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and Figure 23 show similar results for downslope and upslope propagation for typical winter conditions represented by a sound speed profile measured in the month of February. For downslope conditions the sound level decrease rapidly with increasing depth and much more rapidly with shear wave conversion (Figure 24a) than without shear (Figure 24b). With upslope propagation (Figure 23) the sound levels are near independent of shear conversion except at the very long rages where the water depth becomes constant. The examples demonstrate that sound propagation in the ocean is strongly influenced by both by the oceanographic conditions and the geophysical properties of the bottom. Reliable prediction of acoustic propagation condition requires modeling tool that can that can handle both bottom and water properties.
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... Westwood (1992) proposed a ray method for establishing a two-dimensional point source sound field, and extended it to a threedimensional model. Hovem (2013) proposed a simulation model of hydroacoustic propagation based on the ray method and achieved better results. Luo et al. (2012) proposed a stepwise coupled-mode model with the use of the direct global matrix approach. ...
An acoustic solver is implemented based on a generic framework for solving PDEs. • Large-scale problems can be efficiently simulated through parallel computing. • Our method shows high accuracy and the ability to support complex scenarios. Graphical abstract and Research highlights will be displayed in online search result lists, the online contents list and the online article, but will not appear in the article PDF file or print unless it is mentioned in the journal specific style requirement. They are displayed in the proof pdf for review purpose only. A B S T R A C T Numerical simulations based on the Finite Volume Method (FVM) solve the wave equation directly to provide highly accurate predictions of the sound field in ocean waveguides without model simplification and loss of generality. However, the huge computational complexity severely limits the application range of direct numerical solvers for underwater acoustic waves. In this paper, a highly scalable parallel algorithm for solving acoustic models using the cell-centred finite volume method was proposed, and a corresponding direct numerical solver was developed and validated. The paper has three main findings as follows. Firstly, the acoustic solver was implemented based on a generic framework for solving partial differential equations (PDEs), so that more complicated models such as fluid-sound coupling and sound-structure coupling could be easily extended. Secondly, the large spatial scales of underwater sound propagation can be efficiently calculated by parallel computing. Thirdly, our approach demonstrated high computational accuracy and the ability to support complex scenarios. We verify the accuracy and efficiency of the solver through experiments, and prove the solving ability of the solver in large-scale, complex structures and complex configurations.
... We used ray tracing and the BELLHOP package to calculate the signal arrival times and amplitudes [36], [37]. Ray tracing is applied in underwater propagation problems, for which it is well known and recognized [38], [39]. ...
Underwater localization and tracking is a challenging problem and Time-of-Arrival and Time-Difference-of-Arrival approaches are commonly used. However, the performance difference between these approaches is not well understood or analysed adequately. There are some analytical studies for terrestrial applications with the assumption that the signal arrival times are not correlated. However, this assumption is not valid for underwater propagation. To present the distinct nature of the problem under the water, a high-fidelity simulation is required. In this study, Time-of-Arrival and Time-Difference-of-Arrival approaches are compared using a ray tracing based propagation model. Moreover, basic methods to mitigate the multipath propagation problem are also implemented for Bernoulli filters. Since the Bernoulli filter is a joint detection and tracking filter, the detection performance is also analysed. Comparisons are done for all combinations of filter and measurement approaches. The results can help to design underwater localization systems better suited to the needs.
... Thus, time complexity gets reduced as compared to conventional ray theory and normal mode theory. This theory is applicable to static tank channels where the properties of channel are not changing much wrt time [5]. ...
... Where Combination no. 10 shows perfect agreement between theory and experimental results. Deviation of simulated TL for combination no.3,5,8,10,12,13,14, &15 varies from -3% to 8% of experimental TL.Whereas from remaining 9 combinations, combination no 2,6,7,9,11,16, &17 shows that simulated values of TL are -0.18 to 0.19 times experimental transmission loss. It shows that agreement between theoretical and experimental TL is within reasonable limit. ...
The propagation loss varies with underwater channel conditions which might considered to be random phenomena. Modeling propagation loss will become meaningful iff mathematical model includes parameters namely viz projector's transmit voltage response (TVR), hydrophone's open circuit receiving response (OCRR), directivity patterns of both, channel parameters such as salinity, temperature, pressure, enclosure boundary conditions along with placements of Tx & Rx nodes & their operating frequency. To best of our knowledge, existing simulators are unable to trace eigen rays for very short range i.e. less than 0.1 km and therefore they are not suitable for computation of such short-range propagation losses. We have made an attempt to overcome limitations of existing simulators wherein we proposed mathematical model SWARA which includes parameters as mentioned above to study very short-range propagation losses using plane wave theory. To validate simulated propagation loss, we conducted tank trials at UWAA Lab, CARE, IIT Delhi to investigate effects of placements of projector & hydrophone on occurrence of transmission loss. The Simulated results of SWARA mathematical model shows that simulated maximum transmission loss is -0.18 to 0.10 times experimental maximum transmission loss, whereas simulated minimum transmission loss is -0.36 to 0.19 times experimental minimum transmission loss for placements of projector (ITC 1042) and hydrophone (Keltron 8240000001) at depths varying from 0.3m-1.2m & range varying from 2m-3.2m in uw tank facility of 3.85m long 2.4 wide 2m deep for 30kHz chirp signal (10kHz bandwidth) under static channel conditions.
