共查询到20条相似文献，搜索用时 15 毫秒
1.
Solutions of the eikonal equation and the first two transport equations are derived for problems involving ray chaos. The solution of the eikonal equation approximates the phase. The solutions of the transport equations approximate the amplitude as an asymptotic series in ω-1. Examples are presented to illustrate that the second term in the series grows relative to the first term along some rags. This secular behavior is associated with the exponential decay of amplitude, which occurs along chaotic rays. The results suggest that chaotic ray solutions (including ray paths, phases, and amplitudes) break down rapidly with range. Although the analysis is limited to a special case that is free of caustics, the results bring into question the use of chaotic ray solutions for long-range propagation 相似文献
2.
This paper deals with the basic modeling problem in underwater acoustics that is the characterization of the channel between a transmitter and a receiver. The problem is analyzed here using an array of sensors that receive PSK signals emitted by several sources. Data come from an experiment realized by a physical system situated in the Mediterranean Sea. In order to identify the multipath channel, we need to access the propagation time delay and the angle of arrival of each propagation ray. However, many of these acoustic ray paths are too close to be separated by classic processing methods (matched filter, beamforming, etc.); new methods with better resolution must be applied in order to analyze the experimental signals and to determine their arrival time on the array of sensors. After a presentation of this problem, we will first discuss high-resolution methods that are usually applied in the localization problem; we will then focus on wavelet packet analysis which provides good results by improving the temporal resolution of acoustic signals 相似文献
3.
This paper presents an adaptive hybrid algorithm to invert ocean acoustic field measurements for seabed geoacoustic parameters. The inversion combines a global search (simulated annealing) and a local method (downhill simplex), employing an adaptive approach to control the trade off between random variation and gradient-based information in the inversion. The result is an efficient and effective algorithm that successfully navigates challenging parameter spaces including large numbers of local minima, strongly correlated parameters, and a wide range of parameter sensitivities. The algorithm is applied to a set of benchmark test cases, which includes inversion of simulated measurements with and without noise, and cases where the model parameterization is known and where the parameterization most be determined as part of the inversion. For accurate data, the adaptive inversion often produces a model with a Bartlett mismatch lower than the numerical error of the propagation model used to compute the replica fields. For noisy synthetic data, the inversion produces a model with a mismatch that is lower than that for the true parameters. Comparison with previous inversions indicates that the adaptive hybrid method provides the best results to date for the benchmark cases 相似文献
4.
Traditionally, matched-field processing (MFP) has been used to localize low-frequency sources (e.g., <300 Hz) from their acoustic signals received on long vertical arrays. However, some sources emit acoustic signals of much higher frequency. Applying MFP to signals in the mid-frequency range (e.g., 1-4 kHz) is a very challenging problem because MFP's sensitivity to environmental parameter mismatch becomes more severe with increasing frequency. Robust MFP techniques are required to process signals in the mid-frequency range. As a practical issue, short vertical arrays are more convenient to work with than are long vertical arrays; they are easier to deploy and are less prone to large amounts of deformation. However, short vertical arrays undersample the water column, which can result in severely degraded MFP performance. In this paper, we present experimental data results for this nonconventional paradigm. Using the environmentally robust broad-band L/sub /spl infin//-norm estimator, MFP results are given using shallow-water experimental data. This data consisted of broad-band signals in the 3-4-kHz band collected on an eight-element 2.13-m vertical array. These results serve to demonstrate that good localization performance can be attained for this difficult problem. Guidelines on the appropriate use of ray and normal-mode propagation models are also presented. 相似文献
5.
Long-range source localization is shown to be affected by a mesoscale eddy whose realization is solely a cyclonic current (no thermal manifestation). The sensitivity of a matched-field type processor (known ocean) to an eddy is demonstrated, as well as its sensitivity to a mismatch between the parameters of the eddy and the processor assumptions. Optimum uncertain field processing techniques are used to overcome these sensitivities by incorporating uncertainties about the environment into the processor. These processors operate on data produced by a special 3-D ray tracer using actual sound speed data and two different models for eddy current structure 相似文献
6.
