Reverberation vertical coherence and sea-bottom geoacoustic inversion in shallow water

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.     

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     

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.     

Benchmarking geoacoustic inversion methods for range-dependent waveguides

Inversion methods have been developed over the past decade to extract information about unknown ocean-bottom environments from acoustic field data. This paper summarizes results from the Office of Naval Research/Space and Naval Warfare Systems Command (SPAWAR) Geoacoustic Inversion Techniques Workshop, which was designed to benchmark present-day inversion methods. The format of the workshop was a blind test to estimate unknown geoacoustic profiles by inversion of synthetic acoustic field data. The fields were calculated using a high-angle parabolic approximation and verified using coupled normal modes for three range-dependent shallow-water test cases: a monotonic slope; a shelf break; and a fault intrusion in the sediment. Geoacoustic profiles were generated to simulate sand, silt, and mud sediments in these environments. Several different approaches for inverting the acoustic field data were presented at the workshop: model-based matched-field methods; perturbation methods; methods using transmission loss data; and methods using horizontal array information. An effective inversion must provide both an estimate of the bottom parameters and a measure of the uncertainty of the estimated values. New methods were presented at the workshop to formalize the measure of uncertainty in the inversion. Comparisons between the different inversions are discussed in terms of a metric-based transmission loss calculated using the inverted profiles. The results demonstrate the effectiveness of present-day inversion techniques and indicate the limits of their capabilities for range-dependent waveguides.     

Matched-field inversion for geoacoustic model parameters in shallowwater

Matched-field inversion is used to, estimate geoacoustic properties from data obtained in an experiment with a vertical line array (VLA). The experiment was carried out using broad-band sources (shots) in water depths of about 200 m on the continental shelf off Vancouver Island. The data were processed to obtain spectral components of the field for frequencies near the bubble frequency for the shot. The ocean bottom in this region consists of a layer of mainly sandy sediments (about 100 m thick) overlying older consolidated material. Consequently, the inversion was designed to estimate the parameters of a two-layer elastic sediment model. In the inversion, an adaptive global search algorithm was used to investigate the multidimensional space of geoacoustic models in order to determine the set of values corresponding to the best replica field. Convergence is driven by adaptively guiding the search to regions of the parameter space associated with above-average values of the matched field correlation between the measured and replica fields. The geoacoustic profile estimated by the inversion consisted of a 125-m layer with compressional speed ~1700 m/s and shear speed ~400 m/s, overlying a layer with compressional speed ~1900 m/s. This model is consistent with the results from conventional seismic experiments carried out in the same region     

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.     

Effective parameters for matched field geoacoustic inversion in range-dependent environments

Using the well-established technique of geoacoustic inversion, one can estimate a set of acoustic sea-bed parameters from sonar array data. Simultaneously, one can search for geometric parameters such as range, water depth, and hydrophone depth. When the technique is applied in a range-dependent environment, there is a potentially much larger set of parameters to match, unless one has perfect knowledge of the bathymetry. From the point of view of optimization, one needs to handle uncertainties in bathymetry without hugely increasing the amount of computation. A simple time-domain view (which is shown to be equivalent to the adiabatic approximation) suggests that it is sufficient to use a range-independent model with an empirical "effective" depth even when the bottom is not flat. In fact, there is a set of effective environments that will suffice; one can choose whichever is the most convenient. The success of this concept is demonstrated with some test cases from a recent Geoacoustic Inversion Techniques Workshop.     

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     

Modal evolution and inversion for seabed geoacoustic properties inweakly range-dependent shallow-water waveguides

In a shallow-water ocean environment, the range dependent variation of the geoacoustic properties of the seabed is one of the crucial factors affecting sound propagation. Since the local modes of propagation depend on the spatial changes in the bottom sediments, the local eigenvalues of these modes are useful as tools for examining the range dependence of the sediment properties. In order to extract the local eigenvalues from measurements of the pressure field in a laterally inhomogeneous waveguide, the zeroth-order asymptotic Hankel transform with a short sliding window is utilized. The local peak positions in the output spectra differ from the local eigenvalues due to both the range variation of the local modes and the interference of adjacent modes. The departure due to the former factor is evaluated analytically by using the stationary phase method. In order to reduce the error induced by the latter factor, mode filtering is utilized by incorporating data from a fixed vertical array of receivers. The methods developed are applied to simulated pressure field data as well as experimental field data, and it is shown that the range evolution of the local modes can be successfully estimated. In addition, field measurements are used to demonstrate that the modal trajectories in range can be used to infer the range-dependent geoacoustic properties of the seabed     

