Shallow Water Reverberation Research Articles

Propagation measurement using source tow and moored vertical line arrays during TREX13

The objective of TREX13 (Target and Reverberation EXperiment 2013) is to investigate shallow water reverberation by concurrently measuring propagation, local backscatter, and reverberation, as well as sufficient environmental parameters needed to achieve unambiguous model/data comparison. During TREX13 the Marine Physical Laboratory (MPL) conducted propagation and forward scatter measurements. The MPL effort during TREX13 included deploying three, 32-element (0.2 m element spacing), vertical line arrays along the Main Reverberation Track at a bearing of ~128° and ranges ~2.4 km, ~4.2 km, and ~0.5 km from the R/V Sharp, where reverberation measurements were being made. In addition, MPL carried out repeated source tows in the band of 2–9 kHz along the Main Reverberation Track, using tonal and LFM waveforms. The experimental procedure is described and the resulting source-tow data is examined in the context of Transmission Loss and its implications for reverberation.

SONAR Equation perspective on TREX13 measurements

Modeling shallow water reverberation is a problem that can be approximated as two-way propagation (including multiple forward scatter) and a single backward scatter. This can be effectively expressed in terms of the SONAR equation: RL = SL-2 x TL + SS, where RL is reverberation level, SL is the source level, TL is the one way transmission loss, and SS is the integrated scattering strength. In order to understand the reverberation problem at the basic research level, both propagation and scattering physics need to be properly addressed. A major goal of TREX13 (Target and Reverberation EXperiment 2013) is to quantitatively investigate reverberation with sufficient environmental measurement to support full modeling of reverberation data. Along a particular reverberation track at the TREX13 site, TL and direct-path backscatter were separately measured. Environmental data were extensively collected along this track. This talk will bring together all the components of the SONAR equation measured separately at the TREX13 site to provide an assessment of the reverberation process along with environmental factors impacting each of the components.

Overview of the Target and Reverberation EXperiment 2013 (TREX13)

The Target and Reverberation Experiment (TREX13) is a shallow water reverberation experiment that endeavored to measure contemporaneously acoustic and adequate environmental data so detailed model/data comparison can be achieved and important environmental factors can be identified for different applications. TREX13 was sponsored by ONR Ocean Acoustics and the SERDP DoD programs. It was conducted during April to June of 2013 off the coast of Panama City, Florida, with participation from multiple institutions and involving three research vessels: The R/V Sharp and R/V Walton Smith from the United States, and the Canadian Force Auxiliary Vessel Quest. From a SONAR viewpoint, reverberation consists of two-way propagation and a single backscatter. Therefore, reverberation, transmission loss, and bottom backscatter were repeatedly measured over a time period of several weeks in the frequency band of 2–10 kHz, along with extensive environmental measurements. Discussed will be planning and execution of the field experiments, strategies and steps for data analysis, and modeling efforts.

Volume scattering and reverberation in shallow water: A simplified modeling approach

A simplified physics-based approach is described that allows significantly faster yet reasonably accurate estimations of volume reverberation in complex shallow water environments. An integral expression is presented for scattering intensity with a factorized integrand comprised of two kernels, the double propagator and local volume scattering coefficient. The propagator describes the local intensity and can be calculated using available models, such as PE, normal modes, or ray approximations. The scattering kernel can be specified using available volume scattering models for continuous or discrete heterogeneity of sea-water column and seabed caused by spatial fluctuations of compressibility and density, or randomly distributed discrete targets, such as bubbles, fish, shells, and others. The approach is more general than and can be used for verification of existing reverberation models. For instance, calculation of bottom reverberation is not based on using the equivalent surface scattering strength (although considers it as a particular case). Numerical examples for shallow water reverberation time series, based on a PE propagation model, are presented to estimate potential contributions of different mechanisms of scattering. The estimations provide a comparison of relative contributions of scatterers with the same scattering strengths but located at different depths in water column or in the sediment. [Work supported by the US Office of Naval Research.]

Issues in reverberation modeling

Reverberation usually consists of two-way propagation, or forward scatter, and a single backscatter. The scattering cross section is often employed to couple the two-way propagation to obtain approximate reverberation strength. Because this approach is inherently incoherent and heuristic in nature, certain limitations to its applicability need to be elucidated. In particular, unlike backscatter problems in half-space, reverberation in shallow water involves coherent incident fields at different wavenumbers. Starting with the fundamental definition of scattering T-matrix and through examples, this paper intends to address the following issues: (1) how to incorporate coherent component of reverberation into simulations, (2) how to rigorously relate time to range for given bandwidth, and (3) how to increase computation speed through proper smoothing. [Work supported by ONR.]

