Ocean Acoustic Environment Research Articles

Numerical simulation of wave propagation in ice-covered ocean environments based on the equivalent-source method

Accurate modeling of sound propagation in ice-covered ocean environments can help with interpreting discrepancies between predictions and experimental observations in the changing Arctic Ocean; this is advantageous for environmental conservation, resource exploration, and naval applications. Building on the recent development of the equivalent-source (ES) method (ESM), herein, an ESM-based sub-ice model (ESM-SUBICE) is presented for wave propagation in an ice-covered ocean acoustic environment. The presented model solves exact governing equations for acoustic–elastic propagation in an ice-covered waveguide by expressing the wave solution in terms of a field superposition produced by several sets of ESs. Their unknown amplitudes are solved by strictly enforcing additional ice-layer boundary conditions. ESM-SUBICE achieves high efficiency using a water–seabed Green's function to automatically satisfy the boundary conditions at this interface. By further dividing the ocean environment into layers, ESM-SUBICE is extended for more general situations including stratified sound-speed structures and seabed range dependencies. ESM-SUBICE is benchmarked against a finite-element model, and it is found to produce high-quality solutions with high efficiency. Transmission-loss predictions for elastic, fluid, and free-surface ice representations in different ocean environments are compared to examine the effect of ice elasticity on propagation and scattering. The results suggest that the fluid representation is adequate for deep-water environments where the seabed is soft and the surface duct effect is insignificant; otherwise, for accurate predictions, the ice elasticity should be considered.

Effects of modelling assumptions of complex ocean floor topography on marine life

Passive acoustic monitoring (PAM) is an important technique to assess the presence of marine mammals and, if necessary, mitigate the effects of anthropogenic noise sources upon them. The complexity of the ocean acoustic environment makes accurate localisation of marine mammal vocalisations difficult. It is, therefore, important to be able to predict the detection and localisation performance of PAM to ensure sensors are optimally placed and any resulting actions are suitably informed. A variety of acoustic models can be used to predict the performance of a sensor field ranging from simple 1D models up to full 3D models. Each type of model has advantages and disadvantages. Simple 1D models offer the most advantages in terms of solution time and minimal inputs but their predictions can have reduced accuracy. This is especially true in areas of complex ocean floor topography, such as on a shelf break, where complex 3D acoustic effects can occur. This study investigates the variation in the predicted acoustic field from 1D up to 3D models on a shelf break and how with increasing model fidelity the picture of the acoustic environment changes and its impact on acoustic monitoring advice. © Crown copyright (2022), Dstl.

Adaptive ship-radiated noise recognition with learnable fine-grained wavelet transform

Analyzing the ocean acoustic environment is a tricky task. Background noise and variable channel transmission environment make it complicated to implement accurate ship-radiated noise recognition. Existing recognition systems are weak in addressing the variable underwater environment, thus leading to disappointing performance in practical application. In order to keep the recognition system robust in various underwater environments, this work proposes an adaptive generalized recognition system — AGNet (Adaptive Generalized Network). By converting fixed wavelet parameters into fine-grained learnable parameters, AGNet learns the characteristics of underwater sound at different frequencies. Its flexible and fine-grained design is conducive to capturing more background acoustic information (e.g., background noise, underwater transmission channel). To utilize the implicit information in wavelet spectrograms, AGNet adopts the convolutional neural network with parallel convolution attention modules as the classifier. Experiments reveal that our AGNet outperforms all baseline methods on several underwater acoustic datasets, and AGNet could benefit more from transfer learning. Moreover, AGNet shows robust performance against various interference factors.

A Novel FDTD–PML Scheme for Noise Propagation Generated by Biomimetic Flapping Thrusters in the Ocean Environment

Biomimetic flapping-foil thrusters can operate efficiently while offering desirable levels of thrust required for the propulsion of a small vessel or an Autonomous Underwater Vehicle (AUV). These systems have been studied both as main propulsion devices and for augmenting ship propulsion in waves. In this work, the unsteady hydrofoil loads are used to calculate the source terms of the Ffowcs Williams–Hawkings (FW-H) equation which is applied to model noise propagation in the underwater ocean acoustic environment. The solution provided by a simplified version of the Farassat formulation in free space is extended to account for a bounded domain and an inhomogeneous medium, characterizing the sea acoustic waveguide. Assuming the simplicity azimuthal symmetry of the environmental parameters, a numerical model is developed based on a Finite Difference Time Domain (FDTD) scheme, incorporating free-surface and seabed effects, in the presence of a variable sound speed profile. For the treatment of the outgoing radiating field, a Perfectly Matched Layer (PML) technique is implemented. Numerical results are presented illustrating the applicability of the method.

