Sound Propagation In Shallow Water Research Articles

High-resolution observations of shallow-water acoustic propagation with distributed acoustic sensing.

Distributed acoustic sensing (DAS), converting fiber-optic cables into dense acoustic sensors, is a promising technology that offers a cost-effective and scalable solution for long-term, high-resolution studies in ocean acoustics. In this paper, the telecommunication cable of Martha's Vineyard Coastal Observatory (MVCO) is used to explore the feasibility of cable localization and shallow-water sound propagation with a mobile acoustic source. The MVCO DAS array records coherent, high-quality acoustic signals in the frequency band of 105-160 Hz, and a two-step inversion method is used to improve the location accuracy of DAS channels, reducing the location uncertainty to ∼2 m. The DAS array with refined channel positions enables the high-resolution observation of acoustic modal interference. Numerical simulations that reproduce the observed interference pattern suggest a compressional speed of 1750 m/s in the sediment, which is consistent with previous in situ geoacoustic measurements. These findings demonstrate the long-term potential of DAS for high-resolution ocean acoustic studies.

Low frequency inversion method based on multi-layer holizontal variation shallow seafloor model

Sound propagation in shallow water is significantly influenced by geoacoustic properties. Estimating these geoacoustic parameters is essential for sound field analysis and sonar performance assessment. As a common practice, the seafloor is often treated as a single-layer or two-layer range-independent geoacoustic model to reduce the number of involved parameters. However, acoustic parameters inverted through these two geoacoustic models are typically limited in their applicability to a specific frequency range, thus posing challenges when applied across a broader frequency range. A range-dependent multi-layer geoacoustic model based on experimental measurements obtained with a sub-bottom profiler is proposed in this study. The inversion scheme combines three inversion methods to estimate geoacoustic parameters, considering the different sensitivities of geoacoustic parameters to different physical parameters within the acoustic field. Firstly, modal dispersion is used to invert the geoacoustic parameters of each layer, with the dispersion curve obtained through warping transform and the Wigner-Ville distribution. After that, both the localization using matched field processing and the dispersion curve fitting demonstrate the effectiveness of the inversion results for each layer, although the peak of the probability distribution of sound speed in the first layer is broader than in others. Secondly, matched field processing is employed to invert the geoacoustic parameters of the first layer. This method is based on the theory that as frequency increases, the depth of sound rays penetrating the seabed decreases, revealing changes in the first layer's sound speed with the seabed depth. Lastly, bottom attenuation coefficients at different frequencies are inverted by the transmission loss (TL), and a fitting relationship between the attenuation coefficient and the frequency is derived. The inversion results obtained by using the range-dependent multi-layer geoacoustic model are compared with results estimated by the single-layer geoacoustic model. The findings indicate that the transmission loss (TL) error from the range-dependent multi-layer geoacoustic model in this study is smaller than that from the single-layer geoacoustic model, especially in the lower frequency band. The range-dependent multi-layer geoacoustic model proves to be suitable for a broader frequency range, providing better precision in explaining various acoustic phenomena.

Study on pile driving and sound propagation in shallow water using physics-informed neural network

A novel, effective, and precise physical-informed neural network (PINN) is presented in this paper to learn and investigate pile driving and sound propagation in shallow water. The validation of the proposed PINN framework is proved by comparing analytical and predicted displacements of the pile and underwater sound pressure radiated from pile driving. The optimization of the PINN algorithms is studied in detail by tuning the hyperparameters. And then, a fast computing PINN framework with good inverse-solving abilities is constructed. A suggestion to help the neural network learn partial differential equations better by adding non-zero boundary conditions is put forward. It is shown that the neural network made up of a small number of neurons, a shallow network with a few layers and the activation of the sine function is reliable and satisfying. The identification of unknown parameters in the PINN framework is carried out and discussed via various tests, and the PINN method accurately predicts the solution of motion of pile driving and approximates the underwater sound pressure in shallow water for a wide range of frequencies and propagation distance. The advantage of the proposed PINN method is that it is a powerful tool to deal with inverse problems and is independent of the frequency and the distance.

