Sonar Performance Models Research Articles

Forty-year review of developments in underwater acoustic modeling

This is the sixth paper in a series of reviews presented at eight-year intervals starting in 1979 [J. Acoust. Soc. Am. 65, S42 (1979); 82, S102 (1987); 97, 3312 (1995); 114, 2430 (2003); 129, 2631 (2011)]. All surveys covered basic acoustic models and sonar performance models. Basic acoustic models included 143 propagation, 29 noise, and 32 reverberation models. Propagation models were categorized according to ray theory, multipath expansion, normal mode, fast field, and parabolic approximation formulations; further distinctions were made between range-independent and range-dependent solutions. Noise models included ambient noise and beam-noise statistics models. Reverberation models included cell and point-scattering formulations. The 44 sonar performance models included active sonar models, model operating systems, and tactical decision aids. Active sonar models integrated basic acoustic models, signal-processing models, and supporting databases into cohesive operating systems organized to solve the sonar equations. Model operating systems provided a framework for the direct linkage of data-management software with computer-implemented codes of acoustic models. Tactical decision aids represented a form of engagement-level simulation that blended environmental and sonar performance information with tactical rules. The overall inventory increased by 5 models per year; historical references are maintained in the fifth edition of Underwater Acoustic Modeling and Simulation.

Use of Landsat 8 data for characterizing dynamic changes in physical and acoustical properties of coastal lagoon and estuarine waters

This study intends to develop methodologies that use high resolution satellite data from Landsat 8 (Operational Land Imager) OLI and (Thermal Infrared Sensor) TIRS sensors for characterizing spatial and temporal changes in physical and acoustical properties of coastal lagoon and estuarine waters. It employs multiple steps to achieve this possibility: a novel atmospheric correction algorithm is applied to OLI spectral data to retrieve water-leaving radiances which are key inputs for the applied models; appropriate parameterizations are developed for the OLI bands and used in conjugation with a hybrid model to produce the spectral absorption coefficients of coloured dissolved organic matter (aCDOM) and to derive surface salinity fields which inversely correspond with the aCDOM values; an efficient algorithm is employed to estimate surface water temperature using thermal infrared bands, and well-known models are employed with the satellite-derived products to determine the acoustical properties (sound attenuation and speed). Results from the above methodology were evaluated using in-situ data and Landsat 8 OLI matchup data acquired over the coastal lagoon systems (e.g., Chilika Lagoon on the coast of Bay of Bengal) during monsoon and non-monsoon seasons. The uncertainties associated with the derived products such as CDOM, salinity, temperature, sound attenuation and speed were found to be within the desirable mission goal. Recognizing the importance of the salinity gradient that plays a unique and fundamental role in defining a transitional ecosystem, spatial and temporal patterns in the structure of the salinity gradient were examined together with the CDOM patterns. High resolution OLI products exhibited a general horizontal gradient with salinity decreasing from the lagoon mouth in the eastern and central sectors to the river mouth in the northern sector and a near uniform gradient with moderate salinity in the adjacent locations (southern sector) of the lagoon. The time-series of OLI products further showed that spatial and temporal structures of the salinity are modulated by the terrestrially delivered freshwater inputs, tidal forcing at the lagoon mouth, mixing of these two waters sources, and local geomorphology. Surface water temperature products derived from the TIRS sensor for the lagoon and its adjoining locations depicted a well pronounced seasonal cycle with warmer temperatures modulated by reduced mixing and increased solar heating and stratification during non-monsoon, summer months and cooler temperatures during monsoon, winter months. The effect of salinity and temperature on the sound attenuation and sound speed was prominent in the locations of the freshwater discharge and tidal mixing regimes, where the salinity exerted a greater influence on both sound attenuation and speed despite the opposing effects of surface water temperatures. In areas of surface heating and stratification, both salinity and temperature increased causing an increase in sound attenuation and speed over the ranges found in the lagoon. These results are important for sonar performance modelling and operation of acoustic devices in such shallow water environments impacted by the terrestrial and ocean forcing factors.

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.

