Geoacoustic Inversion via Genetic Algorithm and Its Application to Manganese Sediment Identification

An acoustic inversion method using a wide-band signal and two near field receivers is proposed and applied to multiple layered seabed models including a manganese sediment. The inversion problem can be formulated into a probabilistic model comprised of signals, a forward model, and additive noise. The forward model simulates wide-band signals, such as chirp signals, and is chosen to be the source-waveletconvolution plane wave modeling method. The wavelet matching technique, using weighted least-squares fitting, estimates the sediment sound-speed and thickness on which determination of the possible numerical ranges for a priori uniform distribution is based. The genetic algorithm is applied to a global optimization problem to find a maximum a posteriori solution for determined a priori search space. Here the object function is defined by an L 2 norm of the difference between measured and modeled signals. Not only the marginal pdf but also its statistics are calculated by numerical evaluation of integrals using the samples selected during importance sampling process of the genetic algorithm.     

A hybrid simplex genetic algorithm for estimating geoacousticparameters using matched-field inversion

Matched-fieId inversion (MFI) undertakes to estimate the geometric and geoacoustic parameters in an ocean acoustic scenario by matching acoustic field data recorded at hydrophone array with numerical calculations of the field. The model which provides the best fit to the data is the estimate of the actual experimental scenario. MFI provides a comparatively inexpensive method for estimating ocean bottom parameters over an extensive area. The basic components of the inversion process are a sound propagation model and matching (minimization) algorithm. Since a typical MFI problem requires a large number of computationally intensive sound propagation calculations, both of these components have to be efficient. In this study, a hybrid inversion algorithm which uses a parabolic equation propagation model and combines the downhill simplex algorithm with genetic algorithms is introduced. The algorithm is demonstrated on synthetic range-dependent shallow-water data generated using the parabolic equation propagation model. The performance for estimating the model parameters is compared for realistic signal-to-noise ratios in the synthetic data     

A numerical tidal and storm surge model of the North Sea

Abstract

A two‐dimensional, vertically integrated, numerical model has been used to compute the M₂ and S₂ tidal components for the North Sea. The model has also been applied for simulation of two storm surge situations. The results of the computations are compared with sea level observations from coastal stations, and for one of the simulations a comparison with current measurements is also performed.     

XGBoost算法在多光谱遥感浅海水深反演中的应用

在多光谱遥感浅海水深反演过程中,考虑到水体和底质影响,水深值和海水表面辐射亮度之间的线性关系不成立。本文以甘泉岛南部0～25m范围的沙质区域为研究区域,利用GeoEye-1多光谱遥感影像和多波束实测水深数据构建XGBoost非线性水深反演模型,研究了XGBoost算法用于水深反演的性能。以决定系数(R~2),均方误差(MSE)和平均绝对误差(MAE)作为评价指标,并与3种传统线性回归模型进行了对比分析。结果表明, XGBoost非线性水深反演模型的R~2、MSE和MAE分别为0.991、0.33m和0.44m,拟合程度最好,精度优于线性回归模型。为进一步探究各模型在不同水深的反演精度,将水深范围分成3段(0～8 m, 8～15 m, 15～25 m)分别进行精度验证和误差分析。结果表明, XGBoost模型在各分段的精度均优于线性回归模型, MSE依次为0.56 m, 0.14 m和0.43 m。可见,在单一底质区域下XGBoost模型的水深反演精度更高,且反演效果更稳定。     

Prediction of berm geometry using a set of laboratory tests combined with teaching–learning-based optimization and artificial bee colony algorithms

Understanding sediment movement in coastal areas is crucial in planning the stability of coastal structures, the recovery of coastal areas, and the formation of new coast. Accretion or erosion profiles form as a result of sediment movement. The characteristics of these profiles depend on the bed slope, wave conditions, and sediment properties. Here, experimental studies were performed in a wave flume with regular waves, considering different values for the wave height (H₀), wave period (T), bed slope (m), and mean sediment diameter (d₅₀). Accretion profiles developed in these experiments, and the geometric parameters of the resulting berms were determined. Teaching–learning-based optimization (TLBO) and artificial bee colony (ABC) algorithms were applied to regression functions of the data from the physical model. Dimensional and dimensionless equations were found for each parameter. These equations were compared to data from the physical model, to determine the best equation for each parameter and to evaluate the performances of the TLBO and ABC algorithms in the estimation of the berm parameters. Compared to the ABC algorithm, the TLBO algorithm provided better accuracy in estimating the berm parameters. Overall, the equations successfully determined the berm parameters.     

