利用海底反射信号进行地声参数反演的方法

针对现有反演方法的缺点,提出了一种基于海底反射信号的地声参数高分辨反演方法.它利用短距离声源在不同深度上发射宽带线性调频信号,采用垂直阵进行接收,首先通过匹配滤波方法提取多径到达信息,然后利用海底反射损失曲线,反演海底表层的声速和密度,最后利用浅底层反射信号估计沉积层参数.由于海水中直达波受到内波的强烈影响,选择海底表面反射作为参考,用以可靠地计算浅底层反射的相对到达时间和幅度,从而估计出沉积层的厚度、速度和衰减系数.通过海上实验,验证了利用浅底层反射信号反演参数的有效性. 关键词： 海底参数 反演 浅底层反射信号     

Quantifying the uncertainty of geoacoustic parameter estimates for the New Jersey shelf by inverting air gun data

This paper describes geoacoustic inversion of low frequency air gun data acquired during an experiment on the New Jersey shelf. Hybrid optimization and Bayesian inversion techniques based on matched field processing were applied to multiple shots from three air gun data sets recorded by a vertical line array in a long-range shallow water geometry. For the Bayesian inversions, full data error covariance matrix was estimated from a set of consecutive shots that had high temporal coherence and small spatial variation in source position. The effect of different data error information on the geoacoustic parameter uncertainty estimates was investigated by using the full data error covariance matrix, a diagonalized version of the full error covariance, and a diagonal matrix with identical variances. The comparison demonstrated that inversion using the full data error information provided the most reliable parameter uncertainty estimates. The inversions were highly sensitive to the near sea floor geoacoustic parameters, including sediment attenuation, of a simple single-layer geoacoustic model. The estimated parameter values of the model were consistent with depth averaged values (over wavelength scales) of a high resolution geoacoustic model developed from extensive ground truth information. The interpretation of the frequency dependence of the estimated attenuation is also discussed.     

Bayesian geoacoustic inversion of ship noise on a horizontal array

This paper applies geoacoustic inversion to low-frequency narrow-band acoustic data from a quiet surface ship recorded on a bottom-moored horizontal line array in shallow water. A Bayesian matched-field inversion method is employed which quantifies geoacoustic uncertainties and allows for meaningful comparison of inversion results from different data sets. Geoacoustic inversion results for ship-noise data are compared with inversion results for multitone data from a towed controlled source collected in the same experiment, and with independent geophysical measurements. To increase the information content of low-level ship-noise data, the effect of including multiple, independent data segments in the inversion is investigated and shown to significantly reduce geoacoustic parameter uncertainties. Geoacoustic uncertainties are also shown to depend on ship range and orientation, with increased uncertainties for long ranges and for the ship stern oriented away from the array.     

Statistical geoacoustic inversion from vertical correlation of shallow water reverberation

Statistical geoacoustic inversion results based on reverberation vertical correlation （RVC） data from 300 Hz to 800 Hz are presented. The data were obtained during Yellow Sea Experiment-2005 （YSE-05）. An uncertainty analysis is carried out. It is found that the inversion sea bottom sound velocities decrease when the frequency increases. So it is difficult to determine the sea bottom sound velocity. In order to solve this problem, a two-layer bottom model is assumed, and a multi-frequency inversion approach based on Genetic Algorithm is proposed. The approach is demonstrated using YSE-05 experiment data. Both RVC and normal modes depth functions （NMDFs） calculated using the inverted geoacoustic parameters are in good agreement with the experimental measurements.     

一种地声参数的联合反演方法

根据地声参数对不同声场物理量影响不同,提出了一种利用简正波频散特征结合声传播损失反演地声参数的联合反演方法。首先,考虑到简正波的频散特性(群速度)对海底的密度和声速较为敏感,而对海底吸收系数不敏感,利用自适应时频分析方法,获得不同频率不同号数简正波的到达时间差,以此作为代价函数,采用全局优化算法,反演出海底密度和海底声速的分层结构,并用概率统计的方法评价反演结果的有效性。反演出海底密度和声速后,利用实验船辐射噪声得到随距离连续的声传播损失来反演出海底吸收系数。最后,把反演的参数很好的用于声源匹配定位验证了反演结果的有效性。     

Effects of incoherent and coherent source spectral information in geoacoustic inversion