海底沉积物的基本地声结构与地声模型

为准确建立海底地声模型,本文探讨地声模型的基本组成和基本结构。通过样品实验室测量,分析南海海底表层沉积物的密度、孔隙度与声速随着埋深变化的关系,得出海底实际存在的低声速表面–声速缓慢变化类型、低声速表面–声速增大类型、高声速表面–声速缓慢变化类型和高声速表面–声速增大类型4种典型地声结构;对比钻探测量,分析黄海海底沉积物的密度、孔隙度与声速随埋深变化关系,得出海底地声模型分层特征与地声结构组合特征。研究表明,地声模型可以归结为4种基本地声结构的组合,通过与底层海水声速、同层内声速剖面以及与上层海底沉积物下表面声速的比较,可以建立各种海底地声模型;基于实验室测量法建立的地声模型可以作为参考地声模型,但需要考虑实际海底温度和压力梯度以及海底沉积物的频散特性等,借助于声速比校正法和频散性理论模型进行计算及修正。     

A comparison of complete and approximate solutions for second-order diffraction loads on arrays of vertical ci cular cylinders

This paper presents a comparison between two theoretical methods for computing the second-order diffraction loads on arrays of bottom-mounted, surface-piercing vertical circular cylinders in regular waves. One method presents a complete solution for the second-order hydrodynamic loads on the cylinder array via a numerical integration over the mean fluid free-surface. The other method is based on a large spacing approximation between the array members and involves the solution of a set of equivalent isolated body problems to obtain estimates for the second-order hydrodynamic loads. Numerical results for a pair of cylinders indicate very good agreement between the two methods at center-to-center spacing of both three and five radii, indicating that the approximate method may be sufficient to compute hydrodynamic interference effects to the second-order in many practical engineering situations.     

Transmitted and reflected coefficients for horizontal or vertical plate type breakwater

Surface or submerged horizontal or vertical plate can be considered as a new concept breakwater.This paper investigates the wave-plate interaction of this type of breakwater by use of the boundary element method.The relationships of wave transmitted and reflected among plate thickness,submergence and length are carefully studied by numerical simulation.It is shown that:(1) The transmitted coefficients of submerged horizontal plate or vertical plate will become larger with the increase of plate thickness and reduce rapidly with the decrease of plate submergence.(2) Both surface horizontal and vertical plate are efficient for intermediate and short wave elimination,but vertical plate is more effective.(3) Submerged horizontal plate can act more effectively than submerged vertical plate does.With all wave frequencies,the vertical plate almost has no wave elimination effect.     

Geoacoustic inversion by matched-field processing combined with vertical reflection coefficients and vertical correlation

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.     

Sand-mediated divergence between shallow reef communities on horizontal and vertical substrata in the western Indian Ocean

Distinctions are rarely made between vertical and horizontal surfaces when assessing reef community composition, yet physical differences are expected because of hydrodynamic differences and sediment accumulation on flat surfaces. As sand often diminishes biotic cover, we hypothesised that vertical surfaces will support a greater biomass but have lower diversity due to domination by a few species. To test this, we quantified sessile communities on vertical and horizontal surfaces at three sites in the Delagoa Bioregion on the east coast of South Africa. Community composition consistently differed: vertical communities were dominated by various filter feeders, especially the ascidian Pyura stolonifera, whereas those on horizontal reef comprised a mixture of filter feeders and various algae. The total number of species and all diversity metrics were significantly greater for horizontal reef surfaces. Contrastingly, Simpson’s dominance and biomass were significantly greater for vertical reef surfaces. Percentage cover of sand explained much of the variation in community composition whereas depth did not. Small-scale topographic differences in substratum orientation associated with differences in sand inundation will therefore influence both α and β diversity. Coastal developments and activities that alter sand movements and delivery to the coastal zone are therefore likely to have a profound influence on the maintenance and diversity of shallow subtidal communities.     

Second-order vertical and horizontal wave forces on a circular dock

A second-order potential solution is presented for the diffraction of a nonlinear progressive wave in finite-depth water, incident on a fixed circular dock. The usual perturbation analysis is used to produce first- and second-order subproblems. The mathematical method is based on the assumption that inner and outer solutions exist and these are matched by the requirements of continuity for mass flux and pressure between adjacent regions. It is shown that the solutions for the second-order problem can be derived in the same manner as in the first-order theory.     