The relationship between tidal current field and sediment transport in the Huanghai Sea and the Bohai Sea

-Based on a two-dimensional numerical M2 tidal model of the Huanghai and Bohai Seas, sediment transport of particles with different sizes in the computational area is given. It is concluded that tidal currents play an important role in sediment transport and deposition in the Huanghai and Bohai Seas.     

A NURBS-based high-order panel method for three-dimensional radiation and diffraction problems with forward speed

A high-order Rankine panel method based on Non-Uniform Rational B-Spline (NURBS) is developed for solving the three-dimensional radiation and diffraction problems with forward speed. A NURBS surface is used to precisely represent the body geometry. Velocity potential on the body surface is described by B-spline after the source density distribution on the boundary surface is determined. A collocation approach is applied to numerical computation and the integral equations are evaluated by applying Gauss–Legendre quadrature. The m_j-terms are evaluated by a desingularized method which utilizes NURBS technique. In order to verify the method proposed, it is firstly applied to the unbounded flow problem of a sphere and spheroids. The numerical results are found to be in good agreement with analytical solutions. Then the method is used to solve the radiation and diffraction problems of a sphere and the diffraction problem of a spheroid moving with a forward speed beneath the free surface in frequency domain. The numerical results are satisfactory in comparison with the published analytical results and experimental results.     

Two-dimensional,two-phase granular sediment transport model with applications to scouring downstream of an apron

We present a two-dimensional, two-phase model for non-cohesive sediment transport. This model solves concentration-weighted averaged equations of motion for both fluid and sediment phases. The model accounts for the interphase momentum transfer by considering drag forces. A collisional theory is used to compute the sediment stresses, while a two-equation (k–ε) fluid turbulence closure is implemented. A benchmark sediment transport problem concerning the scouring downstream of an apron is carried out as an example and numerical results agree with existing experimental data.     

High-precision array element localization for vertical line arraysin the Arctic Ocean

Array element localization (AEL) surveys are often required to accurately localize acoustic instruments (transponders or sensors) in the ocean. These are typically based on transmitting or recording acoustic signals from or at a set of well-known positions. A significant limiting factor in many AEL surveys is the uncertainty inherent in these “known” positions. In this paper, an inversion algorithm is developed which properly treats both transponder and sensor positions as unknowns, subject to available a priori information in the form of position estimates and uncertainties. The algorithm essentially consists of an iterative linearized inversion of the raytracing equations employing the method of regularization. The approach is applied to independently localize transponders and vertical line array (VLA) sensors that form part of a three-dimensional sensor array in the Arctic Ocean. Confidence limits estimated via Monte Carlo simulation indicate that transponders and sensors are localized to less than 1 m in three dimensions. The VLA sensor motion, monitored over a seven-week period, appears to be predominately driven by tidal currents and is consistent with historical current measurements for the region     

Influence of viscosity on one-dimensional thermal consolidation of marine clay

ABSTRACT

The elastic mechanical response of porous materials under a heat source has many applications in civil engineering and has received considerable attention in the geotechnical literature. In this paper, a Kelvin viscoelastic model is combined with the thermohydromechanical governing equations for marine clay and solved using a numerical inversion of the inverse Laplace transform in the time domain. After validation against existing analytical solutions, numerical parametric studies are conducted to investigate the influence of viscosity on temperature, excess pore pressure, and displacement. It is shown that viscosity has little influence on temperature, a modest influence on displacements, and a quite significant influence on excess pore pressure.     

潮致泥沙全沙净输运解析模式

泥沙运动作为水流和底床相互作用的纽带,是河流、河口及海岸工程研究的重要内容。在潮波作用明显的河口、海岸地区,周期性的动力作用下的泥沙运动具有往复和可逆的特征,因此研究这类水域的泥沙的净输运更具有实际的意义。基于泥沙输运和流速呈指数关系假设,建立潮流环境下的泥沙全沙净输运的解析解公式,并对该公式的计算结果和数值计算以及数学模型的结果进行了检验和验证,结果表明本研究提出的公式能较好地反应潮流环境下的泥沙净输运。由此,基于本公式采用潮流分潮调和常数可计算得到全沙净输运,并可以分析各分潮流及其相互作用与泥沙净输运的关系。研究结果显示,在受径流影响较大的半日潮河口,S2、MS4、M2三潮相互作用对全沙净输运的贡献显著高于通常的潮流不对称作用,即M2、M4的相互作用。此外,河口区域径流导致的余流对泥沙净输运的贡献不可忽略,特别是在洪季,大径流量条件下往往导致余流较大,其对泥沙净输运的贡献所占比例较大。     

