Sonar Performance Prediction Research Articles

Probabilistic acoustic predictions leveraging NESBA and ARGO measurements

Ocean acoustic propagation models used for sonar performance prediction often rely on global or regional ocean modeling and data assimilation, or monthly climatologic averages, for estimating the 4D ocean temperature and salinity (T/S) field. Ocean models rely heavily on satellite-based ocean surface measurements, e.g., sea surface temperature (SST) and height (SSH), plus typically vastly less resolved in space and time vertical water column measurements derived using ship-based and/or ARGO float instruments. Systems also exist that create 3D synthetic T/S fields from satellite SST/SSH measurements, e.g., the Modular Ocean Data Assimilation System (MODAS) and the more recent Improved Synthetic Ocean Profile (ISOP) system. It is unclear, however, how well these synthetic T/S fields capture significant ocean ducting conditions. Similarly, climatologic averages combine historical vertical T/S profile data in spatial cells by month; but the averaging process tends to blur out important features. This talk will address the use of 2021 New England Shelf Break Acoustics (NESBA) experiment and ARGO vertical profile data for: (1) testing the degree to which ocean models or climatology capture important acoustic features under a range of conditions; and (2) providing more realistic sonar performance prediction means and uncertainties. [Work supported by the Office of Naval Research].

Convert path planning for underwater vehicle based on sonar detection probability

Underwater vehicle is essential for many tasks such as underwater exploration, equipment deployment and covert reconnaissance. Many underwater missions must be carried out with stealth to avoid being detected by enemy sonars, and safety and security are required in such high-risk situations. In this paper, a covert path planning algorithm for underwater vehicle is proposed based on the sonar detection probability analysis. First the sonar performance prediction model is constructed with sonar parameters, marine environment and underwater vehicle acoustic properties. Afterwards an improved ant colony algorithm is established by minimizing the cumulative detection probability. The evaluation indicate that the optimal covert path obtained for underwater vehicle is effective for reducing the cumulative detection probability.

New England Shelf Break Acoustic (NESBA) experiment: Stochastic acoustic prediction

Sonar performance prediction has been a topic of interest for decades. Predictions are based on acoustic propagation modeling that is typically deterministic, i.e., lacking quantified confidence or uncertainty bounds. While this may be adequate in some locations, in oceanographically complex regions it can result in misleading predictions and conclusions that sonar performance predictions are unreliable. A better approach includes explicit calculations of the underlying source, receiver, and environmental uncertainties to provide a prediction confidence level. In oceanographically complex areas, the resulting uncertainties are likely to be high; however, knowledge of this allows uncertainties to be managed. The NESBA Signals and Noise experiment was conducted in April–May 2021. The goal of the experiment was to assess the potential for sonar prediction effectiveness gains given improved environmental awareness. This talk addresses the NESBA sub-goal of demonstrating that high-resolution ocean modeling with ensemble forecasts (in this case, Regional NCOM) could be leveraged for stochastic acoustic predictions, resulting in increased accuracy in performance predictions with quantified uncertainty bounds. NESBA results will be presented leveraging the high-resolution ocean model forecasts with ensembles run concurrently with NESBA and the numerous temperature/salinity profiles taken during the experiment. [Work supported by the Office of Naval Research.]

Three Dimensional Volume Coverage in Multistatic Sonar Sensor Networks

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.

Rapid estimation of empirical Green’s functions for passive acoustic characterization of dynamic shallow-water environments

Applications of acoustic noise interferometry to passive remote sensing of the ocean and improvement of sonar performance predictions rely on retrieval of empirical Green’s functions (EGFs) from synchronized measurements of ambient sound at spatially separated points. Usually, long noise averaging times of many hours and even days are employed to achieve precise passive measurements of the ray or normal mode travel times that are necessary for evaluation of the environmental parameters with oceanographically relevant accuracy. However, long averaging times have limited utility when nonlinear internal waves or other oceanographic processes cause significant, time-dependent variations in sound propagation conditions. Analysis of the noise records acquired during the Shallow Water 2006 experiment reveals that EGFs can be reliably retrieved from cross-correlations of noise recorded over periods as short as 10 minutes by moored receivers horizontally separated by 40–50 ocean depths. The measured EGFs evolve as the sound speed field changes. Suitability of the EGFs for acoustic remote sensing is demonstrated by geoacoustic inversion of the passively measured normal mode dispersion curves and comparison to previously reported inversion results. Rapid EGF retrieval has far-reaching implications for operational use of the underwater acoustic noise interferometry. [Work supported by NSF.]

