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《地球物理学报》2021,64(6)
极低频电磁探测方法(WEM)是一种新型人工源低频电磁探测技术,目前尚未见极低频电磁探测方法的海区研究,缺少相关经验.为测试WEM在海洋环境中的电磁响应特征,本文将"地-电离层"全空间电磁波传播理论应用到含海水介质的"地-电离层"全空间模型,由地表处数值格林函数出发,利用空气层与海水层界面谢昆诺夫势函数满足的边界条件,推导了海底界面的场强表达式.采用高采样密度的Hankel数字滤波方法进行数值模拟计算.为探究WEM在海洋油气资源探测中的适用性,建立简化海洋地电模型,通过分析WEM对不含异常体的背景场和含高阻油气异常体时的电磁响应曲线差异来检测WEM探测海底高阻油气层的可行性.并讨论不同观测方式、不同收发距以及海水层厚度、油气层埋深和油气藏厚度对WEM电磁响应特征的影响.研究结果表明,WEM可在数百千米收发距下探测浅海区域海底下方电性异常体的存在. 相似文献
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地球物理学中关于电磁波勘探研究通常采用的是地球半空间模型.然而,对于几十公里的有限长电缆源（长偶极源）,远距离电磁波场探测必须要考虑电离层的影响,它是一个全空间问题.关于包含电离层、空气层和地球介质（我们称“地-电离层”模式）的电磁波场特征的研究在国外较少,国内几乎是空白.本文采用全空间积分方程法首先对小尺度的可控源电磁波场特征进行了研究,由于此时电离层的影响可忽略,它应该和半空间成熟的CSAMT模拟结果一致,对比结果表明,二者是一致的,验证了全空间模拟方法的可靠性和有效性.随后进行了50 km长电缆电离层和空气层高度都为100 km的“地-电离层”模式大尺度电磁波场模拟,以探讨大尺度可控源电磁波场的特征.给定频率的“地-电离层”模式电磁场的衰减曲线表明长电缆远距离电磁波场由于受电离层的作用存在衰减逐渐变小的过渡场和衰减变小的波导场.为了探讨复杂介质“地-电离层”模式电磁波特征,对“地-电离层”模式的典型地盾和地台多层介质模型进行了数值模拟,得到了偶极源长度50 km、电流200A、收发距离远达1600 km和2500 km的合理的电磁场结果.最后,对一简单含油储层结构模型进行了长偶极、大功率、远距离电磁波场响应计算.储层横向不均匀复杂结构模拟的结果表明,考虑电离层和大气层的“地-电离层”模式大尺度深层复杂介质模拟时,电磁场对深部目标体仍有很好的异常响应. 相似文献
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针对我国地震监测预报中地电阻率定点连续观测中存在的偶极接地线的干扰问题, 本文将台站区域地层简化为3层均匀介质模型, 将接地线等效为偶极接地的电阻体, 建立了接地线干扰地电阻率观测的耦合物理模型. 通过有限元分析软件ANSYS模拟分析不同电性断面情况下接地线对地电阻率观测的影响, 同时分析这种干扰的产生机理, 并结合实际观测中存在的干扰问题作了对比验证分析. 结果表明: ① 接地线使得供电电极产生的地下对称性电场分布发生局部调整, 从而影响地电阻率观测; ② 接地线对地电阻率观测的影响主要取决于线缆的位置及方位角的大小; ③ 适当增大电极埋深可以减小其对地电阻率观测的影响; ④ 电性结构的差异性决定干扰变化幅度的大小. 本文结果对相关台站地电阻率观测异常分析落实及干扰源避让和观测系统改造具有参考意义. 相似文献
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为了探索我国海域油气和水合物等高阻目标体CSEM勘探的可行性和方法技术,本文研究了在海水中水平电性源激励下有限水深海洋地电模型的频率域电磁响应,为进一步的1D和3D仿真计算奠定了理论基础.在推导电磁响应公式时,首先给出了各层介质的Lorentz势,然后根据Coulomb势与Lorentz势的关系,得到了各层介质的Coulomb势.各层介质中的电磁场均可以由Lorentz势或者Coulomb势计算得到,但在有限元计算时Coulomb势具有优势.长导线源的电磁场和势函数可以由电偶源的电磁场和势函数沿导线长度积分得到.文中具体给出了海水中水平电偶源和长导线源在海水层的电磁场公式,并根据该公式计算了不同水深环境下海底表面的电磁场分布,分析了海水深度对海底油气储层电磁异常的影响.结果表明,随着水深减小,异常幅度和形态特征发生明显变化.当水深很浅时(如50 m),只有同线方向的Ex和Ez两个电场分量存在明显异常.最后,以两个已知海底油田为例,计算了不同水深环境下可观测到的电场异常,展示了电性源频率域CSEM在海底勘探中(包括浅海环境)的良好应用前景.对于该方法实用化过程中还需进一步解决的问题,文中结尾部分也进行了初步探讨. 相似文献
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分布式被动源电磁法系统 ,简称DPEM。DPEM利用天然电磁场或人文电磁场作为源场 ,研究大地的电磁响应 ,探测地下电性分布及地质结构 ,可适应各种复杂的地理地质环境。主要特点有 :同步阵列遥测不同地理坐标点的电磁场 ,具有高信噪比、高分辨率和高效率 ;主机 -子机系统 ,为非封闭挂接式 ,可按使用要求配置子机数量 (坐标点数 ) ,一次实现面积性同步覆盖测量 ;频率范围 16kHz～ 0Hz ,实现浅自近地表、深至 1km、直至上地幔的不同地质目标的探测 ;可提供视电阻率、阻抗相位、异常相位等地电学参数。文中介绍了 3个工作实例 相似文献
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倾子是航空大地电磁方法的主要参数,在地下异常体的探测中有着重要的作用和显著优势.本文简要概括了倾子测量的基本原理;基于有限单元法进行了二维大地结构的倾子建模仿真,分析了影响倾子响应特征的主要因素,包括空间性参数以及地下介质参数.通过对倾子响应特征影响因素的分析,提出了一种利用倾子及其梯度的异常位置和异常幅度,对地下二维异常体的电性特征、水平位置、埋深以及电阻率进行快速、定量识别的方法.最后,利用已知模型的倾子响应对异常体参数进行了试算,通过对试算结果和已知模型参数的对比,验证了该方法的可行性. 相似文献
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诸多研究表明我国深部资源潜力巨大，但目前的开发开采深度普遍停留在500 m以浅，开展"攻深探盲"是构建国家资源安全体系的有效途径.应用最先进的科学技术手段，提取深部地质信息，已成为我国当前地球物理科学研究的发展方向.作为地球物理学的重要分支，电磁法是矿产资源探查的主体手段之一.在分析我国现阶段航空、地面及海洋电磁探测技术进展的基础上，本文重点说明了极低频电磁法（简称WEM法），多通道瞬变电磁法（简称MTEM）和电性源短偏移瞬变电磁法（简称SOTEM）等电磁探测新技术.WEM法建立一套包括岩石层、大气层和电离层在内的全空间电磁传播理论，通过新研制的观测系统，获取地下10 km的地电信息；MTEM方法是地下埋深4 km目标体精细勘查的有效手段；SOTEM实现地下1.5 km深度范围内目标体的精细探测.通过多种电磁探测技术组合，可实现地下10 km深度范围内多尺度探测，达到"望远镜+放大镜+显微镜"探测效果.同时，本文指出进一步研发与新方法配套的装备、资料处理技术和大数据人工智能识别等将是我国电磁法未来的发展方向. 相似文献
