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1.
Shallow-water bottom reverberation measurements 总被引：2，自引：0，他引：2
Stanic S. Goodman R.R. Briggs K.B. Chotiros N.P. Kennedy E.T. 《Oceanic Engineering, IEEE Journal of》1998,23(3):203-210
High-frequency bottom reverberation measurements were made at an experimental site in the Gulf of Mexico. The acoustic data were taken as a function of frequency (40-180 kHz) and grazing angle (40-33°). The measured acoustic reverberation results are compared to predictions made by models developed by Jackson et al. (1986, 1996) and Boyle and Chotiros (1995). The models used inputs from the analysis of sediment cores and stereophotography. The model predictions show differences from each other and from the data. The results show reverberation-level variabilities as a function of frequency that cannot be accurately predicted by these models 相似文献
2.
High-frequency bistatic sediment scattering experiment was conducted in the shallow waters off the east coasts of Korea. Acoustic data were taken as a function of grazing angle (30°, 45°, and 60°), scattered angle (30°, 45°, and 60°), and bistatic (azimuthal) angle (0°, 60°, and 120°). Besides a flat bottom it was artificially raked so as to produce directional ripples. The measured scattering strengths for a flat bottom were compared to model predictions of D.R. Jackson et al. (1986). The surface reverberation component is seen to dominate over the volume scattering part at the frequency of 240 kHz. Compared to the flat bottom case, the scattering strengths for directional ripples showed lower and higher variation depending on the ripple's orientation 相似文献
3.
Broad-band forward loss and backscattering measurements were made at low to moderate grazing angles in shallow water off San Diego using pulses extending from 1 to 6 kHz in bandwidth. For forward bounce measurements, these large bandwidths achieved time resolutions as small as 0.25 ms, and revealed fine-scale subbottom layering with separations down to approximately 50 cm. The forward loss values show large fluctuations (>10 dB) over translation distances of 20-50 m in some cases or between two measurement runs separated by a few hundred meters in other cases. This observation, along with associated variations in the extent and number of subbottom arrivals, indicates a distinct patchiness in surficial sediment type. Previous measurements made in nearby locales also evidenced strong variations in bottom loss, but lacked the spatial resolution to discern interface reflections from subbottom contributions. Broad-band backscattering strength measured at 20-40° grazing was quite homogeneous over the entire region, probably because the critical angle is below 20°, as inferred from forward loss measurements. Theory suggests that scattering at angles above critical is from subbottom inhomogeneities rather than boundary roughness. The grazing angle and frequency dependence of these backscattering data are relatively weak 相似文献
4.
在南黄海某一典型的砂质海底区域,采用全向性声源和全向性接收水听器开展了频率范围为6-24 kHz的海底反向声散射测量。测量结果表明,在避免海面散射干扰并满足远场条件的情况下,本次实验获得了掠射角范围为18~80°的海底反向声散射强度,其数值为-41.1~24.4 dB。在有效掠射角范围内,声散射强度总体上随掠射角的增大呈现出增大趋势,但对于不同的频率,其变化趋势有所不同,反映出不同的散射机理。在20°、40°和60°掠射角处,在6-24 kHz的频率范围内反向声散射强度总体上呈现出正相关的频率依赖性,其线性相关斜率分别为0.2229 dB/kHz、0.5130 dB/kHz、0.1746 dB/kHz。在最大掠射角80°处,反向声散射强度未呈现出明显的频率相关性。 相似文献
5.
Cable P.G. Yudichak T.W. Dorfman Y. Knobles D.P. Renhe Zhang Zhaohui Peng Fenghua Li Zhenglin Li 《Oceanic Engineering, IEEE Journal of》2006,31(1):145-155
Determinations of bottom scattering strength in the decade below 1 kHz under downward refracting conditions have been made using acoustic reverberation and transmission data from the 2001 East China Sea Asian Seas International Acoustic Experiment (ASIAEX). The measurements were performed using explosive sources and receiving hydrophones in ship-suspended vertical-line arrays. The focus of this paper has been the dependence of bottom scattering strength on the frequency and characterization of the uncertainties associated with the extraction of scattering strength from reverberation. The derived bottom scattering strength gradually rises with frequency from 100-300 Hz and then more rapidly above 300 Hz. A potential explanation suggests that the frequency variation results from two scattering mechanisms, rough layer scattering at the low end of the band and sediment near-surface volume scattering at the high end. The spatial extrapolation of these results is explored by comparing them with similarly derived scattering strengths using data obtained under the Navy's Harsh Environments Program at a somewhat separated site (56 km) under environmental conditions similar to those during ASIAEX. In the ASIAEX analysis, it has been found that the largest source of uncertainty in the scattering-strength frequency dependence arises from persistence of finite-amplitude effects associated with the source signal. 相似文献
6.
