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Towed array reverberation beam time series can provide information about the underwater acoustic environment by providing a snapshot of the scattering in both range and bearing. By mapping time into range and beam angle into azimuth, John Preston pioneered the use of polar plots to survey an area [Preston et al., J. Acoust. Soc. Am., 87,119–134 (1991)]. When the polar plot is superimposed on the bathymetry of an area, a scattering map can be made. High scattering is generally associated with bottom features. With additional analysis and modeling, bottom loss and scattering strengths can be obtained [Preston and Ellis, J. Marine Systems 78, S359–S371 (2009)]. This talk emphasizes results from the rapid environmental assessment (REA) Rapid Response exercises 1996–1998, a multi-nation collaboration organized by NATO MILOC (military oceanography). While transmission loss experiments along a single radial could take hours, a single charge dropped near the towed array gave information on all radials in just minutes. Anomalies could be immediately identified, for investigation by more precise techniques. The REA exercises attracted interest and led to the Boundary Characterization and Clutter Joint Research Projects between CMRE (the NATO Centre for Maritime Research and Experimentation), Canada, and US. [Work supported by ONR.]

The Target and Reverberation Experiment was conducted in the Gulf of Mexico, off Panama City, Florida. Reverberation, noise, and target echo measurements were carried out for more than a month during April and May 2013, using a fixed source and fixed horizontal array receiver deployed in about 20 m of water. To support modeling efforts, a considerable number of ancillary environmental and transmission loss measurements were made. Various pulses in the 1.8–3.6 kHz frequency band were transmitted day and night, and have produced a rich data set. Initial results by various authors were published in the recent TREX13 Special Issue of the IEEE Journal of Oceanic Engineering. This work extends the analysis, concentrating on the variability of the echoes from various targets: a vertical hose, vertical arrays, a towed echo repeater, the hull of the towing ship, and other persistent bottom scatters, such as shipwrecks. In addition to the ping-to-ping variability of the echoes there are consistent trends, which are not understood, though probably due to oceanographic effects. An attempt will be made to relate the echo variability to measured and modeled reverberation and transmission loss. [Work supported by ONR, Ocean Acoustics, Code 22.]

Ray-based propagation models are often used to simulate underwater acoustic communications signals. For longer-range low frequency acoustic communications, particularly under ice cover, it is necessary to determine if a ray-based model can give accurate results. An elastic ice reflection algorithm was added to the Bellhop Gaussian beam model, and the results for mid- and low-frequency propagation were compared to results obtained using a normal modes model adapted for an elastic ice reflection coefficient, and to the OASES wavenumber integration model. The effects of simplified boundary conditions, frequency, and ice thickness on amplitude and phase matching are examined.

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 symm...

Acoustic backscatter data were collected from the ocean bottom at four sites on the Sohm Abyssal Plain. The bottom sediment at the four sites varied from mud/clay to silt/gravel. Using a simple array consisting of a free-flooding-ring projector which was omnidirectional in azimuth, and an omnidirectional hydrophone, data were collected over the frequency range of 800–2400 Hz [Hines and Barry, J. Acoust. Soc. Am. 92, 315–323 (1992)]. To obtain sufficient signal-to-noise ratio without impairing spatial resolution, 200-Hz-wide linear FM pulses were employed. By deploying the array at a height of 500 m above the seabed, bottom backscatter data were obtained at grazing angles down to 4 degrees before the onset of the first surface return. For data arriving after the first-bottom–first-surface interaction, the geometry approximates an in-plane bistatic experiment. Estimates of the in-plane bistatic scattering strength were obtained for pairs of incident and scattered grazing angles (φi,φs)

The Five Octave Research Array (FORA) [K.M. Becker and J. R. Preston, Proceedings Oceans 2003, 2607-2610] includes a triplet section in which the elements are three omnidirectional sensors. The elements can be combined as a standard linear array to produce conical beam patterns, or the triplets can be used to form directional beams that allow for “left-right” discrimination. The beam output can then be matched filtered to produce high-temporal-resolution beams over the full 360° azimuth, and identify scattering features in reverberation data. To extract scattering strengths, the calibration of the beams and the matched filter output must be known accurately. Here we look at results from the 2013 Target and Reverberation Experiment (TREX13), where data were received on the FORA array, then beamformed and matched filtered by various procedures that had different normalizations. The broadside beam and several natural beams of the linear array can readily be calibrated against a simple “exact” procedure. No s...