Evaluation of ADCP backscatter inversion to suspended sediment concentration in estuarine environments

Acoustic Doppler Current Profilers (ADCP), designed for measuring velocity profiles, are widely used for the estimation of suspended sediment concentration from acoustic backscatter strength, but its application to estuarine environments requires further refinement. In this study, we examined the inversion capability of two ADCPs with 600 and 1200 kHz in three Korean estuaries: the supra-macrotidal Han River Estuary (HRE), microtidal Nakdong River Estuary (NRE), and anthropogenically altered macrotidal Yeongsan River Estuary (YRE). In particular, we examined the relative importance of the sound attenuations due to water (α_w) and sediment (α_s) in response to sediment characteristics (size and concentration) as well as changing salinity and temperature. The inverted concentration was compared with reference concentrations obtained either from water samples or Optical Backscatter Sensors. In NRE and YRE, where suspended sediment concentrations were less than 0.2 g/l, the acoustic inversion performed poorly only with α_s (r = 0.20 and 0.38 for NRE and YRE, respectively), but well with α_w (r = 0.66 and 0.42 for NRE and YRE, respectively). Thus, it is important to accurately constrain α_w in low-concentration estuarine environments. However, we did not find that the varying α_w performed considerably better than the constant α_w. On the other hand, the acoustic inversion was poorest at HRE regardless of α_w and α_s (r = 0.71 and mean relative error = 45%). The large discrepancy appears to result from the poorly constrained, spatially and temporally varying sediment characteristics (grain size, density and concentration) due to non-local sediment transport in the macrotidal HRE.     

Organic matter and surface properties of solid particles in the estuarine mixing zone

Two physicochemical parameters: specific surface area (SSA) and heat-of-wetting (Δ_wH) were measured in order to characterize the surface of sediments from river and marine environments. The data for the sediment samples of the Krka River Estuary were compared with polluted river and shelf marine sediment samples. It was found that organic free samples from the polluted river sediments show two to three times larger SSA compared to native samples. At the same time, Δ_wH was two to three times higher for native samples. The inverse of this trend was found for marine sediment samples. In the Krka River Estuary a narrow transition zone is evident, where such an inversion occurs. This inversion is attributed to significant compositional changes of organic matter within the estuary.     

Constraining Navigation by Matching Swath Bathymetry and Gravity Measurements at Ship Track Crossovers

A method which utilizes the lateral offset information obtained by comparing swath bathymetric data at track crossover points as a further constraint on the navigation is presented. The method, based on generalized least squares inversion theory, derives a new navigational solution that minimizes the overall misfit between the pairs of topography at crossovers while trying to remain smooth and close to the starting model. To achieve a high numerical efficiency during inversions of large matrices, we employed sparse matrix algorithms. The inversion scheme was applied to a set of Sea Beam data collected over the East Pacific Rise near 9° 30' N in early 1988 at the time when the Global Positioning System had limited coverage. The starting model was constructed by taking evenly spaced samples of positions along the tracklines. For each one of the 361 crossovers, we gridded the bathymetric data around the crossover point compared the gridded maps, and calculated the offset and uncertainty associated with this estimation. A suite of inversion solutions were obtained depending on the choice of three free parameters (that is, the a priori model variance, the correlation interval of a priori model, and the trade-off coefficient between fitting the data and remaining close to the a priori model). The best solution was chosen as one that minimizes both the Sea Beam topography and free-air gravity anomaly differences at crossovers. The improvement was significant; the initial rms mismatch between the tracks and free-air gravity anomalies at crossovers was reduced from 610m to 75m and from 2.5mGal to 1.9mGal, respectively.     

Acoustic tracking of a freely drifting sonobuoy field: experimental results

This paper describes a regularized acoustic inversion algorithm for tracking individual elements of a freely drifting sonobuoy field using measured acoustic arrival times from a series of impulsive sources. The acoustic experiment involved 11 sonobuoys distributed over an 8/spl times/6-km field, with a total of six sources deployed over 72 min. The inversion solves for an independent track for each sonobuoy (parameterized by the sonobuoy positions at the time of each source transmission), as well as for the source positions and transmission instants. Although this is a strongly under-determined problem, meaningful solutions are obtained by incorporating a priori information consisting of prior estimates (with uncertainties) for the source positions and initial sonobuoy positions and a physical model for sonobuoy motion along preferentially smooth tracks. The inversion results indicate that the sonobuoys move approximately 260-700 m during the source-deployment period. Closely spaced sonobuoys move along similar tracks; however, there is considerable variability in track directions over the entire field. Positioning uncertainties in horizontal coordinates are estimated using a Monte Carlo appraisal procedure to be approximately 100 m in an absolute sense and 65 m in a relative sense. A sensitivity study indicates that the uncertainties of the a priori position estimates are the limiting factor for track accuracy, rather than data uncertainties or source configuration.     