Acoustic scattering from ocean sediment layers with multiple rough interfaces using finite elements

Acoustic scattering from the ocean bottom is a major component in shallow water reverberation and propagation as well as having a significant effect on the transmission of acoustic communication. However, boundary element models can only model scattering from a single rough interface. While some scattering models, such as the GeoAcoustic Bottom Interaction Model (GABIM), have considered scattering from layered sediments, these models are normally constrained to only one rough interface. Finite element models have been shown to accurately model scattering from both fluid and elastic boundaries, and, unlike conventional models based solely on the Helmholtz-Kirchhoff integral, are not limited to boundary interactions. In this study, a two-dimensional finite element model for scattering from two fluid layers and a fluid layer over an elastic layer is compared with perturbation theory and Kirchhoff approximation models to test the validity of considering the considering underlying interfaces flat. [Work sponsored by ONR, Ocean Acoustics.]

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.

Comparison of transport theory predictions with measurements of the decrease in shallow water reverberation level as the sea state increases

Transport theory has been developed for modeling shallow water propagation and reverberation at mid frequencies (1–10 kHz) where forward scattering from a rough sea surface is taken into account in a computationally efficient manner. The method is based on a decomposition of the field in terms of unperturbed modes, and forward scattering at the sea surface leads to mode coupling that is treated with perturbation theory. Reverberation measurements made during ASIAEX in 2001 provide a useful test of transport theory predictions. Modeling indicates that the measured reverberation was dominated by bottom reverberation, and the reverberation level at 1 and 2 kHz was observed to decrease as the sea surface conditions increased from a low sea state to a higher sea state. This suggests that surface forward scattering was responsible for the change in reverberation level. By modeling the difference in reverberation as the sea state changes, the sensitivity to environmental conditions other than the sea surface roughness is much reduced. Transport theory predictions for the reverberation difference are found to be in good agreement with measurements. [Work supported by the U.S. Office of Naval Research, 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.

Planning for a reverberation field experiment

A basic research reverberation experiment, supported by the US Office of Naval Research, is planned for 2013. Measurement issues that arise when planning such an experiment are discussed. The fundamental requirement for this basic research experiment is that the environment is characterized in sufficient detail to allow accurate numerical modeling of the acoustical results based on the environmental description. The main goal is to measure mid-frequency shallow water reverberation with full companion environmental measurements so that model/data can be compared without ambiguity. Included in the goal is to make statistical estimates of the uncertainties associated with all the environmental conditions. The frequency range of interest is 1-10 kHz with emphasis at 3 kHz. A pilot field experiment was conducted off the coast of Panama City, Florida. Data from the pilot experiment will be discussed in light of the forthcoming main experiment, including simulations on both propagation/forward-scatter and reverberation for given noise background on both vertical and horizontal arrays which will be deployed in the main experiment.

The horizontal correlation of long range bottom reverberation in shallow water with inclined sea floor

The performance of active sonar system is seriously influenced by bottom reverberation in shallow water waveguide. It is important to understand the horizontal correlation of bottom reverberation for active towed-array processing techniques in shallow sea. However, little work had been done for the research on horizontal correlation of distant bottom reverberation. In this paper, a coupled mode reverberation model was applied for the horizontal correlation, and it was investigated as a function of receiving position, time and frequency. Calculations show that transverse correlation is greater than the longitudinal correlation in horizontal space for distant bottom reverberation. The adiabatic mode solution is introduced to derive the mathematic mode for horizontal correlation in the range-dependent waveguide with varying depth and the numerical results indicate that the influence of inclined sea floor on the horizontal correlation should be considered.

Reverberation modeling with transport theory

Transport theory has been developed for modeling shallow water propagation at mid frequencies (1-10 kHz) where forward scattering from a rough sea surface is taken into account in a computationally efficient manner. The method is based on a decomposition of the field in terms of unperturbed modes, and forward scattering at the sea surface leads to mode coupling that is treated with perturbation theory. Transport theory has recently been extended to model shallow water reverberation, including the effect of forward scattering from the sea surface. Transport theory results will be compared with other solutions for reverberation examples taken from ONR Reverberation Modeling Workshop problems. These comparisons show the importance of properly accounting for multiple forward scattering in shallow water reverberation modeling. [Work supported by ONR Ocean Acoustics.]

Low frequency seabed scattering at low grazing angles

Low-frequency (LF) seabed scattering at low grazing angles (LGA) is almost impossible to directly measure in shallow water (SW), except through inversion from reverberation. The energy flux method for SW reverberation is briefly introduced in this paper. The closed-form expressions of reverberation in an isovelocity waveguide, derived from this method, indicate that in the three-halves law range interval multimode/ray sea bottom scattering with different incident and scattering angles in forming the reverberation may equivalently be represented by the bottom backscattering at a single range-dependent angle. This equivalent relationship is used to derive the bottom backscattering strength (BBS) as a function of angle and frequency. The LF&LGA BBS is derived in a frequency band of 200-2500 Hz and in a grazing angle range of 1.1°-14.0° from reverberation measurements at three sites with sandy bottoms. This is based on three previous works: (1) The closed-form expressions of SW reverberation [Zhou, (Chinese) Acta Acustica 5, 86-99 (1980)]; (2) the effective geo-acoustic model of sandy bottoms that follows the Biot model [Zhou et al., J. Acoust. Soc. Am. 125, 2847-2866 (2009)] and (3) A quality database of wideband reverberation level normalized to source level [Zhou and Zhang, IEEE J. Oceanic Eng. 30, 832-842 (2005)].

