Shallow water reverberation: normal-mode model predictions comparedwith bistatic towed-array measurements

A normal-mode model for calculating reverberation in shallow water is presented. Some illustrative calculations are given for the bistatic case and for vertical and horizontal line-array receivers. Emphasis is on comparison with measurements of bistatic reverberation obtained at a shallow-water area in the Mediterranean. The data are from explosive sources received by a towed array, analyzed in one-tenth-decade frequency bands at subkilohertz frequencies. Model calculations for a flat-bottomed environment indicate a strong dependence on propagation conditions and a weak dependence on beam steering direction. Preliminary comparisons give quite good agreement between measured reverberation and model predictions, but point to the need for extending modeling efforts to handle range-dependent environments     

浅海均匀层远程混响的垂直相干性

混响是主动声纳在浅海环境中的一种干扰,有关其空间相关特性,Urick和Lund发表了两篇实验性报告[7].本文根据浅海平均声场角度谱分析法[3],从理论上计算了浅海均匀层远程混响垂直相关特性与界面反射、散射等环境参数之间的关系,及其随距离、水听器间隔的变化,供声纳设计或在浅海环境中研究低频、小掠角的散射特性时参考.     

Shallow-water reverberation level: measurement technique and initial reference values

A quality database of reverberation is absolutely essential if one is to understand the shallow-water reverberation problem. However, to get wideband reverberation levels (RL) simultaneously for both short and long ranges at low- and mid-frequencies is a delicate task that can be subject to errors. This paper introduces a simple method to get RL for the Asian Sea International Acoustics Experiment in the East China Sea (ASIAEX01). Special attention is paid to the measurements of the RL at short- and mid-ranges. With this method, one does not need to accurately calibrate hydrophones and measurement systems, or to measure absolute source level (SL). It can avoid signal overflow and saturation problems caused by powerful sound sources. The RL (relative to SL) at 1 s (or at 2 s) after an explosive source is detonated is defined as the initial reference reverberation level (IRRL). The IRRLs from four sites with different sandy sediments and different water depths have been given as a function of frequency in the 150-2500 Hz range. A mathematical model gives a physical explanation of the measured IRRL data. The resultant RL and IRRL may offer some reference values for the design of reverberation measurements or numerical simulations of shallow-water reverberation and bottom scattering.     

Reverberation rejection via modeforming with a vertical line array

The concept of modeforming with a vertical line array for active target detection in a reverberation-limited shallow-water environment is explored. The concept is based on the hypothesis that, in certain environments, the reverberation field is due to subbottom scattering and the reverberant energy is carried by the higher order modes; on the other hand, the targets in the water column are contained in the lower order modes. Under these conditions, isolation of a low-order mode, or a selected set of modes, with a vertical array could substantially increase the target-to-reverberation ratio. The optimum mode depends greatly on the environmental conditions and the depth of the target. Modeforming is achieved by deriving a spatial filter function that depends on the receiving array and the local environment     

A model/data comparison for shallow-water reverberation

A bottom-scattering model based on sediment small scatterers, single scattering approximation is presented, which is combined with a normal-mode-based reverberation model. The combined model (for total reverberation) is compared with measurements of shallow-water reverberation from the 2001 Asian Sea International Acoustic Experiment (ASIAEX) in the East China Sea. Reverberation intensities as a function of time and frequency are compared with theoretical predictions with reasonable agreement. The effects of the rough sea surface on the reverberation are also discussed.     

Data-model comparisons of reverberation at three shallow-watersites

Reverberation measurements made by the SACLANT Undersea Research Centre at three shallow-water sites (130-190-m depth) are compared with each other and with estimates from the DREA normal-mode reverberation model OGOPOGO. The experiments over silt-clay and sand seabeds were conducted at slightly bistatic geometries (0.7-6.0-km source-receiver separation), using explosive sources detonated at mid-water depths. The signals were received on hydrophones of either a vertical or horizontal array and analyzed in one-tenth-decade frequency bands from 25 to 1000 Hz. The data are compared with each other to investigate the site differences and frequency dependencies, and with the estimates from the reverberation model OGOPOGO to interpret the data and to obtain a qualitative measure of the scattering. For modeling purposes, geoacoustic models of the seabed were assumed, and the reverberation data were fitted by adjusting the Lambert bottom scattering coefficients. Good model agreement was obtained with both individual hydrophone and data. Though somewhat sensitive to the geoacoustic the Lambert coefficients give a measure of the frequency dependence of the scattering. For the silt-clay bottom, the scattering is weak but is independent of frequency; for the sand bottoms, the scattering is stronger and increases with frequency. These results are compared with estimates from other experiments     

