Reverberation Experiment Research Articles

A Matlab and normal mode based reverberation model and some data/model comparisons

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.]

Reverberation in the shallow water and its physical explanation

An oscillation phenomenon of the low frequency reverberation intensity was observed in several shallow water reverberation experiments. This phenomenon cannot be explained by the widely used incoherent reverberation theory. In this paper, to explain the observed oscillation phenomenon, a normal mode based coherent reverberation theory is presented. The theoretical analysis and numerical results show that modal interference can cause the regular oscillation phenomenon of the low frequency reverberation intensity, and the oscillation frequency is determined by the normal mode eigen-values. A new method to estimate the bottom sound speed based on the oscillation frequency of reverberation intensity was presented in this paper. The experimental results at three different sites indicate that the bottom sound speed estimated from the oscillation frequency of reverberation intensity agrees with that inverted from Matched Field Processing (MFP) well.

Multistatic broadband sonar (MSBA): Principles and measurements

Shallow-water (SW) coastal areas with depths between 40 and 200 m with bottom sediment layers (<25 m) formed by deposition and sea level variations are known to have frequency-dependent sound transmission and reverberation characteristics especially under the adverse conditions of downward refraction. Coastal ocean-dynamics complicates these environments by coupling the surface, volume, and bottom characteristics and can dramatically affect the temporal and spatial coherency of the signal. The broad-band signal and processing smear these frequency-dependent effects as well as signal excess fluctuations. Furthermore, recent experiments have shown that despite these complexities, explosive sources coupled with directional receivers have the capability to detect, localize, and classify sonar targets in the presence of clutter most likely due to bottom features, fish schools, and sunken objects. Active search sonar operations in SW are typically reverberation limited and experiments have shown that MSBA sonar when employed in bistatic source-receiver geometries can be effective. This paper reviews the principles underlying the MSBA sonar in shallow water and presents results from several experiments that demonstrated the utility of MSBA. The results are discussed in terms of a classical sonar model with basic statistical considerations to define limits on the detection, classification, and localization with multiple receivers.

Time-dependent freezing of water under dynamic compression

Using shock wave reverberation experiments, water samples were quasi-isentropically compressed between silica and sapphire plates to peak pressures of 1–5 GPa on nanosecond time scales. Real time optical transmission measurements were used to examine changes in the compressed samples. Although the ice VII phase is thermodynamically favored above 2 GPa, the liquid state was initially preserved and subsequent freezing occurred over hundreds of nanoseconds only for the silica cells. Images detailing the formation and growth of the solid phase were obtained. These results provide unambiguous evidence of bulk water freezing on such short time scales.

Shallow-water reverberation highlights and bottom parameter extractions from the STRATAFORM

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 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.]

Scattering strength uncertainty

A serious weakness in modeling shallow water reverberation is the uncertainty in bottom scattering strength and its angle-dependence. If the bottom scattering law is assumed to be a separable function of an incoming and outgoing angle it follows that the reverberation contains separable incoming and outgoing propagation terms. Thus the returning multipaths from a scattering patch are weighted directly by (the outgoing part of) the scattering law. This means that comparisons of reverberation and propagation angle-dependence on a vertical receiving array have the potential to reveal the scattering law directly. In this paper we discuss a reverberation experiment with complementary propagation measurements using a VLA and a broadband source to deduce scattering law angle-dependence and absolute scattering strength. The approach is justified by some analysis, and findings are compared with the numerical results of a new multistatic sonar model, SUPREMO. The experiment was conducted in a fairly flat bottomed part of the Mediterranean south of Sicily during BOUNDARY2002.

AN OSCILLATION PHENOMENON OF REVERBERATION IN THE SHALLOW WATER WITH A THERMOCLINE

An oscillation phenomenon of the reverberation intensity was observed in a recent shallow water reverberation experiment. This phenomenon cannot be explained by the widely used incoherent reverberation theory. In this paper, to explain the observed oscillation phenomenon, the modal interference of reverberation in the shallow water with a thermocline is discussed on the basis of the ray-mode theory. The theoretical analysis and numerical results show that modal interference can cause the regular oscillation phenomenon of the reverberation intensity, and the oscillation phenomenon of the reverberation intensity can be used to test different reverberation models.