Seismic airgun sound propagation in shallow water of the East Siberian shelf and its prediction with the measured source signature

Seismic airgun sound was measured with an autonomous passive acoustic recorder as a function of distance from 18.6 to 164.2 km in shallow water (<70 m) at the continental shelf of the East Siberian Sea in September 2019. The least-square regression curves were derived in the zero-to-peak sound pressure level, sound exposure level, and band level in a frequency range between 10 and 300 Hz using the initial amplitude scaled from the near-field hydrophone data. In addition, propagation modeling based on the parabolic equation with the measured source spectrum was performed for range-dependent bathymetry, and the results were compared with the band level of the measurements. The sediment structure of the measurement area was a thin layer of iceberg-scoured postglacial mud overlying a fast bottom with high density based on grounding events of past ice masses. The observed precursor arrivals, modal dispersion, and rapid decrease in spectrum level at low frequencies can be explained by the condition of the high-velocity sediment. Our results can be applied to studies on the inversion of ocean boundary conditions and measurement geometry and basic data for noise impact assessment.

Comparison of semi-analytical and numerical methods for the scattering of acoustic waves by rough, sloped seabed geometries

The seabed possesses variations in its material properties, layering structure, slope and roughness making prediction, and analysis of sound propagation in shallow water highly complex. For certain frequency ranges, the effect of roughness becomes significant. Numerous models have been adopted to account for the scattering of acoustic waves by rough surfaces, including Kirchhoff approximation (for relatively large scales of roughness), small perturbation theory (for relatively small scales of roughness), and small slope approximation, which bridges across the two regimes under certain conditions. Although grounded in analytical theory, these approaches invariably involve the computation of numerical integrals so additional approximation theories have been explored including stationary phase methods and Taylor series approximations. In this study, these semi-analytical methods are incorporated within a new sound propagation model and compared with a numerical graphics processing unit (GPU)-accelerated finite element model incorporating roughness parameters (root mean square height and correlation length) for rough, sloped seabed geometries (with gradients ranging from 0° to 45°) in shallow water environments. The effect of large- and small-scale roughness on sound propagation is investigated in two- and three-dimensional space. The finite element modelling is used to validate the optimal choice of semi-analytical method for the different environments considered.

The Effects of Bubble Scattering on Sound Propagation in Shallow Water

As sea waves break, a bubble layer forms beneath the sea surface. The bubble scattering affects sound propagation, thus influencing the accuracy of sound field prediction. This paper aims to investigate the effects of bubble scattering on the statistical characteristics of the sound field, the distribution of transmission loss (TL), and the average scattering attenuation in shallow water. A bubble layer model based on the bubble spectrum and a parallel Parabolic Equation (PE) model are combined to calculate and analyse the sound field in the marine environment with bubbles. The effects of the bubble layer are then compared with those of the fluctuant sea surface. The results show that the bubble scattering causes additional energy loss and random fluctuations of the sound field. The TL distribution properties and the average scattering attenuation are related to the wind speed, range, frequency, and source position relative to the negative gradient sound speed layer in shallow water. The comparison demonstrates that the random variation caused by the fluctuation of the sea surface is more significant than that caused by bubbles, and the energy loss caused by bubble scattering is more significant than the fluctuant sea surface under strong wind conditions.

Sequential Parameter Estimation of Modal Dispersion Curves with an Adaptive Particle Filter in Shallow Water: Experimental Results

An adaptive particle filter method is presented for performing sequential geoacoustic estimation of a shallow water acoustic environment using the explosive sound sources. This approach treats environmental parameters and source–receiver distance as unknown random variables that evolve as the source moves. As a sequential estimation method, this approach reduces the expense of computation than genetic algorithm and yields results with the same accuracy. Comparing with standard Particle filter, proposed method can adjust control parameters to adapt to a rapidly changing environment. This approach is demonstrated on the shallow water sound propagation data which was collected during the ASIAEX 2001 experiment. The results indicate that the geoacoustic parameters are well estimated and source–receiver distance are also well determined.

