Geoacoustic characterization of a range-dependent ocean environment using towed array data

This paper examines geoacoustic inversion over a range-dependent multiple-layer seabed using a towed acoustic source and towed horizontal array. The approach is based on combining the results of a series of short-range, range-independent inversions to form a range-dependent representation of the environment. The data were collected in the Strait of Sicily during the MAPEX 2000 experiment. Issues such as the resolvability of multilayer structure and the sensitivity of various geoacoustic parameters are investigated by inversion of simulated data and by comparison of the MAPEX 2000 inversion results to a high-resolution seismic profile and to sediment core measurements. It appears that two, and in some cases possibly three, sediment layers can be resolved.     

Estimates of geoacoustic model parameters from inversions of horizontal and vertical line array data

This paper describes results from geoacoustic inversion of low-frequency acoustic data recorded at a receiving array divided into two sections, a sparse bottom laid horizontal array (HLA) and a vertical array (VLA) deployed in shallow water. The data are from an experiment conducted by the Norwegian Defence Research Establishment (FFI) in the Barents Sea, using broadband explosives (shot) sources. A two-layer range-independent geoacoustic model, consistent with seismic profiles from the area, described the environment. Inversion for geoacoustic model parameters was carried out using a fast implementation of the hybrid adaptive simplex simulated annealing (ASSA) inversion algorithm, with replica fields computed by the ORCA normal mode code. Low-frequency (40-128 Hz) data from six shot sources at ranges 3-9 km from the array were considered. Estimates of sediment and substrate p-wave velocities and sediment thickness were found to be consistent between independent inversions of data from the two sections of the array.     

An adaptive-hybrid algorithm for geoacoustic inversion

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     

Using the adaptive simulated annealing algorithm to estimate ocean-bottom geoacoustic properties from measured and synthetic transmission loss data

This paper presents the results obtained using the adaptive simulated annealing (ASA) algorithm to invert the test cases from the Geoacoustic Inversion Techniques Workshop held in May 2001. The ASA algorithm was chosen for use in our inversion software for its speed and robustness when searching the geoacoustic parameter solution space to minimize the difference between the observed and the modeled transmission loss (TL). Earlier work has shown that the ASA algorithm is approximately 15 times faster than a modified Boltzmann annealing algorithm, used in prior versions of our TL inversion software, with comparable fits to the measured data. Results are shown for the synthetic test cases, 0 through 3, and for the measured data cases, 4 and 5. The inversion results from the synthetic test cases showed that subtle differences between range-dependent acoustic model version 1.5, used to generate the test cases, and parabolic equation (PE) 5.0, used as the propagation loss model for the inversion, were significant enough to result in the inversion algorithm finding a geoacoustic environment that produced a better match to the synthetic data than the true environment. The measured data cases resulted in better fits using ASTRAL automated signal excess prediction system TL 5.0 than using the more sophisticated PE 5.0 as a result of the inherent range averaging present in the ASTRAL 5.0 predictions.     

Multistep matched-field inversion for broad-band data from ASIAEX2001

This paper discusses the results of geoacoustic inversion carried out using explosive charge data from the Asian Seas International Acoustic Experiment (ASIAEX) East China Sea (ECS) Experiment. A multifrequency incoherent matched-field inversion processor and a genetic algorithm (GA) are used for the inversion. A multistep matched field inversion approach is presented, which makes use of the varying sensitivities of wave fields at various frequencies to reduce the inversion problem into a sequence of smaller inversions with fewer unknowns to estimate at each stage. Different parameters are estimated using data at different frequencies according to their sensitivities. Inversion results for different areas in the ECS region are summarized and compared with core data.     

Geoacoustic inversion using a rotated coordinate system and simulated annealing

An efficient method for geoacoustic inversions in a range-dependent ocean waveguide is implemented and tested with synthetic data. This method combines a simulated annealing search with an optimal coordinate rotation that increases the efficiency of navigating parameter landscapes for which parameter coupling is important. The coordinate rotation associated with the parameter couplings also provides information about which parameters are resolvable for a particular inversion frequency and array geometry. Using this information, results from several single-frequency inversions can be combined to obtain an estimate for the sediment parameters.     