This paper examines the fluctuations in low-grazing-angle 100-kHz backscatter from a rocky limestone sea bed near Copenhagen, Denmark, at horizontal ranges up to 420 m. The sea-bed reverberation was characterized by strong short spatial-scale variations in scattering strength and statistical parameters. The measured areal backscatter strengths were in the range from -50 to -24 dB at grazing angles less than 3/spl deg/, showing a strong local variability and grazing-angle dependence definitely not in accordance with Lambert's law. The observed echo-amplitude distributions varied between log-normal and Rayleigh models, with more Rayleigh-like probability density functions having higher scintillation indices and skewness (approaching values of 1.0 and 0.63, respectively). The scintillation index and skewness parameters were found to increase mildly with both horizontal range and water-current magnitude. A simple model using the coherent superposition of multiple scatterers was proposed to explain the observed scattering statistics. This model is based on the assumption that the sea bed is effectively immobile, with water-borne scintillation and micro-multipaths providing fluctuations in scatterer phase. This simple model shows that echo-amplitude fluctuations can deviate from the Rayleigh model through two mechanisms: 1) decreasing the levels of water-borne phase fluctuation and 2) increasing the nonuniformity of the sea-bed scatterer amplitudes. 相似文献
7.
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 相似文献
8.
Modeling Propagation and Reverberation Sensitivity to Oceanographic and Seabed Variability 总被引：1，自引：0，他引：1
《Oceanic Engineering, IEEE Journal of》2006,31(2):402-412
The propagation of bottom and oceanographic variability through to the variability of acoustic transmissions and reverberation is evaluated with a simple adiabatic model interacting with Gaussian distributed uncertainty in a narrow frequency band. Results show that there is significant sensitivity of time series and reverberation uncertainty to different types of environmental uncertainty. For propagation over uncertain bottoms, it is shown that it is that later part of the time series, corresponding to the highest angle energy reflecting most often off the surface and bottom, that is most sensitive to bottom uncertainty. This implies that the larger reverberation contributions from the highest grazing angles with the largest scattering strength is also the most uncertain. Conversely, it is the lowest angle arrivals which are most sensitive to uncertainty in the sound-speed profile. These behaviors are predicted analytically by the theory [K.D. LePage, in “Impact of Littoral Environmental Variability on Acoustic Predictions and Sonar Performance,” Kluwer, 2002, pp. 353-360]. 相似文献
9.
Acoustic backscattering from a sandy seabed was measured at a frequency of 5.5 kHz at a wide range of grazing angles. The measurement system used was the University of Miami's sonar tower, consisting of an omni-directional broadband source and two 16-channel hydrophone receiver arrays. A volume scattering model, which combines a fluid model with reflection/transmission coefficients derived from the Biot theory, is used. This model allows energy penetration into the bottom, calculations of the volume scattering at all grazing angles, and the frequency dependence of the sound speed in the water-saturated sediment. In the model, rather than assume sound-speed correlation length in sedimentary volume, core data were used to assimilate a 3-D fluctuation spectrum of the density. The numerical results showed excellent agreement with the measurement at lower grazing angles. We concluded that the interface roughness scattering was dominant at lower grazing angles, while the volume scattering is dominant at higher grazing angles at the sandy site. The border of the dominance of the interface and volume scattering was the so-called critical angle at this frequency. The frequency dependence of sound speeds is also discussed. 相似文献
10.
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 相似文献
11.
Ji-Xun Zhou Xue-Zhen Zhang 《Oceanic Engineering, IEEE Journal of》2005,30(4):832-842
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. 相似文献
12.
浅海均匀层远程混响的垂直相干性 总被引：1，自引：0，他引：1
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. 相似文献
13.
Cable P.G. Frech K.D. O'Connor J.C. Steele J.M. 《Oceanic Engineering, IEEE Journal of》1997,22(3):534-540
Determinations of acoustic scattering strength for sand bottoms have been made at several different shallow-water areas under downward refracting sound propagation conditions in the frequency decade below 1 kHz. The measurements have been made using explosive sources detonated at mid-water depth and bottom-mounted vertical and horizontal hydrophone line arrays as receivers. The ubiquitous presence of multipaths in shallow water prevents a direct-path scattering geometry, and scattering strength must be extracted from the full reverberation field, which complicates the determination of bottom grazing angle dependence of scattering. The major focus of this paper has been the variation of scattering strength with frequency (integrated over participating bottom angles), though estimates of the angular dependence of scattering strength have been made using the vertical receiving array. Typically the integrated scattering strength for sand bottoms reported (and elsewhere) are found to decrease below 1 kHz and in some instances to exhibit a minimum in the several hundred hertz range. Sand bottom scattering strengths below 1 kHz are significantly lower than those predicted by the Mackenzie formula and the limited angular dependence determinations have been found to be consistent with Lambert's law 相似文献
14.
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 相似文献
15.