Sound propagation in shallow water with periodic rough bottom

The rough sea bottom has a large effect on underwater acoustic propagation and underwater acoustic detection applications. By using the typical shallow water environment from the Yellow Sea, the acoustic propagation characteristics under the condition of both periodic rough sea bottom and strong negative thermocline layer are systematically analyzed by using the parabolic equation model RAM (where RAM stands for range-dependent acoustic model) and ray theory. For a low-frequency and short-range acoustic source, the transmission loss (TL) increases up to about 5–30 dB due to the existence of the periodic rough bottom. Abnormal TLs and pulse arrival structures with different source depths, different periods and heights of the rough bottom are analyzed and summarized. Specifically, when the period of the rough bottom is constant, TL increases with the height of the rough bottom increasing. When the height of the rough bottom is constant, the effect of the rough bottom on the sound propagation becomes smaller with the increase of the period. The mechanism of the TL difference caused by rough bottom is explained by using the ray theory. The incidence and reflection angle of the sound ray on the sea bottom are changed due to the periodic rough bottom, which makes small grazing angles of some of the rays incident at sea bottom become large grazing angles, and the bottom loss increases. On the other hand, the change of the reflection angle increases the number of ray interaction with the sea bottom, causing the reversion propagation. Therefore, the energy of the sound field will attenuate with range increasing. The influence of the periodic rough bottom on the sound pulse propagation is mainly reflected in the energy conversion between sound rays (or normal modes) with different angles, the increasing of energy attenuation of some sound rays with large angles, and the decreasing of multipath structure. The change of the arrival time and relative amplitude of the multipath structure affect the frequency spectrum of the sound field, which will affect the performance of the method based on matching field localization. Most of existing studies focus on the influence of the change in large scale sea bottom topography on the sound field, but there are few studies on small scale periodic sea bottom fluctuations, and the relevant summary of the law of sound propagation is lacking. When sonar is used in the actual shallow water environment, more attention should be paid to the influence of the periodic rough bottom. In addition, the present research results also have important reference significance for the spatial accuracy of surveying and mapping of sea bottom topography.

Effect of Sound-Speed Inhomogeneities in Sea Bottom on the Acoustic Wave Propagation in Shallow Water

The effect of volume inhomogeneities in the sea bottom upper layer (sediments) on the long-range sound propagation in shallow water is analyzed. The experimental basis for developing inhomogeneous bottom models are the data of 3D seismic survey on the spatial structure of sediments in the Kara Sea, where variations of ~10% in the average sound speed in the bottom are observed at ranges of ~1 km. The mode coupling due to bottom volume inhomogeneities is revealed within the normal mode approach. Coupled mode equations are derived not only for propagating modes but also for leaky modes and quasimodes; including these types of modes may be urgent in analysis of the waveguides where the sound speed in the bottom (c1) is less or approximately equal to the sound speed in water (c). Numerical simulations show that the coupling between energy-carrying modes is maximum at c1 ≈ c. Calculations of the attenuation of depth-averaged sound field intensity in a water layer, performed for the Kara Sea at ranges of up to 5 km, show that the mode coupling due to bottom volume inhomogeneities can be neglected, and the depth-averaged transmission loss can be calculated using the adiabatic approximation at frequencies below a few hundred hertz. When analyzing the depth-averaged intensity, the effects of mode coupling on bottom inhomogeneities can be observed only by implementing special conditions, for example, using mode arrays providing maximally pure (free of other modes) radiation of a specified mode.

Effects of rough surface on sound propagation in shallow water**Project supported by the National Natural Science Foundation of China (Grant Nos. 11434012, 11874061, and 41561144006).