The Geoclutter experiment: Remotely imaging sub-bottom geomorphology in shallow water

Preliminary results of the Geoclutter experiment of April–May 2001 are presented. This bistatic reverberation experiment was conducted at the New Jersey Strataform site in continental shelf waters. Roughly 3000 waveforms were transmitted from vertical source arrays in the 390–440 Hz band and received by a horizontally towed array. On average roughly 10–100 discrete clutter events were registered per transmission for a total of at least 30<th>000 geoclutter events that could be confused with a discrete target over the period of the experiment. Bathymetry throughout the New Jersey Strataform area is extremely benign with few discrete features. The vast majority of clutter returns appeared in level areas where absolutely no bathymetric features are present. The clutter events often registered well with buried river channel networks previously characterized in the Geoclutter program. Extensive probing of river-channel-infested areas revealed that the number and level of clutter events is highly dependent on bistatic orientation. The registrations suggest that bistatic aspects that produce ‘‘glints’’ from large projected areas along river channel axes yield the strongest and most frequent targetlike returns. These results are consistent with those predicted by a unified model for reverberation and submerged-target scattering [J. Acoust. Soc. Am. 109, 909–941 (2001)].

Rapid assessment of gently varying bathymetry using TRM reverberation

A concept for rapidly probing shallow water bathymetry [B. E. McDonald and Charles Holland, J. Acoust. Soc. Am. 109, 2495 (2001)] using reverberation from a time reversed mirror (TRM) is further developed. Adiabatic modal analysis and WKB methods applied to a mildly range-dependent shallow water waveguide lead to a simple relation between TRM focal shifts and range integrated bathymetric variations. This relation is compared with RAM simulations carried out in conjunction with the July 1999 Saclantcen–Scripps bottom reverberation experiment near Elba. Our integral expression is found to agree satisfactorily with focal spot shifts predicted by RAM. Simulation studies using the Saclantcen prosim reverberation model are also presented to help explain details of the July 1999 data. Experimental implications for rapid assessment of unknown or shifting shallow bathymetry are discussed. [Work supported by NRL.]

Some shallow water reverberation highlights and bottom parameter extractions in the 80–4000 Hz region from the boundary characterization and geoclutter experiments

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.]

Low-frequency long-range propagation and reverberation in the central Arctic: Analysis of experimental results

BBN and the ASW Environmental Acoustic Support program office of ONR conducted a low-frequency reverberation and transmission loss experiment in the central Arctic during the spring of 1992. In this article we report analysis of these data which extracts the controlling propagation and reverberation characteristics. The recorded source levels are combined with the received levels recorded at range to estimate the dependence of the scattering strength of the ice canopy on incident and backscattered grazing angle in frequency bands between 10 and 70 Hz. Results show that the monostatic backscattering strength is roughly proportional to frequency and angle squared, with a nominal value of −47 dB at 40 Hz and 8.5° grazing.

Shallow-water reverberation from a time reversed mirror: Data-model comparison

A July 1999 bottom reverberation experiment and accompanying simulation results will be discussed. The experiment was carried out as part of the Focused Acoustic Fields joint program between Saclantcen and Scripps Institution of Oceanography. A 3.5-kHz probe source 1 m off the bottom just north of Elba produced a signal which was recorded on a vertical source/ receive array (SRA) at ranges between 3 and 7.4 km. The SRA then time reversed and retransmitted the signal, resulting in a focal annulus near the bottom at the probe source range. Reverberation from the SRA signal was recorded at the SRA. A distinct maximum in reverberation at the time corresponding to the probe source range indicates selective bottom ensonification near the focus. Simulation studies carried out using RAM and the Saclantcen PROSIM reverberation models reproduced salient features of the experiment and led to suggestions for future experiments. Model sensitivity to array aperture and focal range will be discussed. Results imply that reverberation from a time reversed mirror may be useful in rapidly probing mildly varying bathymetry. [Work supported by Saclantcen.]

