Acoustic Propagation In Shallow Water Research Articles

Reconstructions of time-evolving sound-speed fields perturbed by deformed and dispersive internal solitary waves in shallow water

The high-fidelity reconstruction of sound speeds is crucial for predicting acoustic propagation in shallow water where internal solitary waves (ISWs) are prevalent. Mapping temperatures from time series to spatial fields is an approach widely used to reproduce the sound speed perturbed by deformed internal waves. However, wave-shape distortions are inherent in the modeling results. This paper analyzes the formation mechanism and dynamic behavior of the distorted waveform that is shown to arise from the mismatch between the modeled and real propagation speeds of individual solitons within an ISW packet. To mitigate distortions, a reconstruction method incorporating the dispersion property of an ISW train is proposed here. The principle is to assign each soliton a real speed observed in the experiment. Then, the modeled solitons propagate at their intrinsic speeds, and the packet disperses naturally with time. The method is applied to reconstruct the sound speed perturbed by ISWs in the South China Sea. The mean and median of the root-mean-square error between the reconstructed and measured sound speeds are below 2 m/s. The modeled shape deformations and packet dispersion agree well with observations, and the waveform distortion is reduced compared with the original method. This work ensures the high fidelity of waveguide-environment reconstructions and facilitates the investigation of sound propagation in the future.

Broadband attenuation-based measurements of bubble size distributions

Oceanic bubbles generated by breaking waves are a key mechanism for air-sea gas exchange. Dense bubble plumes can also interfere with acoustic propagation in shallow water due to their high scattering and attenuation. Most studies of oceanic bubbles have used narrow-band acoustical and optical techniques to quantify bubble size distributions under breaking waves in open water. To study bubble distributions at estuarine tidal fronts, where localized downwelling and advection can generate large bubbles plumes, we developed a measurement system for estimating bubble size distributions via broadband excess attenuation. The system consists of two transducers (3–30 kHz and 30–110 kHz) and two hydrophones mounted on a towable frame. Testing was performed in a laboratory wave tank to measure bubble distributions below a field-scale (1 + meter) breaking wave. This presentation will discuss the bubble size distributions observed at different locations along the breaking wave crest in the direction of propagation and their associated time scales and sound speed anomalies. We also compare these distributions with those previously observed in open water and will present preliminary results from a field deployment of the system in the Connecticut River estuary.

The effect of ocean turbulence on acoustic propagation in shallow water

This work devoted to research the temperature and salinity distribution in ocean turbulence. The sound velocity when ocean turbulence presented was calculated by the function relation between temperature, salinity, depth and sound velocity. The sound transmission loss was calculated when the ocean turbulence was presented or not and compared them. The sound field fluctuation resulted by ocean turbulence was analyzed. It was very significant to comprehend the mechanism and variation rule of ocean turbulence. Thus it was further study the effect of ocean turbulence on acoustic propagation. As an example, there was thermocline in the ocean water. The sound transmission loss was calculated when presented ocean turbulence or not. It analyzed the effect of ocean turbulence on acoustic propagation. The sound field fluctuation caused by ocean turbulence could reach 8dB.

Study on the influence of sound speed profiles on the optimum depth of shallow water acoustic propagation

Study on the influence of sound speed profiles on the optimum depth of shallow water acoustic propagation

Development and Application of a Pre-Corrected Fast Fourier Transform Accelerated Multi-Layer Boundary Element Method for the Simulation of Shallow Water Acoustic Propagation

