The Effect of an Ocean on Magnetic Diurnal Variations

Summary. Theoretical calculations have been made for the idealized case in which an ocean is approximated by an infinitely long rectangular conducting sheet. The results indicate that even in the case of magnetic variations with periods as long as 24 hr one should expect a significant and easily observable enhancement in the vicinity of the edge of an ocean. When the inducing field is a function of local time only, it is found that the amplification of the variations is greater at the eastern edge of the ocean than at the western edge.
Laboratory model studies have been used to investigate the effect of irregularities in the shape of a coastline. Results which have been obtained from a model of the Pacific Ocean in the neighbourhood of Japan indicate that it is reasonable to attribute at least a part of the anomalous diurnal variations observed in central Japan to electric currents induced in the ocean.     

H-polarization induction over an ocean edge coupled to the mantle by a conducting crust

Summary. The Wiener—Hopf technique is used to obtain an exact analytical solution for the problem of H -polarization induction over the edge of a perfectly conducting thin sheet, representing an ocean, electrically connected to a perfectly conducting mantle through a slab of finite conductivity and thickness, which represents the Earths crust. It is shown that the induced currents resulting from this type of induction process are drawn up into the sea from the cust and mantle with the greatest concentration of current near the ocean edge. The surface impedance over the land surface, is calculated for various mantle depths and is shown to increase sharply as the coastline is approached. The magnetic field along the ocean floor is also plotted as a function of distance from the coastline, and the results are found to agree very well with those calculated previously by approximate and numerical methods.     

Electromagnetic response of an ocean-coast model to E-polarization induction

summary . An ocean-coast model which consists of a uniformly conducting half-space screened by a perfectly conducting half-plane (the model ocean) is studied. On the land the electric field decreases continuously to zero as the coast is approached. The horizontal magnetic field component is found to vary rapidly, but remains finite; the vertical component on the other hand, increases to infinity at the coast. On the surface of the model ocean as well as on the sea floor, electric field and vertical magnetic field are both nil, but the horizontal magnetic field becomes singular as the seashore is approached. This horizontal magnetic field however, is different on the sea floor and at the ocean surface, because the integrated ocean current is finite, even growing to infinity as the shore is approached. The very large ocean currents near the shore act as an extremely long line antenna, which radiates far afield. This antenna feature explains the very long range of the ocean-coast effects observed under E -polarization induction, compared to the corresponding H -polarization effects where no such antenna-like feature occurs. A similarly large difference of ranges can be expected for all shallow structures with large lateral conductivity contrasts. The present study may therefore be of some interest in relation to geomagnetic depth soundings by the inductive and magnetotelluric methods, as well as in understanding the ocean-coast effect known for some time from records of coastal observatories.     

Electromagnetic induction fields in the deep ocean north-east of Hawaii: implications for mantle conductivity and source fields

Summary. We have analysed a thirty-six day recording of the natural electric and magnetic field variations obtained on the deep ocean floor north-east of Hawaii. The electromagnetic fields are dominated by tides which have an appreciable oceanic component, especially in the east electric and north magnetic components. The techniques of data analysis included singular value decomposition (SVD) to remove uncorrelated noise. There are three degrees of freedom in the data set for periods longer than five hours, indicating a correlation of the vertical magnetic field and the horizontal components, suggesting source field inhomogeneity. Tensor response functions were calculated using spectral band averaging with both SVD and least squares techniques and rotated to the principal direction. One diagonal component, determined mainly by the north electric and east magnetic fields, is not interpretable as a one-dimensional induction phenomenon. The other diagonal term of the response function indicates a rapid rise in conductivity to 0.05 mho m⁻¹ near 160 km. No decrease in conductivity below this depth is resolvable. Polarization analysis of the magnetic field indicates moving source fields with a wavelength near 5000 km. Model studies suggest that the two dimensionality in the response function may be caused by motion in the ionospheric current system.     

Induction in arbitrarily shaped oceans — IV. Sq for a simple case

Summary. World curves are presented of electric currents induced in electrically insulated oceans by Sq using a perfectly conducting sphere to simulate the underlying mantle. The curves are valid everywhere away from those coastal regions at which the sea bed rises steeply. In such cases an edge correction may be needed. The numerical computations are for periods of 6, 8, 12 and 24 hr and comparison is made with results obtained by others for a 24-hr period. Circulation conditions around the island are ignored in this note.     