Estimation of Seabed Properties Using Ambient Noise from Shallow Water Sites of the Indian Continental Shelf

ABSTRACTIn this study an attempt has been made to extract sediment geoacoustic properties using ambient noise measured from a vertical hydrophone array. Time series noise data recorded from three shallow water sites (Chennai, Cuddalore and Cochin) along the Indian continental shelf were used for the analysis. The compressional sound speed of sediment for all the sites was estimated from the vertical directionality of ambient noise. Using the value of the compressional sound speed remaining wave properties and material properties were deduced from the Grain-Shearing (G-S) theory of wave propagation in saturated granular media. The type of sediment extracted from the G-S theory correlates well with the results obtained from sieve and particle size analysis of grab samples, collected from all the sites. The study clearly shows the application of ambient noise in extracting environmental information in shallow water, and further applying it to improve sonar performance modeling.

Acoustical terminology in the Sonar Modelling Handbook

The UK Sonar Modelling Handbook (SMH) defines the passive and active Sonar Equations, and their individual terms and units, which are extensively used for sonar performance modelling. The new Underwater Acoustical Terminology ISO standard, which is currently being developed by the ISO working group TC43/SC3/WG2 to standardize terminology will have an impact on the SMH definitions. Work will be presented comparing the current SMH terminology with both the future ISO standard and other well-known definitions to highlight the similarities and differences between each of these.

Finding Acoustically Stable Areas Through Empirical Orthogonal Function (EOF) Classification

Validity of sonar performance models is generally limited by environmental uncertainty, and particularly uncertainty in the sound-speed profile (SSP). Rapid environmental assessment (REA) missions, e.g., using gliders, and advanced ocean models may be used to reduce this uncertainty before sonar operation in hostile waters. This work shows how data from ocean models may be used for planning of REA missions. The area of operation is divided into oceanographically stable subareas using empirical orthogonal functions (EOFs) and different methods of clustering analyses on SSPs from the ocean model. The acoustic stability of each subarea is assessed using sonar performance modeling. Acoustically unstable areas are divided into smaller subareas. Acoustically stable groups are represented by a single SSP. A map of acoustically stable areas in the area of operation is the main output. Large, geographically contiguous groups indicate acoustically stable areas where frequent SSP measurements are unnecessary, e.g., low concentration of gliders. Small and noncontiguous groups indicate the opposite. Other applications include determination of suitable locations for sonar tests that require stable sonar conditions, and efficient optimization of sonar operation in acoustically stable areas.

Classification of acoustically stable areas using empirical orthogonal functions

Sonar performance modeling is crucial for submarine and anti–submarine operations. The validity of sonar performance models is generally limited by environmental uncertainty, and particularly uncertainty in the vertical sound speed profile (SSP). Rapid environmental assessment (REA) products, such as oceanographic surveys and ocean models may be used to reduce this uncertainty prior to sonar operations. Empirical orthogonal functions (EOF) applied on the SSPs inherently take into account the vertical gradients and therefore the acoustic properties. We present a method that employs EOFs and a grouping algorithm to divide a large group of SSPs from an ocean model simulation into smaller groups with similar SSP characteristics. Such groups are henceforth called acoustically stable groups. Each group represents a subset in space and time within the ocean model domain. Regions with low acoustic variability contain large and geographically contiguous acoustically stable groups. In contrast, small or fragmented acoustically stable groups are found in regions with high acoustic variability. The main output is a map of the group distribution. This is a REA product in itself, but the map may also be used as a planning aid for REA survey missions.

The effect of wind-generated bubbles on sea-surface backscattering at 940 Hz

Reliable predictions of sea-surface backscattering strength are required for sonar performance modeling. These are, however, difficult to obtain as measurements of sea-surface backscattering are not available at small grazing angles relevant to low-frequency active sonar (1-3 kHz). Accurate theoretical predictions of scattering strength require a good understanding of physical mechanisms giving rise to the scattering and the relative importance of these. In this paper, scattering from individual resonant bubbles is introduced as a potential mechanism and a scattering model is derived that incorporates the contribution from these together with that of rough surface scattering. The model results are fitted to Critical Sea Test (CST) measurements at a frequency of 940 Hz, treating the number of large bubbles, parameterized through the spectral slope of the size spectrum for bubbles whose radii exceed 1 mm, as a free parameter. This procedure illustrates that the CST data can be explained by scattering from a small number of large resonant bubbles, indicating that these provide an alternative mechanism to that of scattering from bubble clouds.

Accounting for spatiotemporal variability of shallow water waveguides in the geoacoustic inverse problem.