This paper examines the effect on matched-field geoacoustic inversion of including source spectral information, as can be available in controlled-source acoustic surveys. Source information can consist of relative or absolute knowledge of the source amplitude and/or phase spectra, and can allow frequency-coherent processing of spatial acoustic-field data. A number of multi-frequency acoustic processors, appropriate for specific types of source information, are defined based on the likelihood function for complex acoustic-field data with Gaussian noise. The information content of the various processors is quantified in terms of marginal probability distributions and highest-probability density intervals for the unknown geoacoustic and geometric parameters, which define the accuracy expected in inversion. Marginal distributions are estimated using a fast Gibbs sampler approach to Bayesian inversion, which provides an efficient, unbiased sampling of the multi-dimensional posterior probability density. The analysis is illustrated for incoherent and coherent processors corresponding to several types of source knowledge ranging from complete information to no information, and the results are considered as a function of the spatial and frequency sampling of the acoustic fields.     

Geoacoustic inversion with ships as sources

Estimation of geoacoustic parameters using acoustic data from a surface ship was performed for a shallow water region in the Gulf of Mexico. The data were recorded from hydrophones in a bottom mounted, horizontal line array (HLA). The techniques developed to produce the geoacoustic inversion are described, and an efficient method for geoacoustic inversion with broadband beam cross-spectral data is demonstrated. The performance of cost functions that involve coherent or incoherent sums over frequency and one or multiple time segments is discussed. Successful inversions for the first sediment layer sound speed and thickness and some of the parameters for the deeper layers were obtained with the surface ship at nominal ranges of 20, 30, or 50 water depths. The data for these inversions were beam cross-spectra from four subapertures of the HLA spanning a little more than two water depths. The subaperture beams included ten frequencies equally spaced in the 120-200 Hz band. The values of the geoacoustic parameters from the inversions are validated by comparisons with geophysical observations and with the parameter values from previous inversions by other invesigators, and by comparing transmission loss (TL) measured in the experiment with modeled TL based on the inverted geoacoustic parameters.     

Data error covariance in matched-field geoacoustic inversion

Many approaches to geoacoustic inversion are based implicitly on the assumptions that data errors are Gaussian-distributed and spatially uncorrelated (i.e., have a diagonal covariance matrix). However, the latter assumption is often not valid due to theory errors, and can lead to reduced accuracy for geoacoustic parameter estimates and underestimation of parameter uncertainties. This paper examines the effects of data error (residual) covariance in matched-field geoacoustic inversion. An inversion approach is developed based on a nonparametric method of estimating the full covariance matrix (including off-diagonal terms) from the data residuals and explicitly including this covariance in the misfit function. Qualitative and quantitative statistical tests for Gaussianity and for correlations in complex residuals are considered to validate the inversion results. The approach is illustrated for Bayesian geoacoustic inversion of broadband, vertical-array acoustic data measured in the Mediterranean Sea.     

Trans-dimensional geoacoustic inversion

This paper develops a general trans-dimensional Bayesian methodology for geoacoustic inversion. Trans-dimensional inverse problems are a generalization of fixed-dimensional inversion that includes the number and type of model parameters as unknowns in the problem. By extending the inversion state space to multiple subspaces of different dimensions, the posterior probability density quantifies the state of knowledge regarding inversion parameters, including effects due to limited knowledge about appropriate parametrization of the environment and error processes. The inversion is implemented here using a reversible-jump Markov chain Monte Carlo algorithm and the seabed is parametrized using a partition model. Unknown data errors are addressed by including a data-error model. Jumps between dimensions are implemented with a birth-death methodology that allows transitions between dimensions by adding or removing interfaces while maintaining detailed balance in the Markov chain. Trans-dimensional inversion results in an inherently parsimonious solution while partition modeling provides a naturally self-regularizing algorithm based on data information content, not on subjective regularization functions. Together, this results in environmental estimates that quantify appropriate seabed structure as supported by the data, allowing sharp discontinuities while approximating smooth transitions where needed. This approach applies generally to geoacoustic inversion and is illustrated here with seabed reflection-coefficient data.     

