Long-range Reverberation Research Articles

Study of the Distribution of Reverberation over Doppler Frequencies in a Deep-Sea Bistatic Experiment

The paper describes the results of an experiment on studying long-range surface reverberation in the deep-water part of the Black Sea. The experiment employed tone pulses with a filling frequency of 2 kHz and a duration sufficient for narrowband spectral analysis. The source and receiver of acoustic signals were spatially offset in the water area; therefore, directional reception and time strobing made it possible to study different situations: forward scattering, bistatic scattering, and monostatic (back) scattering. The paper analyzes the Doppler scattering spectra for different values of the bistatic scattering angle.

Modeling of Long-Range Reverberation Signal for Rough Bottom Consisting of Polygon Facets

To acquire a stable reverberation signal from an irregular ocean bottom, we derive the analytic surface integral of a scattered signal using Stokes’ theorem while approximating the bottom using a combination of polygon facets. In this approach, the delay difference in the elemental scattering area is considered, while the representative delay is used for the elemental scattering area in the standard reverberation model. Two different reverberation models are applied to a randomly generated rough bottom, which is composed of triangular facets. Their results are compared, and the scheme using analytic integration shows a converged reverberation signal, even with a large elemental scattering area, at the cost of an additional computational burden caused by a higher order approximation in the surface integral of the scattered signals.

Low-frequency reverberation estimates using elastic parabolic equation solutions for ice-covered and ice-free Arctic environments

A computational model of reverberation at low frequencies in an ice-covered environment is developed. The model is built on a full-field perturbation approach and includes elastic parabolic equation solutions for the acoustic field and its horizontal and vertical derivatives near water-ice and water-air interfaces. Our previous work demonstrated that average reverberation intensity is sensitive to both elasticity and thickness of the ice at mid-frequencies, where the ice layer thickness is on the order of or larger than both compressional and shear wavelengths. Here we address what happens to the reverberation when ice thickness approaches zero. Reverberation estimates for rough free surface and those from rough ice-water interface with increasing ice layer thickness are compared to determine at what ice thicknesses and acoustic frequencies the long-range reverberation distinguishes between the two cases. To isolate effects of ice thickness, we assume roughness is the same in both environments. Frequencies will be varied from very low, where the ice layer is practically transparent, to moderately low where the presence of the ice becomes noticeable in the reverberation. Numerical examples for reverberation in a typical Arctic environment with upward refracting sound-speed profile are presented and discussed. [Work supported by ONR.]A computational model of reverberation at low frequencies in an ice-covered environment is developed. The model is built on a full-field perturbation approach and includes elastic parabolic equation solutions for the acoustic field and its horizontal and vertical derivatives near water-ice and water-air interfaces. Our previous work demonstrated that average reverberation intensity is sensitive to both elasticity and thickness of the ice at mid-frequencies, where the ice layer thickness is on the order of or larger than both compressional and shear wavelengths. Here we address what happens to the reverberation when ice thickness approaches zero. Reverberation estimates for rough free surface and those from rough ice-water interface with increasing ice layer thickness are compared to determine at what ice thicknesses and acoustic frequencies the long-range reverberation distinguishes between the two cases. To isolate effects of ice thickness, we assume roughness is the same in both environments. Frequencie...

Long-range reverberation in an Arctic environment: Effects of ice thickness and elasticity.

A full-field perturbation approach is modified for an ice-covered ocean and applied to estimating narrowband long-range reverberation caused by roughness of the ice-water interface. First-order approximation of the approach is used which requires the roughness amplitudes be small compared to the acoustic wavelength. To obtain the zeroth-order Green's function and transmission loss field used in the reverberation model, elastic parabolic equation solutions are generated in range-independent environments. Ice is represented by an isospeed layer on top of a linear transition layer. Effects of ice properties are discussed and demonstrated by comparing reverberation calculated for different ice layer thicknesses and wave speeds for typical ice values.

A full-field scattering coefficient for rough interfaces and a modified sonar equation for long-range reverberation

A full-field perturbation approach [A.N. Ivakin (2016), J. Acoust. Soc. Am., 140(1), 657–665] allows modifying two terms of sonar equation for reverberation in complex environments and waveguides with rough interfaces. The first one describes a two-way transmission loss for the full-field intensity in the vicinity of the interface. The second term is a full-field scattering coefficient related to but different from the conventional scattering cross-section per unit area. In particular, the coefficient has a non-zero limit value at small grazing angles of incident and scattered waves defined by the contrast of acoustic parameters at the interface and the power spectrum of roughness. The modified sonar equation allows extremely fast estimating the long-range reverberation based on only one (central) frequency calculation of the transmission loss averaged over a certain span of ranges defined by the used frequency bandwidth, and only one value of the scattering coefficient (at zero grazing angles). No multip...