An equivalent source CVIS method and its application in predicting structural vibration and acoustic radiation in ocean acoustic channe

The acoustic wave propagating in ocean is severely distorted due to the sound speed profile of sea water and the reflection and absorption of seabed. The FEM/BEM is a preferable method to solve the fluid-structure interactive vibration and sound radiation problem of underwater structure in ocean acoustic environment, since the Green function can be applied in the integral equation. The simple source integral method is more preferable compared with Helmholtz integral because the calculation of normal derivative of Green function is avoided. In order to remove the irregular frequency problem, Zhou (Zhou et al., 2010) proposed a CVIS theory, which however introduced the normal derivative of Green function on CVIS surface and limited its application into ocean acoustical environment. Therefore, an equivalent source formulation of CVIS is proposed based on the definition of normal derivative so that the normal derivative of Green function on the integration of CVIS is replaced. The radiated SPL of spherical shell is calculated in shallow and deep sea with various sound speed profiles. It is shown that the equivalent source CVIS can be used to eliminate the irregular frequency problem of the far field radiated sound pressure of underwater elastic structure in ocean acoustic environment.

Study on calculation methods for acoustic radiation of axisymmetric structures in finite water depth

In this paper, a kind of FEM–WSM (Finite Element Method–Wave​ Superposition Method) is used to calculate the acoustic radiation of axisymmetric structures in finite water depth. FEM is used to solve the dry modes of axisymmetric structures, and WSM is applied along with the dry mode method to consider fluid–structure interaction effects and calculate the acoustic radiated field. This method combines the advantages of FEM and WSM. On one hand, it is suitable for complex or large axisymmetric structures on the one hand. On the other hand, it has higher computational efficiency than the FEM, and the computational domain size for the water is not limited. As long as the Green’s function is tailored for the boundary condition, the acoustic radiated field of axisymmetric structures in more complex ocean acoustic environments can be calculated by using this method. Besides, a least-square method is used to reduce the distortion resulting from computational errors of the modal estimates. The influence of the number of source and field points and the finite element mesh density on the calculation accuracy are discussed, eliciting some disciplinary conclusions. Using a spherical shell and a capsule shell as models, the results from the present approach, a semi-analytical method, and the crude FEM are compared to verify the correctness and efficiency. Based on numerical examples, the influence of the sea surface and the seafloor on the acoustic radiated field of structures in finite water depth is also analyzed.

Calculation method of acoustic radiation for floating bodies in shallow sea considering complex ocean acoustic environments

For floating bodies in shallow sea, the prediction of acoustic radiation requires consideration of complex acoustic environments. Generally, the influence of the sea surface, seabed and sound speed profile cannot be neglected. In this paper, a relevant calculation method in shallow sea considering the sound speed profile is developed by introducing the Green's function corresponding to a complex ocean acoustic environment into the three-dimensional sono-elasticity theory for ships. The inviscid, irrotational and compressible fluid, which can be regarded as an acoustic medium, is considered. The fluid load is taken into account by hydrodynamic coefficients during the fluid-structure coupling vibration analysis. The structural vibration response is obtained by the modal superposition method. The simple-source method is employed to calculate the acoustic field. The mirror method and the normal mode method are applied in the calculation of near-field and far-field Green's functions respectively. A series of numerical examples are given and the results are compared with those of the full FEM approach to verify the accuracy and applicability of the presented method.

A mixed analytical-numerical method for the acoustic radiation of a spherical double shell in the ocean-acoustic environment

This study presents a mixed analytical-numerical method based on the analytical method and the wave superposition method to predict the acoustic radiation of a spherical double shell immersed in the ocean-acoustic environment. Using the Green's function, the fluid structure coupled vibration as well as the near and far field acoustic radiations can be simultaneously analyzed. Different forms of Green's functions are utilized to deal with the near and far field distribution of sound pressure. To reduce the complexity of the computation, the image-source method is used in the near field while the normal mode method is adopted in the far field. The acoustic radiation fields of the spherical double shell in finite depth ocean-acoustic environment with positive and negative gradient sound speed profiles are calculated using the proposed method. Numerical results compared with results using the finite element method obtained by COMSOL verify that the proposed method improves the computational efficiency without losing computational accuracy. Further, taking the spherical double shell as an example, the effects of seabed, sea surface, and sound speed profile of seawater on the fluid structure coupled vibration, acoustic radiation, and sound propagation of underwater structures are analyzed. Some figures are also included to show these effects clearly. The main idea proposed in this paper can be applied to the integrated calculation of fluid structure coupled vibration and the near and far field acoustic radiations of arbitrary three-dimensional elastic floating bodies in the ocean-acoustic environment.