Reverberation fluctuations from a smooth seafloor

High-frequency shallow-water reverberation statistics were measured from a smooth, sandy, featureless seafloor. The reverberation statistics are presented as a function of source frequency (20-180 kHz), grazing angle (30°, 20°, 9.5°), and source beamwidths (1.2°-2.75°). Generally, the reverberation statistics did not follow a Rayleigh fading model. The model dependence of the reverberation statistics exhibited a complex behavior that ranged from near Gaussian to beyond log-normal. The results show that small changes in the source frequency, grazing angles, and beamwidths caused large variations in the model dependence of the reverberation statistics     

Simulation of non-Rayleigh reverberation and clutter

The simulation of active sonar reverberation time series has traditionally been done using either a computationally intensive point-scatterer model or a Rayleigh-distributed reverberation-envelope model with a time-varying power level. Although adequate in scenarios where reverberation arises from a multitude of scatterers, the Rayleigh model is not representative of the target-like non-Rayleigh reverberation or clutter commonly observed with modern high-resolution sonar systems operating in shallow-water environments. In this paper, techniques for simulating non-Rayleigh reverberation are developed within the context of the finite-number-of-scatterers representation of K-distributed reverberation, which allows control of the reverberation-envelope statistics as a function of system (beamwidth and bandwidth) and environmental (scatterer density and size) parameters. To avoid the high computational effort of the point-scatterer model, reverberation is simulated at the output of the matched filter and is generated using efficient approximate methods for forming K-distributed random variables. Finite impulse response filters are used to introduce the effects of multipath propagation and the shape of the reverberation power spectrum, the latter of which requires the development of a prewarping of the K distribution parameters to control the reverberation-envelope statistics. The simulation methods presented in this paper will be useful in the testing and evaluation of active sonar signal processing algorithms, as well as for simulation-based research on the effects of the sonar system and environment on the reverberation-envelope probability density function.     

Signal recovery in a reverberation-limited environment

The problem of recovering signals masked by reverberation is considered. Reverberation data from a shallow-water active sonar experiment in conjunction with simulated echoes are used to examine the potential for signal recovery offered by adaptive filtering and prediction. The deterministic least squares lattice filter is the central adaptive estimator of choice. The prediction error lattice is used to selectively "whiten" the composite process by controlling the algorithm adaptation speed. This is shown to result in significant signal enhancement for low-Doppler echoes masked by reverberation. Adaptive noise canceling with multiple reference beams is shown to be successful in extracting even zero-Doppler echoes from the reverberation background.     

多波束测深海底混响仿真研究

海底混响是海洋混响的重要组成部分,采用模拟仿真进行验证分析,是仿真技术的一项重要应用。采用单元散射模型,研究在单发射阵元下,分布在同一直线上多个接收阵元接收的海底回波,忽略声波传播的相位起伏,只考虑振幅起伏,将传播损失、声吸收系数、海底反射损失、海底沉积层密度等参数带入海底混响仿真数学模型,仿真海底混响,使其更加接近海底的实际情况。     

Enhanced Kalman Filter Algorithm Using the Invariance Principle

Target tracking in multistatic active sonar systems is often limited in shallow-water environments due to the high level of bottom reverberation that produces false detections. Past research has shown that these false alarms may be mitigated when complete knowledge of the environment is available for discrimination, but these methods are not robust to environmental uncertainty. Recent work has demonstrated the existence of a waveguide invariant for active sonar geometries. Since this parameter is independent of specifics of the environment, it may be used when the environment is poorly known. In this paper, the invariance extended Kalman filter (IEKF) is proposed as a new tracking algorithm that incorporates dynamic frequency information in the state vector and uses the invariance relation to improve tracker discrimination. IEKF performance is quantified with both simulated and experimental sonar data and results show that the IEKF tracks the target better than the conventional extended Kalman filter (CEKF) in the presence of false detections.      