Predicting Acoustic Variability: Pragmatic Considerations for Selecting a Stochastic or Deterministic Approach

Sonar performance prediction models require environmental inputs, including bottom depth and composition, sound-speed profile (SSP), and wave or wind conditions. Temporal and spatial variation and measurement uncertainties in environmental inputs result in uncertain model outputs. Ideally, acoustic fluctuations could be predicted from input parameter variability using a suitable modeling framework. A data set consisting of 460 SSPs and acoustic transmission loss (TL) measurements between 300 and 900 Hz was acquired during GLISTEN15, a 2015 Mediterranean Sea trial led by North Atlantic Treaty Organization's Centre for Maritime Research and Experimentation. The TL was measured in 5-min sampling periods at fixed source-to-receiver ranges of 0.5-10 km over two separate days. A gradual change in TL as large as 15 dB over 5 min was observed at some receivers and frequencies. Predicted TL variability was compared to measured TL variability by using the SSPs as inputs to an acoustic propagation model. Purely stochastic modeling either over- or underpredicted variability, depending on the temporal and spatial separation of the SSPs used as inputs. In this article, an internal wave model derived from the SSP time series was used to demonstrate the possible effect of internal waves propagating perpendicular to the source-receiver direction. The best agreement between modeled and measured variability was obtained for internal waves of 1536-s period and 0.8-1.6-m amplitude. In this way, an approach that included a physically plausible model of internal wave dynamics grounded in a few relevant in situ measurements provided good predictions of observed TL variability and a reasonable explanation of its origin.

Acoustically measured transmission loss improves effectiveness of sonar search

The Sonar Equation is the heart of the process for predicting the effectiveness of an Antisubmarine Warfare (ASW) ship's sonar in the underwater detection of submarines. The equation's determination of Ranges associated with probabilities of detection and false alarms is the fundamental input for the selection of the most effective sonar modes in an individual ship and, at the broader tactical scale, in the selection of ASW search plans. The Sonar Equation's predictions of sonar performance, however, are subject to considerable error when employed tactically at sea in ASW operations. A major source of the error is the input, Transmission Loss, usually derived from XBT “snapshots” of temperature versus water depth. The resulting predictions of sonar performance are projected uniformly over space and time, leading to predictions that are stale and inaccurate. Accuracy of the Sonar Equation's output, however, can be improved by measuring Transmission Loss acoustically and continuously. A stream of pulses, periodically broadcast from a specially designed expendable sound source, is measured at the front end of the ship's sonar. A large population of Transmission Loss values is built up from which strong statistical inferences regarding detection can be drawn. The power of a continuously updated population of Transmission Loss values leads to more accurate inferences of the local acoustic environment.

The update of Urick’s Principles of Underwater Sound

For over 50 years Principles of Underwater Sound by Robert Urick has been a widely used book on underwater acoustics and sonar for practicing engineers, scientists, technicians, project managers, teachers and students and it is in the last stages of an extensive update to be released this spring. The book encapsulates the fundamentals and phenomena of underwater sound as applied to the Sonar Equation, the heart of prediction of sonar performance, and the book’s contents lie squarely in the middle between theory at one end and practical technology at the other. The editor and contributors of the update have attempted to imbue the new edition with the same “blue collar” spirit of Urick’s original work but with so much new technology to address since the present edition was published in 1983, it’s been difficult to keep the updated printed edition to a convenient size. Therefore, in addition to the traditional printed book edition, we are introducing a new e-book edition which will provide more detail than the printed edition. This presentation includes: (1) Descriptions of the two new formats (2) Examples of updated technology, and (3) New subjects: sonar transmissions’ effect on the health of marine mammals; sea floor mapping; locating downed airliners; and search plans.