Estimating the influence of ice thickness and elasticity on long-range narrow-band reverberation in a representative Arctic environment

A full-field perturbation approach [A. Ivakin (2016), J. Acoust. Soc. Am., 140(1), 657-665] is modified for an ice-covered ocean and applied to estimating narrow-band long-range reverberation caused by roughness of the ice-water interface. First-order approximation of the approach is used which requires the roughness amplitudes be small compared to the acoustic wavelength. An upward refracting sound speed water column profile typical for Arctic conditions is assumed. To obtain the zeroth-order Green’s function and transmission loss field used in the reverberation model, elastic parabolic equation solutions [J. Collis, S. Frank, et al. (2016), J. Acoust. Soc. Am., 139(5), 2672-2682] are generated in range-independent environments. Ice is represented by two layers. The first approximates acoustic properties of a relatively thin, water-saturated transitional ice layer and is described as a stratified fluid with sound speed increasing with the distance from the ice-water interface. Second layer is introduced as a homogeneous isotropic elastic medium with fixed complex shear and longitudinal wave speeds. Effects of ice properties are discussed and demonstrated by comparing reverberation calculated for different ice layer thicknesses and shear speeds varying from zero (for a fluid model) to typical ice values. [Work supported by ONR.]

Investigation of Sea Surface Effect on Shallow Water Reverberation by Coupled Mode Method

The effects of rough sea surface on the long-range bottom reverberation in shallow seas are studied by the coupled mode reverberation theory. The scattering effect caused by irregular rough sea surface is described by couple coefficients. The decaying rules of long-range bottom reverberation level are simulated at different sea states, and the rough sea surface effect on the coherence of distant bottom reverberation is also discussed. It is indicated that irregular upper boundary has changed the propagation effect of the shallow water waveguide, and bottom reverberation, which is dominated among other kinds of reverberation in shallow water, is affected by the sea surface scattering as the increasing sea state. Compared with other literatures, the emphasis of this paper is to present the mechanism of rough sea surface scattering by describing the transfer of energy between different modes, and the details of energy transitions between different modes which are caused by sea surface scattering are presented for different sea states. With the increasing sea state, stronger mode coupling caused by surface scattering would affect the intensity and its space coherence of bottom reverberation obviously.

Efficient Algorithm for Long-Range Monostatic Reverberation in Shallow Water Using Geometrical Ray-Bundle

An efficient algorithm for long-range monostatic reverberation is developed using geometrical ray-bundle as its propagation model. In the standard approach for reverberation modeling, scattered signals are numerically integrated along the rough surface. In this work, the integration along the rough surface is converted to an integration along the delay time of source signal. It can be expressed in an analytic form under the assumption of ignoring time-dispersion effect. Repetitive eigen-ray searches for calculating scattered signals are avoided by using a split-step marching algorithm for the range based on the geometrical ray-bundle. When the bottom is range-dependent, arrival times of scattered signals from the rough bottom at same range is different according to azimuthal angles and they cannot be directly summed in time domain. The reverberation signals along the azimuthal angles are interpolated to obtain new reverberation signals at certain times. They are summed to compute a time series reverberation signal from the rough sloping bottom. The algorithm is applied to problems of reverberation modeling workshop, and its results are compared with those from the standard reverberation algorithm.

Long-Range Reverberation of Sound Signals Generated by Sequential Underwater Explosions

Long-Range Reverberation of Sound Signals Generated by Sequential Underwater Explosions

Efficient algorithm for long-range monostatic reverberation in shallow water using geometrical ray-bundle

An efficient algorithm for long-range monostatic reverberation is developed using geometrical ray-bundle as its propagation model. In the standard approach for reverberation modeling, scattered signals are numerically integrated along the rough surface. In this work, the integration along the rough surface is converted to an integration along the delay time of source signal. It can be expressed in an analytic form under the assumption of ignoring time-dispersion effect. Repetitive eigen-ray searches for calculating scattered signals are avoided by using a split-step marching algorithm for the range based on the geometrical ray-bundle. When the bottom is range-dependent, arrival times of scattered signals from the rough bottom at same range is different according to azimuthal angles and they cannot be directly summed in time domain. The reverberation signals along the azimuthal angles are interpolated to obtain new reverberation signals at certain times. They are summed to compute a time series reverberation signal from the rough sloping bottom. The algorithm is applied to problems of reverberation modeling workshop, and its results are compared with those from the standard reverberation algorithm.

