The offset‐midpoint travel‐time pyramid for P‐wave in 2D transversely isotropic media with a tilted symmetry axis

For pre‐stack phase‐shift migration in homogeneous isotropic media, the offset‐midpoint travel time is represented by the double‐square‐root equation. The travel time as a function of offset and midpoint resembles the shape of Cheops’ pyramid. This is also valid for transversely isotropic media with a vertical symmetry axis. In this study, we extend the offset‐midpoint travel‐time pyramid to the case of 2D transversely isotropic media with a tilted symmetry axis. The P‐wave analytical travel‐time pyramid is derived under the assumption of weak anelliptical property of the tilted transverse isotropy media. The travel‐time equation for the dip‐constrained transversely isotropic model is obtained from the depth‐domain travel‐time pyramid. The potential applications of the derived offset‐midpoint travel‐time equation include pre‐stack Kirchhoff migration, anisotropic parameter estimation, and travel‐time calculation in transversely isotropic media with a tilted symmetry axis.     

τ-p深度偏移

本文介绍一种基于τ-P变换的新的叠后深度偏移方法。这一方法是在均匀介质条件下根据波动方程推导出的。其基本思想是:在τ-p域中,当波场分解为它的平面波成分时,改变其τ、p值,再通过τ-p反变换回到深度域中,从而完成了地下界面的偏移归位。 方法所需的计算时间基本是与完成τ-p正、反变换所需时间相同,是一种快速的偏移方法。它可用于任意形状的地下界面,甚至在强干扰背景的条件下,也能提高资料的信噪比和改善信号的连续性。这一方法是在深度域中进行τ-p变换,故它能适应速度存在纵、横变化的地区。为了论证本方法的效果,本文给出了不同的正演理论模型和偏移结果的例子。     

Migration and urban survival strategies in Windhoek, Namibia

Based on recent research in Windhoek, this paper demonstrates that it is the well established and widespread social linkages that persist between rural and urban households that are central to the ability of poor urban households to survive. Whereas informal coping mechanisms that include borrowing, piecework and credit are pervasive in rural areas of Namibia, intra-urban sources of food are poorly developed, and outside of kinship circles, social networks within Windhoek are used sparingly, even in times of greatest need. In addition, urban agriculture plays a very limited role at present in urban food supply. The research demonstrates that urban households that are most vulnerable to hunger are those that have limited social connections to the rural areas, and must rely on intra-urban opportunities to get food (including borrowing, begging, piecework and crime). The most vulnerable are female- and male-headed households with no access to farm income and limited/irregular urban incomes. In contrast, those with active rural-urban linkages enjoy significant transfers of food from rural areas that offset hunger and vulnerability in the urban context. Urban-rural reciprocity is therefore not only a one-way movement of people and resources from the urban to the rural areas, but also a transfer of food from rural to urban households.     

环太平洋大震的时空特征分析

本文基于聚类分析方法和聚类树图, 对1890年以来环太平洋周边7.5级以上大震的时间间隔特征进行分析, 发现起始震级低于8.0级的地震分类特征一致, 而起始震级在8.0级以上地震分类特征比较详细: 在初步分为地震活跃期和地震平静期时, 在地震活跃期内, 又可细分为地震活动相对高活跃期(时间间隔1年左右)和地震活动相对低活跃期(时间间隔6年左右)。 进而, 以8.0级以上地震最后一个活跃期为例, 揭示了在地震活跃期内, 地震的空间迁移表现出逆时针旋转的特征。     