A practical model to compute shallow‐water boundary reverberation is described. Normal modes are used to calculate the acoustic energy propagating from the source to the scattering area, and from the scattering area to the receiver. At the scattering patch each mode is decomposed into up‐ and down‐going waves, then ray‐mode analogies and empirical scattering functions can be used to compute the scattered energy. The method was first described by Bucker and Morris [J. Acoust. Soc. Am. 44, 827–828 (1968)], and papers which first appeared in the Chinese literature. Their work is extended here by using group velocities to obtain the travel times for each mode pair, and by further developing the ray‐mode analogy. The effects of summing the modes coherently or incoherently and of including the time spreading due to the modal group velocities are examined. This paper deals with the range‐independent monostatic case, although the technique is extendible to bistatic geometries and range‐dependent environments. Calculations show excellent agreement with some ray‐based models, and, using the Lambert bottom‐scattering coefficient as the only adjustable parameter, good agreement is obtained with some measured shallow‐water reverberation.

Shallow‐water seabeds are often varied and complex, and are known to have a strong effect on acoustic propagation. Some of these seabeds can be modeled successfully as fluid or solid half‐spaces. However, unexpectedly high propagation loss with respect to these models has been measured in several regions with rough, partially exposed, hard‐rock seabeds. It is shown that the high propagation loss in these areas can be modeled successfully by introducing a thin layer of elastic–solid sediment over the hard‐rock substrate. Propagation loss predictions using the safari fast‐field program exhibit bands of high loss at regularly spaced frequencies. Normal‐mode calculations show resonance phenomena, with large peaks in the modal attenuation coefficients at these same frequencies, and with rapid changes in the mode wavenumbers. Bottom reflection loss calculations indicate that the high propagation loss is due to absorption of shear waves in the sediment layer.

A range-dependent shallow-water reverberation model using adiabatic normal modes has been previously developed [Ellis et al., ISURC Conference, La Spezia, 2008] to handle bottom scattering and clutter echoes in a range-dependent environment. It has now been extended to handle volume scattering from the water column and volume scattering from the sub-bottom. Beam time series similar to that from a horizontal line array can be produced. Comparisons can then be made directly with data, and area scattering maps created. Of particular interest will be data obtained on the triplet line array during the 2013 TREX experiment in the Gulf of Mexico off Panama City, Florida. Predictions will be compared with data for average reverberation from a number of pings, as well as ping-to-ping variability on a particular run. [Work supported in part by U.S. Office of Naval Research, Code 32.]

Reverberation measurements were made in the Gulf of Mexico off Panama City, Florida, USA in April 2012 in preparation for the main Target and Reverberation Experiment (TREX) in May 2013. The data were gathered using the triplet section of the ONR Five Octave Research Array (FORA), deployed as a fixed receiver. By steering cardioid beams to the right or left the array can reduce ambiguity. Beamformed data from the 2012 trial show background noise with high directionality and variability due to nearby shipping. Model predictions of reverberation and target are compared with data using a range-dependent Clutter Model, which uses adiabatic normal modes as the computational engine. The initial predictions use isovelocity water, over a sandy bottom halfspace with Lambert scattering, and bathymetry from the GEBCO_08 database. These initial results will be presented, hopefully supplemented by improved predictions with better environmental inputs and additional clutter data obtained during the May 2013 experiment. [Work supported by ONR Code 322 OA].

