High-order Bragg forward scattering and frequency shift of low-frequency underwater acoustic field by moving rough sea surface

Acoustic scattering modulation caused by an undulating sea surface on the space-time dimension seriously affects underwater detection and target recognition.Herein,underwater acoustic scattering modulation from a moving rough sea surface is studied based on integral equation and parabolic equation.And with the principles of grating and constructive interference,the mechanism of this acoustic scattering modulation is explained.The periodicity of the interference of moving rough sea surface will lead to the interference of the scattering field at a series of discrete angles,which will form comb-like and frequency-shift characteristics on the intensity and the frequency spectrum of the acoustic scattering field,respectively,which is a high-order Bragg scattering phenomenon.Unlike the conventional Doppler effect,the frequency shifts of the Bragg scattering phenomenon are multiples of the undulating sea surface frequency and are independent of the incident sound wave frequency.Therefore,even if a low-frequency underwater acoustic field is incident,it will produce obvious frequency shifts.Moreover,under the action of ideal sinusoidal waves,swells,fully grown wind waves,unsteady wind waves,or mixed waves,different moving rough sea surfaces create different acoustic scattering processes and possess different frequency shift characteristics.For the swell wave,which tends to be a single harmonic wave,the moving rough sea surface produces more obvious high-order scattering and frequency shifts.The same phenomena are observed on the sea surface under fully grown wind waves,however,the frequency shift slightly offsets the multiple peak frequencies of the wind wave spectrum.Comparing with the swell and fully-grown wind waves,the acoustic scattering and frequency shift are not obvious for the sea surface under unsteady wind waves.

An ensemble learning method to retrieve sea ice roughness from Sentinel-1 SAR images

Sea ice surface roughness(SIR)affects the energy transfer between the atmosphere and the ocean,and it is also an important indicator for sea ice characteristics.To obtain a small-scale SIR with high spatial resolution,a novel method is proposed to retrieve SIR from Sentinel-1 synthetic aperture radar(SAR)images,utilizing an ensemble learning method.Firstly,the two-dimensional continuous wavelet transform is applied to obtain the spatial information of sea ice,including the scale and direction of ice patterns.Secondly,a model is developed using the Adaboost Regression model to establish a relationship among SIR,radar backscatter and the spatial information of sea ice.The proposed method is validated by using the SIR retrieved from SAR images and comparing it to the measurements obtained by the Airborne Topographic Mapper(ATM)in the summer Beaufort Sea.The determination of coefficient,mean absolute error,root-mean-square error and mean absolute percentage error of the testing data are 0.91,1.71 cm,2.82 cm,and 36.37%,respectively,which are reasonable.Moreover,K-fold cross-validation and learning curves are analyzed,which also demonstrate the method’s applicability in retrieving SIR from SAR images.

Sea Surface Roughness Derivation from Wind Speed Estimated by Satellite Altimeter

For open sea conditions the sea surface roughness is described as a function of surface stress and wind speed over sea surface by Charnock relation. The sea surface roughnessn in the North-west Pacific Ocean is derived successfully using wind speed data estimated by the TOPEX satellite altimeter. From the results we find that： （1） the mean sea surface roughness in winter is greater than in summer; （2） compared with other sea areas, the sea surface roughness in the sea area east of Japan （ N30°- 40°, E135°- 150°） is larger than in other sea areas; （3） sea surface roughness in the South China Sea changes more greatly than that in the Bohai Sea, Yellow Sea and East China Sea.

Application of the method of equivalent edge currents to composite scattering from the cone-cylinder above a dielectric rough sea surface

Compared with scattering from a rough surface only, composite scattering from a target above a rough surface has become so practical that it is a subject of great interest. At present, this problem has been solved by some numerical methods which will produce an enormous calculation amount. In order to overcome this shortcoming, the reciprocity theorem （RT） and the method of equivalent edge currents （MEC） are used in this paper. Due to the advantage of RT, the difficulty in computing the secondary scattered fields is reduced. Simultaneously, MEC, a high-frequency method with edge diffraction considered, is used to calculate the scattered field from the cone-cylinder target with a high accuracy and efficiency. The backscattered field and the polarization currents of the rough sea surface are evaluated by the Kirchhoff approximation （KA） method and physical optics （PO） method, respectively. The effects of the backscattering radar cross section （RCS） and the Doppler spectrum on the size of the target and the windspeed of the sea surface for different incident angles are analysed in detail.