秋季大气质量海陆间迁移与南北涛动年际变化不同位相配置及其对中国气温异常的影响

利用NCEP/NCAR逐月再分析资料,通过构建秋季欧亚大陆-太平洋区域的大气质量海陆间迁移(MAMLO)指数和大气质量南北涛动(IHO)指数,对大气质量海陆间迁移和南北涛动的不同位相配置下环流特征及其对中国秋季气温的影响进行了研究。结果表明,大气质量海陆间迁移与南北涛动指数相关系数仅为-0.06,二者在统计学意义上相互独立。二者存在4种位相配置类型,即大气质量海陆间迁移和南北涛动均为正位相(第Ⅰ类配置);大气质量海陆间迁移和南北涛动均为负位相(第Ⅱ类配置);大气质量海陆间迁移为正位相而南北涛动为负位相(第Ⅲ类配置);大气质量海陆间迁移为负位相而南北涛动为正位相(第Ⅳ类配置)。大气质量海陆间迁移和南北涛动呈现不同位相配置类型时,大气质量的重新分布造成了不同的地表气压场分布并显著地影响到了中国北方地区秋季气温异常分布。当二者为第Ⅰ类配置时,南北涛动削弱北太平洋地区大气质量的负异常,减弱了该区域的异常低压,增强欧亚地区大气质量正异常并加强了欧亚地区的地面高压环流,共同造成中国秋季气温异常成东西反位相型分布,东部暖而西部冷;当二者为第Ⅱ类配置时,南北涛动削弱欧亚大陆大气质量的正异常,增强北太平洋地区大气质量负异常。此时大气质量海陆间迁移和南北涛动通过影响地表风场使得气温异常成全国一致型分布,全区偏暖;而第Ⅲ类配置与第Ⅰ类配置对气温的影响类似,但为东部冷而西部暖,第Ⅳ类配置与第Ⅱ类配置的影响类似。这些表明大气质量海陆间迁移对秋季欧亚地区气温异常起到主要作用,而南北涛动则在大气质量海陆间迁移影响中国北方气温异常分布中起到干扰作用。这些结果对深刻认识秋季大气环流变化机理及中国北方乃至欧亚地区秋季气温异常具有重要意义。     

基于小振幅波方程的起伏海面构建及其对地震波场传播特性的影响

海面起伏状态对波场传播以及模型空间成像有着非常重要的影响, 在常规海洋地震数据采集和处理中, 都是将海面作为水平界面处理, 实际海洋地震数据采集中, 由于海水受风浪、洋流等的影响, 海面呈现高低起伏状态, 这样来自底层反射界面的反射波经过海水起伏界面反射后会引起波场的强烈散射, 从而造成地震记录反射波同相轴抖动和畸变, 最终影响成像效果.为了更加深入了解起伏海面存在对波场传播以及偏移成像的影响, 本文首先基于流体力学小振幅波方程, 从边界条件出发, 给出包含海浪谱信息的动态边界条件, 并在此基础上构建起伏海面速度模型; 然后在构建的起伏海面速度模型上通过冻结不同时刻海面开展波场传播特征分析, 研究起伏海面在不同冻结时刻下波场传播规律以及对反射地震记录和偏移成像的影响.研究结果表明, 不管是对于简单的层状介质模型还是海底较为复杂的大水深模型, 海面不同形态的起伏变化直接影响着波场记录以及波场快照形态, 同时会对偏移成像结果造成较大影响, 随着海水深度的增加, 该影响对于简单模型影响逐渐变小, 但对于复杂模型, 影响依然很严重.

     

基于目标区的局部角度域方向照明分析及其在偏移成像中的应用

叠前深度偏移已成为近年来地震偏移成像领域广泛应用的技术之一,其中复杂盐丘下方目标区域结构的成像始终是偏移成像中较难解决的问题.局部角度域波场分解可以提供在空间和方向上的双重局部化信息,因而这一技术被广泛地应用在方向照明分析、成像振幅校正等方面.本文在采用局部指数标架小波束进行角度域方向照明分析的基础上,研究采集系统对复杂盐丘下部层状结构成像质量的影响;同时通过分析目标区对应于相对地表采集系统分布的照明能量,确定叠前炮集数据中对盐下目标区成像起重要贡献的部分,并将这些数据用来对目标区成像,从而达到提高盐下结构成像质量的目的.本文将二维SEG-EAGE盐丘模型中盐下反射体作为目标区,分别计算其照明分析和由照明能量分布确定的炮集数据对目标区的偏移成像结果,同时通过与成像振幅校正前后的成像结果对比,说明了该方法对提高目标区结构成像质量的有效性.     