Seafloor sub-bottom Imaging along the TREX reverberation track

The Buried Object Scanning Sonar (BOSS) integrated into a Bluefin12 autonomous underwater vehicle was used to collect seafloor sub-bottom data along the TREX reverberation track. BOSS is a downward looking sonar and employs an omni-directional source to transmit a 3 to 20 kHz linear frequency modulated (LFM) pulse. Backscattered signals are received by two 20-channel linear hydrophone arrays. The BOSS survey was carried out to support long-range reverberation measurements at 3 kHz. The data were beamformed in three dimensions and processed into 10cm x 10cm x 10cm voxel maps of backscattering to a depth of 1 m. Comparison of the BOSS imagery with 400 kHz multibeam sonar imagery of the seafloor allows muddy regions to be identified and shows differences rationalized by the differences in sediment penetration of the two frequency ranges utilized. Processed BOSS data are consistent with observations from diver cores and the reverberation data collected by stationary arrays deployed on the seafloor. Specifically, stronger and deeper backscattering from muddy regions is observed (relative to near-by sandy regions). This correlates well with the large amounts of detritus (e.g., shell fragments) and complicated vertical layering within cores, and the enhanced reverberation, from those regions. [Work supported by ONR.]

Integrating the energy flux method for reverberation with physics-based seabed scattering models: Modeling and inversion

During the past 30 years, one of the major accomplishments in ocean acoustics is the improvement of understanding seabed scattering, resulting from a significant effort of both at-sea measurement and theoretical modeling. [Jackson and Richardson: High-Frequency Seafloor Acoustics, 2007]. Benefiting from this accomplishment, this paper integrates the energy flux method for shallow-water (SW) reverberation [Zhou, (Chinese) Acta Acust. 5, 86-99 (1980)] with the physics-based seabed scattering models. This integration directly and intuitively results in general expressions for SW reverberation in the angular and modal domains. The latter expression is the same as the modal reverberation expression derived from the Green's function and boundary perturbation method by Tracey and Schmidt [IEEE J. Ocean. Eng. 22, 317-331(1997)]. The integration also results in a simple relationship between the classic boundary scattering cross sections and the modal scattering matrix in SW waveguides. The bottom roughness spectrum and sediment volume scattering cross section at low grazing angles are inverted in a frequency range of 150-2500 Hz from the wideband long-range reverberation data by using the Biot seabed geoacoustic model. The results may offer some reference data sets for future analysis of the low-frequency seabed scattering mechanisms.

Evidence for a common scale O(0.1) m that controls seabed scattering and reverberation in shallow water regions

Analysis of the spectral content of long-range reverberation yields two observations. First, there is a remarkably similar scale, O(0.1)m, between three diverse continental shelf regions. This is surprising given general understanding of the complexity and diversity of geologic processes. Second, there is strong evidence that the scale is associated with heterogeneities within the sediment. Thus, sediment volume scattering, not interface scattering, controls long-range reverberation from a few hundred Hertz to several kilohertz. This is also unexpected given that at long-ranges the vertical grazing angles are less than the critical angle, and hence, the penetration of the acoustic field into the sub-bottom is expected to be modest. The consistency of the scale, O(0.1)m, suggests an underlying feature or mechanism that is consistent across many ostensibly diverse geological settings. Neither the feature nor mechanism is known at this time. Several hypotheses will be presented. [Work supported by ONR Ocean Acoustics.]

Review of long-range bottom reverberation in shallow water

Theoretical models of long range bottom reverberation in shallow water are reviewed. Firstly, several classical models are summarized based on the empirical scattering function and physical scattering mechanism. Then, the development of theoretical model and experimental measurement about bi-static shallow water reverberation are outlined in the second part. During summarization, the researching work of the group from HEU is also introduced. Bi-static reverberation in range dependent waveguide is the emphasis of their research, and the decaying rule of reverberation intensity in wedged environment is presented in paper.