Geoacoustic inversion of range-dependent data with added Gaussian noise

A rotated coordinates inversion algorithm is used on subsets of the Inversion Techniques 2001 Geoacoustic Workshop data, to which white Gaussian noise is added. The resulting data sets are equivalent to noisy broad-band signals received on a horizontal line array (HLA) during a single integration time interval. The inversions are performed using a technique called systematic decoupling using rotated coordinates (SDRC), which expands the original idea of rotated coordinates by using multiple sets of rotated coordinates, each corresponding to a different set of bounds, to systematically decouple the unknowns in a series of efficient simulated annealing inversions. The cost function minimized in the inversion is based on the coherent broad-band correlation between data and model cross spectra, which increases the coherence gain of the signal relative to incoherent noise. Using the coherent broad-band cost function with sparse HLA-like data sets, the SDRC inversion method yields good estimates for the sensitive environmental parameters for signal-to-noise ratios as low as -15 dB.     

Bayesian geoacoustic inversion for the Inversion Techniques 2001 Workshop

This paper applies a Bayesian formulation to range-dependent geoacoustic inverse problems. Two inversion methods, a hybrid optimization algorithm and a Bayesian sampling algorithm, are applied to some of the 2001 Inversion Techniques Workshop benchmark data. The hybrid inversion combines the local (gradient-based) method of downhill simplex with the global search method of simulated annealing in an adaptive algorithm. The Bayesian inversion algorithm uses a Gibbs sampler to estimate properties of the posterior probability density, such as mean and maximum a posteriori parameter estimates, marginal probability distributions, highest-probability density intervals, and the model covariance matrix. The methods are applied to noise-free and noisy benchmark data from shallow ocean environments with range-dependent geophysical and geometric properties. An under-parameterized approach is applied to determine the optimal model parameterization consistent with the resolving power of the acoustic data. The Bayesian inversion method provides a complete solution including quantitative uncertainty estimates and correlations, while the hybrid inversion method provides parameter estimates in a fraction of the computation time.     

Description of the Hamilton-Bachman smart rule (HBSR) inversion technique (IT) applied to impulsive source data from the 2001 IT workshop

An inversion technique (IT) is developed and preliminarily tested using data from the 2001 IT Workshop. This technique was developed using TL versus range data collected by the harsh environments program (HEP) and provided through the workshop. However, the IT developed here applies to all sonar systems, active and passive. The sonar-independent portion of the IT consists of a simulated annealing algorithm to be developed by Neumann et al. constrained by an expert system called the Hamilton-Bachman Smart Rules (HBSR), which was developed by the authors. This expert system constrains the geoacoustic model being inverted to realistic Hamilton-Bachman-type values, curve shapes, etc. The sonar-specific module of the IT is chosen based on the sonar frequency and models available to run at those frequencies. Two measured data cases from the workshop are presented and, due to the HBSR, good solutions were acquired in less than 50 iterations.     

基于模拟退火和区域划分的海表流场反演算法研究

经典的海洋表面流场迭代反演算法采用固定的校正系数对探测区域进行整体分析和计算,因此耗时较长且反演精度也受到了一定程度的限制。本文提出利用模拟退火算法对反演过程中的校正系数进行优化,使其能根据每一次反演的结果进行自适应的调整和改变,从而减少迭代次数;同时根据流场分布的特点,提出划分探测区域的反演方法,最终,在经典算法的基础上给出了改进的海表流场反演算法。仿真反演实验结果表明,改进后的反演算法,其时效性和精度都有了明显提高。     

Full waveform inversion of field sonar returns for a visco-acousticEarth; a comparison of linearized and fully nonlinear methods

The techniques of linearized least squares inversion (LLSI) and simulated annealing (SA) are both used to invert a series of synthetic and real normal-incidence, geo-acoustic sonar returns for estimates of impedance versus two-way travel time in the top several meters of ocean floor sediment. The objective is to determine the better (faster, more accurate) method for inverting this class of data. LLSI uses an over parameterized earth, i.e., one composed of layers whose thickness corresponds to a travel time equal to the sample interval. This makes the inverse problem quite large, but also makes it nearly linear. SA uses a more efficient parameterization, one whose layers have variable thickness as well as variable impedance. Because of the relatively narrow frequency band (~1 octave at 20 dB down from the peak) the time domain signal is oscillatory and inversion for layer thickness is nonlinear. Results show greater time efficiency in solving the large linear problem (LLSI) than in solving the small nonlinear problem (SA). However, in both cases almost all of the waveform energy was modeled, indicating that essentially all the information in the data had been successfully recovered. The inversions are applied to 10-20 kHz field data acquired offshore Florida, and several techniques are employed to enhance the effectiveness of each inversion method     