One of the most challenging issues in active sonar is the discrimination of targets from clutter. In bottom-limited areas, one of the primary sources of clutter is from the seabed. An important source of seabed clutter is discrete objects that may lie on or under the sediment interface. It is clear that target-like scattered returns should arise from discrete features that have a spatial scale, or scattered response with time scales of order of the target of interest. The intent of this research is to show that in certain cases, slowly and continuously varying sediment layers can also lead to a target-like response. This is not an intuitive cause of clutter, and the physical mechanisms that lead to the target-like response are expounded.

The requirement by modern navies to predict sonar performance in shallow water, whether for use in research, planning, or operations, led to an initiative for the validation of reverberation models in the form of two reverberation modeling Workshops at the University of Texas at Austin [J. S. Perkins and E. I. Thorsos, J. Acoust. Soc. Am. 126, 2208 (2009)]. The scenario considered here (Problem XI, from the first workshop) requires the computation of reverberation versus time in a Pekeris waveguide with Lambert scattering from the seabed. Simple analytical methods are presented that provide insight into the dominant propagation paths giving rise to the reverberation and hence establish regimes of validity for various computer models making different approximations or assumptions. Results from eigenray, normal mode, and hybrid continuum methods are compared with each other and with the analytical solutions. Numerical predictions are shown to overlap to within a few tenths of a decibel in regions where the ...

Most shallow water environments are variable over relatively short ranges. Low‐frequency reverberation is typically dominated by bottom scattering, and therefore has considerable dependence on range and azimuth. Directional receivers, such as towed line arrays, are an important tool for measuring this azimuthal dependence. Not many reverberation models are capable of handling these beam patterns, but two which have proved useful are the Generic Sonar Model developed at NUWC and the OGOPOGO normal‐mode model developed at DREA. By adjusting the bottom scattering strength and sometimes the bottom loss, good agreement has been achieved between the reverberation data and model predictions in relatively flat areas. Differences between the ‘‘flat‐bottom’’ model predictions and data are generally correlated with bottom scattering features. The talk will focus on comparisons of model predictions with towed array data obtained in joint trials with the SACLANT Undersea Research Center. The comparisons can be used to...

The shallow‐water normal‐mode reverberation approach [D. D. Ellis, J. Acoust. Soc. Am. 97, 2804–2814 (1995)] can be extended to handle the echo from a target or discrete clutter objects. Normal modes are used to handle the propagation, and ray‐mode analogies are used to handle the echo and scattering. The time spreading of the echo can be approximated using the modal group velocities. This description for the time spreading is approximate, but the computations are quite efficient, since the normal modes need only be calculated at one frequency. The target can be point‐like and mode functions can be used, or the target can be considered more extended and the mode amplitude envelopes can be used. The formulation will be presented, along with a number of calculations based on problems from the ONR Reverberation Modeling Workshops. [Work supported in part by ONR, Code No. 321OA.]

A number of problems were developed for, and presented at, a 2006 Reverberation Modeling Workshop sponsored by the US Office of Naval Research. The simplest of these known to the participants as Problem 11, was a Pekeris waveguide (isospeed water over a flat bottom half‐space) with Lambert bottom scattering. The water depth was 100 m and frequencies of 250, 1,000, and 3,500 Hz were specified. A number of source‐receiver combinations were specified, but the reverberation predictions are quite insensitive to sensor depth except at 250 Hz. With some benefit from hindsight and the results from other models, we compare our results from ray, normal‐mode, and energy‐flux approaches. All three approaches agree at intermediate times, say 3 to 50 s. At short times, the steep‐angle paths and fathometer returns cause the mode and energy‐flux models to underpredict the reverberation. At longer times, the ray models run out of steam: i.e., there are too many contributing ray paths for them to handle so they underpredict the reverberation. By combining the model predictions together with analytical results from an energy‐flux model, we propose a composite benchmark solution. [Work supported in part by ONR.]