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工程电法勘探中，常需要探测埋深不大、规模较小、分布复杂的目标体，对探测方法的精度要求较高.由于三维高密度电阻率法数据采集密度大，可对目标体进行多方位观测，是目前工程探测中常用的方法之一.本文对三维高密度电阻率E SCAN法的视电阻率异常特征进行研究.设计几种典型地质体模型，利用有限单元法进行正演计算，并分析其异常特征和分布规律.算例结果表明，E SCAN法对低阻体的分辨能力强于高阻体.无需对所有电极进行扫描供电观测，即可分辨地质体电性特征及水平位置，但观测结果难以分辨地质体纵向延伸. 相似文献
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对井下地电阻率观测的探测深度进行了研究，计算了均匀半空间和给定结构参数的水平层状介质模型在不同装置电极埋深下的探测深度，分析了探测深度与装置电极埋深和介质电阻率结构之间的关系，得到如下结果:①与地表观测相比，在供电极距为1 km左右时，探测深度随装置电极埋深的增大而增大，且增大的速度与装置电极埋深密切相关；当装置电极埋深h < 100 m时，探测深度的增大速度远小于装置电极埋深h≥100 m时. ②当装置电极埋深h < 50 m时，与地表观测相比探测深度增加很小，不超过10 m；当装置电极埋深相同时，供电极距越大，与地表观测相比探测深度增加得越小. ③对于水平层状电阻率均匀分层结构，在装置电极埋深相同的情况下，下伏低阻结构的探测深度显著大于下伏高阻结构.本文的研究结果表明，为了观测到深部电阻率的变化情况，首先需要查明测区电性结构，再进行综合分析，以确定井下地电阻率观测的装置电极埋深，其结果为深部电阻率变化研究提供了理论基础. 相似文献
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M. Poddar 《Pure and Applied Geophysics》1996,147(3):551-566
There is growing interest in the use of transient electromagnetic (TEM) sounding for shallow geotechnical, environmental and groundwater investigations. Two commonly used transmitterreceiver configurations for TEM sounding are 1) loop-loop or its variation, in-loop configuration and 2) wire-loop configuration. The less common configuration of a horizontal electric dipole (HED) transmitter and receiver is treated in this study and called wire-wire configuration.Two important problems of shallow investigation in hard and soft rocks respectively are, defining 1) a fractured/fissured zone of medium resistivity, sandwiched between an overlying surface weathered rock of low resistivity and an underlying fresh compact rock of high resistivity and 2) a body of resistive sand buried in conductive clay. Lateral change in the middle layer resistivity is modeled by including a 3-D body of anomalous resistivity. The effect of perturbing the resistivity of the 3-D inclusion and the host middle layer for the wire-wire configuration is compared with that of the commonly used loop-loop configuration. The wire-wire configuration is found more sensitive to the model perturbations than the loop-loop configuration.1-D inversion of synthetic 1-D data sets for the wire-wire configuration finds resolution and estimation errors to be less than 10 percent for all the model parameters. For 3-D models, 1-D inversion results give a resolution error of 10 percent or less for the depth to, resistivity and thickness of, the 3-D inclusion. The estimate is within 10 percent of the true value for the first parameter but 40 percent for the other two. Resolution as well as estimation of the basement resistivity is always very poor.Using the wire-wire configuration, it is theoretically possible to define a buried resistive layer and any lateral change in its resistivity, subject to the above limitations of 1-D inversion. However, the basement resistivity cannot be estimated with reasonable accuracy in the presence of a lateral inhomogeneity in the overlying layer. 相似文献