Reliable acoustic field predictions are essential for sonar performance modeling and matched field processing, particularly related to the geoacoustic inverse problem. On continental shelves and slopes, properties of the water column can vary widely in both time and space, while the seabed can be heterogeneous over scales of only a few kilometers and sometimes less. This combined spatiotemporal variability of the acoustic waveguide was a hallmark of the Shallow Water Experiment 2006 (SW06). This is illustrated using data collected during SW06 for an acoustic field measured on a fixed receiver array for a source towed along a radial track and repeated several times. In these experiments, the large degree of variability in the observed acoustic fields was attributed to large-scale changes in the water column that occurred over the course of a day. Properly accounting for this variability proved to be essential for obtaining robust geoacoustic inversion results for the seabed. Specifically, the spatiotemporal properties of the water column had to be accounted for in the forward models from which objective functions were derived. Methods for capturing the observed water-column variability are discussed as used in the inverse problem as well as for the more general problem of model-data comparison.

Bayesian acoustic prediction assimilating oceanographic and acoustically inverted data

The prediction of the acoustic field evolution on a day to week frame, in a given oceanic area, is an important issue in sonar performance modeling. It relies primarily on acoustic propagation models, which convert water column and geometric/geoacoustic parameters to ‘instantaneous’ acoustic field estimates. In practice, to model the acoustic field, even the most accurate acoustic models have to be fed with simplified environmental descriptions, due to computational issues and to a limited knowledge of the environment. This is a limitation, for example, in acoustic inversion methods, in which, by maximizing the proximity between measured and modeled acoustic signals, the estimated environmental parameters are deviated from reality, forming what is normally called an ‘acoustically equivalent environment’. This problem arises also in standard acoustic prediction, in which, the oceanographic forecasts and bottom data (typically from archives) are fed directly to an acoustic model. The claim in the present work is that, by converting the oceanographic prediction and the bottom properties to ‘acoustically equivalent’ counterparts, the acoustic prediction can be obtained in an optimal way, adapted to the environmental model at hand. Here, acoustic prediction is formulated as a Bayesian estimation problem, in which, the observables are oceanographic forecasts, a set of known bottom parameters, a set of acoustic data, and a set of water column data. The predictive posterior PDF of the future acoustic signal is written as a function of elementary PDF functions relating these observables and ‘acoustically equivalent’ environmental parameters. The latter are obtained by inversion of acoustic data. The concept is tested on simulated data based on water column measurements and forecasts for the MREA'03 sea trial.

Computational Predictions of sonar performance based on full wave acoustic propagation modeling

For fishery acoustics, sonars offer the advantage of inferring the biomass of a given ocean volume at a lower operational cost of traditional echo-sounders, but wave guide effects which induce multi-pathing yield unknown variances in bio-mass estimates. A sonar performance model has recently been developed to assist in understanding this variance based on the para-axial approximation to the acoustic wave equation. The model takes into account the geometry of the sonar deployment (source depth, transmit/receive beam patterns, pulse shape) and the acoustic environment (water column and geo-acoustic bottom) and creates a data base of two-way transfer functions for a chosen set of range-depth cells. Then, given a hypothetical aggregation or school of fish, each with its own target strength, as well as noise and reverberation levels, one can create intensity maps which can be used to perform sensitivity studies to ascertain the performance characteristics of a fishery sonar. This talk will discuss the theoretical development of the model, its constraints, and provide an example which highlights its potential for future work. The example will be in a shallow water (100 m) environment using acoustic frequencies of around 10 kilohertz for imaging aggregations of pelagic fish. The Naval Ocean Partnership Program is gratefully acknowledged for this work.

Coherent effects of flow- and pressure-hull of a generic submarine on target scattering in an active sonar performance model

Since the late eighties the sonar performance model ALMOST for active and passive sonar is under development at TNO. For active detection performance, first a point target was used, with a single target strength value dependent on parameters like aspect angle, based on measurements or other sources. However by now there is a demand for TS of ships and wakes with realistic dimensions and characteristics. A generic sub was modeled with additional software routines, as a pixel file. A newly developed time domain model for hull reflection was implemented, also using scattering pixels, assuming multiple scattering with damping in the metal hull layer. This extension of the pixel modelling is evaluated versus literature data. Some modeling results of target strength computations are shown, for a generic submarine with pressure hull, with aspect angle, frequency and bandwidth as parameters.

