A numerical method to calculate the electromagnetic field of a horizontal current dipole

Summary A computer program is described to calculate the field of an oscillating current dipole over a horizontally stratified medium for the quasi-static case. Multi-layer master curves for frequency sounding can be calculated with this program. Examples of these curves are presented.Contribution No. 90 of the Geophysical Institute, Swiss Federal Institute of Technology, Zürich.     

电磁测深方法研究

为能够实施电磁测深新方法的基本原理,并为研制探测仪器提供一些依据,分析和讨论了源信号为尖脉冲时,入射波与地层反射波通过二阶带通滤波器的输出波形;利用反射波与入射波前沿瞬态叠加的波形,反演地层的电导率和厚度的方法及与探测仪器有关的主要问题等内容. 结果表明,反演仍能给出惟一解,并具有可实践性.     

高分辨电磁法在B隧道勘查中的应用

本文介绍了在B隧道勘查中应用高分辨电磁成像技术探测隐伏构造的一个成功实例。数据采集采用高频电磁成像系统（STRATAGEM EH4，频率范围从1Hz到90kHz）。野外采集正交的电磁场分量，从地磁成像剖面中提取目标体有关电磁学信息。为获取高质量的采集数据，野外采用包含天然场源和全张量可控源的混合场源。B隧道坐落在位于中国中部的湖北省的西部，隧道埋深不到200m，但区域地质作用导致它的地质机构十分复杂。第一次勘查过程中，勘探人员误把一个脱落体的露头当成基岩面，施工过程中出现了冒顶。第二次勘探时采用高分辨电磁法和折射地震法，这次勘探找到了隐伏的基岩面和一个隐伏断裂。勘探结果与后来隧道挖掘揭示的构造吻合。     

A calibrated,non-invasive method for measuring the internal interface height field at high resolution in the rotating,two-layer annulus

We describe a remote sensing method for measuring the internal interface height field in a rotating, two-layer annulus laboratory experiment. The method is non-invasive, avoiding the possibility of an interaction between the flow and the measurement device. The height fields retrieved are accurate and highly resolved in both space and time. The technique is based on a flow visualization method developed by previous workers, and relies upon the optical rotation properties of the working liquids. The previous methods returned only qualitative interface maps, however. In the present study, a technique is developed for deriving quantitative maps by calibrating height against the colour fields registered by a camera which views the flow from above. We use a layer-wise torque balance analysis to determine the equilibrium interface height field analytically, in order to derive the calibration curves. With the current system, viewing an annulus of outer radius 125?mm and depth 250?mm from a distance of 2?m, the inferred height fields have horizontal, vertical and temporal resolutions of up to 0.2?mm, 1?mm and 0.04?s, respectively.     

电流磁场法在厚覆盖区探测陡倾斜金矿的试验效果

覆盖区寻找隐伏矿成为当前找矿工作的热点问题,尤以寻找厚覆盖层下陡倾斜无磁性脉状金属矿难度较大.由于某些陡倾斜金属矿具有良好的导电性,并与围岩存在较大的电性差异,利用大功率场源发射系统,向地下发射不同频率电流,同时利用接收仪器在场源附近分别测量不同频率供电(或供电前后)时矿体上方磁场强度,可根据矿体内外的电流密度分布情况,判断金属矿体存在与否及其位置.在五河金矿整装勘查区开展了电流磁场法穿透低阻厚覆盖层精准探测金矿体的找矿方法试验.通过分析电流磁场异常特征,在陡倾斜脉状金矿体上方发现了极具意义的"笋状"多频电流磁场组合异常,说明电流磁场法在厚覆盖区寻找陡倾斜脉状金属矿具有较好的成果.     

Comparison of analogue model and field station EM responses on Southern Vancouver Island

Measurements of geomagnetic induction responses for a laboratory analogue model, that includes a simulation of the subducting Juan de Fuca Plate, are compared with those obtained at three sites on Vancouver Island. Good agreement between model and field responses at the central Vancouver Island site is observed over the period range 3–60 min, while at the east and west coastal sites, good agreement is achieved only for periods greater than 20 min. At shorter periods, departures of the observed responses from those of the model are possibly the result of upper crust inhomogeneities not replicated in the simulation, and the complex strait with its numerous small islands and its irregular coastlines inadequately simulated in the course model. Nevertheless, the analogue model results are consistent with the premise of a Juan de Fuca plate, underlain by the conductive asthenosphere, subducting at a shallow depth beneath Vancouver Island.     

接地导线源电磁场全域有效趋肤深度

为了更好地推进广域电磁法的发展和应用,本文以接地长导线源为例,研究了可控源电磁场全场域的有效趋肤深度.利用频域电偶极源在均匀半空间产生电磁场的闭合表达式,计算了不同电磁场分量定义的有效趋肤深度,并讨论了在不同频率、不同偏移距、不同电导率情况下,有效趋肤深度的变化特性.根据有效趋肤深度随偏移距的变化特征以及与平面波趋肤深度之间的关系,利用多项式拟合的办法在五个不同频率范围内给出了适用于全场域的有效趋肤深度快速估算公式.研究结果表明：不同分量定义的有效趋肤深度是不同的,但是它们随偏移距、频率、电导率等参数的变化趋势是类似的,而且在一定范围内都趋近于平面波趋肤深度.理论模型的研究表明,有效趋肤深度可以作为测量参数选取和数据解释工作的参考依据.     