Propagation theory for bi-static long-range bottom reverberation in shallow water waveguide

Bi-static long-range bottom reverberation in shallow water is investigated, and the impulse response function is derived using the linear theory of sound channel. For the case of almost layered medium, the adiabatic mode solution is introduced for the derivation of transfer function. The reverberation intensity is calculated for the range-dependent waveguide, and then both the intensities and the decaying rules are compared with their corresponding counterparts in different wedged sea areas. It is shown that the propagation effect on intensity of distant bottom reverberation can not be neglected for the bi-static active sonar in shallow water waveguide.

Quantifying seismic survey reverberation off the Alaskan North Slope

Shallow-water airgun survey activities off the North Slope of Alaska generate impulsive sounds that are the focus of much regulatory attention. Reverberation from repetitive airgun shots, however, can also increase background noise levels, which can decrease the detection range of nearby passive acoustic monitoring (PAM) systems. Typical acoustic metrics for impulsive signals provide no quantitative information about reverberation or its relative effect on the ambient acoustic environment. Here, two conservative metrics are defined for quantifying reverberation: a minimum level metric measures reverberation levels that exist between airgun pulse arrivals, while a reverberation metric estimates the relative magnitude of reverberation vs expected ambient levels in the hypothetical absence of airgun activity, using satellite-measured wind data. The metrics are applied to acoustic data measured by autonomous recorders in the Alaskan Beaufort Sea in 2008 and demonstrate how seismic surveys can increase the background noise over natural ambient levels by 30-45 dB within 1 km of the activity, by 10-25 dB within 15 km of the activity, and by a few dB at 128 km range. These results suggest that shallow-water reverberation would reduce the performance of nearby PAM systems when monitoring for marine mammals within a few kilometers of shallow-water seismic surveys.

Importance of surface forward scattering on reverberation level

Transport theory has been developed for modeling shallow water propagation at mid frequencies (1–10 kHz) where forward scattering from a rough sea surface is taken into account in a computationally efficient manner. The method is based on a decomposition of the field in terms of unperturbed modes, and forward scattering at the sea surface leads to mode coupling that is treated with perturbation theory. Transport theory has recently been extended to model shallow water reverberation, and the effect of forward scattering from the sea surface is found to be very important in accurately modeling the reverberation level under winter-like (nearly isovelocity) sound speed conditions. At a frequency of 3 kHz and a wind speed of 15 knots (7.7 m/s) with a fully developed sea, the reverberation level from sea surface scattering is found to be up to 20 dB greater than when a coherent reflection loss is used to account for the effect of surface roughness during the propagation out to and back from the primary backscatter source of reverberation. Examples illustrating the importance of sea surface forward scattering on reverberation level are shown. [Work supported by ONR Ocean Acoustics.]

Integrating the energy flux method of shallow-water reverberation with physics-based seabed scattering models.

The energy flux (angular spectrum) method for shallow-water (SW) reverberation was first presented 3 decades ago in archived Chinese journals. These included closed form expressions for reverberation in SW with a down refraction or isovelocity profile. [Zhou, Acta Acust. 5, 86–99 (1980); Chin. J. Acoust. 1, 54–63 (1982)]. More papers on SW reverberation using this method have recently been published in JASA and IEEE JOE. However, most of these papers are based on semiempirical seabed scattering models, which do not have a physical basis. In the past 30 years, one of the major accomplishments in ocean acoustics is the improvement in our understanding of 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 model of SW reverberation with the physics-based seabed scattering models in the angular domain. This integration directly and intuitively results in a general expression for SW reverberation in terms of seabed physical/scattering parameters, and this expression is almost identical with the expression derived from the boundary perturbation method. The results may be used to estimate the bottom roughness or sediment inhomogeneity from SW reverberation measurements.[Work supported by the ONR and CAS].

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.

The interplay of acoustic signals with boundary and volume scattering phenomena: An overview.

Active sonar systems use acoustic sources and receivers coupled with signal processing to detect, classify, localize, and track undersea targets. The limiting influence of scattering phenomena, manifesting as reverberation and clutter, on sonar performance has long been recognized, with acoustic echoes from the waveguide boundaries and biologics capable of both masking desired signals and creating false targets/tracks. This talk provides a broad overview of reverberation and clutter phenomena in both deep and shallow water in the context of their impact on sonar performance for frequencies of 50 Hz to 5 kHz. The interplay of system (geometry, transmitter/receiver design, frequency, waveform, and signal processing) and environmental (oceanography, geology, ice, and biology) factors will be discussed, with a series of data and model examples demonstrating the importance of using high‐fidelity, physics‐based approaches to both characterize the acoustic field and guide the signal processing. While the environment presents many challenges to active sonar systems because of its spatiotemporal variability and strong coupling between scattering and propagation structures, it should also be viewed as both potentially manageable and presenting opportunities. If the mechanisms are understood, they can be dealt with, and even exploited. [Work supported by ONR.]