Matched Field Processing in Phase Space

The traditional method of matched field processing is based on comparing the complex amplitudes of the measured and calculated sound fields in an underwater waveguide. Because of the high sensitivity of the wave field to variations in environmental parameters, the use of this approach requires accurate knowledge of the ocean-acoustic environment. In this paper, it is shown that under conditions of uncertain environment, instead of comparing the depth dependencies of complex field amplitudes, it is advisable to compare their field amplitude distributions in the phase plane “grazing angle-depth.” Such distributions are calculated using the coherent state expansion borrowed from quantum mechanics. Due to the absence of multipath in the phase space, the amplitudes of the coherent states are less sensitive to variations in the environmental parameters than the total wave field. This makes it possible to construct the similarity coefficients of measured and calculated fields that almost “do not notice” the differences of the compared fields caused by weak sound-speed variations, and “react” only to differences caused by strong changes in the sound-speed field and/or source position.

Study of integrated calculation method of fluid-structure coupling vibrations, acoustic radiation, and propagation for axisymmetric structures in ocean acoustic environment

Based on a combination of the finite element method and wave superposition method (FEM-WSM), an integrated calculation method of fluid-structure coupling vibrations, acoustic radiation, and propagation for axisymmetric structures in ocean acoustic environments is proposed. The FEM was applied to study the dry mode of axisymmetric structures in a vacuum, and WSM was used to consider the fluid-structure interaction effects and calculate the acoustic radiation field in ocean acoustic environments. The method proposed combines the advantages of FEM and WSM. This method is suited for complex or large axisymmetric structures. Furthermore, it is more efficient than FEM, and the calculation range of the water is not limited. To simplify the calculation, the Green's functions corresponding to the propagation of specific ocean environments were calculated differently in the near and far fields. Using spherical and elliptical axisymmetric shells as examples, the results from the method proposed and COMSOL were compared to verify the correctness of the proposed model. In the calculation of the acoustic radiation field, the numerical errors from matrices, including partial derivatives of the Green's function with huge condition numbers, required the use Tikhonov regularization, and the efficiency was not influenced.

Range-dependent impacts of ocean acoustic propagation on automated classification of transmitted bowhead and humpback whale vocalizations.

Significant effort has been made over the last few decades to develop automated passive acoustic monitoring (PAM) systems capable of classifying cetaceans at the species level. The utility of such systems depends on the systems' ability to operate across a wide range of ocean acoustic environments; however, anecdotal evidence suggests that site-specific propagation characteristics impact the performance of PAM systems. Variability in propagation characteristics leads to differences in how each cetacean vocalization is altered as it propagates along the source-receiver path. A propagation experiment was conducted in the Gulf of Mexico to investigate the range-dependent impacts of acoustic propagation on the performance of an automated classifier. Modified bowhead and humpback vocalizations were transmitted over ranges from 1 to 10 km. When the classifier was trained with signals collected near the sound source, it was found that the performance decreased with increasing transmission range-this appeared to be largely explained by decreasing signal-to-noise ratio (SNR). Generation of performance matrices showed that one method to develop a classifier that maintains high performance across many ranges is to include a varied assortment of ranges in the training data; however, if the training set is limited, it is best to train on relatively low SNR vocalizations.

Study on the integrated calculation method of fluid–structure interaction vibration, acoustic radiation, and propagation from an elastic spherical shell in ocean acoustic environments

A modeling method based on the wave superposition method is studied in this paper. Different underwater acoustic propagation models are used in the near and far field. Taking the acoustic radiation field calculation of an elastic spherical shell as an example, the near and far field can be analyzed as a unified system, using this method. Many studies use FEM/BEM (finite element method/boundary element method) to calculate acoustic radiation fields of elastic structures, but only a few of these methods can be applied when considering finite ocean depth, seafloor reflection loss, and sound velocity profile. In this study, the Green function is calculated differently in the near and far field to significantly simplify calculations. The virtual source method is used to calculate the acoustic radiation field in the near field and source strength, while the normal mode method is used in the far field. The results are compared with those obtained using the COMSOL finite element software, which showed that this method was both computationally efficient and accurate. Based on numerical examples, the influence of sea surface, seafloor, and sound velocity on the acoustic radiation field of elastic structures in an ocean acoustic environment is quantitatively analyzed.