Particle filtering for dispersion curve tracking in ocean acoustics

A particle filtering method is developed for dispersion curve extraction from spectrograms of broadband acoustic signals propagating in underwater media. The goal is to obtain accurate representation of modal dispersion which can be employed for source localization and geoacoustic inversion. Results are presented from the application of the method to synthetic data, demonstrating the potential of the approach for accurate estimation of waveguide dispersion characteristics. The method outperforms simple time-frequency analysis providing estimates that are very close to numerically calculated dispersion curves. The method also provides uncertainty information on modal arrival time estimates, typically unavailable when traditional methods are used.     

Broadband sound propagation in shallow water and geoacoustic inversion

Part of an experiment to test a measurement package in a shallow water region in the Gulf of Mexico was designed to gather broadband acoustic data suitable for inversion to estimate seabed geoacoustic parameters. Continuous wave tow acoustic signals at multiple frequencies and broadband impulsive source signals were recorded on a horizontal line array in a high-noise environment. Simulated annealing with a normal mode forward propagation model is utilized to invert for a geoacoustic representation of the seabed. Several inversions are made from different data samples of two light bulb implosions, the measured sound speed profiles at the HLA and at the positions of the light bulb deployments, and for two different cost functions. The different cost functions, measured sound speed profiles, and measured time series result in different inverted geoacoustic profiles from which transmission loss is generated for comparison with measurements. On the basis of physical consistency and from the comparison of the transmission loss and time series, a best estimate geoacoustic profile is selected and compared to those obtained from previously reported inversions. Uncertainties in the sound speed profile are shown to affect the uncertainties of the estimated seabed parameters.     

Tabu for matched-field source localization and geoacoustic inversion

Tabu is a global optimization technique that has been very successful in operations research. In this paper, a Tabu-based method is developed for source localization and geoacoustic inversion with underwater sound data; the method relies on memory to guide the multiparameter search. Tabu is evaluated through a comparison to simulating annealing. Both methods are tested by inverting synthetic data for various numbers of unknown parameters. Tabu is found to be superior to the simulated annealing variant implemented here in terms both of accuracy and efficiency. Inversion results from the SWellEX-96 data set are also presented.     

Geoacoustic inversion of ambient noise: a simple method

The vertical directionality of ambient noise is strongly influenced by seabed reflections. Therefore, potentially, geoacoustic parameters can be inferred by inversion of the noise. In this approach, using vertical array measurements, the reflection loss is found directly by comparing the upward- with the downward-going noise. Theory suggests that this simple ratio is, in fact, the power reflection coefficient-potentially a function of angle and frequency. Modeling and parameter searching are minimized, and the method does not require a detailed knowledge of the noise source distribution. The approach can handle stratified environments and is believed to tolerate range dependence. Experimental data from five sites, four in the Mediterranean, one on the New Jersey Shelf, are described. Most of the Mediterranean sites had temporally varying noise directionality, yet yielded the same reflection properties, as one would hope. One site was visited in conditions of very low surface noise. This paper concentrates on an experimental demonstration of the feasibility of the method and data quality issues rather than automatic search techniques for geoacoustic parameters.     

Subbottom profiling using a ship towed line array and geoacoustic inversion

Bottom profiling traditionally uses broadband signals received on a line array at long ranges to estimate the bottom layer structure and thickness. In this paper, a subbottom profiling method is developed and applied to a ship-towed line array using the same ship towed source to estimate the subbottom layer structure and thickness. A ship towed line-array system can be used to estimate bottom properties using geoacoustic inversion and can cover a wide area in a short time. It needs some prior information about the subbottom structure and layer thickness, without which the solution can be ambiguous and even erratic when resolving parameters over a wide area. It is shown that the required subbottom information can be obtained from the time-angle relation by beamforming the same acoustic signal data used for geoacoustic inversion. The time-angle analysis is used to expose the prevalent physics intrinsic to geoacoustic inversion. One finds that the tau-p relation of the bottom and the bottom reflection coefficients, sampled at discrete angles associated with bottom and multiple surface-bottom returns, are often adequate, for many practical applications, to uniquely determine the geoacoustic bottom at low (< or =1 kHz) frequencies.     