基于动力有限差分法的典型海域自由场二维地震反应分析

以我国近海海域工程场地为研究对象,充分考虑上覆海水的自重影响,构建典型的饱和海底自由场计算模型,运用动力有限差分法开展二维地震反应分析,探讨以不同幅值的SV波、P波作为基底输入条件下上覆海水厚度对海底地震动峰值和反应谱的影响,总结上覆有水和无水场地的地震动结果差异,并分析差异产生的原因。结果表明:当基底输入SV波时,上覆有水场地海床表面峰值加速度小于上覆无水场地地表峰值加速度,海水层厚度对峰值加速度的影响可以忽略;当基底输入P波时,上覆有水场地海床表面峰值加速度大于上覆无水场地地表峰值加速度,且随着海水层厚度的增大,海床表面峰值加速度逐渐减小。      

3D anisotropic modeling and identification for airborne EM systems based on the spectral-element method

The airborne electromagnetic (AEM) method has a high sampling rate and survey flexibility. However, traditional numerical modeling approaches must use high-resolution physical grids to guarantee modeling accuracy, especially for complex geological structures such as anisotropic earth. This can lead to huge computational costs. To solve this problem, we propose a spectral-element (SE) method for 3D AEM anisotropic modeling, which combines the advantages of spectral and finite-element methods. Thus, the SE method has accuracy as high as that of the spectral method and the ability to model complex geology inherited from the finite-element method. The SE method can improve the modeling accuracy within discrete grids and reduce the dependence of modeling results on the grids. This helps achieve high-accuracy anisotropic AEM modeling. We first introduced a rotating tensor of anisotropic conductivity to Maxwell’s equations and described the electrical field via SE basis functions based on GLL interpolation polynomials. We used the Galerkin weighted residual method to establish the linear equation system for the SE method, and we took a vertical magnetic dipole as the transmission source for our AEM modeling. We then applied fourth-order SE calculations with coarse physical grids to check the accuracy of our modeling results against a 1D semi-analytical solution for an anisotropic half-space model and verified the high accuracy of the SE. Moreover, we conducted AEM modeling for different anisotropic 3D abnormal bodies using two physical grid scales and three orders of SE to obtain the convergence conditions for different anisotropic abnormal bodies. Finally, we studied the identification of anisotropy for single anisotropic abnormal bodies, anisotropic surrounding rock, and single anisotropic abnormal body embedded in an anisotropic surrounding rock. This approach will play a key role in the inversion and interpretation of AEM data collected in regions with anisotropic geology.     

基于非结构有限元的时间域海洋电磁三维反演

海洋电磁法是一种有效的钻前储层评价手段,可识别出海底构造是否储油气,从而减少干井率降低勘探成本.近年来,频率域三维海洋电磁反演得到了快速发展,但受到空气波的影响,其在浅海环境中无法取得很好的效果.为解决这一问题,本文研究时间域海洋电磁数据三维正反演.正演模拟和计算中,我们选择基于非结构网格的矢量有限元方法.该算法中空间离散采用非结构四面体网格,可很好地拟合复杂海底地形条件和地下复杂结构;而对于时间离散,我们采用无条件稳定的后推欧拉方法,以确保任意时间步长数值计算的稳定性.反演计算中,灵敏度信息采用伴随正演隐式进行计算;同时,依据时间域反演方法的特点采用L-BFGS方法计算模型修正量.通过利用合成数据反演结果证明了本文提出的三维时间域反演方法可用于复杂海底环境,特别是在浅海环境下的有效性.

     

Two and three dipoles approximating the earth's main magnetic field

Summary Two and three dipoles approximating the earth's main magnetic field are determined minimizing a sum of squares of differences between the dipole fields and the earth's magnetic field. Numerical results have been obtained by the aid of a digital computer for epochs 1932, 1942, 1950 and 1960.     

A note on the free-surface effect on the topographically induced vorticity field in a homogeneous rotating flow

Abstract

The vorticity field induced by a topography in a homogeneous rotating fluid bounded by a free-surface is investigated. It is found that the integrated strength of the topographically bound vortex is zero and that the volume of the free-surface perturbations is equal to the volume of the topography.     

直升机吊舱姿态变化对电磁场测量的影响规律及其校正方法研究

在直升机航空电磁法中,吊舱的姿态变化（如摆动、倾斜、偏航等）会使磁偶极子相对正常飞行时在方向和位置上发生变化,使得接收线圈处的电磁响应产生误差.传统的基于重叠偶极模型的姿态校正方法只考虑了磁偶极子方向上的变化,而忽略了磁偶极子位置上的变化,因此校正效果受到了一定的限制.本文分析了五种不同电性参数的均匀半空间模型中线圈系在姿态变化前后的电磁响应比,进而提出了一种新的姿态校正方法,该方法综合考虑了磁偶极子方向和位置变化对姿态变化前后的电磁响应比的影响,并推导出总姿态变化电磁响应比与方向和位置单独变化时电磁响应比的乘积近似成固定比例关系.为了验证本文提出的总姿态校正方法,本文分别使用本方案及基于重叠偶极模型的姿态校正方法对五组具有代表性的合成数据进行了校正.对比结果表明,本文的总姿态校正方法较之重叠偶极姿态校正方法能较大地提高校正精度.     