The magnetic field of an infinite plane current sheet uniform except for two circular insertions of different uniform conductivities

Summary. The electrical system of currents excited by a uniform electric field of arbitrary direction in an infinite plane sheet of uniform conductivity except for two non-overlapping circular areas is obtained analytically. Using the method introduced by Ashour, the magnetic field of the system is also obtained. The components of this additional field are expressed as line integrals which are suitable for computation. The results reduce, in the special case of one insertion, to those obtained earlier by Ashour & Chapman.
As an illustration, numerical results are obtained for the special case of two equal insertions of zero conductivity.
The analysis and results obtained are useful in estimating the modification of the currents flowing in an ocean and their magnetic field by two islands.     

Electric field at the seafloor due to a two-dimensional ionospheric current

The relation between the seafloor electric field and the surface magnetic field is studied. It is assumed that the fields are created by a 2-D ionospheric current distribution resulting in the E-polarization. The layered earth below the sea water is characterized by a surface impedance. The electric field at the seafloor can be expressed either as an inverse Fourier transform integral over the wavenumber or as a spatial convolution integral. In both integrals the surface magnetic field is multiplied by a function that depends on the depth and conductivity of the sea water and on the properties of the basement. The fact that surface magnetic data are usually available on land, not at the sea surface, is also considered. Test computations demonstrate that the numerical inaccuracies involved in the convolution method are negligible. The theoretical equations are applied to calculate the seafloor electric fields due to an ionospheric line current or associated with real magnetic data collected by the IMAGE magnetometer array in northern Europe. Two different sea depths are considered: 100 m (the continental shelf) and 5 km (the deep ocean). It is seen that the dependence of the electric field on the oscillation period is weaker in the 5 km case than for 100 m.     

Three-dimensional topography corrections of magnetotelluric data

Topographic effects due to irregular surface terrain may prevent accurate interpretation of magnetotelluric (MT) data. Three-dimensional (3-D) topographic effects have been investigated for a trapezoidal hill model using an edge finite-element method. The 3-D topography generates significant MT anomalies, and has both galvanic and inductive effects in any polarization. This paper presents two different correction algorithms, which are applied to the impedance tensor and to both electric and magnetic fields, respectively, to reduce topographic effects on MT data. The correction procedures using a homogeneous background resistivity derived from a simple averaging method effectively decrease distortions caused by surface topography, and improve the quality of subsurface interpretation. Nonlinear least-squares inversion of topography-corrected data successfully recovers most of structures including a conductive or resistive dyke.     

Three-dimensional gravity inversion for Moho depth at rifted continental margins incorporating a lithosphere thermal gravity anomaly correction

This paper describes a method for determining Moho depth, lithosphere thinning factor (γ= 1 − 1/β) and the location of the ocean–continent transition at rifted continental margins using 3-D gravity inversion which includes a correction for the large negative lithosphere thermal gravity anomaly within continental margin lithosphere. The lateral density changes caused by the elevated geotherm in thinned continental margin and adjacent ocean basin lithosphere produce a significant lithosphere thermal gravity anomaly which may be in excess of −100 mGal, and for which a correction must be made in order to determine Moho depth accurately from gravity inversion. We describe a method of iteratively calculating the lithosphere thermal gravity anomaly using a lithosphere thermal model to give the present-day temperature field from which we calculate the lithosphere thermal density and gravity anomalies. For continental margin lithosphere, the lithosphere thermal perturbation is calculated from the lithosphere thinning factor (γ= 1 − 1/β) obtained from crustal thinning determined by gravity inversion and breakup age for thermal re-equilibration time. For oceanic lithosphere, the lithosphere thermal model used to predict the lithosphere thermal gravity anomaly may be conditioned using ocean isochrons from plate reconstruction models to provide the age and location of oceanic lithosphere. A correction is made for crustal melt addition due to decompression melting during continental breakup and seafloor spreading. We investigate the sensitivity of the lithosphere thermal gravity anomaly and the predicted Moho depth from gravity inversion at continental rifted margins to the methods used to calculate and condition the lithosphere thermal model using both synthetic models and examples from the North Atlantic.     