Because of the complexities associated with the domain geometry and environments, accurate prediction of acoustics propagation and scattering in realistic shallow water environments by direct numerical simulation is challenging. Based on the pre-corrected Fast Fourier Transform (PFFT) method, we accelerated the classical boundary element method (BEM) to predict the acoustic propagation in a multi-layer shallow water environment. The classical boundary element method formulate the acoustics propagation problem as a linear equation system in the form of [A]{x}={b}, where [A] is an N×N dense matrix composed of influence coefficients. Solving such linear equation system requires O(N2/N3) computational cost for iterative/direct methods. The developed method, PFFT-BEM, can effectively reduce the computational efforts for direct numerical simulations from O(N2~3) to O(Nlog N), where N is the total number of boundary unknowns. To numerically simulate the sound propagation in a shallow water environment, we applied the first-order non-reflecting boundary condition in the truncated numerical domain boundary to eliminate the errors due to reflected waves. Multi-layer coupled formulation was used to include the environment inhomogeneity in PFFT-BEM. Through multiple convergence tests on the number of layers and elements, we validated and quantified the accuracy of PFFT-BEM. To demonstrate the usefulness and capability of the developed PFFT-BEM, we simulated three-dimensional (3D) underwater sound propagation through 3D geometries to check the efficacy of the established classical method: the 3D Parabolic equation model. Finally, PFFT-BEM was employed to simulate sound propagation through a complex multi-layer shallow water environment with internal waves. The “3D+T” results obtained by PFFT-BEM compared well with the physical test, thereby proving the capability and correctness of this method.

Experimental investigation of acoustic channel reciprocity in shallow water

Channel reciprocity is recognized as one of the most fundamental properties of underwater acoustic propagation. The theoretical proof of channel reciprocity assumes that the ocean environment is static during the course of acoustic propagation. Since this assumption might not be valid for the case of shallow-water acoustic propagation, a question to be raised is: does channel reciprocity remain true in shallow water? A sea-going experiment was recently conducted to study the shallow-water channel reciprocity. In the experiment, two custom-built software-defined acoustic modems were used to probe their shared channel from two opposite directions. The channel reciprocity was examined by comparing the two probed channels directly. The probing signals and handshaking protocols were judiciously designed to minimize the effects of channel dynamics under the constraint of half-duplex acoustic modems. This paper will provide the details of this sea experiment (including its design, setups and execution) and report interesting findings. [Work supported by US Office of Naval Research.]

Effect of Shear on Modal Arrival Times

The sea bottom is generally modeled as a fluid for many of the shallow water acoustic propagation modeling applications. The inherent assumption is that the bottom does not support any shear wave propagation. This study explores the impact of this assumption on the dispersion behavior of acoustic normal modes. A sensitivity study is performed to investigate the effect of density, shear, and compressional wave speeds on modal dispersion. This study focuses on low-frequency (less than 100 Hz) and lower order modes (mode four and lower) in shallow water (70–90-m water depth). These modes and frequency bands are characterized by deeper penetration and larger depth averaging scales. This enabled the use of a simple half-space elastic model of the sea bottom for most of the analyses in this study. A depth-dependent bottom model was used to investigate the effect of a mud layer on modal dispersion. Sediment shear wave speeds were also estimated using broadband data from two shallow water experiments: Shelf Break Primer (1996) and Seabed Characterization Experiment (2017). The inversion results from these two experiments are compared and they are also compared with deep core data and prior inversions.

A fast multi-layer boundary element method for direct numerical simulation of sound propagation in shallow water environments

Direct three-dimensional (3D) numerical simulations of acoustic fields in range-dependent shallow water environments remains a challenge due to environmental complexities and large computational cost. We develop an efficient 3D boundary element method (BEM) to solve the Helmholtz equation for shallow water acoustic propagation, which utilizes a Pre-corrected Fast Fourier Transform (PFFT) approach to reduce the computational effort from O(N2∼3) to O(Nlog⁡N) where N is the total number of boundary unknowns. To account for inhomogeneous media, the method allows for arbitrary number of coupled multi-layer BEM sub-domains. With O(Nlog⁡N) efficiency and the use of massively parallel high-performance computing platforms, we are able to conduct multi-layer 3D direct numerical simulations of low-mid frequency acoustics over kilometer ranges. We perform extensive validations of the method and provide two shallow water waveguide examples benchmarked against theoretical solutions. To illustrate the efficacy and usefulness of the PFFT-BEM method, we perform 3D large-scale direct numerical simulations to assess the performance of two established canonical models: axisymmetric coupled mode model for 3D seamount; and Kirchhoff approximation and perturbation theory for 3D rough surface scattering.

