基于小波变换和数学形态学的潮沟提取方法研究

潮沟是潮滩的主要地貌类型之一,潮沟信息的检测对湿地生态环境的监测和保护有着重要意义。本文以长江口九段沙下沙北部发育比较复杂的潮沟为研究对象,使用2016年1月26日Landsat8全色波段数据,采用了一种融合小波变换、最大类间方差法和数学形态学的方法对潮沟边缘信息进行了检测。小波变换中通过减小高层小波系数,达到弱化潮滩灰度变化的目的;通过增大低层小波系数,达到增强潮沟信息的目的;利用数学形态学和图像之间的几何(加减乘)运算,完成潮沟的提取;最后,分别在原始数据和检测结果相应的位置取样进行精度验证,样方潮沟面积一致性精度平均为92.1%。     

Hydroelastic response of marine risers subjected to internal slug-flow

It is the purpose of this study to investigate the dynamic behaviour of catenary pipelines for marine applications, assuming the combined effect of harmonic motions imposed at the top, and the internal slug-flow. The analysis is based on the assumption of a steady slug-flow inside the pipe that results in a relatively simplified model for the formulation of the internal flow. The slug-flow model is described using several assumptions and empirical correlations which attempt to reveal the ill-understood and concealed properties of the slug-flow. The pipeline dynamics are investigated in the two dimensional space omitting the out-of-plane vibrations. The system of differential equations is generic and accounts for the steady effect of the internal liquid as is conveyed through the structure.The two models, those of the internal slug-flow and the pipeline’s dynamical model, are properly combined through the internal flow terms of the dynamic equilibrium system. The solution provided is achieved using a frequency domain technique which is applied to the linearized governing set. The effect of the slug-flow is assessed through comparative computations with and without internal flow effects. The conclusions are drawn having the structure excited under axial and normal motions paying particular attention to the variation of the dynamic components along the complete length of the pipeline.     

基于磁偶极子的频率域电磁系统几何误差分析

基于磁偶极子模型的频率域电磁法仪器在进行大地探测的过程中,由于收发线圈的安装误差、晃动以及材料的几何形变等因素影响,收发线圈的位置及姿态角发生变化,从而产生几何误差,而几何误差的定量分析对频率域电磁法仪器在制作、野外实验操作以及校正方面均具有很强的指导意义。本文根据误差来源的不同,将几何误差分为一次场误差和二次场误差两类;通过引入一次场张量矩阵和二次场张量矩阵,基于均匀大地模型正演得到9种线圈架构接收线圈接收到的一次场及二次场,并以微分的方式定量地分析了9种线圈架构收发线圈位置及姿态角变化引起几何误差的变化特征。对9种线圈架构的几何误差分析可知:在几何形变引入的一次场误差中,仅有PERyz、PERzy线圈架构不包含位置变化的一次项误差,仅有VCA、VCP及HCP线圈架构不包含姿态角变化的一次项误差;在几何形变引入的二次场误差中,PERzx、PERxz线圈架构最大,而VCA、VCP及HCP线圈架构受高度及收发距变化的影响很大,需要实时测量并进行校正,VCP线圈架构受收发距变化以及姿态角变化的影响最小。     

基于新的差分结构的时-空域高阶有限差分波动方程数值模拟方法

传统的高阶有限差分波动方程数值模拟方法采用高阶差分算子近似空间偏导数,能有效抑制空间频散.然而,传统的有限差分法仅采用二阶差分算子近似时间偏导数,这使得地震波场沿时间外推的精度较低.当采用较大的时间采样间隔,传统的有限差分法模拟波场会出现明显的时间频散,甚至不稳定.本文基于新的差分结构和中心网格剖分,发展了一种空间任意偶数阶精度、时间四阶和六阶精度的时空域有限差分方法.基于对离散后的频散关系进行泰勒展开,本文推导了时空域高阶有限差分算子的差分系数.相速度分析表明时间四阶、六阶精度的差分方法能显著地减小传统时间二阶精度差分方法的时间频散.在相同的精度下与传统差分法比较,本文发展的时间四阶、六阶有限差分方法的计算效率比传统方法高.均匀和非匀均介质中的波场数值模拟实验进一步证实本文研究的时空高阶有限差分方法的优越性.     

适用于声波方程数值模拟的时间-空间域隐式有限差分算子优化方法

声波方程数值模拟已广泛应用于理论地震计算,同时构成了地震逆时偏移成像技术的基础.对于有限差分法而言,在满足一定的稳定性条件时,普遍存在着因网格化而形成的数值频散效应.如何有效地缓解或压制数值频散是有限差分方法研究的关键所在.有限差分格式分为显式有限差分和隐式有限差分.隐式有限差分能够进一步压制数值频散效应.因此本文提出了给定频率范围满足时间-空间域隐式有限差分频散关系的方法,并根据震源频率、波速和网格间距确定波数范围,在此基础上建立方程确定了相应的隐式有限差分系数,使得差分系数能在更大频率范围符合波场传播规律.通过频散分析和正演模拟,验证了本文方法的有效性.     