The effect of polar wander on the tides of a hemispherical ocean

Summary. A numerical model is constructed of the tides in a hemispherical ocean driven by the forces corresponding to the Y₂^–2 equilibrium tide. The model is used to study how tidal dissipation is affected by changes in the position of the ocean relative to the Earth's rotational axis and to test a hypothesis concerning the Gerstenkorn event.
As the position of the Earth's axis is varied with respect to the ocean, the model shows changes in the dissipation rate due to the changing position and importance of individual resonances of the ocean. However, a cooperative effect is also observed which results, for an ocean of depth 4400 m, in broad frequency bands near 10 rad day⁻¹ and-6 rad day⁻¹ in which the dissipation rate remains high.
The cooperative effect is found to arise from the existence, in an unbounded ocean, of resonances at these frequencies which match the tidal forces. When ocean boundaries are introduced, the new resonances near these frequencies contain a large component of the underlying resonance and as a result are themselves a good match to the driving forces.
For the real ocean, these findings imply that changes in the position of the pole, and also possibly changes in the shape of the ocean, will on average have little effect on the energy dissipated by the tides. However in the past changes in the mean depth and area of the ocean or the increased rotation rate of the Earth may have resulted in a smaller dissipation rate.     

Electromagnetic induction over the edge of a perfectly conducting ocean: theH-polarization case

Summary. Using the Wiener—Hopf technique, a solution is obtained for the problem of H -polarization electromagnetic induction at an ocean edge. The ocean is represented by a perfectly conducting thin sheet. The implications of the solution for magnetotelluric and geomagnetic sounding are discussed.     

南极中山站固体潮观测分析

中国南极中山站位于南极拉斯曼丘陵地区（λ＝７６°２２′Ｅ，φ＝６９°２２′Ｓ），１９９１年３月至１９９３年２月，用ＬａｃｏｓｔｅＥＴ型重力仪，在这个地区进行了为期近两年的观测，取得了６００多天的有效资料，经调和分析和流变模型、海潮负荷改正后得到：δｏ１＝１．２２５３±０．００３１，Δφｏ１＝０．７３°±０．１４°；δｍ２＝１．０７８５±０．００４５，Δφｍ２＝－３．５５°±０．２９°从资料分析的精度来看，观测资料是比较好的；但由于海潮负荷改正缺少附近海域的资料，会对最终结果造成较大的影响     

中国海陆过渡带——岸海洋环境特征与变化研究

海岸海洋是陆地与海洋过渡的地带,位居亚洲与太平洋之交的中国海岸类型丰富,河海交互作用与人类活动影响使海岸海洋环境与作用过程独具特色.河流的贯通作用,使得全球变化与人类活动对河流流域的作用,亦在海岸海洋地区表现出来,并对海洋环境形成显著的影响.历史与现实实例表明,在海岸海洋的开发利用中需重视研究海陆交互过渡带环境特征,尽...     

Three-dimensional induction in a non-uniform thin sheet at the surface of a uniformly conducting earth

Summary. A new method for solving problems in three-dimensional electromagnetic induction in which the Earth is represented by a uniformly conducting half-space overlain by a surface layer of variable conductance is presented. Unlike previous treatments of this type of problem the method does not require the fields to be separated into their normal and anomalous parts, nor is it necessary to assume that the anomalous region is surrounded by a uniform structure; the model may approach either an E- or a B -polarization configuration at infinity. The solution is expressed as a vector integral equation in the horizontal electric field at the surface. The kernel of the integral is a Green's tensor which is expressed in terms of elementary functions that are independent of the conductance. The method is applied to an illustrative model representing an island near a bent coastline which extends to infinity in perpendicular directions.     

塔克拉玛干沙漠的流场特征与风沙活动强度的关系

近年来, 人们对于塔克拉玛干沙漠的风沙活动方向与活动强度, 以及由此而造成的沙漠化的严重程度争议较大, 特别是在沙漠的南缘和西南缘。本文利用沙漠周围气象站的风向和风速自记记录, 计算了最大可能输沙强度及其合成方向, 并结合该地区不同高度的高空流场资料, 对塔克拉玛干沙漠的风沙活动方向与活动强度进行分析研究, 结果表明: 最大可能输沙强度及其合成方向与沙丘移动方向和强度相当吻合。合成输沙强度的水平梯度是沙漠的形成、分布与演变的充分而必要的条件。特别是对高大复合型的沙丘的形成提供了有说服力的科学依据。     

UT variation of internal Sq currents and the oceanic effect during 1980 March 1–18

Summary. UT variation of the internal part of S _q currents is examined using the geomagnetic data during 1980 March 1–18, and the effect of the ocean is found. The ratio of the internal currents to the external increases when the external current vortex comes above the Atlantic and Pacific Oceans, and the internal current vortex shifts to the oceans when the external vortex approaches the edge of the oceans. The existence of the ocean increases the total induced current by about 30 per cent and this amount is consistent with model calculations for S _q by previous workers.     