Diurnal Fluctuation of Shallow-Water Acoustic Propagation in the Cold Dome Off Northeastern Taiwan in Spring

The diurnal fluctuation of acoustic propagation in a cold dome region was studied by using the output of the high-resolution Finite Volume Coastal Ocean Model. The 3-D acoustic propagation in this region was studied first, and the results suggest that the diurnal fluctuation of the ocean environment results in the fluctuation of acoustic propagation. However, the variation pattern cannot be determined with only few outputs, and comprehensive analysis was needed. Then, a feature model for the thermal front on the edge of the cold dome was proposed. By analyzing the output of the ocean model, fluctuations were found to mainly occur in the two aspects of the feature model: first, horizontal excursion of the thermal front and second, vertical motion of the isothermal lines in the cold dome. The variation pattern of acoustic propagation over the two kinds of fluctuations was quantified. Results suggest that both kinds of fluctuations of model parameters result in great transmission loss (TL) variation in some cases. Typically, acoustic propagation with high frequency and deep source experience great influence from the fluctuations. The TL versus range usually experience influence from the fluctuations of the first kind from an approximately 40-km range and from the start for the second kind. The reason is that the vertical motion of the isothermal lines of the cold dome induces variation over the temperature profile from the start until the end, whereas the horizontal excursion of the thermal front brings influence mainly over the temperature profiles near the thermal front.

Shallow water acoustic propagation prediction

A set of propagation loss curves, extracted from measured reverberation in an environment with water depths of 200–250 ft is used to investigate the crossover region between deep and shallow water using two normal mode implementations designed to predict propagation in deep and shallow water, respectively. A third range-recursive calculation which is applicable in situations for which cycle mixing with range results in randomization of phase interference between modes is also used for comparison. The impact of forward boundary scattering on reproducing the measured levels at multiple receiver depths is demonstrated.

The preliminary study on spatial correlation of ocean sound field

The sound field temporal correlation and spatial correlation, which are the foundation of the investigation of underwater signal space-time high-order characteristics, have important value in the underwater acoustic application. The spatial correlation is studied based on the shallow water acoustic propagation experiment data acquired in the northern South China Sea in 2017, and the deep water acoustic propagation experiment data acquired in the western Pacific in 2013. As for the explosive sound signals in shallow water, time domain waveform cross-correlation coefficients between signals from different propagation distance are calculated. In contrast, the linear frequency modulated signals in deep water need additional matched filtering. The signal processing results shows that, the overall spatial correlation is poor and the correlation radius is relatively small in shallow water, the convergence zone has an obviously better spatial correlation than the shadow zone for the deep water situation. The processing result is verified by simulation and analysis.

Analysis of mode mismatch in uncertain shallow ocean environment

The actual ocean is an uncertain acoustic propagation environment. For the localization algorithms that rely on the precise ocean environmental parameters, the environmental mismatch problems will exist and performance degradation may be very serious. In the uncertain ocean environment, the uncertainties of sound field will have different effects on different normal modes propagating in the sound field, thus analysis of the mismatch characteristics of normal modes affected by uncertain environmental parameters can provide technical guidance for practical engineering applications. Based on the shallow-water acoustic propagation model, this paper simulates and analyzes the mismatch results of the modal depth eigenfunction and the horizontal wave number of each normal mode under conditions of environmental mismatch. Research indicates that the influence of different environmental parameters on normal modes in the sound field is not exactly the same. It was found that the sound-speed profile and seawater depth affect significantly, followed by sediment sound speed, the other parameters appear to be relatively minor importance.

Performance Analysis of Single-Receiver Matched-Mode Localization

Acoustic propagation in shallow water at low frequency is characterized by a few propagating modes. When the source is impulsive or short enough, the modes can be extracted from the signal received on a single sensor using a warping operator. This opens the door to single-receiver matched-mode processing (SR-MMP) as a means to estimate source location and/or ocean environmental parameters. While the applicability of SR-MMP has been demonstrated through several experiments, prediction of its achievable performance has not been fully investigated. In this paper, performance analysis of SR-MMP is carried out using numerical simulations of a typical shallow water environment, incorporating possible environmental mismatch as well as degradations resulting from nonideal modal filtering. SR-MMP is a nonlinear estimation problem that presents three regions of operation: the high SNR asymptotic region driven by local errors, the intermediate SNR threshold region driven by sidelobe ambiguities and the low SNR no-information region. The method of interval errors, which gives computationally efficient and reliable mean squared error performance prediction, is used to conduct the analysis. The results suggest that the SR-MMP performance depends strongly on the source/receiver depth. A significant loss in performance is observed when the receiver is located at a node common to two modes. Receiver depth must therefore be chosen with care. SR-MMP seems to be quite robust to mismatch on the seabed properties alone but does not handle well the combined effect of seabed and water column mismatches. Nonideal modal filtering has a moderate impact on performance.