... As the frequency increases, the computational costs do not rise. Moreover, ray-based models can offer a good approximation, even at low frequencies, if the source and the receiver are located at least at half-wavelength above the seabed, as some authors have pointed out [39]. It must be underscored that the aim of this work is not to perform a study of different propagation methods but to establish a methodology that combines sound maps related to marine traffic and the population density of marine mammals with the aim to infer the possibility of individuals being affected by masking. ...
Over the last decade, national authorities and European administrations have made great efforts to establish methodological standards for the assessment of underwater continuous noise, especially under the requirements set by the Marine Strategy Framework Directive (MSFD). Through the MSFD implementation across EU Member States Marine Reporting Units (MRUs), it is intended to establish the Good Environmental Status (GES) whether it is achieved or not. The evaluation of the Sound Pressure Level (SPL) at the local or regional scale for 1/3 octave band of 63 Hz and 125 Hz and the identification of long temporary trends were considered to be a priority due to the valuable information they can offer in relation to continuous low-frequency noise. Nevertheless, the methodology to determine threshold values from which to evaluate the GES has become difficult to define, and new approaches and considerations are currently being discussed by groups of experts, such as the technical subgroup on underwater acoustics (TGnoise) and regional commissions (e.g., OSPAR). This work presents a methodology to perform the assessment of a given area, providing a risk index that is related to potential appearance of masking effect due to the underwater noise produced by marine traffic. The risk index is hinged on the calculation of area under curves defined by the density of animals and a variable related to underwater noise SPL, defined as percentage of communication distance reduction. At this stage, the methodology presented does not consider physiological or behavioral mechanisms to overcome the masking by animals. The methodology presented has been applied to the bottlenose dolphin (Tursiops truncatus) inhabiting the ABIES—NOR marine demarcation to illustrate the possible use of risk-based models to manage marine areas related to human pressures, such as marine traffic, with the potential adverse impact on a given species (e.g., masking effect).
... Also, a reflection coefficient is used to achieve the interaction of the ray with the ocean. [128][129][130]. ...
The oceans are home to millions of Earth’s plants and animals. It is our life source, supporting humanity’s food and that of every other organism on earth. The ocean produces at least 50% of the planet’s oxygen. It is the home to most of the earth’s biodiversity and is the main source of protein for more than a billion people around the world. The ocean is the key to our economy with an estimated 40 million people being employed by ocean-based industries. Despite regular discoveries about the ocean and its occupants, much remains unknown. More than 80% of the ocean is unmapped and unexplored.
Maritime surveillance and strategies are of great significance to ensure that all events on the oceans are carried out carefully and that national and international security is upheld without any disputes. Today, due to several reasons, the worldwide geopolitical changes have created substantial awareness in the defence research for continuous improvements and the modernisation of surveillance systems to tackle unlawful activities. Recent history recounts that one of the susceptible zones for carrying out these unlawful activities is the maritime domain. Refugee shipping, illegal migrant ships, sea pirates, loss of lives due to accidents, attacks, and robberies, illegal fishing and infiltration, and terror activities are some of the unlawful activities to name a few in this domain. This has led to several national and multinational initiatives in maritime surveillance to know all coastal and high-seas activities relevant to national security. As 71% of the earth surface is enclosed by the ocean, the dependency of maritime surveillance on technologies and research services is inevitable.
The surveillance needs to cover the above-water and underwater environments together for successful maritime surveillance. Most of the technologies that are operative in an above-water environment are not operable underwater. There are not many comparable technologies in the underwater environment as the ocean is a very lossy medium for the propagation of electromagnetic energy because of the high electrical conductivity of seawater. However, it supports acoustic propagation enabling sonar technologies bright for underwater environments. The physics of acoustic propagation say aloud that the sonar technologies cannot offer a long-range or wide-area surveillance coverage as offered by either the radar technologies or space-borne satellite surveillance systems. Therefore, underwater sonar technologies cannot be used in the same way as the ocean environment is unfriendly most of the time.