频率慢度域自适应迭代反演算法压制海上倾斜缆鬼波方法及其应用

海上倾斜缆采集技术具有多样的陷波特征,通过去鬼波处理可获得宽频数据.针对海水面波浪起伏及缆深误差引起的鬼波延迟时间估计误差以及崎岖海底和目的层深度变化使得鬼波和一次反射波的振幅差异系数随偏移距的变化而难以给定一个固定值的问题,本文推导出频率慢度域中鬼波滤波算子以及自适应迭代反演求解上行波算法,该鬼波滤波算子与不同水平慢度对应的鬼波和一次反射波的振幅差异系数以及鬼波延迟时间有关.并基于计算出的理论下行波与实际下行波之间的平方误差最小理论实现自适应反演迭代最优计算该振幅差异系数和鬼波延迟时间.合成的及某海上采集的倾斜缆数据去鬼波处理结果表明,频率慢度域自适应迭代反演算法能较好地去除海上变深度缆鬼波,能达到拓宽地震记录频带目的.     

角度域剩余深度对剩余速度的敏感性分析

本文针对地震勘探深度域偏移速度建模研究,利用角度域共成像点道集(ADCIGS)建立了以剩余速度为自变量,剩余深度为目标函数的关系式,及目标函数的梯度公式.利用导出的两个公式分别对剩余深度与剩余速度的关系进行了定量分析.通过理论分析和模型试算证明初始速度模型的误差具有方向敏感性,即正误差较负误差对速度建模迭代收敛更敏感.利用此结论进行深度域速度建模既可以提高计算效率也可以提高建模精度.     

Sharp spatially constrained inversion with applications to transient electromagnetic data

Time‐domain electromagnetic data are conveniently inverted by using smoothly varying 1D models with fixed vertical discretization. The vertical smoothness of the obtained models stems from the application of Occam‐type regularization constraints, which are meant to address the ill‐posedness of the problem. An important side effect of such regularization, however, is that horizontal layer boundaries can no longer be accurately reproduced as the model is required to be smooth. This issue can be overcome by inverting for fewer layers with variable thicknesses; nevertheless, to decide on a particular and constant number of layers for the parameterization of a large survey inversion can be equally problematic. Here, we present a focusing regularization technique to obtain the best of both methodologies. The new focusing approach allows for accurate reconstruction of resistivity distributions using a fixed vertical discretization while preserving the capability to reproduce horizontal boundaries. The formulation is flexible and can be coupled with traditional lateral/spatial smoothness constraints in order to resolve interfaces in stratified soils with no additional hypothesis about the number of layers. The method relies on minimizing the number of layers of non‐vanishing resistivity gradient, instead of minimizing the norm of the model variation itself. This approach ensures that the results are consistent with the measured data while favouring, at the same time, the retrieval of horizontal abrupt changes. In addition, the focusing regularization can also be applied in the horizontal direction in order to promote the reconstruction of lateral boundaries such as faults. We present the theoretical framework of our regularization methodology and illustrate its capabilities by means of both synthetic and field data sets. We further demonstrate how the concept has been integrated in our existing spatially constrained inversion formalism and show its application to large‐scale time‐domain electromagnetic data inversions.     

混合ADI-FDTD亚网格技术在探地雷达频散媒质中的高效正演

提出混合ADI-FDTD亚网格技术开展频散介质GPR正演,即在物性参数变化剧烈局部区域采用细网格剖分ADI-FDTD计算,其他的区域采用粗网格剖分常规FDTD计算,ADI-FDTD突破了CFL条件的限制,可选取与粗网格一致的大时间步长,有效地提高了计算效率.本文首先基于Debye方程,推导了粗网格FDTD及细网格ADI-FDTD频散介质差分格式,着重对粗细两种网格结合的场值交换方式进行了深入探讨,给出了该算法的计算流程.然后以一个薄层模型为例,分别应用粗网格、细网格、混合ADI-FDTD亚网格算法对该模型进行正演,计算资源的占用及模拟精度说明了混合ADI-FDTD亚网格算法的优势.最后,建立频散介质与非频散介质的组合模型,应用3种方法对该模型进行正演,对比3种方法优劣,分析雷达剖面中非频散介质及频散介质中波形特征,有效地指导雷达资料的精确解释.     

地震记录的时间域反褶积、仿真及在地震计方位角相对测量中的应用

介绍了一种用冲击不变法从速度响应平坦地震计二阶传递函数设计得到的递归滤波方法,比用双线性变换得到的递归滤波系数少,因此使用更方便.作者用此方法作了反褶积与仿真的数值实验和实际地震记录的短周期仿真试验,仿真结果与实际仪器记录符合较好;并将此法应用于地震计方位角的相关分析法相对测量,结果与不仿真的相关分析测量相比更接近寻北仪的绝对测量结果.