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1.
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  相似文献   

2.
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.   相似文献   

3.
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   

4.
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  相似文献   

5.
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   

6.
In many strategic shallow water areas, the geoacoustic properties of the sub-bottom are largely unknown. This paper demonstrates that inverse theory and measured data from a single hydrophone can be used to accurately deduce the geoacoustic properties of the sub-bottom, even when the initial background geoacoustic model is a highly inaccurate estimate. Since propagation in shallow water is very sensitive to the geoacoustic properties of the sub-bottom the inverse technique is a vitally important, practical, and inexpensive means to improve sonar performance prediction in a potentially hostile environment. To provide ground truth for the inverse technique, measured data collected during Project GEMINI were compared to the inverse solutions. Detailed, site-specific geoacoustic models were developed for two array locations and the finite-element parabolic equation (FEPE) model was used to estimate transmission loss (TL). The model estimates from FEPE compared well with the measured data and the detailed geoacoustic models were considered as “ground truth.” To test the efficacy of the technique, initial background geoacoustic models were constructed assuming no a priori information of the bottom. The resultant inverse solution was used to predict the geoacoustic properties at each of the sites. The final results were in excellent agreement with the measured data and the resulting TL estimates derived from the inverse technique were as good or better than the TL estimates obtained from the detailed, site-specific geoacoustic models  相似文献   

7.
The wide-band source (WBS) signals measured in the Asian Seas International Acoustics Experiment (ASIAEX) in the East China Sea (ECS) were used to invert for geoacoustic parameters. Sound speed and density were inverted using the matched-field processing method combined with the vertical reflection coefficients and sea-bed attenuation coefficients were inverted from the vertical correlation data. For a half infinite liquid sea-bottom model, the inverted equivalent bottom sound speed is 1610/spl plusmn/12 m/s and the bottom density is 1.86 g/cm/sup 3/. The inverted attenuation coefficients are well described by a nonlinear relationship of the form /spl alpha//sub b/=0.28f/sup 1.58/ dB/m (f is in units of kilohertz) in the frequency range of 100-600 Hz.  相似文献   

8.
Current models used to predict the backscattering strength of the ocean floor are either very involved, requiring geoacoustic parameters usually unavailable for the site in practical applications, or overly simplistic, relying mainly on empirical terms such as Lambert's law. In any case, solutions are very approximate and the problem is still far from being solved. In this paper, a model is presented that avoids empirical functional forms yet requires only a few physical parameters to describe the surficial sediments, often tabulated for typical sediments. The aim of this paper is to develop a simple algorithm for operational prediction of bottom reverberation with only one free parameter, i.e., the volume scattering coefficient. The algorithm combines a two scale surface scattering model with scattered contributions originating from inhomogeneities within the sediments, talking into consideration the rough interface. No specific mechanism is assumed for scattering at the volume inhomogeneities; however, the inhomogeneities are assumed to be uniform and isotropic. The volume scattering coefficient, combined with the bottom attenuation and density and referenced to the surface, plays a role similar to the Lambert's constant in empirical models. The model is exercised on a variety of published datasets for low and moderately high frequency. In general, the model performs very well for both fast and slow sediments, showing a definite improvement over Lambert's law  相似文献   

9.
To obtain the bistatic scattering function on the sandy ripple bottom, high-frequency bistatic sea-floor scattering measurements were made in the shallow waters off the east coast of Korea. A sand ripple field was present at the site, with wavelength generally in the 10-20-cm range. The mean ripple orientation relative to the direction of wave propagation was estimated to be roughly 20/spl deg/-30/spl deg/. Field experiments were made to measure forward (in-plane) and out-of-plane scattering from the ripple bottom. The measured scattering strengths were compared to the predictions of the APL-UW bistatic scattering model. Overall, forward-scattering strength measurements showed favorable comparison with the model predictions. The global scattering characteristics for the ripple bottom gave an augmented out-of-plane scattering.  相似文献   

10.
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   

11.
声波在海底界面的反射和透射是海底散射、海底混响、海底目标探测的重要问题。利用Biot多孔介质声传播理论对声波在水-多孔海底界面上的反射和透射进行了分析,具体给出反射声波的反射系数,3种透射声波的透射系数以及声能透射系数随入射波入射角和频率(10~40 kHz)的变化关系,分析了各种透射波对透射声能的贡献。多孔海底介质参数分别采用Stoll和Chotiros给出的2组参数进行理论计算。  相似文献   