The impact of water column variability on horizontal wave number estimation and mode based geoacoustic inversion results

The influence of water column variability on low-frequency, shallow water geoacoustic inversion results is considered. The data are estimates of modal eigenvalues obtained from measurements of a point source acoustic field using a horizontal aperture array in the water column. The inversion algorithm is based on perturbations to a background waveguide model with seabed properties consistent with the measured eigenvalues. Water column properties in the background model are assumed to be known, as would be obtained from conductivity, temperature, and depth measurements. The scope of this work in addressing the impact of water column variability on inversion is twofold. Range-dependent propagation effects as they pertain to eigenvalue estimation are first considered. It is shown that mode coupling is important even for weak internal waves and can enhance modal eigenvalue estimates. Second, the effect of the choice of background sound speed profile in the water column is considered for its impact on the estimated bottom acoustic properties. It is shown that a range-averaged sound velocity profile yields the best geoacoustic parameter estimates.     

Analysis of the structure of acoustic emission signals of the audible range by the sparse approximation method

A new approach for the time-frequency analysis of acoustic emission of the audible frequency range is proposed. The approach is based on the sparse approximation method. A basis dictionary based on Berlage functions is constructed with allowance for the characteristics of geoacoustic signals. It is shown that application of the developed method in analyzing real data makes it possible to reveal the internal geoacoustic pulse structure caused by the features of their sources. The results can be used for diagnosing deformation processes in natural media.     

舰船辐射噪声贝叶斯方法反演浅海底层结构及地声参数EI北大核心CSCD

针对以舰船辐射噪声为参考声源的浅海海底分层结构及地声参数反演问题,研究了一种基于贝叶斯理论的浅海多层海底地声参数反演方法。反演中以舰船辐射噪声的线谱成分为研究对象,进而采用非线性贝叶斯反演方法反演浅海底层结构、层中声速、声速衰减和密度,并对反演结果的不确定性进行分析。反演结果的最大后验概率估计值和边缘概率分布分别通过拨正模拟退火算法和Metropolis-Hastings采样法在各参数先验区间内计算获得,并根据贝叶斯信息准则确定最佳海底分层结构。海上实验表明:根据该方法反演获得海底分层结构及地声参数,计算得到的声压场与实测舰船辐射噪声传播损失误差不超过10%,反演结果能够准确表征实验海区海底特征。反演结果不确定性分析表明:海底纵波声速、横波声速以及密度的不确定性更小,对声压场变化更加敏感,反演结果更有效、准确。     

Bayesian geoacoustic inversion in a dynamic shallow water environment

This paper presents results for matched field Bayesian geoacoustic inversion of multitonal continuous wave data collected on the New Jersey continental shelf. To account for effects of significant spatial and temporal variation of the water column sound speed, the sound speed profile was represented by empirical orthogonal functions. Data error information for the inversion was estimated from multiple time windows of the data. Inversion results for the sediment sound speeds at three ranges are in excellent agreement with the ground truth.     

基于等效密度流体模型的地声参数分析

目前的地声参数反演多采用液态海底模型,但是实际海底为多孔弹性海底。该文在等效密度流体模型基础上,通过计算液态海底和多孔弹性海底的反射系数及传播损失,给出了等效密度流体模型和液态海底模型的等效性分析。数值仿真结果表明在低频情况下,多孔弹性海底给出的快纵波声速与等效密度流体模型给出的声速以及等效密度流体的实部与真实的海底密度基本一致。将等效密度流体模型近似看作液态海底模型进行反射系数和传播损失计算,在小掠射角和远距离时,计算结果表明与多孔弹性海底计算结果具有较好一致性,从而以此为依据确定海底为液态,进行地声参数反演。     

Analysis and modeling of broadband airgun data influenced by nonlinear internal waves

To investigate acoustic effects of nonlinear internal waves, the two southwest tracks of the SWARM 95 experiment are considered. An airgun source produced broadband acoustic signals while a packet of large nonlinear internal waves passed between the source and two vertical linear arrays. The broadband data and its frequency range (10-180 Hz) distinguish this study from previous work. Models are developed for the internal wave environment, the geoacoustic parameters, and the airgun source signature. Parabolic equation simulations demonstrate that observed variations in intensity and wavelet time-frequency plots can be attributed to nonlinear internal waves. Empirical tests are provided of the internal wave-acoustic resonance condition that is the apparent theoretical mechanism responsible for the variations. Peaks of the effective internal wave spectrum are shown to coincide with differences in dominant acoustic wavenumbers comprising the airgun signal. The robustness of these relationships is investigated by simulations for a variety of geoacoustic and nonlinear internal wave model parameters.