Spatial and temporal correlations of mid-frequency acoustic signals on the Chukchi Sea Shelf

The spatial and temporal correlations of 1.5–4 kHz LFM signals were measured using a 64-element billboard array and a mid-frequency acoustic source, deployed on the Chuckchi Sea Shelf during October 2016 to March 2017. The billboard array was deployed at 150 m water depth and the acoustic source was deployed at 320 m water depth near the shelf break to measure the acoustic signals propagating along a 20 km cross-shelf path. Both receiver depth (100 m) and source depth (200 m) were chosen to provide an effective transmission in a sound channel bounded by Pacific Summer Water at 40–100 m and warm Atlantic Water at 140-320 m below the surface. It was observed that signal amplitude variations and correlations were strongly affected by a combination of highly dynamic oceanography and Arctic sea ice. The effects of measured ocean and sea-ice variabilities were also simulated using a PE acoustic propagation model. The agreement between numerical predictions and measured data provides a framework for a robust mid-frequency sonar performance prediction capability in the Arctic. [Work supported by the ONR.]

Measurements of Midfrequency Acoustic Backscattering From a Sandy Bottom in the South Yellow Sea of China

Acoustic bottom scattering is the main source of reverberation background in target detection and recognition near the seafloor. Accurate estimation of bottom scattering strength is necessary for prediction of sonar performance in the presence of reverberation in shallow-water environments. A measurement of bottom backscattering strength at midfrequency in a shallow-water environment of the South Yellow Sea of China was carried out in April 2016, where the water depth was 39 m. The surface of the seafloor was relatively flat and covered by some shell fragments observed through a digital camera system. Core data showed that the main component of sediment at this experiment site was fine sand. The backscattering strength as a function of grazing angle (20°–70°) in the frequency range of 6–12 kHz was extracted from monostatic backscattering data, employing an omni-directional source and an omni-directional hydrophone. The measured backscattering strength increases with the increase of the grazing angle, and changes more rapidly at large grazing angles (60°–70°). Comparing the data at different frequencies, it is found that the measured backscattering strength slightly increases with the increase of the acoustic frequency. A fit of Lambert's law to the measured data shows that the backscattering strength deviates from the Lambert's law at large grazing angles. Finally, the fitted result of a scattering model to the measured data suggests that seafloor roughness scattering is larger than that from sediment volume heterogeneity in the measurement frequency band.

Ambient Noise Vertical Directionality and Reflection Loss Estimates for a Shallow Tropical Site with a Fast Sound Speed Bottom

For high frequency ocean acoustic modeling applications, seabed reflection loss is a useful alternative input compared to conventional geoacoustic model parameters. Reflection loss can be estimated by comparing the noise intensity of the up and down components of the ambient noise vertical directionality pattern. The potential of this method is demonstrated with experimental data spanning one week, collected off shallow east coast of India using a 21 element vertical hydrophone array. The compact and easily operable vertical array has been designed for high frequency directionality estimation in the band 2–10 kHz. The ambient noise data are beam formed to arrive at the vertical directionality pattern. Further reflection loss values as a function of frequency and grazing angle have been estimated for 1/3 octave bands for a sandy sea bed in warm tropical waters. This has been compared with modeled reflection loss estimates using OASR reflection loss module of OASES. This will serve as inputs to propagation models for applications such as inverse techniques, ambient noise modelling, and sonar system performance prediction.

Estimating channel capacity and sonar performance in the changing arctic

Underwater acoustic processes including ambient noise, propagation, reverberation, and scattering have been studied for over half a century in the Canadian Arctic. Despite this, realistic predictions of communications and sonar performance have proved challenging due to the complex impact of ice cover on these processes, and the relative scarcity of sound speed profiles and information about the seabed. This challenge has been exacerbated in recent years because the rapid change in environmental conditions is reducing the relevance of many historical records. A key component of the present effort is to extract site specific model inputs from the environmental data contained in the literature, with an informed weighting toward more recent measurements. Site specific modelling was enhanced using inputs from recent year-long ambient noise recordings. Model inputs were also influenced by specific source and receiver hardware being developed for use in future Arctic experiments. In this paper, low frequency sonar and communication performance estimates in the frequency band 20–250 Hz, based on site-specific environmental inputs and this low frequency hardware, are presented for several geographical regions of strategic relevance in the Canadian Arctic.