A Matlab and normal mode based reverberation model has been developed that uses Ellis’ algorithm [J. Acoust. Soc. Am. 97, 2804–2814] for reverberation predictions and the ORCA normal mode model [Westwood et al., J. Acoust. Soc. Am. 100, 3631–3645] to compute the eigenvalues and mode functions. The model is currently range independent, but handles bistatic geometries, and towed array beam patterns. Group velocity corrections similar to LePage’s [J. Acoust. Soc. Am. 106, 3240–3254] have recently been added to this model. The matrix formulation in Matlab allows the bistatic calculations to be performed more efficiently. An overview of the model is presented that includes sample model‐model comparisons for some problems from the 2006 ONR Reverberation Workshop. The model predictions are also compared with data obtained with the NURC (formerly the NATO Undersea Research Centre, SACLANTCEN) towed arrays during several reverberation experiments on the Malta Plateau. Select data from these experiments are used to compare with model predictions using best estimates of bottom properties in the area. [Work supported by US Office of Naval Research, Code 321OA, Grant No. N00014‐05‐1‐0156.]

In shallow water environments with a low‐speed sediment layer overlying a higher‐speed sub‐bottom, the observed reverberation may be dominated by scattering from the sub‐bottom. Here, reverberation predictions from normal mode and energy flux models are compared for the case where the scattering arises from a sub‐bottom half‐space under the low‐speed sediment layer. It is shown that in such an environment, the position of the angle of intromission, in addition to the angular dependence of the scattering kernel, is a factor controlling the vertical angle distribution. It is also shown that the reverberation from a sub‐bottom horizon is typically governed by higher grazing angles than the case where the scattering occurs at the water‐sediment interface. There was generally very close agreement between the models as a function of frequency (200–1,600 Hz), layer thickness (0–8 m), and range (1–15 km). The model comparisons, showing some differences, illuminate the effect of different approximations in the two...

A shallow‐water reverberation model based on normal modes and ray‐mode analogies has been previously developed [J. Acoust. Soc. Am. 97, 2804–2814 (1995)]. It was first developed for boundary scattering and recently extended to handle volume reverberation in both the water and sub‐bottom. The volume scattering is formulated in terms of an empirical plane‐wave scattering function, which depends on depth but which can also depend on the incident and scattered angles. Our first approximation for the volume reverberation is to choose the scattering to be isotropic, but to get a better approximation based on physical parameters, the in‐plane bistatic scattering function of Hines is used [J. Acoust. Soc. Am. 99, 836–844 (1996)]. In this paper, calculations of shallow‐water reverberation are made with the Hines scattering function used as input to the normal‐mode model. Results are compared with reverberation assuming isotropic scattering in the bottom, as well as with reverberation due to bottom interface scatte...

Acoustic backscatter data were collected from the ocean bottom at four sites on the Sohm Abyssal Plain. Using a simple array consisting of a free‐flooding ring projector, omnidirectional in azimuth, and an omnidirectional hydrophone, data were collected over the frequency range of 800 to 1700 Hz. To obtain sufficient signal‐to‐noise ratio without impairing spatial resolution, 200‐Hz‐wide linear FM pulses were employed. The array was deployed 500 m above the seabed and bottom backscatter data were obtained at grazing angles down to 4° before the onset of the first surface return. For data arriving after the first‐bottom first‐surface interaction, the geometry approximates an in‐plane bistatic experiment. Therefore, the in‐plane bistatic data and the monostatic data can be compared to examine the validity of using the separable and half‐angle approximations used in some bistatic scattering models [J. Acoust. Soc. Am. 89 (1991)]. In this paper the bistatic geometry is explained and the data are interpreted in light of the bistatic arrivals. Then, an algorithm is presented for extracting the in‐plane bistatic scattering strength from the data. Finally, bistatic and monostatic data are compared using the separable and half‐angle approximations.