It is shown that useful relative backscatter strengths can be calculated from GLORIA long-range side-scan sonar data using a simple acoustic model. The calculation was performed on GLORIA side-scan sonar data collected during 1987 in the southern Indian Ocean. GEOSECS hydrographic information was used to access the effects of refraction (ray bending and aspherical spreading signal losses). Sea Beam bathymetry was used to correct the effective insonified area and compute the grazing angle. A major difficulty in performing this calculation over the terrain chosen (mid-ocean ridge topography) was one of adjusting navigation so that small features in Sea Beam and GLORIA data matched. Preliminary results show a 10-dB falloff in backscatter strength with decreasing grazing angle (10°-40°) at 6.5 kHz over what must presumably be a rough surface (extruded basalts and breccias) 相似文献
16.
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. 相似文献
17.
A two-scale roughness model for bottom backscattering (Novarini and Caruthers) was applied to multibeam sounder data (95 kHz) from Browns Bank (south of Yarmouth, Nova Scotia, Canada). In order to better understand frequency and incident angle dependence of backscattering, acoustic-calibration data (1-6 kHz) were collected from the same area and treated with the same model. The frequency and incident angle dependence of bottom backscattering in the multibeam and acoustic-calibration data were compared. Backscattering due to large-scale roughness was most relevant at near-normal incidence (<7°) and it was more dominant in the low-frequency range, and was strongly dependent on incident angle. Volume scattering was least dependent upon incident angle. It was the dominant factor at the large incident angle. Bragg scattering was the most significant over a very wide frequency range and was more important for high frequency (>5 kHz) and small incidence, but not near-normal incidence 相似文献
18.
Backscattering measurements were performed in shallow water on sand, gravel, and clay bottoms. The equipment included a parametric array that emitted pulses of differential frequencies (8 to 40 kHz) with a 3° directivity. The ranges did not exceed 50 m. The grazing angles varied from 4° to 90°. The bottom backscattering strength does not depend on the emitted pulse type (frequency and length). If one fits a Lambert law to the variations of the backscattering strength versus the grazing angle, the value at the origin fluctuates between-15 and-22 dB without any clear effect from the different bottom types. Statistical tests show that under the experimental measurement conditions: (1) the alternative received signal does not generally follow a normal distribution; (2) among five classical distributions in sonar and radar that have been fitted to the detected-integrated signal (exponential, Weibull, chi-2, log-normal, Rice), the best-fitted law is the log-normal; (3) signals backscattered by separated areas of the same bottom can hardly be regarded as stationary and, even less, homogeneous; and (4) with an anisotropic bottom topography the statistical properties depend on the aspect under which this topography is seen 相似文献
19.
Ji-Xun Zhou Xue-Zhen Zhang Rogers P.H. Simmen J.A. Dahl P.H. Guoliang Jin Zhaohui Peng 《Oceanic Engineering, IEEE Journal of》2004,29(4):988-999
Optimal array-processing techniques in the ocean often require knowledge of the spatial coherence of the reverberation. A mathematical model is derived for the reverberation vertical coherence (RVC) in shallow water (SW). A method for analysis of RVC data is introduced. Measured reverberation cross-correlation coefficients as a function of time and frequency, obtained during the Asian Seas International Acoustic Experiment (ASIAEX) in the East China Sea, are reported. SW reverberation from a single shot provides a continuous spatial sampling of the surrounding sound field up to several tens of kilometers and holds valuable information on the geoacoustic properties of the sea floor over this distance. SW reverberation data can, therefore, be used as the basis for a quick and inexpensive method for geoacoustic inversion and has the obvious advantage that acquiring the data in situ requires only a single platform. This paper considers the use of the vertical coherence of the reverberation as the starting point for such an inversion. Sound speed and attenuation in the sea bottom at the ASIAEX site are obtained over a frequency range of 100-1500 Hz by finding values that provide the best match between the measured and predicted RVC. 相似文献
20.
Computer simulations are carried out to study the feasibility of an adaptive equalizer applied to an hydroacoustic data-transmission channel. The channel is examined with a comprehensive acoustical model to acquire worst-case examples of the ocean acoustic transmission channel. The equalizer performance is investigated by simulations with a computer-generated channel response. Equalizer behavior in a mobile time-variant environment is also studied by use of a simplified, time-discrete multipath channel model. A stochastic gradient lattice equalizer is simulated for a channel which varies due to movement of the transmitter platform. The equalizer was able to track a velocity of up to 0.4 m/s for a favorable transmission geometry, using a transmitter beamwidth of 10°. The results demonstrate the feasibility of coherent modulation schemes for medium-distance ocean acoustic telemetry. It was found that small beamwidths are imperative in maintaining signal coherence and in facilitating adaptive equalization. In particular, narrow-beam transducers will reduce equalizer complexity as well as the frequency spread 相似文献