磁性层状介质的CSAMT电场分量响应特征

在传统的勘探电磁学理论中,往往把岩层磁导率近似看成空气中的磁导率,从而得到简化后的电磁场响应式,主要用于指导非磁性介质情况下的电磁探测.本文开展了磁性介质的电磁响应理论、数值模拟和特征分析等研究,以便实现对磁性矿体的电磁精细探测.首先推导出磁性层情况下的电磁响应表达式;然后,计算了几种典型磁性介质地质断面的响应曲线,并对其响应特征进行了分析;研究结果表明:当磁性层位于地表时,当表层相对磁导率μr1.2,磁性层对电磁测深曲线具体明显的影响;当岩层为中间层时,只要μr1.2,对H型、A型曲线中段会有影响.对H型曲线,主要表现在使极小值处的曲线抬高、变宽,以致使曲线形态发生变化;当磁性层处于最后一层时,不论岩层的磁导率是多少,对各类二、三层曲线来讲,不会影响曲线的形态,只是使渐近线前第一个极值点的位置右移,极值点至渐近线之间的线段变得较为陡峭.     

水平多层大地上垂直磁偶极频率测深的全波视电阻率

本文根据视电阻率定义的原则,以及用不同的场量定义的视电阻率效果不同这一事实,提出一种新的全波视电阻率定义.在全区同时用均匀大地上电磁场的三个分量来分区定义祝电阻率.在远区视电阻率由磁场的水平分量求出,在近区由磁场的垂直分量或其实分量定义,而在过渡区则由电场的水平分量确定.用这种方法定义的视电阻率为电磁响应的单值函数,它随频率变化的曲线显著改善,能直观地反映地层电阻率随深度的变化,数值比较接近地层的真电阻率值,假极值效应明显压低.在计算中用切比雪夫多项式分段拟合均匀大地电磁响应的反函数,并给出一套系数,由此算出的视电阻率误差小于1％.     

电磁法在工程地质勘察中的应用

本文采用有场源电磁法勘探粤东沿海广(州)深(圳)珠(海)及深(圳)汕(头)南线等高速公路工程,勘探使用美国GDP—12及加拿大EM—37电磁数据采集系统。文章论述了时间域与频率域的多种电磁法装置在桥梁、互通、遂道及软土公路工程中的地磁物理应用前提及它们的应用环境、工作效率、勘探深度等,低阻覆盖下的地质分层及断层产状性质等问题是本文探讨要点。文中例举了多种装置的电磁异常场剖面图及其曲线形态:对磨刀门桥址西江断裂的电磁异常剖面曲线形态、特征作了构造地质解释,认为该断裂系由两条主干断层组成的蠕滑断层组。上述工程地质解释结论已作为工程勘察设计依据。有的电磁异常已被工程地震及钻探证实。     

电磁监测台站观测的舒曼谐振背景变化

舒曼谐振是由闪电激发的电离层与地面之间的全球电磁震荡,在地球与电离层空腔中传播,由于共振作用,其电磁波能量明显高于其他频率电磁波能量.舒曼谐振具有稳定的频域参数和频谱结构,近几年的研究发现,地震发生前会使这些固有参数发生扰动,因此利用舒曼谐振异常来进行地震短临监测可能是一种行之有效的手段.要判断舒曼谐振地震电磁异常,了解舒曼谐振电磁场背景变化规律是前提.本文利用极低频电磁台站观测的天然电磁场数据,处理和分析了各台站观测的舒曼谐振频率电磁场功率谱随时间的变化,得到了舒曼谐振随季节和纬度的背景变化规律.并提出将舒曼谐振背景变化规律应用到地震短临监测预报中,能够为今后辨别地震监测中的舒曼谐振异常提供物理依据.     

非均匀电磁波在导电媒质界面反射的横向偏移

由电磁场边值关系给出了非均匀电磁波在导电媒质界面反射时反射系数所产生的附加相角. 利用这一附加相角导出了非均匀电磁波在导电介质界面的横向偏移,并进行了横向偏移的相关计算,绘出了横向偏移随入射角的变化曲线. 结果表明：在导电媒质界面反射电磁波的横向偏移随入射角的增大而增大,当入射角接近90°时,电磁波在界面要传播较长时间后才返回第一种媒质. 在同一入射角电场偏振化方向在入射面的电磁波要比电场偏振化方向垂直于入射面的电磁波的横向偏移大,随着电磁波频率的增加横向偏移变小.     