On Sea Surface Roughness Parameterization and Its Effect on Tropical Cyclone Structure and Intensity

A new parameterization scheme of sea surface momentum roughness length for all wind regimes, including high winds, under tropical cyclone （TC） conditions is constructed based on measurements from Global Positioning System （GPS） dropsonde. It reproduces the observed regime transition, namely, an increase of the drag coefficient with an increase in wind speed up to 40 m s-1 , followed by a decrease with a further increase in wind speed. The effect of this parameterization on the structure and intensity of TCs is evaluated using a newly developed numerical model, TCM4. The results show that the final intensity is increased by 10.5% （8.9%） in the maximum surface wind speed and by 8.1 hPa （5.9 hPa） in the minimum sea surface pressure drop with （without） dissipative heating. This intensity increase is found to be due mainly to the reduced frictional dissipation in the surface layer and little to do with either the surface enthalpy flux or latent heat release in the eyewall convection. The effect of the new parameterization on the storm structure is found to be insignificant and occurs only in the inner core region with the increase in tangential winds in the eyewall and the increase in temperature anomalies in the eye. This is because the difference in drag coefficient appears only in a small area under the eyewall. Implications of the results are briefly discussed.

Investigation on global positioning system signal scattering and propagation over the rough sea surface

This paper is devoted to the study of polarization properties, scattering properties and propagation properties of global positioning system （GPS） scattering signal over the rough sea surface. To investigate the polarization and the scattering properties, the scattering field and the bistatic scattering coefficient of modified Kirchhoff approximation using the tapered incident wave is derived in detail. In modeling the propagation properties of the GPS scattering signal in the evaporation duct, the initial field of parabolic equation traditionally computed by the antenna pattern using fast Fourier transform （FFT） is replaced by the GPS scattering field. And the propagation properties of the GPS scattering signal in the evaporation duct with different evaporation duct heights and elevation angles of GPS are discussed by the improved discrete mixed Fourier transform taking into account the sea surface roughness.

Optimal Stiffness for Flexible Connectors on A Mobile Offshore Base at Rough Sea States

This paper investigates a simplified method to determine the optimal stiffness of flexible connectors on a mobile offshore base(MOB) during the preliminary design stage. A three-module numerical model of an MOB was used as a case study. Numerous constraint forces and relative displacements for the connectors at rough sea states with different wave angles were utilized to determine the optimized stiffness of the flexible connectors. The range of optimal stiffnesses for the connectors was obtained based on the combination and intersection of the optimized stiffness results, and the implementation steps were elaborated in detail. The percentage reductions of the optimized and optimal stiffness of the flexible connector were determined to quantitatively evaluate the decreases of the constraint force and relative displacement of the connectors compared with those calculated by using the original range of the connector stiffnesses. The results indicate the accuracy and feasibility of this method for determining the optimal stiffness of the flexible connectors and demonstrate the rationality and practicability of the optimal stiffness results. The research ideas, calculation process, and solutions for the optimal stiffness of the flexible connectors of an MOB in this paper can provide valuable technical support for the design of the connectors in similar semisubmersible floating structures.