Three-dimensional hydroelasticity theory of ships in waves and ocean-acoustic environment with applications

The three-dimensional (3D) hydroelasticity theory of floating structures established in 1984 embodies the predictions of seakeeping behaviors, structural loads, dynamic strength, fatigue, and vibration of a surface ship or an underwater vehicle travelling in the sea in a unified generalized fluid-structure interaction theory. After more than thirty years of development, this theory has been further extended to two main branches: the 3D hydroelasticity theories for examining the wave excited dynamic responses of ships, and the 3D hydroelasticity theories for investigating the machinery or propeller excited vibrations and underwater acoustic radiations of a ship. The latter is also called the 3D sono-elasticity theory of ships. Drawn by the momentum gained from the comprehensive demand of marine applications, China Ship Scientific Research Center (CSSRC) has been devoted to the systematic innovative research in creating the linear and non-linear 3D hydroelasticity theories of ships, the 3D hydroelasticity theory of floating structures deployed in complicated ocean geographical environment near islands and reefs, the 3D sono-elasticity theory of ships in ocean acoustic environment etc. together with developing the numerical methods and corresponding software. These have been extensively employed to solve the key technical problems, such as motion and structural response predictions, behavior and safety assessment, design check and optimization of surface ships, underwater vehicles and other marine structures, This paper is not to present the state of the art of the worldwide development of hydroelasticity theories, but to concisely described the concept and framework of the 3D hydroelasticity and sono-elasticity theories of ships which are mainly developed by the research group of CSSRC. Typical application examples of the hydroelastic analyses of a 500000 DWT ultra large ore carrier and an 8-module very large floating structure, as well as the sono-elastic analysis of a simplified submarine model are also briefly illustrated. Finally, simply mentioned are the important directions of future research in the field of hydroelasticity and sono-elasticity of ships.

The possible effects of anthropogenic acoustic pollution on marine mammals' reproduction: an emerging threat to animal extinction.

For about 119 species of cetaceans and other aquatic animals, sound is the key source of learning about the environment, navigation, communication, foraging, and avoiding predators. However, in the recent era, the introduction of large quantities of anthropogenic noise into the ocean has significantly altered the ocean's acoustic environment. The anthropogenic noises travel very long distances, blanketing enormous areas. This can affect cetaceans, either by direct killing or compromising hearing, navigation, communication, predation, as well as normal behaviors. It has been suggested that acoustic pollution could possibly negatively affect cetacean reproduction, which is harmful for endangered and threatened species. However, it is still unknown how acoustic pollution can suppress cetacean reproduction. This is the first comprehensive review article, which focuses on the possible consequences affecting the reproduction of marine mammals resulting from acoustic pollution.

A transformation of the CVIS method to eliminate the irregular frequency

The simple-source method implies an interior virtual fluid field for the exterior acoustic radiation problems. Due to the resonance of this virtual fluid field, the emergence of the irregular frequencies (fictitious frequency) will occur on certain frequencies, which lead to the distortion of the results. It is a purely mathematical problem that the boundary integral equation has no solution at irregular frequencies. Based on the closed virtual impedance surface method (CVIS method), a constant transformation factor α is introduced to form a transformation of the CVIS method, the CVIS-α method. Through the method, the irregular frequencies can be effectively eliminated. Meanwhile, the partial derivatives of the Green's function can be ignored in the boundary integral equation, which is for the exterior acoustic problems. This method has natural advantages in the integrated calculation of acoustic radiation and propagation of 3D elastic floating body in the ocean acoustic environment. The validity of the method is verified by the acoustic impedance calculation cases of the spherical and cylindrical floating bodies. Moreover, some optimization principles for building the CVIS are given in consideration of both the computing quantity and the eliminating effect.

Confounding effects of environment-dependent propagation on automated classification of cetaceans

Significant effort has been made over the last few decades to develop automated passive acoustic monitoring (PAM) systems capable of classifying cetaceans at the species level. The utility of such systems depends on their ability to operate across a wide range of ocean acoustic environments; however, anecdotal evidence suggests that site-specific acoustic propagation characteristics impact the performance of PAM systems. This is because properties of the ocean acoustic environment can be markedly different between regions and seasons in which PAM is used to observe cetaceans. Variability in propagation characteristics leads to differences in how each cetacean vocalization is distorted as it propagates along the source-receiver path. Unless these differences are accounted for, the acoustic environment will bias estimates of a PAM system’s performance. The impact of environment-dependent propagation on an aural classifier was assessed using a pulse propagation model. Simulations were conducted that showed the sensitivity of classifier performance to sound speed profile type. Furthermore, it was found that classifier performance was range-dependent, largely due to changes in signal-to-noise ratio (SNR); however, in some environments only 60% of the performance reduction was attributed to decreasing SNR, indicating that signal distortion—resulting from environment-dependent propagation—had a large impact on performance.