Estimation and equalization for shallow water communication channel using geometric encoding of channel multipath

The shallow water acoustic channel is well-known to exhibit rapid fluctuations in its impulse response, which is challenging to estimate and compensate for in real time. In particular, the moving ocean surface and static sea bottom reflect the acoustic signal in unpredictable ways to create rapidly time-varying multipath arrivals. The talk will focus on the nexus of shallow water acoustic propagation paths and how multipath can be discovered and exploited to improve shallow water acoustic communications. In particular, the talk will review sampling strategies for the time-varying shallow water acoustic channel and propose novel geometric encoding techniques that exploit non-uniform channel sampling strategies. We will also demonstrate how geometric encoding can be harnessed for real-time channel estimation as well as semi-blind channel equalization. Results based on experimental field data in recent work as well simulation results will be presented.

Sound speed and attenuation in marine mud seabottom

Acoustic propagation in shallow water is greatly influenced by the properties of the bottom. Comparing the acoustic characteristics in the sediments of sand or silty are well inverted or directly core measured, sound speed and attenuation in the mud sediment still need investigated further. An experiment was performed in the Yellow Sea in 2002 to inversion for the acoustic parameters. In the experiment, six different sediment types: Fine Sand, Silty Sand, Sand Silty, Sand-Silty-Clay, Silty Clay and Mud, are included in an area. A hybrid geoacoustic inversion scheme, which combines several inversion methods together to invert for the bottom parameters, is proposed based on the fact that the bottom acoustic parameters have different sensitivities to the different physical parameters of acoustic field. The inverted bottom parameters could distinguish the atlas marked bottom type quite well. The sound speed and attenuation parameter in the marine sediments of silty clay and mud is mainly discussed. The nonlinear frequency relationship of attenuation in the mud is compared with that of silty sand. [Work supported by the National Natural Science Foundation of China under Grant No. 11434012 and Grant No. 41561144006.]

An overview of the Sediment Characterization Experiment 2015 survey cruise

The acoustics of fine grained marine sediments, how they impact shallow water acoustic propagation, and the use of inverse and statistical inference techniques for sediment characterization are being studied in this multi-year, multi-institutional project, which is focused on an area of the Southern New England continental shelf known as the New England Mud Patch. An overview of the first survey cruise associated with this project, which took place in July and August of 2015, will be presented. Measurements during that cruise included sub-bottom profiling and multi-beam bathymetry surveys of the experimental area, as well as CTD and gravity core, box core, and multicore sampling. A limited number of acoustic propagation measurements were made along with ship-board laboratory measurements of recovered sediment sound speed and attenuation measurements. A preliminary assessment of the presence and effect of ocean bottom biology was conducted. In this talk, an overview of the survey cruise will be presented. ...