Sound Scattering From Rough Bubbly Ocean Surface Based on Modified Sea Surface Acoustic Simulator and Consideration of Various Incident Angles and Sub-surface Bubbles＇ Radii

The aim of the present study is to improve the capabilities and precision of a recently introduced Sea Surface Acoustic Simulator(SSAS) developed based on optimization of the Helmholtz–Kirchhoff–Fresnel(HKF) method. The improved acoustic simulator, hereby known as the Modified SSAS(MSSAS), is capable of determining sound scattering from the sea surface and includes an extended Hall–Novarini model and optimized HKF method. The extended Hall–Novarini model is used for considering the effects of sub-surface bubbles over a wider range of radii of sub-surface bubbles compared to the previous SSAS version. Furthermore, MSSAS has the capability of making a three-dimensional simulation of scattered sound from the rough bubbly sea surface with less error than that of the Critical Sea Tests(CST) experiments. Also, it presents scattered pressure levels from the rough bubbly sea surface based on various incident angles of sound. Wind speed, frequency, incident angle, and pressure level of the sound source are considered as input data, and scattered pressure levels and scattering coefficients are provided. Finally, different parametric studies were conducted on wind speeds, frequencies, and incident angles to indicate that MSSAS is quite capable of simulating sound scattering from the rough bubbly sea surface, according to the scattering mechanisms determined by Ogden and Erskine. Therefore, it is concluded that MSSAS is valid for both scattering mechanisms and the transition region between them that are defined by Ogden and Erskine.

Determination of Arctic melt pond fraction and sea ice roughness from Unmanned Aerial Vehicle (UAV) imagery

Melt ponds on Arctic sea ice are of great significance in the study of the heat balance in the ocean mixed layer, mass and salt balances of Arctic sea ice, and other aspects of the earth-atmosphere system. During the 7th Chinese National Arctic Research Expedition, aerial photographs were taken from an Unmanned Aerial Vehicle over an ice floe in the Canada Basin. Using threshold discrimination and three-dimensional modeling, we estimated a melt pond fraction of 1.63% and a regionally averaged surface roughness of 0.12 for the study area. In view- of the particularly foggy environment of the Arctic, aerial images were defogged using an improved dark channel prior based image defog algorithm, especially adapted for the special conditions of sea ice images. An aerial photo mosaic was generated, melt ponds were identified from the mosaic image and melt pond fractions were calculated. Three-dimensional modeling techniques were used to generate a digital elevation model allowing relative elevation and roughness of the sea ice surface to be estimated. Analysis of the relationship between the distributions of melt ponds and sea ice surface roughness show-s that melt ponds are smaller on sea ice with higher surface roughness, while broader melt ponds usually occur in areas where sea ice surface roughness is lower.

An angular cutoff composite model for investigation on electromagnetic scattering from two-dimensional rough sea surfaces

Based on the local configuration angle division to select the corresponding method for electromagnetic scattering calculation from rough sea surface, this paper presents an angular cutoff composite model： when the local scattered angle is in the specular region that is given by an approximately 20 degrees cone around the specular direction, the Kirchhoff approximation is applied to evaluate the specular reflection, which dominates the total scattering in this region; the small perturbation method is employed to handle the diffuse reflection which is predominant as the local scattered angle is situated out of the specular region. Numerical results are compared with those of experimental and theoretical models in several configurations as a function of incident angle, wind speed, wind direction. The comparison of numerical results of other experimental and theoretical models in several configurations shows that the new composite model is robust to give accurate numerical evaluations for the sea surface scattering.

Comparison among sea surface roughness schemes

Based on the measurements from the US National Data Buoy Center 3-m discus buoy site No. 44004 （38.5°N, 70.47°W） from January 1 to March 31 of 2003, with the COARE algorithm （Version 3.0）, the results from four parameterization schemes developed recently for sea surface aerodynamic roughness length were compared with each other. Calculations of frictional speed u., drag coefficient Ca and wind stress r indicate that the calculated frictional velocities from the four schemes （8.50%-16.20%, the normalized standard error estimate, or NSEE）, the computed drag coefficients and wind stress （respectively 15.08%-28.67% and 17.26%,50.59% NSEE） are reasonable. Schemes YT96 and GW03 are consistent. The 002 scheme gives overestimated values for u, and Ca. Schemes TY01 and GW03 display discontinuous characteristics in handling young wave data.