Relationship Between Integrated Bottom Scattering Strength and Modal Backscattering Matrix

The integrated bottom scattering strength (ISS) and the modal backscattering matrix (MBSM) have been used to describe the bottom backscattering in shallow-water reverberation modeling, and both can be extracted from the reverberation data. The relationship between the ISS and the MBSM based on the same reverberation data in shallow water is discussed. It is shown that the ISS is an angular weighting average of the MBSM in mode space. The weighting factors are range-depth-frequency-dependent and also depend on the sound-speed profile (SSP) in the water column. Thus, the ISS has a complex variation. This complexity of the ISS causes it to be so variable that it provides little basic understanding of the scattering. Numerical examples are conducted to show this complexity.     

Signal excess in K-distributed reverberation

Active sonar systems have recently been developed using larger arrays and broad-band sources to counter the detrimental effects of reverberation in shallow-water operational areas. Increasing array size and transmit waveform bandwidth improve the signal-to-noise ratio-and-reverberation power ratio (SNR) after matched filtering and beamforming by reducing the size of the range-bearing resolution cell and, thus, decreasing reverberation power levels. This can also have the adverse effect of increasing the tails of the probability density function (pdf) of the reverberation envelope, resulting in an increase in the probability of a false alarm. Using a recently developed model relating the number of scatterers in a resolution cell to a K-distributed reverberation envelope, the effect of increasing bandwidth (i.e., reducing the resolution cell size) on detection performance is examined for additive nonfluctuating and fluctuating target models. The probability of detection for the two target models is seen to be well approximated by that for a shifted gamma variate with matching moments. The approximations are then used to obtain the SNR required to meet a probability of detection and false-alarm performance specification (i.e., the detection threshold). The required SNR is then used to determine that, as long as the target and scatterers are not over-resolved, decreasing the size of the resolution cell always results in an improvement in performance. Thus, the increase in SNR obtained by increasing bandwidth outweighs the accompanying increase in false alarms resulting from heavier reverberation distribution tails for K-distributed reverberation. The amount of improvement is then quantified by the signal excess, which is seen to be as low as one decibel per doubling of bandwidth when the reverberation is severely non-Rayleigh, as opposed to the expected 3-dB gain when the reverberation is Rayleigh distributed.     

The calibration of a laser light line scan method for determining local interface roughness of the ocean floor

An accurate model of acoustic interaction with sandy sediments is crucial to the application of SONAR in shallow-water environments. Because acoustic scattering from interface roughness plays a major role in the reverberation from and penetration into sandy sediments, it is imperative to be able to accurately measure the roughness of the sediment/water interface. An interface roughness measurement system has been developed in which a laser light sheet is projected onto the ocean floor. A resulting image can then be analyzed to determine the interface roughness. The system has been shown to achieve a height measurement error of less than 0.9 mm over a spatial frequency range of 15 to 60 cycles/m with about 0.5 mm standard deviation. These spatial frequencies correspond to acoustic Bragg frequencies of 11 to 45 kHz for backscattering applications. The error in wavelength was less than 5 mm with a standard deviation of about 1.0 mm. The system is inexpensive, easily deployable and automated in terms of data extraction. This system could greatly aid in determining the local interface profile for in situ acoustic scattering experiments.     

The range-averaged intensity model: a tool for underwater acousticfield analysis

The range-averaged intensity model (RAIM) long known as a powerful method for transmission loss estimation in waveguides, is extended to other aspects of the acoustic field: impulse response and angular pattern estimations at a receiver, general time spreading in a waveguide, signal fluctuations, reverberation levels, and ambient noise structure. An example of its application to a SOFAR propagation and detection case is presented. This method appears to be a very efficient and reliable analysis tool for many underwater acoustics configurations: particularly long-range horizontal telemetry and shallow-water sonar     

Experimental investigation of the angular dependence of low-frequency seabed reverberation

Linear frequency-modulated (LFM) signals with 600 Hz bandwidth, centered at 1.1 kHz were transmitted from a towed source in a shallow-water environment in the Mediterranean Sea and received at a distant vertical line array (VLA). Arrivals recorded on the VLA were beamformed to give incident intensity as a function of vertical angle. Simultaneous measurements of quasimonostatic reverberation were made on a horizontal line array (HLA), towed close to the source. In this paper, these data are analyzed to study the relation between the angle at which sound arrived at the patch of seabed surrounding the VLA and the intensity of quasimonostatic reverberation returned from that patch. The validity of three candidate relations (Lambert's Law, angle-independent scattering and an intermediate relation) is investigated and scattering strength parameters (Lambert /spl mu/ and equivalents) are also deduced for the patch of seabed surrounding the VLA. Only weak evidence is found to support one of the scattering relations (the intermediate) over the other two and the reasons for this are discussed. The physical processes that may underlie the intermediate relation are also discussed.     