特深地下管线的电磁场特征分析及探测研究

研究了管线的一系列电磁场特征,介绍了特深管线探测应用状况,通过实例分析了从1 m到21 m了不同埋深直线电流及强电流3~21 m情况下的电磁场Hx、Hz、ΔHx 〖WTBZ〗的归一化异常的分布特征 , 提出了对特深管线探测的对策,即频率、电流、远端接地等提高信噪比的技术对策,提出了确定地下金属管线平面位置、埋深的方法,利用水平磁场分量70%法确定管线深度的适用条件,分析了特深管线探测的干扰因素,广州市3个特深管线的探测成功实例,说明了方法的良好效果.     

CENTRAL FREQUENCY SOUNDING IN SHALLOW ENGINEERING AND HYDRO-GEOLOGICAL PROBLEMS*

The paper deals with the early stages of development of a convenient form of electromagnetic induction method of sounding referred to as ‘Central Frequency Sounding’ and abbreviated as CFS. The method is introduced as a rapid and useful technique for investigation of shallow engineering and hydro-geological problems. Sets of theoretical two-layer master curves, suitable for interpretation of field data involving measurement of the vertical magnetic component of the field induced at the center of a loop placed on a two-layer earth, have been presented. The approximate but reasonably accurate solutions for a two-layer earth of any arbitrary resistivity contrast have been considered for the purpose and expressed in a form suitable for computation. The computed results have been presented in sets of curves useful for interpretation of field data.     

油井流动成像束状探测电磁场

为了克服油井流动成像电磁探测的"软场"效应,提高成像质量,本文从测量物理模型出发,采用在轴向上屏蔽和径向上聚焦的方法构建束状探测场,通过有限元仿真计算电势分布以及测量敏感场,发现轴向上屏蔽后的电势分布均匀平坦,从而探测电磁场可以简化为二维场;径向上聚焦后敏感区域主要集中在发射电极和接收电极之间的束状区域内,敏感场呈马鞍状,其中间敏感性明显加强.模拟流动实验测量结果表明,束状探测电磁场的测量信号较强,成像质量明显提高.     

Magnetotelluric profiling of vertical conductors

The presence of lateral contrasts of electrical conductivity modifies the original pattern of electromagnetic fields radiated from remote sources. A magnetic transverse plane wave field, interacting with a vertical conductive and outcropping dike placed between two quarter-spaces of unequal electrical conductivities, creates an anomalous vertical component of the magnetic field. This anomalous field has been analysed by computation, and drafting of master curves. Two case histories are presented to illustrate the application and the effectiveness of the solution. It is concluded that: (i) the response is higher for intermediate values of the conducting body induction number; (ii) the curves can be used for the inter-pretation of magnetotelluric, AFMAG, and VLF exploration data; (iii) it is necessary to develop solutions taking into account the vertical as well as the lateral variation of conductivity.     

地热井的天然源超低频电磁探测影响因素分析

本文使用天然源超低频电磁探测仪对北京大学校园内两口地热井进行了超低频电磁探测试验,获得了1号地热井（京热119号井）连续五天、每天四个时刻的天然源超低频电磁探测数据以及2号地热井（京热168号井）单天的天然源超低频探测的数据.基于所获得的数据,开展了地热井的天然源超低频探测影响因素的研究,这些因素主要包括：天然源超低频电磁场和观测时间的关系、探头摆放方向与接收工频谐波干扰关系以及天气状况对观测数据质量的影响等.研究表明：(1)天然源超低频电磁探测信号存在明显的日夜交替变化,其中相对振幅-频率曲线在不同日期、同一邻近时刻总体的包络线具有较好的重复性和一定的稳定性;(2)工频谐波干扰场方向是影响因素之一,当仪器探头长轴方向与工频谐波干扰场的等效主轴方向平行时噪声影响最小;(3)雨天环境会对天然场源超低频信号中的高频部分造成较大影响.     