During the Boundary 2001 sea trial, a number of long‐range reverberation measurements were made at a site on the Scotian Shelf between Halifax and Sable Island. The water depth was about 80 m over a sandy bottom. The sources were SUS charges dropped from either the NATO research vessel Alliance or the Canadian research vessel CFAV Quest. The receiver was the SACLANTCEN 254‐m towed array aboard Alliance. The data were analyzed in two array apertures and frequencies from 80 Hz to 1400 Hz. Model‐data comparisons were made using the Generic Sonar Model, with bottom parameters being extracted using both a manual procedure and an automated procedure. Direct measures of the scattering and bottom properties were made by other researchers, including Hines, Holland, and Osler. The bottom topography was fairly smooth near the site, but at long ranges there were numerous scattering features. The polar plots of the data, and model‐data differences, are compared with the bathymetric features in the area. [Work supported in part by ONR Code 32, Grant No. N00014‐97‐1‐1034.]

Together with SACLANTCEN, the authors recently participated in the Boundary Characterization Experiment to measure shallow‐water bottom reverberation in the STRATAFORM off New Jersey. SUS charges were used as monostatic sources. The receivers were horizontal arrays. Data were analyzed in bands from 160–1500 Hz. The STRATAFORM is known to have benign surface morphology but contains many buried river channels. Highlights of the reverberant returns are discussed that include returns from over the shelf break. Some comparisons in reverberation characteristics between SUS and coherent pulses are noted. Another objective of these reverberation experiments was to quickly invert for bottom scattering and bottom loss parameters. An automated geo‐acoustic parameter extraction method was used together with the Generic Sonar Model and a Jackson‐Mourad model for scattering. After automatically adjusting bottom loss and scattering strength, good agreement is achieved between the diffuse reverberation data and model pre...

The weakest link in performance prediction for naval systems operating in coastal regions is the environmental data that drive the models. In shallow water downward refracting environments, the seabed properties and morphology often are the controlling environmental factors. In the Boundary 2001 Experiment, seabed, surface, and biologic scattering, seabed reflection, propagation, reverberation, and ambient noise data were collected in order to develop and refine measurement techniques for key environmental model inputs. Both Rapid Environmental Assessment (REA) methods and high‐resolution measurement techniques were employed from 0.1–10 kHz. Supporting oceanographic, geologic, and geophysical data were also collected. The experiment was conducted in May 2001 in two littoral regions: the New Jersey shelf and the Scotian Shelf. This paper provides an overview of the experiment objectives, hypotheses and conduct. [Research supported by NATO SACLANT Undersea Research Centre, ONR, and DRDC‐A.]

Together with SACLANTCEN, the authors recently participated in two trials to measure shallow water bottom reverberation near Sicily and the STRATAFORM off New Jersey. Sources were monostatic SUS charges and coherent pulses. The receivers were horizontal arrays. Data were analyzed in bands from 80 to 4000 Hz. Highlights of the reverberant returns are discussed. The STRATAFORM is known to have benign surface morphology but contains many buried river channels. One author was also involved in the 2001 Geoclutter experiment conducted in STRATAFORM using coherent bistatic sources near 400 Hz. Differences in reverberation characteristics between SUS and Geoclutter results are discussed. Another objective of these reverberation experiments was to quickly invert for scattering and geoacoustic parameters. A simulated annealing (SA) algorithm was used together with the Generic Sonar Model (NUWC). After automatically adjusting bottom loss and scattering strength, good agreement is achieved between the diffuse reverberation data and model predictions in relatively flat areas. Model/data differences are generally correlated with bottom scattering features. Since reverberation typically lasts 10–20 s or more, extracted parameters apply over wide areas. Local bottom loss and backscattering measurements were made by Holland in these areas. A comparison with Holland’s results is given. [Work supported by ONR Code 32, Grant No. N00014‐97‐1‐1034.]