Local suppression of bioturbation and its acoustic consequences

Seafloor roughness in shallow waters can be both spatially and temporally dynamic. While wind-driven waves and bottom currents can generate bedforms such as ripples on sand sediments, bottom-feeding fish tend to limit the lifetime of these anisotropic features. This greatly reduces the time during which high frequency sonars can exploit the ripples to coherently penetrate the sediment at subcritical angles. Sonar performance prediction in these environments therefore relies heavily on sediment transport models to generate roughness predictions that accurately reflect changes on these time scales. The competition between the hydrodynamics and biology of the environment can be further complicated by the natural introduction of materials which can locally suppress the bioturbation. Two examples are discussed here: The first comes from the Sediment Acoustics Experiment in 2004 where a storm deposited a layer of mud over some areas of seafloor protecting the bedforms from fish. The second is from the recent Target and Reverberation Experiment where there is evidence that spatially-varying shell content can also reduce the effects of bioturbation. The consequences for sonar performance in these two cases are discussed. [Work supported by the U.S. Office of Naval Research and the Strategic Environmental Research and Development Program.]

The McKinney-Anderson paper and its impact on sediment acoustics

The earliest comprehensive, in-situ measurements of the acoustic backscattering strength of the seabed as a function of frequency, grazing angle and sediment type were made by McKinney and Anderson, and published in the society’s journal in 1964. It was a landmark paper and laid the groundwork for subsequent measurements and models. It is widely referenced in books and papers where seabed scattering and sonar performance prediction are concerned. Scattering strength and sediment classification, usually quantified in terms of the mean grain diameter, were thought to be closely connected. In the laboratory, using pristine samples of sorted sand, this connection was clearly demonstrated, but in situ it was overwhelmed by other factors, such as uneven size distributions, biological activity and roughness. The underlying nature of the seabed is also reflected in the scattering strength. For the same roughness, density and sound speed, the solid/fluid model consistently overestimates the scattering strength because it cannot account for the relative motion between grains and pore water. Thus, recent advances in poroelastic modeling can shed new light on the measurements of McKinney and Anderson. [Work supported by ONR, Ocean Acoustics Program.]

Seabed Characterization From Ambient Noise Using Short Arrays and Autonomous Vehicles

Reliable SONAR-performance prediction in shallow water requires knowledge of the seabed reflectivity, or its geoacoustic properties, which is expensive and difficult to acquire in situ . This paper illustrates two sea trials conducted in different shallow water areas to investigate the feasibility of acquiring such knowledge efficiently from measurements of naturally occurring ambient noise by an array that is compact enough to be mounted on a small autonomous underwater vehicle. The system relies on a previous technique for passively estimating the bottom reflection loss from the acoustic noise field generated by wind and breaking waves at the sea surface. Results from these experiments supported by numerical modeling are presented and compared with independent measurements of the relevant seabed reflectivity properties. The results obtained from both experiments demonstrate the potential of using autonomous underwater vehicles for seabed characterization.

Tempered particle filters for non-linear model selection and uncertainty quantification of highly informative seabed data

Knowledge about seabed properties is important for many geoscientific and navy applications, such as sediment transport, sonar performance prediction, and detection of unexploded ordnance. Bayesian model selection and uncertainty estimation have been shown to provide detailed, quantitative seabed knowledge that is valuable for these applications. However, the extreme computational cost limits the utility of Bayesian methods for increasingly common big data sets. Here, we consider geoacoustic reflectivity surveys based on towed source and receiver arrays. Such systems produce thousands of data sets with high information content that require non-linear inversion along tracks many kilometers in length and cannot be analyzed by standard Bayesian sampling. A particle filter that includes reversible jump Markov chain Monte Carlo updates is applied here for efficient posterior probability estimation. Efficiency is improved by likelihood tempering of various particle subsets and including information exchange within the particle cloud. The tempering applies to reversible jump updates and leads to significantly improved exploration of the trans-dimensional seabed model which accounts for changes in the number of sediment layers and their properties along the track. For challenging track sections, where data change abruptly, the particle cloud is resampled to increase the number of tempered particles. [Work supported by the U.S. Dept. of Defense, through SERDP, and by ONR Ocean Acoustics.]