Wave-Dependence of Friction Velocity, Roughness Length, and Drag Coefficient over Coastal and Open Water Surfaces by Using Three Databases

The parameterization of friction velocity, roughness length, and the drag coefficient over coastal zones and open water surfaces enables us to better understand the physical processes of air-water interaction. In context of measurements from the Humidity Exchange over the Sea Main Experiment （HEXMAX）, we recently proposed wave-parameter dependent approaches to sea surface friction velocity and the aerodynamic roughness by using the dimensional analysis method. To extend the application of these approaches to a range of natural surface conditions, the present study is to assess this approach by using both coastal shallow （RASEX） and open water surface measurements （Lake Ontario and Grand Banks ERS-1 SAR） where wind speeds were greater than 6.44 m s-1. Friction velocities, the surface aerodynamic roughness, and the neutral drag coefficient estimated by these approaches under moderate wind conditions were compared with the measurements mentioned above. Results showed that the coefficients in these approaches for coastal shallow water surface differ from those for open water surfaces, and that the aerodynamic roughness length in terms of wave age or significant wave height should be treated differently for coastal shallow and open water surfaces.

Doppler spectral analysis for time-evolving sea surfaces using second-order small slope approximation

The second-order small slope approximation （SSA2） method is introduced to study the Doppler characteristics from time-evolving sea surfaces. Simulation results show better agreement between the SSA2 model and the numerical method for both vertical and horizontal polarizations, meaning that SSA2 gives a satisfactory prediction of the spectral difference between two po- larizations; while such discrepancy cannot be captured using the lowest-order SSA （SSA1） model. In particular, the Doppler shifts and spectral widths for different incident angles, wind directions and polarizations are analyzed, demonstrating correct variations with respect to such parameters. Those observations prove that the SSA2 provides an efficient and relatively fast tool for sea surface Doppler spectral analysis.

深水多次波与噪音同时压制技术

海洋地震资料预处理,两大难题就是噪音和多次波。常规的近道多次波压制有预测反褶积和SRME,中远道多次波压制有速度滤波等方法。然而在深水崎岖海底资料中,对于侧面反射、绕射多次波等等,常规的多次波压制方法往往失效,原因在于常规的多次波压制方法的理论多假设地震波是以二维双曲线方式传播,实际上地下地震波传播路径复杂,常规去多次方法有局限。另外对于噪音的压制,技术人员往往是头痛医头脚痛医脚,对不同的噪音采用不同的模块进行压制,即增加了处理步骤,也增加了损伤有效波的风险。本文研究了一种利用剔除拟合法并结合LIFT思想进行深水多次波和噪音压制技术,一次处理流程可以同时压制多种噪音和多种多次波,取得了良好的效果。

南海北部深水坡折带双方位地震采集设计与应用

南海北部深水区是当前油气地震勘探的热点和难点区域,陆架坡折区崎岖海底发育,水深急剧变化,常规的窄方位地震采集难以满足坡折变水深下的地震成像和目标评价的需要。以南海北部琼东南陵水某勘探区为例,针对该区地震地质特点,通过正演模拟和照明统计分析等技术综合分析了地下各目的层上有效覆盖次数和照明能量分布,开展了面向地质目标的窄方位、双方位釆集观测系统评价对比分析,确定了一套适合于靶区的双方位采集设计方案和具体参数。采集应用结果表明,双方位采集有效提高了坡折中深层的地震成像品质,其结果可为其他深水海域勘探提供借鉴指导意义。

三维广义SRME方法及其应用

随着深水地震勘探的进展,压制坡折带和深水崎岖海底等复杂地质构造的自由表面多次波逐渐变为海洋地震资料处理中的难题,而传统的三维自由表面多次波衰减(SRME)技术在数据重构问题上存在很明显的不足。因此,在三维SRME技术的基础上,提出了三维广义SRME技术来压制自由表面多次波。该方法借鉴贡献道思想和稀疏反演方法,结合focal变换和三维曲波变换,优化构建全局自由表面多次波数据,最后通过自适应匹配相减来压制多次波。实际资料应用表明,该方法能有效压制陡倾角复杂地质构造情况下的自由表面多次波,提高成像品质,取得了重大突破。