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The bipole-dipole resistivity technique, which uses a single current source (bipole) to map variations in (apparent) resistivity has been much criticized in the past. A series of 3D models are used to show that the use of two distinct current bipoles in the same location but with different orientations, combined with analysis in the form of a previously defined tensor apparent resistivity, can greatly improve many aspects of bipole-dipole mapping. The model study shows that, for measurement stations more than a few bipole lengths from the current source, the apparent resistivity tensor behaves, to a close approximation, as though the current bipoles are idealized dipoles, and hence is independent of the orientation of the individual current sources used. Any pair of current bipoles (in the same location but with different orientations) can therefore be used to determine the tensor resistivity properties. The invariants of the apparent resistivity tensor have considerable advantages over the normal scalar apparent resistivities. Modelling shows that although the electric field vector corresponding to a single current bipole can be highly perturbed by a local inhomogeneity for some considerable distance beyond the inhomogeneity itself, the tensor invariants are virtually unperturbed beyond the extent of the inhomogeneity. Hence false anomalies, which are a characteristic of apparent resistivity measured using only single current bipole models, are almost completely eliminated by the use of tensor invariants. Of the possible tensor invariants, the invariant given by the square root of the determinant gives the best representation of a buried 3D body. Resistivity anomalies are localized, and occur only over the causative body. Even with complex models involving several buried bodies, the tensor invariants clearly delineate the extent of each body. Outside the bounds of perturbing bodies, the tensor data can be analysed by conventional techniques, for example, to determine layered structure. 相似文献
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Out‐of‐plane effects in 2D borehole‐to‐surface resistivity tomography and applications in mineral exploration 
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下载免费PDF全文 In this paper, we discuss the effects of anomalous out‐of‐plane bodies in two‐dimensional (2D) borehole‐to‐surface electrical resistivity tomography with numerical resistivity modelling and synthetic inversion tests. The results of the two groups of synthetic resistivity model tests illustrate that anomalous bodies out of the plane of interest have an effect on two‐dimensional inversion and that the degree of influence of out‐of‐plane body on inverted images varies. The different influences are derived from two cases. One case is different resistivity models with the same electrode array, and the other case is the same resistivity model with different electrode arrays. Qualitative interpretation based on the inversion tests shows that we cannot find a reasonable electrode array to determine the best inverse solution and reveal the subsurface resistivity distribution for all types of geoelectrical models. Because of the three‐dimensional effect arising from neighbouring anomalous bodies, the qualitative interpretation of inverted images from the two‐dimensional inversion