Effects of time dispersion on echo, reverberation, and echo to reverberation ratio in a range-dependent Pekeris waveguide

In shallow water, active sonar performance is typically limited by reverberation, making the prediction of target echo and reverberation, and their ratio, an important part of sonar performance prediction. In range-dependent shallow water environments, these quantities are often calculated without considering the effect of dispersion. The effect of time dispersion is considered taking examples for a range-dependent bathymetry from the 2010 Weston Memorial Workshop. Using the analytical method of [M. A. Ainslie and D. D. Ellis (in press), IEEE Journal of Oceanic Engineering], combined with normal mode predictions [D. D. Ellis (1995). The Journal of the Acoustical Society of America, 97(5), 2804-2814], neglect of time dispersion is found to result in an error of up to 16 dB in the echo level for a short CW pulse (duration 3 ms). The effect of 3D geometry is considered, and results for a cylindrically symmetric bathymetry are shown to differ by up to 14 dB from the corresponding results with a Cartesian symmetry. The difficulties associated with modeling an LFM pulse are discussed.

A century of sonar performance prediction

Developments in sonar technology went hand in hand with increased scientific understanding of the factors affecting sonar performance The first known reference is from Lichte (Physikalische Zeitschrift 1919) where cylindrical spreading for propagation loss was accounted for in deep water (convergence zone, CZ) propagation. Users of sonar in the 1920s encountered an “afternoon effect”, a temporary dip in performance that occurred during warm summer evenings, was explained in the 1930s by Iselin and Batchelder in terms of vertical temperature gradients in the sea. A significant amount of work occurred in both World War I(Klein 1962, Wood 1965) and World War II to understand, and therefore be able to predict, the capabilities of sonar systems to detect submerged targets. Understanding of sonar performance increased rapidly during WW2 and continued to advance during the Cold War. Advances in the understanding of sonar performance modeling before, during and after the Second World War are described.

Echo, Reverberation, and Echo-to-Reverberation Ratio for a Short Pulse in a Range-Dependent Pekeris Waveguide

In shallow water, active sonar performance is typically limited by reverberation, making the prediction of target echo, reverberation, and echo-to-reverberation ratio an important part of sonar performance prediction. In range-dependent shallow-water environments, the echo is often calculated without considering the effect of dispersion, and reverberation predictions are often limited to Lambert's rule for seabed scattering. In this paper, analytical formulae are derived for the echo, including the effect of time dispersion, and for reverberation for non-Lambert scattering laws. More specifically, the method for calculating the short-pulse echo intensity developed by Harrison and Nielsen is extended to a range-dependent bathymetry. For reverberation, the Zhou–Harrison method is also generalized to cover an arbitrary power law for the seabed scattering coefficient, combined with a range-dependent bathymetry. These formulae are applied to range-dependent test cases from an international workshop, and compared with predictions using a normal mode sum. Neglect of time dispersion is found to result in an error of up to 16 dB. Results for a cylindrically symmetric bathymetry differ by up to 14 dB from the corresponding results with a Cartesian symmetry.

Frequency based noise coherence-function extension and application to passive bottom-loss estimation.

Accurate modeling of acoustic propagation in the ocean waveguide is important to SONAR-performance prediction. Particularly in shallow waters, a crucial contribution to the total transmission loss is the bottom refection loss, which can be estimated passively by beamforming the natural surface-noise acoustic field recorded by a vertical line array of hydrophones. However, the performance in this task of arrays below 2 m of length is problematic for frequencies below 10 kHz It is shown in this paper that, when the data are free of interference from sources other than wind and wave surface noise, data from a shorter array can be used to approximate the coherence function of a longer array. This improves the angular resolution of the estimated bottom loss, often making use of data at frequencies above the array design frequency. Application to simulated and experimental data shows that the technique, rigorously justified for a halfspace bottom, is effective also on more complex bottom types. Dispensing with active sources, small autonomous underwater vehicles equipped with short arrays can be envisioned as compact, efficient seabed-characterization systems. The proposed technique is shown to improve significantly the reflection-loss estimate of an array that would be a candidate for such application.