地面矩形大定源电磁法频率域三维非线性共轭梯度反演

本文实现了地面矩形大定源三维频率域反演.矩形大定源三维模型响应计算采用交错网格有限差分技术.正演的微分方程为异常电场满足的非齐次Helmholtz方程,方程右手边源项中的大定源产生的背景格林函数由虚界面法结合虚框法计算.频率域三维反演采用非线性共轭梯度反演技术.反演的数据类型为垂直磁场的频率域响应H_z的实部和虚部分量.数值结果表明,（1）三维模型正演模拟数值结果与前人一致,为三维反演奠定基础;（2）针对两个三维导电模型,分别进行了三维反演数值试算.反演结果可以清晰恢复出异常体的电阻率和位置信息,表明地面矩形大定源三维频率域非线性共轭梯度反演具有可行性.本文研究的意义在于,在电磁响应时频转换技术的基础上,如果将野外实测的瞬变电磁数据变换为对应的频率响应,则结合本文提出的三维反演技术,可以为矩形大定源瞬变电磁数据的三维解释提供一个新的思路.

     

Ground water modeling applications using the analytic element method

Though powerful and easy to use, applications of the analytic element method are not as widespread as finite-difference or finite-element models due in part to their relative youth. Although reviews that focus primarily on the mathematical development of the method have appeared in the literature, a systematic review of applications of the method is not available. An overview of the general types of applications of analytic elements in ground water modeling is provided in this paper. While not fully encompassing, the applications described here cover areas where the method has been historically applied (regional, two-dimensional steady-state models, analyses of ground water-surface water interaction, quick analyses and screening models, wellhead protection studies) as well as more recent applications (grid sensitivity analyses, estimating effective conductivity and dispersion in highly heterogeneous systems). The review of applications also illustrates areas where more method development is needed (three-dimensional and transient simulations).     

吉林油田大功率直流电场强化采油技术研究进展

针对目前多数油田存在的高含水低渗透油层特点,在电场驱替提高采收率室内研究的基础上,重点介绍了现场试验研究大功率直流电场在清除近井油层污染、提高水淹层原油产量和强化开采等方面的进展情况.结果证明：这种高效开采新技术在工艺上是可行的,具有效率高、供电设备安全性强和故障率低等特点,在增油降水提高采收率应用中前景十分广阔.     

位场数据解释的Theta-Depth法

Theta图是利用位场(重磁)数据识别边界的常用方法,其表达式为重磁异常水平变化与垂直变化的比值函数.该方法计算浅源地质体边界的效果较好,而由于深源位场数据在换算过程中会产生趋同效应,在深源地质体识别应用中计算结果不准确,为此,本文提出Theta-Depth法并进行地质体埋深的计算.首先给出直接利用Theta图像进行场源体深度估算的方法,然后推导出基于Theta导数的线性方程来自动估算场源位置参数,本文方法可有效地利用Theta图像的特征为约束条件来提高反演结果的精度.理论模型试验证明本文提出的Theta-Depth法能有效地计算出场源体位置和深度.将该方法应用于满都拉地区实测磁数据的解释,帮助圈定了矿脉的分布.     

Chaotic behavior of magnetic field lines near simplest current systems

In the context of studying the problem of simulation of magnetic fields on the Sun, the structure of the field in the vicinity of two circular current loops with different mutual arrangement in space is considered. When the symmetry in the arrangement is sufficient, a system of magnetic surfaces created by the closed field lines arises. With a reduction in symmetry, isolated closed lines may exist. For the case of two identical current loops coupled perpendicularly, it is shown that the subsystems of these lines may be ordered in space in a complex manner. At large distances, a system of loops is equivalent to a dipole with a high degree of accuracy, while an approximate winding of the lines on the deformed toroids, encircling each of the loops, occurs at small distances. At intermediate distances, there are regions of both ordered and chaotic behavior of field lines. Results were obtained with the use of the numerical simulation method.     

Acquisition of the tide-induced Lagrangian residual current field by the PIV technique in the laboratory

In this paper, a new particle image velocimetry (PIV)-based measurement method is proposed to obtain the high-resolution tide-induced Lagrangian residual current field in the laboratory. A long gravity wave was generated to simulate the tide in a narrow tank full of water laden with PIV particles. Consecutive charge-coupled device (CCD) images were recorded with the studied layer illuminated with a laser beam. Two images separated by one tidal period were processed by applying the pattern-matching algorithm to get the horizontal tide-induced Lagrangian residual current field. The results coincide with sporadic results from the traditional surface-float tracing method, but with much higher spatial resolution and accuracy. Furthermore, it is found that the direct acquisition of the Lagrangian residual current may reduce the error at least by one order compared with those acquisition methods that require the detailed information of the tidal cycle.