Systematic Errors in the Estimation of Black Hole Masses by Reverberation Mapping

The mass of the central black hole in many active galactic nuclei has been estimated on the basis of the assumption that the dynamics of the broad emission line gas are dominated by the gravity of the black hole. The most commonly employed method is to estimate a characteristic size-scale r* from reverberation mapping experiments and combine it with a characteristic velocity v* taken from the line profiles; the inferred mass is then estimated by r*v/G. We critically discuss the evidence supporting the assumption of gravitational dynamics and find that the arguments are still inconclusive. We then explore the range of possible systematic error if the assumption of gravitational dynamics is granted. Inclination relative to a flattened system may cause a systematic underestimate of the central mass by a factor of ~(h/r)2, where h/r is the aspect ratio of the flattening. The coupled effects of a broad radial emissivity distribution, an unknown angular radiation pattern of line emission, and suboptimal sampling in the reverberation experiment can cause additional systematic errors as large as a factor of 3 or more in either direction.

Inversion of reverberation data for rapid environmental assessment

From 1996 through 1998 SACLANTCEN participated in three rapid environmental assessment (REA) trials, including experiments for bottom reverberation. Eight sites were visited: near Sicily, in the Ionian Sea, and near the Straits of Gibraltar. SUS charges were used as sources. The receivers were horizontal arrays (four apertures) spanning a wide frequency range. Sets of nearly monostatic recordings were analyzed in frequency bands from 80 to 4000 Hz. Previous results by the author [Berlin ASA meeting (1999)] showed polar plots of the beam time series superimposed on bathymetric charts, revealing a number of scattering features not on the charts and that directional reverberation measurements are a useful remote-sensing tool. Another objective of the reverberation experiments was to quickly invert for estimates of scattering and geo-acoustic parameters that can be used in a wider variety of conditions. A manual inversion procedure was originally used to obtain at-sea results. New work uses a simulated annealing algorithm giving more formal inversion results. The horizontal array data are compared with the generic sonar model (GSM) predictions for both the manual and automated inverse schemes. A summary of the estimated geo-acoustic parameters using both methods is presented. [Work supported by ONR and DREA.]

FREQUENCY-SELECTIVE ATTENUATION OF SOUND PROPAGATION AND REVERBERATION IN THE YELLOW SEA

In the summer of 1991, sound propagation and reverberation experiments were carried out in the Yellow Sea. Explosive sound sources and omnidirectional hydrophones were used. The sound speed profile consisted of a warmer surface isovelocity layer, a strong thermocline and a cooler isovelocity bottom layer. When both the source and the receiver were located above the thermocline, the transmission loss along a certain course and the non-directional average reverberation intensity showed similar frequency-selective attenuation at around 1300 Hz. On the basis of experimental results and some circumstantial evidence, the authors believe that the observed frequency-selective attenuation is due to swimbladder-bearing fish (probably, anchovies) living only above the thermocline.

Summary of bottom reverberation findings and model/data comparisons during three rapid environmental assessment trials

From August 1996 through February 1998 the authors participated with SACLANTCEN in three rapid environmental assessment (REA) trials, including experiments for bottom reverberation. Eight sites were visited: three sites south of Sicily, three sites in the Ionian Sea, and two sites in the Atlantic near the Straits of Gibraltar. SUS charges were used as sources. The receivers were a nested horizontal array with 128 elements spaced at 0.5, 1, and 2 m and a second horizontal array with 32 elements spaced at 0.18 m. Sets of nearly monostatic recordings were analyzed in frequency bands from 80 to 4000 Hz. Polar plots of the beam time series were superimposed on bathymetric charts, revealing a number of scattering features not on the charts. The results illustrate the use of directional reverberation measurements as a useful remote-sensing tool. Another objective of the reverberation experiments was to quickly invert for estimates of scattering and geo-acoustic parameters that can be used in a wider variety of conditions. The horizontal array data were compared with the generic sonar model (GSM) predictions for selected cases. A summary of the model parameters found at the eight sites is presented. [Work partially supported by ONR.]

