteristics, which could be explained with the normal mode theory.

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Comparison of Sound Propagation Characteristic between Deep and Shallow Water

Applied Mechanics and Materials ◽

10.4028/www.scientific.net/amm.577.1198 ◽

2014 ◽

Vol 577 ◽

pp. 1198-1201

Author(s):

Zhang Liang ◽

Chun Xia Meng ◽

Hai Tao Xiao

Keyword(s):

Shallow Water ◽

Deep Water ◽

Sound Speed ◽

Sound Propagation ◽

Spherical Wave ◽

Sound Field ◽

Propagation Loss ◽

Speed Profile ◽

Sound Speed Profile ◽

Propagation Law

The physical characteristics are compared between shallow and deep water, in physics and acoustics, respectively. There is a specific sound speed profile in deep water, which is different from which in shallow water, resulting in different sound propagation law between them. In this paper, the sound field distributions are simulated under respective typical sound speed profile. The color figures of sound intensity are obtained, in which the horizontal ordinate is distance, and the vertical ordinate is depth. Then we can get some important characteristics of sound propagation. The results show that the seabed boundary is an important influence on sound propagation in shallow water, and sound propagation loss in deep water convergent zone is visibly less than which in spherical wave spreading. We can realize the remote probing using the acoustic phenomenon.

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