The above paragraphs define the problem statement of underwater maritime surveillance and the need for continuous research and development to face the threats. But, it dictates that the most dominant technology suitable for underwater maritime surveillance is sonar technology. Therefore, it becomes evident that the sonar systems are the only eyes and ears of any submarines for them to move in the ocean. Sonar systems are also the eyes and ears for ships for ocean surveillance. Sonar systems are the essential fitments on high-value surface and subsurface naval platforms to protect them from potential attacks. Beamformers forms the basis for the actual eyes and ears of any sonar system. Therefore, the key objective of this research work is to develop a reliable and robust sonar beamformer. Several research gaps have been identified in sonar array beamforming and this thesis attempted to resolve some of those issues.
Beamforming design always depends on the spatial weighting functions of window functions. But, the different ways of utilizing the conventional window functions are saturated. The development of new window functions is also rare. Therefore, this thesis took up the challenge of blending the existing conventional window functions through the principle of constructive and destructive interference techniques used in optical physics. It necessitated two window functions and through careful investigation, this research work identified the robust rectangular and null-steered Dolph Chebyshev windows for making an efficient beamformer using product beamforming. The technique developed thereby demonstrated the superior quality of narrow beamwidth with very low sidelobe levels. Effectively, the sidelobes are suppressed by more than 25 dB using this novel technique. The technique is found to be more robust and stable against an array element failure or other array deformations. The sidelobe spread is found to be very minimal against array deformations.
Thereafter, the investigation tried to reduce the complexity of sonar architecture by bridging a nested array concept and the developed product beamformer to enhance the overall performance without sacrificing the similar performance obtainable by a large hydrophone array. The investigation demonstrated that multiple targets can be detected with much more accuracy using the present method than the existing methods. The outcome of this method would reduce the complexity of the signal conditioning hardware to a large extent, thereby it allows us to have a simple sonar architecture and hence making installation and maintenance easy. Beamforming alone is not sufficient to execute maritime surveillance with sonars in the ocean. Once after detecting a target, the tracking of the target is very important to make necessary strategic decisions. The tracking of any underwater target is better done with the help of target motion analysis. The output of the beamformers plays a crucial role in estimating target motion parameters. Therefore, the developed beamforming technique is applied to target motion analysis. The technique has demonstrated that the motions of the potential underwater targets can be resolved, which are otherwise not possible with the conventional beamformers. Interestingly, the developed beamforming technique is implemented in several types of sonars. Different open ocean experiments with actual ocean passing targets are conducted. The experimental results validate the efficacy of the inventions made in this thesis.
... , the vertical range can be stated as: For a given z j in Eq. (9), the incremental traveling time and horizontal range equations in [29] can now be stated as: ...
This paper presents an autonomous underwater vehicle (AUV) navigation scheme that pairs an inertial navigation system (INS) and a long baseline (LBL) acoustic positioning system. The INS is assigned to be the main navigation aid because of its faster rate. Meanwhile, the LBL provides position reference for compensation of the INS’ main inherent drawback, i.e., accumulating errors. However, the LBL has to deal with time-of-flight (ToF) measurements that may not be carried out under line-of-sight (LoS) circumstances. This is because the propagation speed of underwater acoustic waves is subject to the sound-speed-profile (SSP) of the area in question. This paper’s contribution is to consider the SSP in ToFs while addressing the above scheme. Specifically, the discrete approach to raytracing was implemented. For a given ToF, the Snell’s parameter of the wave is estimated and subsequently used to compute the horizontal range. The ToF results are then used to estimate the position of the AUV, while the position is obtained from a depth sensor. It was shown by simulation that the estimators can provide navigation with accuracy <0.5 m2, as it manages to compensate for errors. Since the estimation of Snell’s parameter is prone to exhibit imaginary numbers, future work should consider a more robust method to tackle this problem.
... Sonar system detection performance in the underwater environment relies strongly on the relative locations of the source, receiver, and target, as well as ambient conditions [25], [26]. In addition, acoustic wave propagation patterns and depth-dependent sound velocity profiles are important factors in finding the correct detection range and coverage underwater [27]. ...
Recent changes in the design of enemy threats such as submarines and the technological achievements in sensor development have paved the way for multistatic sonar applications, which increase security and situational awareness in underwater tactical operations. Previously, coverage in multistatic sonar sensor networks (MSSN) was studied using Cassini ovals and the traditional sonar detection model in two dimensions without any discussion of the practicability and feasibility in terms of conditions related to the underwater acoustic propagation environment. In this study, a practical three-dimensional MSSN channel model is proposed. The proposed model covers a spectral variation of absorption loss, ambient noise, sound speed profile, and shadow zones. The realistic effects of sound propagation and environmental conditions are modeled and evaluated using Lybin, which is a well-known sonar performance prediction tool. Using the practical MSSN channel model, the number of source-receiver pairs required to cover a three-dimensional MSSN volume is calculated. The impacts of frequency, source-to-receiver distance, and source level are investigated. The results are compared to the Cassini oval and traditional sonar detection models via an error expression derived according to our verified practical model. The results reveal that the inclusion of ambient conditions and sound propagation characteristics in the channel model leads to huge error levels of 4700000% in the Cassini oval model and 170000% in the traditional sonar detection model, depending on frequency. Thus, the applicability of these models in realistic MSSN deployment scenarios is limited.