This paper applies a full-field technique to invert bottom sound profile and bottom reflectivity from simulated acoustic data in a shallow water environment. Bottom sound-speed profile and bottom reflectivity have been traditionally estimated using seismic reflection/refraction techniques when acoustic ray paths and travel time can be identified and measured from the data. However, in shallow water, the many multipaths due to bottom reflection/refraction make such identification and measurement rather difficult. A full-field inversion technique is presented here that uses a broad-band source and a vertical array for bottom sound-speed and reflectivity inversion. The technique is a modified matched field inversion technique referred to as matched beam processing. Matched beam processing uses conventional beamforming processing to transform the field data into the beam domain and correlate that with the replica field also in the beam domain. This allows the analysis to track the acoustic field as a function of incident/reflected angle and minimize contamination or mismatch due to sidelobe leakage 相似文献
7.
《Oceanic Engineering, IEEE Journal of》2006,31(2):325-344
Matched-field methods concern estimation of source locations and/or ocean environmental parameters by exploiting full wave modeling of acoustic waveguide propagation. Typical estimation performance demonstrates two fundamental limitations. First, sidelobe ambiguities dominate the estimation at low signal-to-noise ratio (SNR), leading to a threshold performance behavior. Second, most matched-field algorithms show a strong sensitivity to environmental/system mismatch, introducing biased estimates at high SNR. In this paper, some theoretical developments on matched-field performance analysis are summarized, including Bayesian performance bounds and probabilistic ambiguity analysis, both incorporating environmental/system uncertainty/mismatch. Performance analysis is then implemented for source localization in a typical shallow water environment chosen from the Shallow Water Evaluation Cell Experiments (SWellEX). The performance predictions describe the simulations of the maximum-likelihood estimator (MLE) well, including the mean-square error (MSE) in all SNR regions as well as the bias at high SNR. The threshold SNR and bias predictions are also validated through SWellEX experimental data processing. The results suggest the current environmental, acoustic, and statistical modeling has developed to such a level that the optimum theoretical matched-field performance can be achieved in a well-controlled experiment. 相似文献
8.
水声通信中的鲁棒图像编码研究 总被引：2，自引：1，他引：2
由于受各种因素的影响，水下声信道是一种传输差错率较高的信道。标准化的图像编码系统(例如JPEC；H．263，MPEG等)使用了相似的压缩技术，它们往往存在严重的错误扩散，甚至单个错误比特就可能破坏整幅图像，所以一般不适合作为水下声信道图像传输的编码方案。文章针对常用的图像编码的缺点，利用定长编码技术，提出了一种高鲁棒性的图像压缩方案。实验表明在压缩率1．25比特／象素时，压缩后的图像仍然保持了较好的质量，并且能够较好地抵抗信道误码，提高了水下声信道图像传输的质量。 相似文献
9.
Conventional bearing estimation procedures employ planewave steering vectors as replicas of the true field and seek to resolve in angle by maximizing a power function representing the agreement between actual and replica fields. For vertical arrays in oceanic waveguides the received field depends on range and depth, and it is natural to replace the "look-direction" (theta ) by a "look-position" (r, z ). Thus an environmental model is constructed by specifying ocean depth, sound speed profile, bottom properties, etc., and a propagation model is employed to construct a replica of the field that would be received on the array for a particular source position. The usual estimators (e.g., Bartlett or maximum likelihood) are then used to gauge the agreement between actual and replica fields and the true source position is identified as that position where the agreement is best. The performance of this kind of matched-field processing is strongly affected by the environment. In particular, we demonstrate through simulations that for a deep-water Pacific environment dominated by waterborne paths, ambiguities or sidelobes are associated with convergence zones. In the absence of mismatch between replica and actual fields we find that a 16-element array performs extremely well in low-frequency regimes. Mismatch caused by uncertainties in phone positions, bottom parameters, ocean sound speed, surface and bottom roughness, etc., causes degradation in localization performance. The impact of some of these effects on conventional and maximum likelihood estimators is examined through simulation. 相似文献
10.