Geoacoustic inversion of broad-band acoustic data in shallow water on a single hydrophone

The inversion of broad-band low-frequency acoustic signals received on sparse arrays can lead to robust and efficient estimations of sea-bed properties. This paper describes a shallow-water geoacoustic inversion scheme based on the use of a model-based matched-impulse response on a single hydrophone. Results from the INTIMATE'96 experiment on the Portuguese shelf break are reviewed. In order to minimize the effects of strong time variability due to internal tides, only the time-stable waterborne bottom-surface reflected arrivals are exploited. A quasi-linear inversion algorithm is first applied to refine the geometry of the experiment. Then, inversion of bottom parameters is performed with an objective function that only makes use of the bottom-surface reflected arrivals' amplitudes. The experimental results show that broad-band transmissions (300-800 Hz) received on a single hydrophone, combined with the use of a simple eigenray code, are sufficient to correctly resolve geometrical parameters and bottom features. The analysis of the reflection coefficients both on simulated and real data helps to understand the validity of the inverted parameters and to derive the basis of an equivalent medium concept for geoacoustic inversion based on a "through-the-sensor" approach.     

测量海底声学参数的匹配场反演方法

在测量海底声学参数的实际海洋环境中，声源和接收位置的距离这两个参数常常无法准确测量，在这种情况下，需要采用匹配场反演方法来估计海底的声学参数。一般情况下，匹配场反演方法可以归纳为2个组成部分，即海洋声场的声学预报模型和搜索控制策略。文中采用受控制的穷举方法作为搜索控制策略，对1996年中美远黄海试验的实验数据进行了匹配场反演试验，用以测定海底参数，由此得到的海底声学参数与实验中测量的声场衰减进行对比，一致性很好。     

A New Multistep Algorithm for Geoacoustic Inversion via Subspace Extraction

This communication presents a new multistep matched-field algorithm for geoacoustic inversion by subspace extraction with a threshold. In this algorithm, according to the varying sensitivities of geoacoustic parameters, parameters are separated into several subsets (or subspaces). Then, inversions are carried out in each sensitive subspace using an optimization algorithm, and for each inversion, a sub-subspace is extracted where values of objective functions are lower than a given threshold. Finally, in all the extracted sub-subspaces combined with the subspace of insensitive parameters, an inversion is performed for all parameters to find the optimal solution. After the extracting process, the search space is greatly reduced, and generally, the true parameter values will not be excluded from the sub-subspace if a reasonable threshold is designed. Thus, higher efficiency and accuracy can be obtained when compared with other algorithms. Simulation is carried out on synthetic data and results indicate that the new algorithm's performance is significantly superior to those of other algorithms.      

PAssive COastal TOMography: an Innovative Approach for the Remote Monitoring of Sea Temperature

Abstract. This paper describes a new Ocean Acoustic Tomography (OAT) methodology - a passive tomography - presently in an advanced development phase. This technique has been developed for long-term, extensive, remote monitoring of the seawater temperature spatial distribution, which is estimated from the received noise emitted from ships of opportunity. To test the passive tomographic processor under controlled conditions, the components of the naval noise from different kinds of vessels was analysed and realistic naval noise was simulated. The feasibility of the proposed methodology was confirmed by test-runs on semi-synthetic data; its capability to resolve temperature profiles will be better assessed with the use of real acoustic and environmental data collected during the INTIMATE00 experiment performed in October 2000 in the Atlantic Ocean off the Portuguese coast. An analysis of the space and time variability of the Empirical Orthogonal Function (EOF) decomposition of the sound speed (SSP) in the Mediterranean Sea has been carried out to identify areas where acoustic tomography can be successfully applied. Results from simulations in the South Adriatic Sea, which was identified as a region with a high sound speed variability associated with the seasonal cycle and with the main oceanographic processes, are reported.     

