High-frequency reverberation in shallow water

Monostatic reverberation measurements were collected in shallow water, over a coarse gravel and cobble bottom, 100 m deep, off the coast of Nova Scotia. Data were collected at frequencies of 21, 28, and 36 kHz using linear FM pulses of 2-kHz bandwidth and 0.160-s duration. An anchored, high-frequency active sonar array deployed at a depth of 42 m was used to collect the data. The reverberation measurements were compared with estimates computed with the NUWC generic sonar model (GSM). The data were reasonably well modeled for times greater than 0.2 s after pulse transmission by neglecting surface reverberation and using Lambert's rule for bottom backscattering with a scattering coefficient of -27 dB, independent of frequency. At all three frequencies, the data and model show a peak approximately 0.9 s after pulse transmission. This peak results from a focusing effect that the downward-refracting sound-speed profile has on the interaction of the rays with the bottom     

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.     

Specific features of reverberation in the Black-Sea underwater sound channel

We consider the results of instrumental investigations of specific features of reverberation in the Black Sea and obtain qualitative dependences of the duration of volume reverberation on the parameters of the Black-Sea underwater sound channel, (width, drop of the sound velocity, and dimensions of the inhomogeneities of stratification). We also analyse the behaviour of the intensity of surface reverberation in the far-field zone of acoustic illumination and the influence of bottom reverberation on the detection of underwater objects. Translated by Peter V. Malyshev and Dmitry V. Malyshev     

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.     

混响环境中声场重构的实验研究

针对混响环境非自由声场中声源测量的问题,本文以消声水池和混响水槽为实验环境,以换能器辐射的声场为研究对象,以水听器阵列为测量前端,进行了混响环境非自由声场中声源对象的测量、分析和重构的实验研究。通过单层水听器阵列对非自由声场进行声压分布测量,并对测量结果作声波分离处理,将分离前后的声压分布和在消声水池中测量的声压分布进行比较,给出了声源频率为5 000 Hz和7 000 Hz时,声场重构的误差分析结果。结果表明,基于单层水听器阵列声压测量的声波分离方法,能够较精确地对混响环境中的声场进行重构。     

Morphological and statistical approaches to improve detection in the presence of reverberation

The detection of a target echo in a sonar image is usually a difficult task since the reverberation, consisting of a large number of spurious echoes, generates a lot of false alarms. In this paper, we propose two new detectors derived from image processing algorithms. These detectors are respectively based on a morphological and a statistical contrast. Each detector only requires setting a few parameters. This setting is done using some prior knowledge about the data (shape of the emitted signal and the used antenna, characteristics of the reverberation). Nevertheless, an extensive statistical study of the detection performances proves that the proposed methods are robust and that even an imprecise setting of the parameters leads to satisfactory results. Applied to the real data, these detectors and their sequential combination lead to a significant improvement on the performances: The false alarm rate is drastically reduced while the detection probability is preserved. Based on different contrasts, these detectors have complementary behaviors. Therefore, a further improvement is achieved by a fusion of the different results to classify the remaining echoes as whether spurious or true detection.     

Statistics of broad-band bottom reverberation predictions in shallow-water waveguides

A new coherent reverberation model developed at the Naval Research Laboratory, Washington, DC, and the Supreme Allied Commander Atlantic Undersea Research Centre, La Spezia, Italy, is exercised in the 17-750-Hz band to estimate the degree of non-Rayleighness of shallow-water reverberation envelopes as a function of waveguide multipath, system bandwidth, directivity, and frequency. Findings suggest that reverberation from diffuse, but non-Gaussian, scatterer distributions is significantly more Rayleigh for multipath environments than for equivalent environments excited by a single or small number of modes or for broadside receiver array processing that extracts narrow angles of reception. These findings suggest that the problem of non-Rayleigh reverberation in shallow-water waveguides can be ameliorated through the use of tuned ensonification and reception schemes, which retain high probabilities of detection while reducing the associated probability of false alarm.     

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.     

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     

Frequency-selective attenuation of sound propagation and reverberation in shallow water

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浅海声传播和混响的选频衰减

在强负跃层浅海的爆炸声实验中,发现当声源和接收器都位于跃层之上时,平均混响强度和某一航向的声传播损失在频率1000-2000Hz之间出现强烈的异常衰减现象,而且很有意思的是发射和接收均无指向性的平均混响强度的异常衰减与该航向声传播损失的异常衰减具有中心频率相同、带宽一致、附加衰减值相近的窄带共振或选频衰减特权.显然,这一异常衰减现象无法用各向异性的机理(内波、海面或海底的有规律起伏等)来解释.根据本文实验所得的传播损失和混响强度的深度结构以及一些间接的证据,我们认为这一选频附加衰减是由分散活动干跃居上部的有鳔鱼(极可能是鱼)所引起的.     