12.
Over the past decade, inversion methods have been developed and applied to acoustic field data to provide information about unknown ocean-bottom environments. An effective inversion must provide both an estimate of the bottom parameters and a measure of the uncertainty of the estimated values. This paper summarizes results from the Office of Naval Research (ONR)/Space and Naval Warfare Systems Command (SPAWAR) Geoacoustic Inversion Techniques Workshop, test cases 4 and 5. The workshop was held to benchmark present-day inversion methods for estimating geoacoustic profiles in shallow water. The format of the workshop was a blind test to estimate unknown geoacoustic profiles by inversion of measured acoustic transmission loss data in octave bands and reverberation envelopes. The data sets for test cases 4 and 5 were taken at two locations in shallow water, one in the East China Sea and the other along the southwest coast of Florida. The limitations of the data and the limits to the knowledge of the sites are discussed. In both cases, impulsive sources were used in conjunction with air-deployed sonobuoys. Since the measured data was incoherent, only methods consistent with total energy matching were applicable. Comparisons between the different inversion techniques presented at the workshop are discussed. For test cases 4 and 5, a precise metric was unavailable for comparison.  相似文献   

13.
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   

14.
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   

15.
Shallow-water bottom reverberation measurements   总被引：2，自引：0，他引：2  
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   

16.
Tests of models for high-frequency seafloor backscatter   总被引：3，自引：0，他引：3  
The interaction of high-frequency sound with the seafloor is inherently a stochastic process. Inversion techniques must, therefore employ good stochastic models for bottom acoustic scattering. An assortment of physical models for bottom backscattering strength is tested by comparison with scattering strength data obtained at 40 kHz at three shallow water sites spanning a range of sediment types from fine silt to coarse sand. These acoustic data are accompanied by sediment physical property data obtained by core sample analysis and in situ probes. In addition, stereo photography was used to measure the power spectrum of bottom relief on centimeter scales. These physical data provided the inputs needed to test the backscatter models, which treat scattering from both the rough sediment-water interface and the sediment volume. For the three sites considered here, the perturbation model for scattering from a slightly rough fluid seafloor performs well. Volume scattering is predicted to be weak except at a site having a layer of methane bubbles  相似文献   

17.
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   

18.
Phenomenological and global optimization inversion   总被引：1，自引：0，他引：1  
This paper discusses geoacoustic inversion results based on benchmark range-dependent data using SAGA, a global inversion package, and using phenomenological inversions. In phenomenological inversions, physical and signal-processing approaches are used to enhance the data to extract specific features. The global optimization approach is carried out on complex-valued vertical array data, transmission loss data, and reverberation data. The importance of checking the solution is emphasized by inspecting the match with the data and the error estimates and by checking the solution using data that has not been used in constructing the solution. The results show that we are able to estimate the geoacoustic parameters and that these parameters could be used to predict the field for different frequencies and/or source-receiver geometry than used in the inversion.  相似文献   

19.
This paper describes the simulation of backscattering of high frequency sound from complex objects and sand sea-bottom. Backscattering data from complex objects and seabottom were generated using a Universal High Resolution Imaging Sonar Simulation Toolkit (UHRISST) developed by the authors. Our approach here involves the approximation of objects and the sea-bottom through a series of facets that are small compared to the wavelength. The Target Strength of standard objects like the sphere, cylinder, and cone predicted by our model have been compared with various high frequency approximation formulations and produced a good fit. For the prediction of the backscattering strength from the sea-bottom, we have taken into account most of the small-scale features that represent the sand grain properties, intermediate features that represent seabottom roughness and large features that block propagation. Sea-bottom backscattering predictions made using this model were compared with high frequency scattering strength values measured off Panama City, FL, USA, and produced a moderate fit. Nonparametric statistical analysis was carried out on the bottom backscattering signals from a flat isotropic sea-bottom. The probability distribution function and probability of false alarm function of the bottom backscattering data was measured and found to largely follow the normal distribution  相似文献   

20.
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.  相似文献   

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