Underwater acoustic applications depend critically on the prediction of sound propagation, which can be significantly affected by a rough surface, especially in shallow water. This paper aims to investigate how randomly fluctuating surface influences transmission loss (TL) in shallow water. The one-dimension wind-wave spectrum, Monterey–Miami parabolic equation (MMPE) model, Monte Carlo method, and parallel computing technology are combined to investigate the effects of different sea states on sound propagation. It is shown that TL distribution properties are related to the wind speed, frequency, range, and sound speed profile. In a homogenous waveguide, with wind speed increasing, the TLs are greater and more dispersive. For a negative thermocline waveguide, when the source is above the thermocline and the receiver is below that, the effects of the rough surface are the same and more significant. When the source and receiver are both below the thermocline, the TL distributions are nearly the same for different wind speeds. The mechanism of the different TL distribution properties in the thermocline environment is explained by using ray theory. In conclusion, the statistical characteristics of TL are affected by the relative roughness of the surface, the interaction strength of the sound field with the surface, and the changes of propagating angle due to refraction.

Geoacoustic inversion of 3.5-kHz towed receiver data from the KOREX-17

The Korea Reverberation Experiment 2017 (KOREX-17) was conducted in a shallow water located in the south of the Geoje island, Korea. During the experiment, sound propagation measurements were made using a 3.5-kHz CW signal along different 2 tracks to a distance of ~10 km from a bottom-mounted source, ARMS (Autonomous Reverberation Measurement System). The signals were received by a SRH (Self Recording Hydrophone), which was towed at a depth of ~20 m during the measurements. The sound speed profile was almost iso-velocity, and the sediment at the site was mainly composed of silt having a mean grain size of 6 phi. Since the sound propagation in shallow water is greatly influenced by sound interaction with the sediment, geoacoustic inversion was tried using a genetic algorithm based matched field processing in which the measured acoustic pressure field was compared to the simulated field predicted by a parabolic-equation based propagation model (RAM). The results are compared to the geoacoustic parameters obtained by the empirical relationship to mean grain size and a sediment layering information obtained by a chirp sonar survey. [Work supported by Agency for Defense Development, Korea (UD170014DD).]The Korea Reverberation Experiment 2017 (KOREX-17) was conducted in a shallow water located in the south of the Geoje island, Korea. During the experiment, sound propagation measurements were made using a 3.5-kHz CW signal along different 2 tracks to a distance of ~10 km from a bottom-mounted source, ARMS (Autonomous Reverberation Measurement System). The signals were received by a SRH (Self Recording Hydrophone), which was towed at a depth of ~20 m during the measurements. The sound speed profile was almost iso-velocity, and the sediment at the site was mainly composed of silt having a mean grain size of 6 phi. Since the sound propagation in shallow water is greatly influenced by sound interaction with the sediment, geoacoustic inversion was tried using a genetic algorithm based matched field processing in which the measured acoustic pressure field was compared to the simulated field predicted by a parabolic-equation based propagation model (RAM). The results are compared to the geoacoustic parameters ob...

Measurements and modeling of mid-frequency propagation loss during the Korea Reverberation Experiment 2017 (KOREX-17)

Sound propagation in shallow water is significantly influenced by fluctuation of the water medium and scattering from rough ocean boundaries. To study the effect of the intensity fluctuations caused by the various environmental effects, mid-frequency propagation loss measurements along with the ocean environmental measurements were conducted on May 25–31, 2017, in shallow water off Geoje island, as part of the Korea Reverberation Experiment 2017 (KOREX-17). Continuous wave and linear frequency modulated signal with a center frequency of 3.5 kHz were transmitted by the Autonomous Reverberation Measurement System (ARMS). The Self Recording Hydrophone (SRH) was towed by the R/V Mirae at a speed of approximately 3 knots along two different tracks. Sound speed profiles were measured using the moored CTD chain near the source location, covering the water column between 3 and 50 m. CTD casts were also conducted at the beginning and end of each track. In this talk, the fluctuations of measured propagation losses were presented for both tracks. Finally, the effects of the sound speed variations on the propagation loss will be discussed in comparison with the model predictions obtained using the measured sound speed profiles.