区域地电观测网地电场日变化模型研究

应用泰勒多项式和曲面样条函数法建立了我国地电场观测台网中华东、 华北两个区域台网的地电场日变化(K_p≤5)模型,并对拟合结果进行了对比分析. 结果表明: ① 尽管地电场变化复杂,但用这两种方法建立的日变化模型相当吻合,模型曲线随时间的变化符合样本曲线随时间的变化; ② 用模型曲线可以描述其它年份同农历日期、 同等磁情时实测的地电场日变化; ③ 用模型曲线描述实测地电场日变化时,低磁情时得出的结果要优于高磁情时的结果; ④ 用多日样本曲线得到的模型曲线更能清晰地展示出地电场日变化的两次起伏波形、 幅度以及极值集中在午前午后等变化特点. 今后建立全国各区域台网不同磁情、 不同农历日期的地电场日变化模型可定量评价区域台网的地电场观测资料的质量,同时根据该模型能够分离出地电场的非正常变化,进而研判真实的异常,以服务于地震等灾害事件的预测以及地球电磁环境变化的监测等方面.      

任意方向发射源在柱状多层介质中的电磁场的理论计算

针对柱状多层介质测井模型,本文给出了井轴上沿任意方向的发射源在介质空间的电磁场的理论公式,利用递推方法可以很方便地进行数值计算.文中还分别对轴向与径向的电偶极子源和磁偶极子源共4种情况,给出了井轴上接收区磁场强度与侵入半径的关系曲线.     

水平电偶源频率测深中全区视电阻率定义的新方法

从电磁场的统一性出发，分析了水平电偶源频率测深中等效电阻率的算法原理和具体定义．分析表明，等效电阻率可明显地改善波区视电阻率在非波区场的畸变，较为形象、直观地反映出地电断面的电性变化．数值计算、三维模型计算和实际数据处理表明，等效电阻率具有良好的特性和应用前景．     

应城地电场日变化及干扰特征分析

选取应城地电场观测数据,分析地电场日变化规律和数据干扰特征,得到以下结论：①应城地电场日变形态表现为两峰一谷,日变幅约15—30 mV/km;②应城地电测区环境不稳定,降水及高压直流输电干扰显著;③高压直流及雷电干扰会造成地电场观测数据曲线出现台阶变化,而雷电干扰数据表现为恒定值;电磁暴会引起观测数据曲线发生振荡变化,日变幅甚至可达正常幅度的10倍;降雨会导致观测数据出现突跳和漂移现象。研究结果有助于识别地电场数据干扰,剔除无效信息,更好地为湖北省地震监测服务。     

VLF ANOMALIES FROM A PERFECTLY CONDUCTING HALF PLANE BELOW AN OVERBURDEN*

A theoretical solution to the electromagnetic problem of a perfectly conducting half plane below a conducting overburden has been obtained. The VLF anomalies have been computed for different overburden conductivity and thickness and also for different dip angles of the half plane. In the computations the contribution to the secondary magnetic field from the electric Hertz potential has been neglected. The anomaly curves which are displayed as EM 16 readings, show a fairly complicated behaviour. This is mainly due to the phase shift and attenuation of the field caused by the conductivity of the overburden and the host rock. From the anomaly curves it is possible to define the apparent depth to the top of the conductor as the distance between the peak value and the cross-over of the real component. The apparent depth is usually larger than the actual depth, but it is possible to determine the actual depth to the conductor from the relation between the peak-to-peak anomaly and the apparent depth. When the peak-to-peak anomaly is fairly large, it is also possible to make estimates of the dip angle. However, a complete set of master curves will be a necessary tool for interpretation of VLF data when there is need to obtain more accurate estimates of the half plane parameters. In a specific case the theoretical calculations are shown to be in good agreement with measured data.     

ON THE INTERPRETATION OF AIRBORNE ELECTROMAGNETIC DATA FOR THE TWO-LAYER CASE*

For helicopter-borne electromagnetic systems, the distance between the transmitting and the receiving coils is small compared with the altitude above ground. For this case, a major simplification can be made for the calculation of model curves. Some two-layer curves for the interpretation of frequency measurements are presented. A very simple procedure is demonstrated for the conversion of the relative secondary field into apparent resistivity and apparent distance for the mapping of airborne electromagnetic data. Furthermore, an approximation is described for the determination of the thickness and the resistivity of a layer lying on a perfectly conducting half-space.     

Electromagnetic induction of a three-dimensional conductivity inhomogeneity in a two-layer earth

Summary The results of numerical computations of the electromagnetic field induced in a two-layer model of the Earth with a three-dimensional inhomogeneity (a block) in the subsurface layer are given. Several recommendations are given which have enabled the solution of a complicated system of integral equations and the computation of the field at the Earth's surface in an effective way. The analysis of the obtained solution has proved that, in an anomalous electromagnetic field, the field of the horizontal electric dipole, oriented in the direction of the exciting field, is predominant. A number of practically usable diagrams and approximative formulae is given.     