Constraining Navigation by Matching Swath Bathymetry and Gravity Measurements at Ship Track Crossovers

A method which utilizes the lateral offset information obtained by comparing swath bathymetric data at track crossover points as a further constraint on the navigation is presented. The method, based on generalized least squares inversion theory, derives a new navigational solution that minimizes the overall misfit between the pairs of topography at crossovers while trying to remain smooth and close to the starting model. To achieve a high numerical efficiency during inversions of large matrices, we employed sparse matrix algorithms. The inversion scheme was applied to a set of Sea Beam data collected over the East Pacific Rise near 9° 30' N in early 1988 at the time when the Global Positioning System had limited coverage. The starting model was constructed by taking evenly spaced samples of positions along the tracklines. For each one of the 361 crossovers, we gridded the bathymetric data around the crossover point compared the gridded maps, and calculated the offset and uncertainty associated with this estimation. A suite of inversion solutions were obtained depending on the choice of three free parameters (that is, the a priori model variance, the correlation interval of a priori model, and the trade-off coefficient between fitting the data and remaining close to the a priori model). The best solution was chosen as one that minimizes both the Sea Beam topography and free-air gravity anomaly differences at crossovers. The improvement was significant; the initial rms mismatch between the tracks and free-air gravity anomalies at crossovers was reduced from 610m to 75m and from 2.5mGal to 1.9mGal, respectively.     

Weak wave/tide interaction in suspended sediment-stratified flow: a case study

The formulation of weak wind-wave/low-frequency current interaction is extended to the case of suspended sediment-stratified flow. The influence of suspended sediment stratification on flow dynamics is described in terms of a sediment stratification parameter defined as von Karman's constant times a depth-independent function of the relative friction velocity and the relative settling velocity of suspended particles that is specified by a solution for the problem on the vertical structure of the suspended sediment-stratified near-bottom logarithmic layer. This ‘extended’ formulation is inserted in a two-dimensional non-linear, finite-difference, high-resolution hydrodynamic model and the modified model is applied to clarify the roles of wind-wave/tide interaction and suspended sediment stratification—individually and in combination—in the formation of the M₄ and M₆ overtides in Cádiz Bay. It is shown that the predictions for the M₄ and M₆ overtides have much in common and much in contrast with the M₂ tide. For the M₂ tide the influence of suspended sediment stratification shows up most vividly in the spatial variability of the tidal characteristics, but is not evident in changes in the M₄ and M₆ overtides. On the other hand, the influence of wave-induced changes on the M₂ tidal amplitude and phase is only of minor importance, but for the M₄ and M₆ overtides these changes are quite significant. When taken together, the effects of the two factors under investigation are very nearly balanced. This, however, does not mean that the conventional assumption of ignoring these factors is valid in shallow-water dynamics. Simply, that their resulting effect vanishes.     

Generating interface waves using a freely falling, instrumentedsource

In recent years, interface waves such as the Scholte wave have become important tools in the study of the geoacoustic properties of near-bottom seafloor sediments. Traditionally, these waves have been generated by explosive or pneumatic sources deployed at or near the seafloor and monitored by ocean-bottom seismographs or geophone arrays. While these sources generate the requisite interface waves, they also produce higher frequency compressional waves in the water and sediment that tend to contaminate the surface wave and make inversion of the data difficult in the near field. In this paper, a new source consisting of a freely falling projectile instrumented with an accelerometer is described. When the projectile impacts the bottom, the exact time history of the vertical force applied to the sediment is known and therefore may be convolved with the transfer function of a sediment geoacoustic model to produce accurate synthetic seismograms. Moreover, the vertical force applied to the seafloor is very efficient in generating surface wave motion while producing very little compressional wave energy so that the near-field signals are much more easily analyzed. An example of the use of the new source is presented including inversion of the received signals to obtain shear-wave velocity and attenuation as a function of depth in the near bottom sediments at a shallow-water site