尘卷风结构特征及其电场数值模拟研究

尘卷风是地球上常见的小型风沙灾害输移系统,但在火星上却大的多。而且尘卷风内部的电场对火星探测器产生严重的电磁干扰。通过建立尘卷风及其电场形成的模型,对尘卷风结构特征及电场进行数值计算。研究表明：尘卷风的形成机理可以用热对流泡理论来解释。沙粒在尘卷风中出现分层现象,粒径小的沙尘往往在粒径大的沙尘上面。尘卷风中带正负电荷的沙粒大约各占23.4%,荷质比大约为60 μC·kg^-1时,尘卷风数值模拟结果与野外观测值吻合。在尘卷风发展过程中,尘卷风电场大约需要60 s达到稳定。而且电场关于尘卷风中心基本对称,并且在尘卷风中心电场强度较大,在离尘卷风较远的地方,电场趋于零。在距离尘卷风中心一定距离处电场随高度增大先增大后减小,大约10 m以下电场随高度增大而增大,在10 m以上电场随高度增大而减小。     

金字塔沙丘流场的三维数值模拟

对位于敦煌莫高窟顶部鸣沙山东麓的金字塔沙丘流场形态进行数值模拟,分析了多风向条件下沙丘的压力场和速度场变化规律。使用地面立体摄影技术获取沙丘的控制点坐标,建立了与实地沙丘等比例的三维模型,以解决二维建模不能表达3组主风向差异的问题。使用计算流体动力学（CFD）方法,采用非结构网格划分技术以适应沙丘的高度不规则地形,通过剪切压力传输模型（SST）分析计算并与以往风洞试验和数值模拟结果进行对比。模型很好地预测了沙丘底部停滞区、坡上加速区、背风坡回流区以及沙丘表面压力和风速分布等流场信息,反映了多风向条件下沙丘形态与气流的交互作用。流场中速度场的分布与压力分布直接相关。     

海洋本体论及其视角下的南海海洋国土治理研究

长期以来海洋位于社会边缘,海洋治理不可避免地陷入陆地中心主义。本文在批判海洋国土治理背后的陆地本体及其领土化逻辑的基础上,将海洋重新概念化为体积空间与动态时间邂逅的四维时空,指出海洋具有多维性、流动性、非线性和非客体的特点,并以此构建了以体积国土、流动国土、能动国土为特征的海洋国土分析框架,进而将其运用于南海海洋国土治理的理论和政策认知,重构了长期桎梏于陆地规范的“时间、空间和运动”观念,回应了文化和政治地理学的新唯物主义取向。研究认为：① 在南海的体积管控上,岛礁扩建是有效的策略。岛礁扩建使体积海洋折叠为二维领土,以偏平化控制三维空间,改变了南海的地理和空间性质;② 在南海的流动性管控上,因海洋的流体、尺度和深度特征,以及法律边界模糊性,加剧了维护南海海上安全的复杂性,需要考虑与之匹配的管控非法流动性的监管策略;③ 在南海的能动管控上,需要关注渔民独特的生态伦理思想,海洋保护需要考虑不同本体间的对话和信息转译,使保护行为更加紧密地融入到本土社会文化情境中。本文为海洋国土治理提供了来自政治地理学的批判性视角,同时对人文地理学的人海关系研究亦是积极的推动。     

基于层次格网索引的图幅接边处等高线高程错误识别和自动修正研究

等高线蕴含的历史高程信息可有效延长地形研究的时间序列,有利于深入挖掘地形变化长期规律,然而,图幅接边处的高程属性错误降低了等高线的数据质量,制约着等高线高程信息的实际应用。针对这一问题,该文提出一种基于层次格网索引的图幅接边处等高线高程错误识别和自动修正方法:首先,将层次格网索引与方向性二邻域算法相结合,以减少数据重复计算;然后,利用等高线空间位置标签及快速排序算法构建强空间位置关系,解决图幅接边处等高线匹配的准确性问题;最后,以高程冲突位点为驱动因子进行逻辑判断,实现等高线高程错误的识别及自动修正。实验结果表明:该方法运算效率较未进行效率优化时提高了203倍,接边处等高线高程错误识别与修正精度的最大值分别达97.71%和91.40%;相较于现有方法,该方法在精度和效率方面表现更佳,对区域性错误和变形等高线具有更高的适用性。