of electrical resistivity tomography data without prior information can be misleading. Two‐dimensional inversion with drilling data can decrease the three‐dimensional effect. We employed two‐ and three‐dimensional borehole‐to‐surface electrical resistivity tomography methods with a pole–pole array and a bipole–bipole array for mineral exploration at Abag Banner and Hexigten Banner in Inner Mongolia, China. Different inverse schemes were carried out for different cases. The subsurface resistivity distribution obtained from the two‐dimensional inversion of the field electrical resistivity tomography data with sufficient prior information, such as drilling data and other non‐electrical data, can better describe the actual geological situation. When there is not enough prior information to carry out constrained two‐dimensional inversion, the three‐dimensional electrical resistivity tomography survey is the better choice. 相似文献
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R. SCHULZ 《Geophysical Prospecting》1985,33(8):1240-1253
Gradient measurements in a homogeneous electrical primary field can easily be interpreted for simple models. The simplified solution (conducting or resistant body in a homogeneous space in a homogeneous electrical field) is often sufficiently accurate, as comparisons with the exact solution (body of finite resistivity in a homogeneous half-space in a quasihomogeneous electrical field) show. The exact geometry of the body cannot be determined by gradient measurements; the same anomaly of apparent resistivity can be caused by different bodies. In particular, the similarity between a sphere and a cube of the same volume is very high. There is a distinct influence of the resistivity of the overburden: the higher this resistivity is, the stronger is the effect caused by a buried body. If a deviation of 10% of the apparent resistivity is assumed as the lower boundary at which a buried body can be detected by gradient measurements, the depth of investigation for a three-dimensional body is approximately equal to its width; in the two-dimensional case the thickness of the overburden can be twice the width. If the overburden has a resistivity which is higher than the resistivity of the substratum, these depths are greater. The greatest possible depth is approximately three times the width of the body. 相似文献
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在坑道电阻率超前探测工作中，视电阻率异常极值位置与异常体位置、厚度、电性以及产状相关.设计有效的反演方法对异常体的各种参数综合反演是非常必要的.首先利用有限元数值模拟技术分析并消除坑道空腔对视电阻率的影响，并将坑道超前探测视电阻率曲线转化为全空间视电阻率曲线.然后采用最小二乘法对超前探测视电阻率曲线进行快速反演.反演结果可准确反映围岩电阻率、异常体的电性、位置和产状；而由于同层等值现象，反演结果只能确定异常体的厚度和电阻率的组合值. 相似文献
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IntroductionIn multidisciplinary research of earthquake predication, the geoelectric method is one of the important branches. It takes the electric phenomenon or the electric property changing of the earth medium in seismogenic process as the studying objects, and it is the important composing portion of the earthquake-development comprehensive research. The electric parameters of the earth medium, mostly the resistivity, are the main body in geoelectric research.Experiments indicate (LU, et … 相似文献