"Principal component inverse" algorithm for detection in thepresence of reverberation

Detection in the presence of reverberation is often difficult in active sonar, due to the reflection/diffusion/diffraction of the transmitted signal by the ocean surface, ground, and volume. A modelization of reverberation is often used to improve detection because classical algorithms are inefficient. A commonly used reverberation model is colored and nonstationary noise. This model leads to elaborate detection algorithms which normalize and whiten reverberation. In this paper, we focus on a more deterministic model which considers reverberation as a sum of echoes issued from the transmitted signal. The Principal Component Inverse (PCI) algorithm is used with this model to estimate and delete the reverberation echoes. A rank analysis of the observation matrix shows that PCI is efficient in this configuration under some conditions, such as when the transmitted signal is Frequency Modulated. Both methods are validated with real sonar surface reverberation noise. We show that whitening has poor performance when reverberation and target echo have the same properties, while PCI maintains the same performance whatever the reverberation characteristics. Further, we extend the algorithms to spatio-temporal data. We propose a new algorithm for PCI which allows better echo separation. This new method is shown to be more efficient on real spatio-temporal data     

Statistical characterization of active sonar reverberation using extreme value theory

The statistics of reverberation in active sonar are characterized by non-Rayleigh distributed amplitudes in the normalized matched filter output. Unaccounted for, this property can lead to high false-alarm rates in fixed-threshold detectors. A new approach to modeling threshold-crossing statistics based on extreme value theory is proposed, which uses the generalized Pareto distribution as the unique asymptotic model of the tail distribution, valid at large thresholds. Methods of parameter estimation are discussed and applied to active sonar reverberation collected on a hull-mounted sonar system. The statistics of reverberation in active sonar are found to generally have a power-law behavior in the tails with a shape parameter that is persistent in time and bandwidth dependent. The threshold needed for accurate parameter estimation is generally found to be well below that of typical fixed-threshold detectors.     

The spatial variability of surface reverberation under high and lowwind speeds

Two-dimensional images of broadband ocean reverberation in deep water have been made from explosive source data. The effective spatial resolution is 1 m×10 m for the frequency band of 120-1430 Hz. This paper presents two typical images for wind speeds of 4 and 17 ms^-1 . The high wind case exhibits substantial spatial variability on the scale of 100 meters that is not observed in the low wind case. These results have significance for the evaluation of hypotheses for wind induced surface reverberation     

Wave parameter gradients along the wave ray

Computer methods have been used to track selected deep-water waves along their orthogonals to the surf zone. Three bottom orbital parameters were computed at each step: diameter, maximum velocity and acceleration. These three parameters were then plotted along the several wave rays, thereby providing shoreward gradients.The three gradients have essentially the same geometry, so that any one can be taken as an indicator of the other two. Bottom slope changes are responsible for departure of the gradients from linearity. The bottom orbital gradients are quite different from gradients of wave height and wave length. These results from coasting waves cannot be applied directly to the case of forced, or wind-driven, waves.The non-linear nature of the bottom orbital gradients indicates that the near-shore bottom should exhibit a crude banding, more-or-less parallel with the coast, with different sediment transport and ripple-mark geometry from band to band.     

Estimation of bottom scattering strength from measured and modeledmid-frequency sonar reverberation levels

Hamilton-type geoacoustic models were developed for Area Foxtrot, a shallow water test bed south of Long Island, for emerging active sonar systems where the surface sediment type is highly spatially variable. Reverberation levels (RL) were modeled using the finite-element parabolic equation (FEPE) propagation model to augment the generic sonar model (GSM) propagation model because the bottom loss model in GSM did not estimate transmission loss (TL) accurately in shallow water. FEPE estimates reveal that there is a greater than 15 dB difference between TL for sand and that for silt-day sediments in Area Foxtrot. The comparison between modeled RL and measured RL (from a 1991 active sonar exercise) enabled bottom scattering strength kernels to be developed for Area Foxtrot. Bottom scattering strength was found to be a function of sediment type. Hard sand sediment has a bottom scattering strength which obeys Lambert's law (sin² &thetas;) while that of silt-clay sediment is consistent with sub-bottom volume scattering (sine). The RLs in Area Foxtrot are azimuth-dependent and are a function of TL and bottom scattering strength (and hence bottom sediment type). Sonar beams steered towards the hard sand show higher RLs than for silt-clay, and knowledge of the sediment type and its spatial variation must be known to model RL accurately. A method to determine sediment type using measured RLs and RL slopes is given