Relationship between geoacoustic properties and chemical content of submarine polymetallic crusts from offshore Brazil

Abstract

Marine mining is the ocean’s new exploration frontier, and polymetallic crusts (PMCs) and ferromanganese nodules are considered a strategic resource for the future. Acoustic geophysical methods are a valuable tool for oceanic research and have been employed for several decades in the exploration of marine resources and environmental evaluations. The main objective of this work was to investigate the correlation between the chemical composition of PMCs from three different areas along the Brazilian continental margin and their physico-acoustic attributes (P-wave velocity, density and acoustic impedance). The results show that the geoacoustic properties of PMCs are correlated to their chemical composition. Measurements showed positive and negative trends between acoustic impedance and studied elements (Fe, Mn, Co, Cu, Zn, W and Ni) according to the direction of measurement, indicting high anisotropy levels. Our study sheds initial light on the correlation between acoustic properties and metal content of PMCs. The study facilitates assessments of the acoustic responses of PMCs allowing more efficient prospection and exploitation compared to ship-board geophysical techniques that are too qualitative to identify PMCs. The results can contribute to determining the best procedures and techniques for more efficient future exploration of this resource.     

Study on scattering wave force of horizontal and vertical plate type breakwaters

The interaction between wave and horizontal and vertical plates is investigated by the boundary element method,and the relations of wave exciting force with plate thickness,submergence and length are obtained.It is found that:1) The efficient wave exciting force exists while plate submergence is less than 0.5 m,and the plate is very thin with order O(0.005 m).2) The maximum heave wave exciting force exists,and it is the main factor for surface and submerged horizontal plate while the roll force can be ignored.3) The maximum sway wave exciting force exists,it is the main factor for surface or submerged vertical plate,and the roll force is about 20 times of horizontal plate.     

Spatial smoothing and minimum variance beamforming on data fromlarge aperture vertical line arrays

Various approaches to the beamforming of data from large aperture vertical line arrays are investigated. Attention is focused on the conventional beamforming problem where the angular power spectrum is estimated, in this case by the adaptive minimum variance processor. The data to be processed are 200 Hz CW transmissions collected at sea by a 900 m vertical line array with 120 equally spaced sensors. Correlated multipath arrivals result in signal cancellation for the adaptive processor, and spatial smoothing techniques must be used prior to beamforming. The processing of subapertures is proposed. Full aperture and subaperture processing techniques are used on the 200 Hz data. Multipath arrivals are found to illuminate only parts of the array, thus indicating that the wavefield can be highly inhomogeneous with depth     

Performance bounds for acoustic tomography in a vertical ocean slice

Reciprocal travel-time data along acoustic multipaths between transceivers in the ocean sound channel can be inverted to form estimates of sound speed and horizontal current in the vertical plane of the transceiver pair. We investigate the quality of such estimates in a range-independent environment. Since most of the information gathered along an acoustic path is concentrated near its turning depths, inversions are characterized by strong sidelobe contamination. Estimates at depths in the upper ocean are contaminated by fields in the lower ocean and vice versa. Therefore estimates can be improved by use of a priori knowledge of contaminating fields at the position of the sidelobe. The Backus-Gilbert method is adapted to allow for use of a priori knowledge and three measures of system performance are defined, viz., resolution length, stochastic error, and ratio of sidelobe to primary lobe acceptance. Trade-off relations among these quantities are calculated for 17 eigenrays in a Munk canonical profile for various orders of a priori knowledge. The limit of perfect knowledge gives an absolute bound on system performance. Numerical examples indicate poor performance in the lower ocean; but good estimates of sound speed and acceptable estimates of horizontal current are feasible in the upper ocean.     

Total horizontal and vertical forces of irregular waves on partially perforated caisson breakwaters

In this study, the total horizontal and vertical forces as well as the phase differences of irregular waves on a partially perforated caisson breakwater are investigated. The partially perforated caisson is located on a rubble fill foundation and filled with rock. Based on linear potential theory, a simple semi-analytical solution of the present problem is developed by means of the matched eigenfunction method and the finite element method. Experimental tests are also conducted to validate the theoretical model and examine the wave forces acting on the perforated caisson. The effects of some of the main factors on the total wave forces and the phase difference are examined. Theoretical and experimental studies show that when the total horizontal force reaches its peak, the simultaneous total vertical (upward) force is rather small or even becomes downwards. This is due to the existence of an obvious phase difference between the time histories of the total horizontal and vertical forces, which is an important advantage of perforated caissons over traditional structures.