A study of transient variations in the Earth’s electromagnetic field at equatorial electrojet latitudes in western Africa (Mali and the Ivory Coast)

In the framework of the French-Ivorian participation to the IEEY, a network of 10 electromagnetic stations were installed at African longitudes. The aim of this experiment was twofold: firstly, to study the magnetic signature of the equatorial electrojet on the one hand, and secondly, to characterize the induced electric field variations on the other hand. The first results of the magnetic field investigations were presented by Doumouya and coworkers. Those of the electric field experiment will be discussed in this study. The electromagnetic experiment will be described. The analysis of the electromagnetic transient variations was conducted in accordance with the classical distinction between quiet and disturbed magnetic situations. A morphological analysis of the recordings is given, taking into consideration successively quiet and disturbed magnetic situations, with the results interpreted in terms of the characterization of external and internal sources. Particular attention was paid to the effects of the source characteristics on the induced field of internal origin, and to the bias they may consequently cause to the results of electromagnetic probing of the Earth; the source effect in electromagnetic induction studies. During quiet magnetic situations, our results demonstrated the existence of two different sources. One of these, the S_R^E source, was responsible for most of the magnetic diurnal variation and corresponded to the well-known magnetic signature of the equatorial electrojet. The other source (the S_R*^E source) was responsible for most of the electric diurnal variation, and was also likely to be an ionospheric source. Electric and magnetic diurnal variations are therefore related to different ionospheric sources, and interpreting the electric diurnal variation as induced by the magnetic field diurnal variation is not relevant. Furthermore, the magnetotelluric probing of the upper mantle at dip equator latitudes with the electromagnetic diurnal variation is consequently impossible to perform. In the case of irregular variations, the source effect related to the equatorial electrojet is also discussed. A Gaussian model of equatorial electrojet was considered, and apparent resistivities were computed for two models of stratified Earth corresponding to the average resistive structure of the two tectonic provinces crossed by the profile: a sedimentary basin and a cratonic shield. The apparent resistivity curves were found to depend significantly on both the model used and the distance to the center of the electrojet. These numerical results confirm the existence of a daytime source effect related to the equatorial electrojet. Furthermore, we show that the results account for the observed differences between daytime and night-time apparent resistivity curves. In particular, it was shown that electromagnetic probing of the Earth using the classical Cagniard-Tikhonov magnetotelluric method is impossible with daytime recordings made at dip latitude stations.     

Simple relative space–time scaling of electrical and electromagnetic depth sounding arrays: implications for electrical static shift removal and joint DC-TEM data inversion with the most-squares criterion

A simple scaling relationship is shown to facilitate comparison, correlation and integration of data recorded using the common experimental configurations in electrical and electromagnetic depth sounding. Applications of the scheme to field data from typical geological and landfill environments show that it is robust and, where transient electromagnetic (TEM) data are available, enables easy identification and quantification of electrical static shift (galvanic distortion) in magnetotelluric and direct current (DC) sounding curves. TEM‐based procedures are suggested for both the direct removal of static shift in DC sounding curves and effective joint data inversion with the most‐squares criterion in the presence of static shift. A case study of aquifer characterization using sounding data from borehole sites in the Vale of York in England shows that static shift is a common problem in this glacial‐covered terrain and demonstrates the effectiveness of the proposed joint DC‐TEM inversion strategy in handling distorted soundings.     

A RECTANGULAR LOOP SOURCE OF CURRENT ON A TWO-LAYERED EARTH*

Using the concept of reciprocity and the known solution for the electric field of a vertical oscillating magnetic dipole source placed over a two-layered halfspace, an integral expression for the vertical magnetic field produced by a horizontal rectangular loop, carrying an oscillating current and placed on the surface of the same halfspace, is deduced. This expression is such that it can be evaluated by a combination of straightforward numerical integration and digital linear filter techniques. Displacement currents everywhere in space are neglected. Fields both inside and outside the loop are presented as sounding curves. A little over a decade of frequencies (100–3000 Hz) appears to be the minimum requirement and two decades (100–10000 Hz) are sufficient for most situations. Parametric and geometric sounding curves both show good resolution of subsurface layering. Phase shows better resolution than amplitude. Similarly, resolution is higher when conductivity increases with depth than when it decreases. Besides being useful for the electromagnetic depth sounding for layered earth structures, such computations can be applied to determine normal corrections to Turam observations, whenever a rectangular loop of finite size is used as a transmitter.