Sound Propagation in Shallow Water with an Inhomogeneous GAS-Saturated Bottom

We present the methods and results of numerical experiments studying the low-frequency sound propagation in one of the areas of the Arctic shelf with a randomly inhomogeneous gas-saturated bottom. The characteristics of the upper layer of bottom sedimentary rocks (sediments) used in calculations were obtained during a 3D seismic survey and trial drilling of the seafloor. We demonstrate the possibilities of substituting in numerical simulation a real bottom with a fluid homogeneous half-space where the effective value of the sound speed is equal to the average sound speed in the bottom, with averaging along the sound propagation path to a sediment depth of 0.6 wavelength in the bottom. An original technique is proposed for estimating the sound speed propagation in an upper inhomogeneous sediment layer. The technique is based on measurements of acoustic wave attenuation in water during waveguide propagation.

Three-dimensional shallow water sound propagation and applications toward acoustical oceanography

Underwater sound propagation in areas of the continental shelf, shelf break, and continental slope can encounter strong horizontal reflection, refraction, diffraction, focusing, and/or defocusing due to a variety of environmental factors, including bathymetric variability, water column fluctuations (internal waves and shelf-break fronts), and boundary roughness. Theoretical, numerical, and experimental approaches have been taken to investigate the underlying physics of these three-dimensional (3-D) sound propagation effects and their temporal and spatial variability induced jointly by marine geological features and dynamic oceanographic processes. Some recent work on theoretical analysis, numerical modeling and field work experiments to study 3-D sound propagation in nonlinear internal wave ducts, over the continental slope and in submarine canyons will be reviewed in this talk. The ultimate goal of investing these 3-D sound propagation effects is to improve long-distance acoustical oceanographic technolo...

An overview of the Seabed Characterization Experiment 2017

A multi-institutional, multi-disciplinary ocean acoustics experiment, Seabed Characterization Experiment 2017 (SCE2017), was conducted 95 km south of Martha’s Vineyard, MA, USA, in March and April of 2017. Three research vessels and more than a dozen principle investigators conducted several types of experiments to improve our understanding of the acoustics of fine grained sediments, forward modeling of sound propagation in shallow water over fine grained sediments, and inverse and statistical inference processes used to characterize the seabed in such environments. The measurements included impulsive and tonal source tows received on vertical and horizontal line arrays, direct measurement of sediment bulk acoustic waves and interface waves, and supporting oceanographic observations. The 2017 experiment was preceded by survey cruises in 2016 and 2015 in which more than 200 sediment cores were obtained and detailed sub-bottom profiling was conducted. Here, an overview of the 2017 experiment is presented. [Work supported by ONR.]

Ocean surface wave effect on the spatial characteristics of ambient noise

The ocean ambient noise field experiences a stochastic process of many such noise sources and the respective interactions of their wave fields with the waveguide boundaries. At frequencies of about 1 kHz and higher, forward scattering from surface wave can strongly affect shallow water sound propagation. However, most of the available ambient forecasting models do not consider the effects of multiple forward scattering from surface wave. Therefore, there is a need for an accurate method of predicting ambient noises at middle and high-frequency which can account for surface scatterings. Aiming at such a requirement, a propagation model based on transport theory method is described which yields the second-order moment of the acoustic field. Monte Carlo simulations of acoustic propagation loss are employed to validate the transport theory method. The mode number dependence of mode coupling phenomenon is demonstrated at 1000 Hz via the competing effects of mode coupling and attenuation ranges. Low and middle propagating modes are seen to have a smaller coupling range than the attenuation range, allowing mode coupling effects to take precedence over attenuation effects. The mode energies and the coherences are also examined, and it is found that the mode coupling rate for surface wave is significant, but strongly dependent on mode number. Mode phase randomization by surface waves is found to be dominated by coupling effects. On the basis of transport theory propagation model, connecting with the properties of ambient noise sources, a spatial characteristic model for ambient noise under surface wave is presented. Further, the effects of surface wave on ambient noise intensity, vertical correlation and vertical directionality are analyzed. Simulation results show that the surface wave may result in energy transfer from medium modes to low modes and high modes, the rate of energy transfer depends on the mode energy difference. Since the medium mode plays an important role in noise intensity, the noise intensity decreases with the increase of surface wave. In addition to noise intensity, the vertical correlation of ambient noise also decreases due to mode phase randomization by surface wave. Besides, mode coupling can also lead to a change of vertical beam intensity distribution, positive high-angle beams associated with direct, surface, and bottom-surface-bounced rays become weaker, while negative high-angle beams associated with bottom bounced rays become stronger. Since the vertical directionality is sensitive to surface wave, the model can be applied to ocean surface parameter inversion. In summary, the model provided in this paper is closer to actual ocean waveguide and has future prospect in ocean acoustic engineering application.