Comparison of chlorophyll algorithms in the bohai sea of China

Empirical band-ratio algorithms and artificial neural network techniques to retrieve sea surface chlorophyll concentrations were evaluated in the Bohai Sea of China by using an extensive field observation data set. Bohai Sea represents an example of optically complex case II waters with high concentrations of colored dissolved organic matter (CDOM). The data set includes coincident measurements of radiometric quantities and chlorophyll a concentration (Chl), which were taken on 8 cruises between 2003 and 2005. The data covers a range of variability in Chl in surface waters from 0.3 to 6.5 mg m^-3. The comparison results showed that these empirical algorithms developed for case I and case II waters can not be applied directly to the Bohai Sea of China, because of significant biases. For example, the mean normalized bias (MNB) for OC4V4 product was 1.85 and the root mean square (RMS) error is 2.26.     

Geoacoustic seafloor exploration with a towed array in a shallowwater area of the Strait of Sicily

Acoustic propagation in shallow water is greatly dependent on the geoacoustic properties of the seabottom. This paper exploits this dependence for estimating geoacoustic sediment properties from the bottom acoustic returns of known signals received on a hydrophone line array. There are two major issues in this approach: one is the feasibility of acoustic inversion with a limited aperture line array, the other is related to the knowledge of the geometry of the experimental configuration. To test the feasibility of this approach, a 40-hydrophone-4-m spaced towed array together with a low-frequency acoustic source, was operated at a shallow water site in the Strait of Sicily. In order to estimate the array deformation in real time, it has been equipped with a set of nonacoustic positioning sensors (compasses, tilt-meters, pressure gauges). The acoustic data were inverted using two complementary approaches: a genetic algorithm (GA) like approach and a radial basis functions (RBF) inversion scheme. More traditional methods, based on core sampling, seismic survey and geophone data, together with Hamilton's regression curves, have also been employed on the same tracks, in order to provide a ground truth reference environment. The results of the experiment, can be summarized as follows: 1) the towed array movement is not negligible for the application considered and the use of positioning sensors are essential for a proper acoustic inversion, 2) the inversion with GA and RBF are in good qualitative agreement with the ground truth model, and 3) the GA scheme tends to have better stability properties. On the other hand, repeated in version of successive field measurements requires much less computational effort with RBF     

Matched field inversion for geoacoustic model parameters usingadaptive simulated annealing

A method is described for the estimation of geoacoustic model parameters by the inversion of acoustic field data using a nonlinear optimization procedure based on simulated annealing. The cost function used by the algorithm is the Bartlett matched-field processor (MFP), which related the measured acoustic field with replica fields calculated by the SAFARI fast field program. Model parameters are perturbed randomly, and the algorithm searches the multidimensional parameter space of geoacoustic models to determine the parameter set that optimizes the output of the MFP. Convergence is driven by adaptively guiding the search to regions of the parameter space associated with above-average values of the MFP. The performance of the algorithm is demonstrated for a vertical line array in a shallow water enviornment where the bottom consists of homogeneous elastic solid layers. Simulated data are used to determine the limits on estimation performance due to error in experimental geometry and to noise contamination. The results indicate that reasonable estimates are obtained for moderate conditions of noise and uncertainty in experimental geometry     

Geoacoustic Uncertainties From Viscoelastic Inversion of Seabed Reflection Data

This paper applies nonlinear Bayesian inversion to seabed reflection data to estimate viscoelastic parameters of the upper sediments. The inversion provides maximum a posteriori probability (MAP) parameter estimates with uncertainties quantified in terms of marginal probability distributions, variances, and credibility intervals; interparameter relationships are quantified by correlations and joint marginal distributions. The inversion is applied to high-resolution reflectivity data from two sites in the Strait of Sicily. One site is characterized by low-speed sediments, resulting in data with a well-defined angle of intromission; the second is characterized by high-speed sediments, resulting in a critical angle. Data uncertainties are quantified using several approaches, including maximum-likelihood (ML) estimation, treating uncertainties as nuisance parameters in the inversion, and analysis of experimental errors. Statistical tests are applied to the data residuals to validate the assumed uncertainty distributions. Excellent results (i.e., small uncertainties) are obtained for sediment compressional-wave speed, compressional attenuation, and density; shear parameters are less well determined although low shear-wave speeds are indicated. The Bayesian analysis provides a quantitative comparison of inversion results for the two sites in terms of the resolution of specific geoacoustic parameters, and indicates that the geoacoustic information content is significantly higher for intromission data