Underwater acoustic networks 总被引：8，自引：0，他引：8
With the advances in acoustic modem technology that enabled high-rate reliable communications, current research focuses on communication between various remote instruments within a network environment. Underwater acoustic (UWA) networks are generally formed by acoustically connected ocean-bottom sensors, autonomous underwater vehicles, and a surface station, which provides a link to an on-shore control center. While many applications require long-term monitoring of the deployment area, the battery-powered network nodes limit the lifetime of UWA networks. In addition, shallow-water acoustic channel characteristics, such as low available bandwidth, highly varying multipath, and large propagation delays, restrict the efficiency of UWA networks. Within such an environment, designing an UWA network that maximizes throughput and reliability while minimizing the power consumption becomes a very difficult task. The goal of this paper is to survey the existing network technology and its applicability to underwater acoustic channels. In addition, we present a shallow-water acoustic network example and outline some future research directions 相似文献
11.
M. V. Kalashnik 《Izvestiya Atmospheric and Oceanic Physics》2013,49(2):217-228
The process of propagation and trapping of inertial gravity wave (IGW) packets in oceanic shear flows is studied in the geometric-optics approximation (ray theory). It is shown that wave trapping in strong stable stratification occurs in the region of anticyclonic (horizontal) velocity shear in a narrow frequency range on the left side of the inertial frequency. Beyond this range, the packet (ray) is either reflected from a cyclonic shear layer or propagates freely through the shear layer. The basic equations of ray theory are analyzed qualitatively, and analytical expressions are derived for freely propagating and trapped rays. The influence that vertical shear exerts on the ray behavior is also investigated. It is shown that two-dimensional ray focusing occurs as the velocity profile decreases with depth, so that rays concentrate along a specific latitude. 相似文献
12.
在深海汇聚区声场中,不同初始角的声线在传播过程中因折射程度差异形成特定的焦散结构。根据射线理论推导了线性剖面条件下用F算子表示的声线轨迹模型,并讨论了焦散结构与掠射角及声源-接收深度配置的变化关系。折射型焦散线由0°~5°的小角度声线构成,为波导结构;反射型焦散线由掠射角为±(5°~10°)的声线构成,为折线结构,且上行与下行声线的焦散线结构明显不同。应用BELLHOP模型分析得出了汇聚区增益与声源-接收深度条件的变化关系,并根据射线到达结构和焦散特征提出了一种确定汇聚区位置和范围的方法。 相似文献
13.
《Oceanic Engineering, IEEE Journal of》2009,34(2):170-180
14.
Ozard J.M. Yeremy M.L. Chapman N.R. Wilmut M.J. 《Oceanic Engineering, IEEE Journal of》1996,21(4):377-383
This paper describes matched-field processing (MFP) of data collected in shallow water off the western coast of Vancouver Island in the Northeast Pacific Ocean. The data were collected from a vertical line array (VLA) as part of the PACIFIC SHELF trial carried out on the continental shelf and slope during September 1993, sensors in the 16-element VLA were evenly spaced at depths between 90 and 315 m, while the sound source was towed along radial paths or arcs. In this paper, we present results of the analysis of data from a continuous wave (CW) source which was towed downslope at a depth of 30 m in water from 150 to 375 m deep, in order to model the range-dependence of the acoustic propagation efficiently, the replica fields were calculated using the adiabatic normal mode approximation. This approximation was considered appropriate for the bottom slopes of the environment. Using sparse bathymetric data, a water sound speed profile and estimates of bottom properties, MFP correlations on individual ambiguity surfaces were found to be greater than 0.9 for the strongest signals. On account of environmental mismatch, the source position could not be determined unambiguously from most of the ambiguity surfaces even at high signal-to-noise ratios. Nevertheless, when an efficient linear tracker was applied to the ambiguity surfaces to find tracks, the source track was recovered at both low and high signal-to-noise ratios, this tracker performs the analysis at a constant depth and reports the track with the highest estimated track signal-to-noise ratio 相似文献
15.