深水崎岖海底地震资料叠前深度偏移的必要性

深海区域崎岖海底会造成地震波传播速度横向剧烈变化,从而导致时间偏移剖面中构造形态严重畸变,成像效果差。利用理论模型讨论了用常规时间偏移、叠后深度偏移及叠前深度偏移技术解决深水崎岖海底地震成像问题的有效性。通过对深水模型数据各种偏移结果的对比分析认为,常规时间偏移和叠后深度偏移均不能解决崎岖海底地区地震成像问题,而叠前深度偏移是解决这一问题的有效方法。实际资料处理结果证实了这一认识。

深水崎岖海底对下伏地层反射波特征的影响分析

基于深水崎岖海底地区的地震剖面建立崎岖海底地层地震地质模型;基于射线正演模拟及地震波在弹性分界面上的能量分配,分析研究崎岖海底影响下的下伏地层反射界面上的地震波运动学和动力学特征以及对崎岖海底对下伏地层成像精度的影响,分析结果表明,崎岖海底对下伏地层的反射覆盖次数、反射能量以及入射角范围有很大的影响,导致波的动力学特征发生明显变化,尤其海上长排列加剧振幅的横向变化,指出在复杂崎岖海底地区地震勘探必须选择合适的排列长度和覆盖次数才能确保保幅成像处理。

浅海周期起伏海底环境下的声传播

海底粗糙对水下声传播及水声探测等应用具有重要影响.利用黄海夏季典型海洋环境,分析了同时存在海底周期起伏和强温跃层条件下的声传播特性,结果表明:由于海底周期起伏的存在,对于低频(<1 kHz)、近程(10 km)的声信号,传播损失可增大5—30 dB.总结了声传播损失及脉冲到达结构随声源深度、海底起伏周期及起伏高度等因素变化的规律.当海底起伏周期不变时,起伏高度越大引起的异常声传播的影响随之变大;当起伏高度不变时,随着起伏周期变大,其对声传播的影响逐渐变小.用射线理论分析了其影响机理,由于海底周期起伏改变了声波与海底的入射和反射角度,使得原本小掠射角入射到海底的声线变为大掠射角,导致海底的反射损失增大;另一方面,声线反射角度的改变会使得原本可以到达接收点的声能量,由于与海底作用次数增加或变为反向传播而大幅度衰减.在浅海负跃层环境下,声源位于跃层上比位于跃层下对声传播影响更大.周期起伏海底对脉冲声传播的影响表现在引起不同角度的声线(或简正波号数)之间的能量发生转化,一些大角度声线能量衰减加大,多途结构变少.多途结构到达时间及相对幅度的变化进而影响声场的频谱,会使得基于匹配场定位的方法性能受到影响.所以,声呐在实际浅海环境中应用时,应对起伏海底的影响予以重视.此外,研究结果对海底地形测绘空间精度的提高也具有重要参考意义.

叠前深度偏移初始速度模型建模方法应用研究——以JY工区崎岖海底为例

Dix公式法和速度约束反演法,是地震资料叠前深度偏移建立初始速度模型的两种主要方法。首先利用JY工区深水崎岖海底的资料对这两种速度建模方法的成像能力进行了实验分析,结果表明,这两种方法都不能得到合理的初始速度模型。此外,通过速度约束反演得到的初始速度模型可以沿层位生成平均层速度模型。再利用垂向上地层埋深与速度关系和横向上同一套沉积地层与速度关系来共同修正平均层速度模型,这样可以获得更符合地质条件的初始速度模型。实验结果证明,通过引入地质背景的约束有利于建立起更为准确的初始速度模型,为后续的叠前深度偏移成像提供了有利保障。