Analytical formulas for incoherent transmission loss in shallow water based on effective approximations of seafloor depth and reflectivity

We derive analytical formulas for modelling incoherently averaged transmission losses in shallow water environments by taking into account the effects of energy spreading, in-water absorption, sea surface scattering, and leakage into the seafloor. The formulas are based on the effective depth approximation to include the effect of sound penetration into the seafloor and use effective two-parameter representations of seafloor reflectivity. Comparison with published measurement data at several shallow water sites worldwide shows that the formulas are capable of reproducing the main features of shallow water sound propagation in environments with different types of sediments. The formulas are useful for quick assessments of incoherent transmission losses in generic yet representative underwater environments where detailed environmental knowledge is lacking or the fine details of underwater sound propagation is not required.

Passive acoustic remote sensing in a coastal ocean

Sound propagation in shallow water over ranges large compared to ocean depth involves multiple reflections from the sea surface and seafloor. With the acoustic propagation environment being much more dynamic then in a deep ocean, noise interferometry faces new challenges but can also provide additional insights into physical processes in a coastal ocean. This paper will review the results on passive acoustic characterization of the seafloor and the water column, including ocean currents, that were obtained using the ambient noise data collected in 2012–2103 in the Straits of Florida. [Work supported by NSF and ONR.]

Recording and Analyzing Underwater Noise During Pile Driving for Bridge Construction

Underwater sound generated by pile driving was recorded during the Hong Kong–Zhuhai–Macau Bridge (China) construction in 2014.The important parameters on the source level, the transmission loss, the time-frequency characteristics, and the method of the noise measurement were introduced in this paper. Signal analysis showed that the waveform of pile driving resembles a series of short pulses, composed of many different frequencies with main frequency energy distribution from 100 Hz to 2 kHz; the maximum value occurs at about 200 Hz. The best-fit shallow water sound propagation model gave a root-mean-square source level of 208.2 dB re $$1~\upmu \hbox {Pa}$$ at 1 m, a transmission spreading coefficient of 17.76, and an absorption coefficient of 0.015 dB/m. The value of transmission loss is in good agreement with the results calculated by the Marsh and Schulkin (M&S) semi-empirical expressions above 1 kHz, but is higher than the M&S results under 1 kHz, which may be due to the muddy seabed and turbid sea water in our study area. Although it is difficult to compare our research results with other studies as there is a lack of details on monitoring conditions (including pile size, hammer energy, environmental background, etc.), this paper provides a new method for underwater noise monitoring and analysis for pile driving in shallow water.

The role of water column variability on three-dimensional sound propagation in shallow water

Simultaneous measurements of sound field and the sound speed profile perturbed by passage of internal waves in shallow water and the resulting modal evolution in space-time domain is reported. The waveguide modal behavior for broadband signals centered at 100, 200, and 300 Hz with the bandwidth of 60 Hz during the Shallow Water 2006 (SW06) experiment is studied using an L-shaped hydrophone array. Several cases are considered where the angle of the internal wave front with the acoustic track ranges from −8 to + 83 degrees. Mode filtering obtained from the vertical array and tracking the filtered mode along the horizontal array reveals that individual modes propagating between the source and receiver arrive at different angles. Using the method of vertical modes and horizontal rays the phase of the acoustic field is calculated and the change in the angle of arriving ray at the L-shaped array, that is the horizontal angle of refraction, is obtained. A 3D PE model is used for comparison between the calculations and the experimental data. [Work supported by ONR 322OA.]