探地雷达小波域三维波动方程偏移

阐述了矩阵多分辨分析理论中的标准形式与非标准形式,并以Hilbert算子为例,说明了算子多分辨表示的压缩效果,为小波域偏移算法奠定了理论基础.从三维雷达波动方程出发,利用爆炸反射原理和浮动坐标变换,推导出三维探地雷达波动方程差分格式,并通过方程分裂算法及小波多分辨算法,在小波域求解波场外推矩阵,进而得到探地雷达小波域三维波动方程偏移算法,在此基础上,开发了探地雷达小波域偏移处理程序,并把该程序应用于三个球体空洞的3-D正演结果及实际的雷达数据中,通过对比偏移处理前后的雷达资料,得知该三维偏移算法能使3-D正演剖面中的反射波归位、绕射波收敛,极大地提高了雷达剖面的分辨率,有利于探地雷达资料的地质解释.     

拟空间域弹性波方程交错网格有限差分逆时偏移

弹性波逆时偏移是一种高精度的复杂构造地震成像方法.然而,在传统的基于矩形网格离散化的逆时偏移中,介质界面通常会产生畸变.另外,因使用双程波动方程进行波场延拓,其产生的反射波会在成像过程中产生偏移假象.为解决这些问题,本文提出了一种拟空间域弹性波方程高阶交错网格有限差分格式,并给出了差分格式的稳定性条件,进而实现了高精度的拟空间域弹性波方程有限差分逆时偏移.模型实验表明,若在计算拟空间域采样间隔时引入速度界面信息,则拟空间域弹性波方程高阶交错网格有限差分逆时偏移能够避免常规弹性波方程逆时偏移中弯曲界面形态畸变问题;此外基于该方法进行波场延拓时可有效压制弯曲界面的假散射现象,并能有效压制层间反射波,因此可以减少剖面上的偏移假象,从而显著提高成像的质量.

     

A Review of Some Environmental Issues Affecting Marine Mining

This article reviews information recently available from existing marine and coastal mining for responses to environmental issues affecting marine mining at different depths. It is particularly but not exclusively concerned with those issues affecting seabed biodiversity impact and recovery. Much information has been gathered in the past 10 years from shallow mining operations for construction aggregate, diamonds, and gold, from coastal mines discharging tailings to shallow and deep water, and from experimental deep mining tests. The responses to issues identified are summarized in a series of eight tables intended to facilitate site-specific consideration. Since impacts can spread widely in the surface mixing layer SML, and can affect the biologically productive euphotic zone, the main issues considered arise from the depth of mining relative to the SML of the sea. Where mining is below the SML, the issue is whether it is environmentally better to bring the extraction products to the surface vessel for processing (and waste discharge), or to process the extraction products as much as possible on the seabed. Responses to the issues need to be sitespecific, and dependent on adequate preoperational environmental impact and recovery prediction. For deep tailings disposal from a surface vessel, there are four important environmental unknowns: (1) the possible growth of "marine snow" (bacterial flocs) utilizing the enormous quantities of fine tailings particles (hundreds or thousands of metric tons per day) as nuclei for growth, (2) the possibility that local keystone plankton and nekton species may migrate diurnally down to and beyond the depth of deep discharge and hence be subjected to tailings impact at depth, (3) the burrow-up capability of deep benthos and their ability to survive high rates of tailings deposition, and (4) the pattern and rate of dispersion of a tailings density current through the deep water column from discharge point to seabed. Actions to obtain relevant information in general and site-specifically are suggested.