Four dimensional sound speed environments in ocean acoustic simulations

Current ocean acoustic simulation environments often rely on a single depth-varying sound speed profile. This work introduces a robust and configurable Octave/C + + implementation of a four-dimensional sound speed environment (4D SSP) for use with Bellhop in the MIT/LAMSS software package for acoustic simulations. This tool shows considerable advances in that the 4D SSP environment can be created from MIT GCM model output, CTD data, or manually varied from existing historical profiles. By considering the fluctuations in sound speed in longitude, latitude, depth, and time, output pressure data better approximates experimental data. Two experiments in the Arctic and the Santa Barbara Channel (SBC) are simulated in 1-D and 4-D to compare to experimental data taken. A framework for acoustic data assimilation using this new simulated data environment in tandem with experimental data is shown to derive the true sound speed field. [Work supported by ONR under the Information in Ambient Noise MURI.]

Theory and applications of coupled fluid-structure interactions of ships in waves and ocean acoustic environment

Wave induced motions and structural distortions, and machinery or propeller excited vibrations and acoustic radiations of a ship are two kinds of important fluid-structure interaction problems. The branch of ship science that describes the coupled wave induced dynamic behavior of fluid-structure interaction system is referred to as hydroelasticity. During the past three decades the development of three-dimensional hydroelasticity theories and applications gained great progress. Recently the 3-D hydroelasticity theory was further extended to account for the fluid compressibility and the effect of the ocean acoustic environment with finite water depth. A three-dimensional sono-elasticity theory was then produced. In this paper, the 3-D hydroelasticity theory and the 3-D sono-elasticity theory of ships are briefly described. To illustrate the applicability and feasibility of these theories and the corresponding numerical approaches, several examples are presented including the predictions of wave loads, rigid-body and flexible-body responses, springing and fatigue behaviors, machinery or propeller excited coupled structural vibrations and acoustic radiations, as well as design optimizations for improving safety and acoustic behaviors of ships.

Elastic parabolic equation and normal mode solutions for seismo-acoustic propagation in underwater environments with ice covers

Sound propagation predictions for ice-covered ocean acoustic environments do not match observational data: received levels in nature are less than expected, suggesting that the effects of the ice are substantial. Effects due to elasticity in overlying ice can be significant enough that low-shear approximations, such as effective complex density treatments, may not be appropriate. Building on recent elastic seafloor modeling developments, a range-dependent parabolic equation solution that treats the ice as an elastic medium is presented. The solution is benchmarked against a derived elastic normal mode solution for range-independent underwater acoustic propagation. Results from both solutions accurately predict plate flexural modes that propagate in the ice layer, as well as Scholte interface waves that propagate at the boundary between the water and the seafloor. The parabolic equation solution is used to model a scenario with range-dependent ice thickness and a water sound speed profile similar to those observed during the 2009 Ice Exercise (ICEX) in the Beaufort Sea.

Time varying broadband acoustic response compensation at very low signal to noise ratio

Shallow water ocean acoustic environments are challenging due to their multi-path arrival structure and the various Doppler processes that are associated with platform motion and surface interactions. For moving platforms, each acoustic arrival exhibits a Doppler offset that can be quite significant, which results in an overall response that is doubly spread, one which disperses signals in both time and frequency. A virtual ocean acoustic laboratory based on the Bellhop ray tracing model is used to study the distortion of broadband waveforms through dynamic shallow water ocean acoustic environments. Acoustic response functions are presented in order to illuminate the challenges posed by shallow water environments and mobile platforms. The shared Doppler process of the arrivals is illustrated in order to illuminate the value of bulk Doppler compensation schemes. This virtual acoustic laboratory is used to test spread spectrum communication performance at very low SNR. Bulk Doppler compensation schemes derived from joint estimation of symbols and the acoustic response are compared to the actual path dilation processes. Simulation results are presented for fixed source with moving receiver shallow water environments at diverse center frequencies and bandwidths.