Evidence for a common scale O(0.1) m that controls seabed scattering and reverberation in shallow water

Analysis of the spectral content of long-range reverberation yields two observations. First, there is a remarkably similar scale, O(0.1) m, between three diverse continental shelf regions. This is surprising given the complexity and diversity of geologic processes. Second, there is strong evidence that the scale is associated with heterogeneities within the sediment. Thus, sediment volume scattering, not interface scattering, controls long-range reverberation from a few hundred hertz to several kilohertz. This is also unexpected given that at long ranges the vertical grazing angles are less than the critical angle, and hence the penetration of the acoustic field into the sub-bottom is expected to be modest. The consistency of the scale, O(0.1) m, suggests an underlying feature or mechanism that is consistent across many ostensibly diverse geological settings. Neither the feature nor mechanism is known at this time.

Method of coupled mode for long-range bottom reverberation

The theory of coupled mode is used for modeling the long-range bottom reverberation in shallow water caused by bottom roughness. The distant bottom reverberation level and spatial coherence of impulsive source are both derived. The results agree with those from the classical reverberation model, and are compared with the experimental data. The influence of source bandwidth and the distance between sources and receivers on the intensity of bottom reverberation are particularly discussed. The method is shown to be available for both the mono- and the bi-static cases.

Extracting bottom information from towed-array reverberation data: Part I: Measurement methodology

Long-range reverberation data were gathered on towed arrays during the NATO Rapid Response exercises (1996–1998) and the Boundary Characterization experiments (2000–2004). The primary tools were various SACLANTCEN/NURC arrays towed by the NATO Research Vessel Alliance. Most of the sites were in the Mediterranean, in both deep and shallow water, but there were sites on the east and west Atlantic continental shelves. This paper describes the experimental measurements, data processing, and quick-look analyses that were used in our REA (rapid environmental assessment) approach. Polar plots of the beam time series, overlaid on the bathymetry of the region, were extremely useful in providing an immediate analysis of the area. The reverberation highlights are usually correlated with bottom bathymetry and known scattering features. However, it was found that the polar plots often showed scattering features not on the charts, but which could often be identified by detailed follow-on bottom surveys. Thus the polar-plot technique is a valuable survey tool. Examples from a number of the sites are illustrated. The reverberation beam time series data, from areas with few scattering features, were used with model predictions to extract estimates of bottom loss and backscattering strength. An example is given here, but details of the modelling procedure and parameter extraction are left to a companion paper. [Work supported in part by the US Office of Naval Research, DRDC Atlantic and NATO SACLANTCEN/NURC].

Combined Geoacoustic Inversion of Propagation and Reverberation Data

Sonar performance predictions in shallow water are strongly dependent on good knowledge of the geoacoustic and scattering properties of the seabed. One technique to extract information about the bottom is to use a towed source and a towed horizontal array. This towed system has been shown to be applicable for characterizing the bottom properties locally by inversion of the acoustic signals received directly on the towed array at short ranges. The same towed system has also been applied to extract bottom properties from long-range reverberation data providing effective bottom properties over a large area. However, independent geoacoustic inversion of the short-range propagation and long-range reverberation data can introduce low sensitivity and uncertainty in the extracted bottom properties. An attempt to resolve this low sensitivity and ambiguity is made by a simultaneous geoacoustic inversion of short-range propagation and long-range reverberation data with the intention of constraining the possible solutions of the bottom properties.

Surface reverberation at sound field focusing in shallow water

A numerical experiment is carried out to study the long-range surface reverberation in the presence of intense surface waves for the case of using vertical transmitting arrays providing sound field focusing at different depths. To focus the field, a phase conjugation of acoustic waves from a probe source positioned at the focusing point is used. It is demonstrated that surface waves considerably affect the focusing quality at a distance of several tens of kilometers from the transmitting array. This prevents the efficient suppression of long-range reverberation by increasing the focusing depth.

Long-range reverberation in a randomly inhomogeneous shallow water with the use of focused radiation

The low-frequency bottom reverberation in a randomly inhomogeneous shallow water is investigated within the framework of a numerical experiment using vertical transmitting arrays focusing the acoustic field at various distances from the sea bottom. It is assumed that the main source of sound velocity fluctuations in the medium is represented by background internal waves. To focus the field, a phase conjugation of acoustic waves from a probe source positioned at the focusing point is used. It is demonstrated that the reverberation level is mainly determined by the presence of internal waves and may vary by 5–20 dB as the distance from the focusing point to the sea bottom increases up to H/2, where H is the channel depth.