Array gain for a cylindrical array with baffle scatter effects

Cylindrical arrays used in sonar for passive underwater surveillance often have sensors surrounding a cylindrical metal baffle. In some operational sonars, the phones in each stave (i.e., each line of phones aligned with the cylinder axis) are hardwired together so that the array is equivalent to a baffled circular array of directional elements, where each element corresponds to a line array of omnidirectional phones steered to broadside. In this paper a model is introduced for computing the array gain of such an array at high frequencies, which incorporates baffle scatter using infinite, rigid cylinder scattering theory, and with ambient noise described by an angular spectral density function. In practice the phones are often offset from the baffle surface, and the acoustic field sampled by the staves is distorted at high frequencies due to interference between the incident and scattered fields. Examples are given to illustrate the resulting array gain degradation, using three noise distributions that are frequently used in sonar performance modeling: three-dimensional isotropic, two-dimensional isotropic, and surface dipole noise.

Extrapolation of geoacoustic properties using seismic reflection data: A shallow water example

In areas of the world that are bottom limited, sonar performance models require accurate seabed geoacoustic properties. Geoacoustic properties, e.g., sound speed, density, and attenuation as a function of depth, frequency and geographic position, are expensive to acquire over large areas. Most geoacoustic techniques, including direct sampling (e.g., cores, probes) or acoustic inversion methods (e.g., matched field methods) spatially sample either in 1‐D or 2‐D. Conducting these measurements in a dense grid over large areas is not feasible and as a consequence seabed database developers rely on seismic reflection data coupled with geologic models to spatially interpolate and extrapolate the geoacoustic properties. Seismic reflection data is attractive for this purpose because it has much greater geographic coverage and contains information on underlying geologic processes that ultimately control the geoacoustic properties. The inherent strengths and weaknesses of seismic data for geoacoustic extrapolation are explored using seismic reflection transects in the Straits of Sicily together with ground‐truth geoacoustic properties and their uncertainties (from wide‐angle reflection measurements) at each end of the transect. The uncertainties of the extrapolated geoacoustic properties are determined with and without the ground‐truth data. [Work supported by the Office of Naval Research OA321 and the NATO Undersea Research Centre.]

Robustness and constraints of high-frequency ambient noise inversion for sonar performance estimation

Perhaps the most dominant source of error in sonar performance estimation in shallow waters is the geoacoustic description of the seafloor. Various investigators have studied the possible use of ambient noise to infer some key parameters such as the critical angle, geoacoustic properties, or bottom loss. Most operational sonar performance models and tactical decision aids require the bottom loss as a function of grazing angles and frequency to describe the reflected acoustic energy off the ocean floor. Harrison and Simons [Harrison & Simons, ‘‘Geoacoustics inversion of ambient noise: A simple method,’’ J. Acoust. Soc. Am. 112, 1377 (2002)] described a simple measurement approach to infer the bottom loss from ambient noise measurement on a vertical line array. This approach is well suited as an environmental measurement approach during naval operations. The robustness and operational constraints of this measurement approach are quantified using a high-frequency ambient noise model that accounts for wind, shipping, and thermal noise [Ferat & Arvelo, ‘‘Mid-Frequency Ambient Noise Directivity And Array Response in Range-Dependent Environments,’’ High-frequency Ocean Acoustics Conference Proceedings, 2004)]. The effects of array tilt, nearby shipping noise, water absorption, thermal noise, surface-noise dipole pattern, and array beam response on sonar performance estimation are demonstrated. Mitigating factors to maximize estimated bottom loss accuracy will also be outlined.

Measurements of near-surface bubble plumes in the open ocean with implications for high-frequency sonar performance.

This study examines near-surface bubble data obtained with a self-contained 200-kHz inverted echo-sounder deployed at Ocean Station Papa (NE Pacific, 1400 km west of Vancouver Is.) over an 81-day period in the spring of 1996. The instrument operated continuously, recording calibrated volume scattering profiles from near-surface bubbles with 3-s and 30-cm resolution. The data show the frequent occurrence of bubbles organized into vertical, plume-like structures, presumably drawn downwards by turbulence and other near-surface circulations. Average bubble plume penetrations of up to 15 m were observed, with maximum penetrations up to 25 m. Within the plumes, the backscatter cross section exhibited an exponential decay with depth, with e-folding scale in the range 0.5 to 3 m, increasing proportionally to the square of average plume depth. Using standard models for bubble scattering, and incorporating recent acoustic resonator measurements of bubble-size distributions along with actual bubble plume data, high-frequency near-surface sonar performance models were developed. These models show that on a ping-to-ping basis the bubble plume structures can induce significant spatial variations in the reverberation level and path-integrated extinction losses to near-surface targets.