Range-dependent geoacoustic inversion: results from the Inversion Techniques Workshop

Model-based geoacoustic inversion in range-dependent underwater environments is a challenging task constrained by data quality (synthetic or measured) and propagation-model efficiency and accuracy. The Inversion Techniques Workshop (ITW), held in Gulfport, MS, May 15-18, 2001, was organized for the acoustics community to present state-of-the-art numerical geoacoustic inversion capabilities in range-dependent shallow-water environments. The organizers defined five range-dependent test cases (three synthetic and two experimental cases). Two of the synthetic cases were adopted for geoacoustic inversion in this paper. The first test case (TC1) is a monotonic down-slope bathymetry problem and the adiabatic normal-mode model PROSIM was applied for the inversion in this case. The second test case (TC3) is a flat-bottom case with an intrusion. The forward model used in this case was RAMGEO. The global optimization package SAGA was used for geoacoustic inversion of the synthetically generated reference solutions for TC1 and TC3. In general, the geoacoustic inversion results are in good agreement with the true solutions provided by the organizers. The results obtained demonstrate the feasibility of performing geoacoustic inversion in synthetic range-dependent shallow-water environments. However, results show that the propagation model choice in the inversion is strongly dependent on the specific range-dependent environment.     

Environmental Effects of Waveguide Uncertainty on Coherent Aspects of Propagation, Scattering, and Reverberation

The robustness of the coherence of waveguide propagation to environmental uncertainty becomes an important consideration for systems that seek to exploit coherence for gain. Examples include matched field processing for passive localization and time reversal mirrors (TRMs) for active systems. Here, efficient normal mode representations of midfrequency time-domain propagation using the narrowband and adiabatic approximations are used to explore the deterioration of coherent active system predictability and performance in the presence sound-speed perturbations in the water column. Results show that for TRMs the reverberation level at the focal range is increased, and the scattering from an illuminated object is reduced for ensembles over oceanographic uncertainty. Results are obtained analytically as formal averages and are believed to represent a lower limit on the deterioration of TRM performance in the presence of environmental uncertainty for shallow-water waveguides.     

Adaptive port-starboard beamforming of triplet sonar arrays

For a low-frequency active sonar (LFAS) with a triplet receiver array, it is not clear in advance which signal processing techniques optimize its performance. Here, several advanced beamformers are analyzed theoretically, and the results are compared to experimental data obtained in sea trials. Triplet arrays are single line arrays with three hydrophones on a circular section of the array. The triplet structure provides the ability to solve the notorious port-starboard (PS) ambiguity problem of ordinary single-array receivers. More importantly, the PS rejection can be so strong that it allows to unmask targets in the presence of strong coastal reverberation or traffic noise. The theoretical and experimental performance of triplet array beamformers is determined in terms of two performance indicators: array gain and PS rejection. Results are obtained under several typical acoustic environments: sea noise, flow noise, coastal reverberation, and mixtures of these. A new algorithm for (beam space) adaptive triplet beamforming is implemented and tuned. Its results are compared to those of other triplet beamforming techniques (optimum and cardioid beamforming). These beamformers optimize for only one performance indicator, whereas in theory, the adaptive beamformer gives the best overall performance (in any given environment). The different beamformers are applied to data obtained with an LFAS at sea. Analysis shows that adaptive triplet beamforming outperforms conventional beamforming algorithms. Adaptive triplet beamforming provides strong PS rejection, allowing the unmasking of targets in the presence of strong directional reverberation (e.g., from a coast) and at the same time provides positive array gain in most environments.     

The mineralogy of manganese nodules from a range of marine environments

The mineralogy of 45 managanese nodules from a range of marine environments is described. 10 Å manganite is shown to be the principal mineralogical phase in 9 nodules from shallow-water, continental-margin environments, whereas δ MnO₂ is the principal mineralogical phase in 35 nodules from the Carlsberg Ridge, Indian Ocean. The phase relationships appear to be controlled by the redox characteristics of the sedimentary environment rather than the kinetics of nucleation or mineralogical ageing phenomena. The present data give no evidence to support the influence of submarine volcanism on nodule mineralogy.