천해 지질환경에서의 음파전달 특성 연구를 위한 KIOST-한양대 공동실험 개요

본 논문은 "천해 지질환경과 음파전달환경과 상호 연계 연구"를 목적으로 한국해양과학기술원과 한양대학교가 2013년 4 ~ 5월에 경기만 태안반도 서쪽에 위치한 천해환경에서 공동으로 수행한 지질환경 조사 및 수중음향 실험에 대한 개요를 기술하였다. 실험지역은 강한 조류와 천해의 지형학적 특성에 의해 다양한 종류의 퇴적상과 베드폼이 형성되는 해역이다. 집중적인 지질환경 특성 조사를 위해 다중음향측심기, 천부지층탐사기, 중천부지층탐사기, 그랩을 이용하여 조사해역의 지질환경 특성을 파악하였으며, 저주파수, 중주파수 대역의 음원과 수직선배열청음기를 이용하여 주파수 200 ~ 16,000 Hz 대역에서 1) 저주파수 음파전달, 2) 중주파수 해저면 반사손실, 3) 주변소음의 공간 코히런스 분석, 4) 중주파수 해저면 후방산란에 대한 연구 주제로 해상실험이 수행되었다. 본 논문에서는 연구지역에서 음향실험을 실시한 동기, 음향실험 방법론과 측정된 지질자료를 기반으로 음향자료 해석과, 실험기간 동안 측정된 지질환경, 기상 해양물리 자료를 요약 정리하여 기술하였다. This paper presents an overview of the geological environment investigation and underwater acoustic measurements for the purpose of "Study on the Relationship between the Geological Environment and Acoustic Propagation in Shallow Water", which are jointly carried out by KIOST (Korea Institute of Ocean Science & Technology) and Hanyang University in the western shallow water off the Taean peninsula in the Yellow Sea in April-May 2013. The experimental site was made up of various sediment types and bedforms due to the strong tidal currents and coastal geomorphological characteristics. The geological characteristics of the study area were intensively investigated using multi-beam echo sounder, sub-bottom profiler, sparker system and grab sampler. Acoustic measurements with a wide range of research topics in a frequency range of 20~16,000 Hz: 1) low frequency sound propagation, 2) mid-frequency bottom loss, 3) spatial coherence analysis of ambient noise, and 4) mid- frequency bottom backscattering were performed using low- and mid-frequency sound sources and vertical line array. This paper summarizes the topics that motivated the experiment, methodologies of the acoustic measurements, and acoustic data analysis based on the measured geological characteristics, and describes summary results of the geological, meteorological, and oceanographic conditions found during the experiments.

Initial considerations on effects of braided river beds on long-range acoustic propagation in shallow water

A braided river consists of multiple small channels that divide and recombine numerous times. They form when the sediment load and transporting energies are such that coarser sediment can be deposited as shifting islands or bars between the channels. Braided rivers formed on continental shelves during sea level lowstands during the last 700,000 years when the shelf was sub-aerially exposed to greater than 125 m below present day sea level. A salient feature is that the stream bed (which can be many kilometers wide) is expected to exhibit extremely high geoacoustic variability, given that the islands and bars exhibit much coarser material than that in the channels. The variability is expected both laterally (highest variability or smallest scales perpendicular to the stream flow) and also vertically inasmuch as the river channels experience different avulsion patterns over time. Chirp sonar and core data provide some insight into the underlying geologic processes and the associated scales of variability. Modeling gives some initial clues about the effects of braided river beds on acoustic propagation. [Work supported by the Naval Oceanographic Office and the Office of Naval Research, Ocean Acoustics program.]

동중국해 북부해역에서 음파전달 특성의 시공간적 변동성

Acoustic propagation in shallow water with changing environments is a major concern of navy. Temporal and spatial variability of acoustic propagation in the northern East China Sea (ECS) is studied, using the 11 years hydrographic data and the Bellhop acoustic model. Acoustic propagation in the northern ECS is highly variable due to extensive interaction of various ocean currents and boundaries. Seasonal variations of transmission loss (TL) with various source depths are highly affected by sharp gradient of sound speed and bottoms interaction. Especially, various bottom sediment types lead to severely degrading a waterborne propagation with bottom loss. In particular, the highly increased TL near the ocean front depends on the source position, and the direction of sound propagation.

Experimental study of ultra shallow water acoustic wave propagation

The sound propagation in ultra shallow water was studied experimentally. The investigation was carried out in the nature pond approximately 300 × 20 m in size. Its depth was about 1 m. For such a pond critical frequency is about 700 Hz that is easy to emit and receive. The main purpose was to study the process of acoustic wave propagation in case of strong bottom influence and to restore some bottom parameters. The signal was radiated by the transducer and recorded by a set of point receivers located at different distances from the transducer. The signal was linear frequency modulated in 100 Hz–10 kHz band and was 10 s in duration. Its correlation with the recorded signal defines the pond response function. The modes representation was used. The group velocity frequency dependence was obtained by use of this method and it was very close to the theoretical one considering a soft bottom. The critical frequency also corresponded to the soft bottom model. The attenuation of sound was also studied in a wide frequency range including non-propagating mode. The results are very sensitive to the signal-to-noise ratio. It was shown that the used correlation method significantly improved the quality of obtained characteristics.