超短基线定位解算中的距离观测值是指换能器与水下应答器之间的直线距离,而海水声速的不均匀分布导致声波在海水中的实际传播路径为连续弯曲的曲线,需要结合实测声速剖面进行声线修正。根据声速在分层介质中的传播特性,本文提出了一种基于二次多项式拟合的声线跟踪算法,采用线性插值方法对声速剖面数据进行合理加密并按等深度进行分层,设定每层声速梯度是不断变化的,用二次多项式拟合声速,基于运动学原理建立了完整的数学解算模型。仿真结果表明,该方法修正后的水下目标分布具有明显的收敛性,且优于等梯度声线跟踪算法和等效声速剖面法,显著提高了超短基线水声定位系统的定位精度。 相似文献
16.
One goal of seafloor geodesy is to measure horizontal deformation of the seafloor with millimeter resolution. A common technique precisely times an acoustic signal propagating between two points to estimate distance and then repeats the measurement over time. The accuracy of the distance estimate depends upon the travel time resolution, sound speed uncertainty, and the degree to which the path computed from propagation equations replicates the actual path traveled by the signal. In this paper, we address the error from ray propagation equations by comparing three approximations to Snell's Law with ellipsoidal geometry. 相似文献
17.
The Mandovi–Zuari estuarine network on the west coast of India consists of shallow strongly converging channels, that receive large seasonal influx of fresh water due to the monsoons. The main channels, the Mandovi and Zuari estuaries, connect the network to the Arabian Sea. Observations show that tidal amplitude in the channels remains unchanged over large distances (|mS40 km) from the mouths of the main channels and then decays rapidly over approximately 10 km near the head. To understand the dynamics behind this behaviour, a numerical model for tidal propagation has been used that simulates the observed tidal elevations well. Momentum balance in the model is predominantly between pressure gradient and friction. In the region of undamped propagation, the model behaviour is consistent with the theory that geometric amplification balances frictional decay leaving the tide unchanged. This balance breaks down near the upstream end, where channels are narrowest, and mean velocity associated with freshwater influx is sufficiently large to prevent upstream propagation of tide. This leads to rapid decay in tidal amplitude. The model also shows that the mean water-level rises in the upstream direction, in the region of the decay. 相似文献
18.
19.
S. F. Dotsenko 《Physical Oceanography》1994,5(2):113-118
This paper provides calculations of the time of long surface gravity wave propagation in the Black Sea. The ray technique is used by the numerical model to calculate wave fronts with the seabed relief prescribed in tabular form on a rectangular grid. It is speculated how the derived estimates can be employed to predict tsunamis in the Black Sea.Translated by Vladimir A. Puchkin. 相似文献
20.
Deterministic three-dimensional analysis of long-range soundpropagation through internal-wave fields
A Munk profile and a set of propagating internal-wave modes are used to construct a three-dimensional time-varying ocean sound-speed model. Three-dimensional ray tracing is employed to simulate long-range sound propagation of a broadband acoustic signal. Methods are developed to convert three-dimensional ray-tracing results to acoustic time-domain amplitude and phase measurements. The ocean sound-speed model is defined deterministically, and the model acoustic receptions are analyzed deterministically. A single internal-wave mode that is “spatially synchronizes” to an arrival can coherently focus and defocus the acoustic energy. These internal waves can cause an arrival's amplitude fluctuation to mimic Rayleigh fading; however, the time-domain phase is stable, in contradiction to the classical Rayleigh fading environment where the received phase is uniformly distributed. For example, the received power attributed to an early arrival propagated over a 750-km range can fluctuate over 40 dB, while the time-domain phase remains within a quarter of a 75 Hz cycle. The characteristics of the time-domain phase are important for establishing coherent integration times at the receiver 相似文献