Recent developments in underwater acoustic modeling

This is the fourth paper in an ongoing series of research reviews presented at eight-year intervals [P. C. Etter and R. S. Flum, J. Acoust. Soc. Am. Suppl. 1 65, S42 (1979); P. C. Etter, ibid. 82, S102 (1987); J. Acoust. Soc. Am. 97, 3312 (1995)]. This review of international developments in underwater acoustic modeling reveals an inventory containing 114 propagation models, 17 noise models, 17 reverberation models and 26 sonar performance models representing a 38% increase over the 1995 inventory. As in previous reviews, older models have been retained to provide an historical perspective that enables researchers to pose well-informed questions regarding the need for future developments. When executed in higher-level simulations, these models generate predictive and diagnostic outputs that are useful to sonar technologists or acoustical oceanographers in the analysis of complex systems operating in the undersea environment. Recent applications of underwater acoustic models in naval operations, offshore industries and oceanographic research will be discussed. This review coincides with the appearance of the third edition of Underwater Acoustic Modeling and Simulation (Spon Press, London, 2003), which provides a detailed account of research conducted over the past four decades.

Validity of backscatter strength models at high (110–170 kHz) frequencies

Sonar performance models use several low-level acoustic models to predict the effect of the environment on reverberation and noise. Many of these acoustic models, particularly those predicting seabed scattering strength, were developed at lower frequencies than those currently used by both the scientific and mine-hunting communities. This paper examines the popular Applied Physics Laboratory of the University of Washington (APL-UW) acoustic backscatter strength model and tests its validity in the extended frequency band of 110–170 kHz. Acoustic and environmental data (gathered as part of a joint program with NATO SACLANTCEN) from several sites are collated and compared in this paper. This provides compelling evidence regarding the validity of the APL-UW backscatter model at these higher frequencies.

RECENT ADVANCES IN UNDERWATER ACOUSTIC MODELLING AND SIMULATION

A comprehensive review of international developments in underwater acoustic modelling is used to construct an updated technology baseline containing 107 propagation models, 16 noise models, 17 reverberation models and 25 sonar performance models. This updated technology baseline represents a 30% increase over a previous baseline published in 1996. When executed in higher-level simulations, these models can generate predictive and diagnostic outputs that are useful to acoustical oceanographers or sonar technologists in the analysis of complex systems operating in the undersea environment. Recent modelling developments described in the technical literature suggest two principal areas of application: low-frequency, inverse acoustics in deep water; and high-frequency, bottom-interacting acoustics in coastal regions. Rapid changes in global geopolitics have opened new avenues for collaboration, thereby facilitating the transfer of modelling and simulation technologies among members of the international community. This accelerated technology transfer has created new imperatives for international standards in modelling and simulation architectures. National and international activities to promote interoperability among modelling and simulation efforts in government, industry and academia are reviewed and discussed.

Updated technology baseline in underwater acoustic modeling, simulation, and analysis

A comprehensive review of international developments in underwater acoustic modeling is used to construct an updated technology baseline containing 106 propagation models, 16 noise models, 17 reverberation models, and 25 sonar performance models. This updated technology baseline represents a 30% increase over the previous baseline published in 1996 [P. C. Etter, Underwater Acoustic Modeling: Principles, Techniques and Applications, 2nd ed. (E & FN Spon, London, 1996)]. When executed in higher-level simulations, these models can generate predictive and diagnostic outputs that are useful to acoustical oceanographers or sonar technologists in the analysis of complex systems operating in the undersea environment. Recent modeling developments described in the technical literature suggest two major areas of application: low-frequency, inverse acoustics in deep water; and high-frequency, bottom-interacting acoustics in coastal regions. Rapid changes in global geopolitics have opened new avenues for collaboration, thereby facilitating the transfer of modeling and simulation technologies among members of the international community. This accelerated technology transfer has created new imperatives for international standards in modeling and simulation architectures. National and international activities to promote interoperability among modeling and simulation efforts in government, industry, and academia will be reviewed. The implications of this updated baseline for technology planners will also be discussed.