... The analytical transmission loss model, otherwise known as the Urick model, takes a mathematical approach by expressing spreading loss as a function of the transmission distance between transceivers and absorption loss as a function of the transmission frequency (Urick, 1996). The third category consists of models based on beam/ray tracing techniques (Hovem, 2013), examples of which include BELLHOP (Porter, 2011) and World Ocean Simulation System (WOSS) (Guerra et al., 2009). Ray tracing calculates the trajectories of sound rays emanating from a source. ...
Underwater wireless communication (UWC) networks have attracted substantial attention in recent years. UWC can facilitate many critical emerging services including the ones relying on communications, such as Autonomous Underwater Vehicles (AUVs) and Remotely Operated Vehicles (ROVs), environmental monitoring, underwater surveillance, navigation, and exploration. Most UWC devices are battery-powered, where recharging/swapping batteries is not straightforward. Since the reliability of a UWC network depends on limited energy storage, energy-efficient communication is critical for its operation. Whilst communication range and data rate in UWC have been popular research topics, until recently, this energy-efficiency aspect of UWC has not received the same level of attention. In this paper, we provide a comprehensive literature review on existing contributions in the area of energy-efficient UWC. At first, we provide a detailed overview of UWC network architectures and relevant challenges for UWC networks. Next, we discuss and compare the most commonly-used UWC physical layer technologies, reviewing the existing physical layer power-saving techniques proposed in the literature. Then, we review various energy-saving techniques in the upper layers, including Medium Access Control (MAC) protocols, routing protocols, and localization techniques. Lastly, we provide a detailed discussion of alternative energy sources in UWC networks before highlighting future research directions in the field and challenges related to the widespread adoption of the Internet of Underwater Things (IoUT).
... For propagation of acoustic frequencies above 1 kHz a model based on ray theory was used [60,61], modified to include absorption loss by sediment and frequency dependent absorption by ocean water [59]. The accuracy in the wave equation for a receiver depth of 2 m would be compromised below 400 Hz [62]; therefore, for frequencies between 10 and 1000 Hz, a widely used range-dependent acoustic model (GeoRAM), based on the U.S. Navy's standard split-step Fourier parabolic Equation (PE) model was used [63,64]. The models were run using the measured sound speed profiles from the CTD casts at the mooring location, as well as sound speed profiles derived from the SalishSeaCast model, and tidal timing as well as elevation data obtained from the WebTide model [65]. ...
Oceanic acoustic environments are dynamic, shaped by the spatiotemporal variability in transmission losses and sound propagation pathways of natural and human-derived noise sources. Here we used recordings of an experimental noise source combined with transmission loss modeling to investigate changes in the received levels of vessel noise over space and time as a result of natural water column variability. Recordings were made in the Juan de Fuca Strait, on the west coast of Vancouver Island, a biologically productive coastal region that hosts several cetacean species. Significant variability in noise levels was observed due to changing water masses, tied to seasonal temperature variation and, on a finer scale, tidal movements. Comparisons of interpreted received noise levels through the water column indicated that vessel noise recorded by bottom-stationed monitoring devices might not accurately represent those received by whales in near-surface waters. Vertical and temporal differences of 3–5 dB were commonly observed in both the recorded and modeled data. This has implications in estimating the success of noise mitigation measures, and our understanding of the change in sound fields experienced by target species for conservation.
... Among them, normal mode theory solves the Sturm-Liouville eigenvalue problem of the acoustic wave; parabolic equation theory and finite-element theory as numerical methods could accurately solve the three-dimensional underwater acoustic problem. Both the parabolic equation theory and the ray theory are effective methods for predicting underwater sound propagation to solve the range-dependent ocean [26], however parabolic equation theory reduce timeliness and intuitiveness compared with ray theory. Ray tracking theory is based on the generalized Snell's law [27], which adapts to study the sound deflection caused by the continuous variation of sound speed. ...
Oceanic fronts involved by ocean currents led to strong gradients of temperature, density and salinity, which have significant effects on underwater sound propagation. This paper focuses on the impact of the oceanic front on three-dimensional underwater sound propagation. A joint experiment of ocean acoustic and physical oceanography at the western North Pacific fronts is introduced. The measurement data for sound waves passed through the oceanic front is processed. The results are analysed and compared with the numerical simulation. It was found that transmission loss presented some difference when the source was located in the front centre and sound waves propagated towards water mass on opposite sides of the front centre. And when the sound field is excited by the underwater explosion at a depth of 200 m, the effects of the horizontal refraction cannot be neglected. On the other hand, the transmission loss for sound pressure fell sharply and rose rapidly at the side of cold water masses.