Environmental factors for site-to-site differences in low-frequency surface scatter

Acoustic reverberation experiments conducted during the extensive Critical Sea Test (CST) Program have indicated that near-surface microbubble clouds are the primary source of low-frequency (0–1000 Hz) acoustic surface scatter during elevated winds. A crucial issue remaining from CST, however, is the explanation of large (greater than 15 dB) site-to-site differences in surface scattering strength. Using the extensive CST environmental observation set, as well as satellite remote sensing products available over the World Wide Web, we have investigated the hypothesis that site-to-site differences are controlled by processes related to the supply, mixing, and removal of near-surface bubbles in the ocean. It is found that much of the variability can be empirically explained by considering the mean ocean temperature, biological productivity, and wave conditions at each site. These results provide important clues as to the controlling factors for surface scatter in shallow water areas, where biological productivity is highest. The implications for improving an operational surface scattering prediction capability, using readily available (e.g., satellite observations) environmental inputs, will be discussed. [Work supported by the Office of Naval Research/Ocean Acoustics.]

Shallow-water in-plane bistatic scattering experiment

Every reverberation experiment done in shallow water is by its nature, bistatic at long ranges. That is to say, in addition to simple monostatic returns, the received signal is composed of energy arriving along bottom bounce paths for which incident and scattered angles are different. Therefore, it is possible to measure in-plane bistatic scattering strength using a monostatic geometry [Hines, Crowe, and Ellis, J. Acoust. Soc. Am. (in review)]. Noting this, a simple array consisting of a pair of free-flooding-ring projectors and two hydrophones was used to measure monostatic and bistatic scatter in 100 m of water on the Scotian Shelf. A series of linear FM (LFM) pulses was transmitted in four frequency bands over the range 900 to 2100 Hz. The reverberation data between the first bottom return and the first surface return provided estimates of monostatic backscatter strength for grazing angles down to 10°. Data arriving after the first-bottom–first-surface interaction provided estimates of in-plane bistatic scattering strength for pairs of incident and scattered grazing angles (φi, φs) down to (7°, 57°). In this paper, the experiment and data are described in light of the monostatic and bistatic arrivals.

Sound-speed measurements in the surface-wave layer

Wave breaking at the surface of the ocean entrains bubbles, significantly modifying the phase speed and attenuation of acoustic waves propagating through the resulting two-phase medium. An autonomous buoy system was developed that directly measures sound speed at 3.33, 5, and 10 kHz at seven depths ranging from 0.7 to 7 m through the use of a travel-time technique. Simultaneous measurements at each depth are obtained at a 2-Hz rate, allowing observation of the unsteady sound-speed field from individual bubble injection events, as well as the calculation of mean sound speeds. The travel-time technique allows a direct measurement of the sound speed, eliminating the uncertainties common with inferring sound speeds from bubble population data. The sound speed buoy was deployed in the North Atlantic during the winter of 1993–94 as part of the Acoustic Surface Reverberation Experiment (ASREX). Our aim was to characterize the highly variable near-surface sound-speed field under varying environmental conditions. Forty-three days of data were obtained spanning several storm cycles with wind speeds and significant wave heights reaching 20 m/s and 8 m, respectively. During periods of intense wave breaking, average sound speeds below 1000 m/s were observed at the 0.7-m measurement depth while instantaneous sound speeds during individual events approached values as low as 300 m/s. Furthermore, the data suggest that the dispersive effects of bubbles may extend to frequencies as low as 5 kHz near the surface during storms. Strong correlations of the mean and rms sound speed with the overlying wind and wave fields were found.

A comparison of quasi-continuous wave and broadband travel time techniques in the prediction of long-range reverberation

Previous work has shown that the general structure of deep ocean reverberation can be reasonably modeled using single-frequency propagation techniques. Recent results indicate that secondary, multipath influences can affect reverberation levels significantly. Primarily, these influences are observed in shadow regions of the primary, direct path ensonification. Temporal separation of these paths is required in order to distinguish these effects. This suggests that regions of the reverberation signal may not be well predicted by single-frequency quasi-cw (continuous wave) methods. In this paper, a quasi-cw method of predicting long-range reverberation is compared to an exact, two-way travel time technique. Each model assumes a simple, constant plane-wave scattering strength, thereby emphasizing the influence of propagation. Results from both models are compared to measured reverberation data from an ONR-SRP bottom reverberation experiment conducted near the Mid-Atlantic Ridge. It is shown that the single-frequency method underestimates reverberation levels on the backside of large ridge structures which act as shadow zones for all propagation paths. Returns from multipaths, however, begin to fill in these regions in time and may explain the lack of such dramatic signal drop-outs from measured data.