ELECTROMAGNETIC SOUNDING OF THE KAPURDI LIGNITE DEPOSIT IN WESTERN RAJASTHAN,INDIA*

In the Kapurdi block of the Barmer lignite field in western Rajasthan, India, lignite occurs as a number of discontinuous seams of varying thickness up to a few m. The country rock is a succession of clay, sand and various members of the clay-sand family. The overburden, comprising sand, Fuller's earth, bentonitic clay, clayey sand/sandy clay and clay, is thick and, electrically, highly conductive. Both its thickness (40–90 m) and conductivity (0.5–1.0 S/m) are highly variable. The lignite seams may occur anywhere in the clay-sand sequence below this overburden. A ten-layered electrical model of the Kapurdi area was parameterized on the basis of galvanic resistivity sounding and core drilling results. This model was then validated by electromagnetic sounding measurements made with a 200 × 200 m square loop transmitter and energizing current of variable frequency (80 Hz to 11 kHz), using central induction sounding techniques. With a loop of this size, only formations in the overburden could be mapped. Subsequent computer modeling has shown that a larger loop of size, say, 800 m and a frequency band of 30–500 Hz are required to map formations below–as well as those within–the overburden. Even then only various clay-sand interfaces can be mapped, i.e., only over-burden thickness and the likely depth range of lignite seams (if they occur) can be estimated. Direct mapping of lignite seams is not feasible in practice. This is true of loop-dipole, as well as central induction sounding. It is interesting that skin depth need not always be a factor limiting depth of exploration in electromagnetic prospecting.     

ELECTROMAGNETIC DEPTH SOUNDING EXPERIMENT*

An electromagnetic frequency sounding experiment with a rigid horizontal transmitter coil carrying a stabilized oscillating current was carried out in South Tunesia. The field data were interpreted in terms of the mutual impedance ratio in the horizontal coils system. Where the measurements were sufficiently accurate they generally could be interpreted to a high degree of fit. It is concluded that a vehicle-mounted electromagnetic frequency sounding system is suitable for a fast survey.     

漠河地区天然气水合物远景调查之——电法探测岩石性永久冻层的应用研究

在东北天然气水合物远景调查中,解决多年永久性冻土的空间分布是一个关键性问题.但是利用电测方法确定基岩性永久冻土的空间分布比较复杂.为有效利用电测方法探测漠河地区永久冻土的空间分布,首先对漠河地区4口钻井岩芯进行了不同温度条件下的物性测试,并以此建立正演模型进行了正演模拟研究;在工作区永久冻层已知点做了方法有效性实验,获得了永久冻层与上、下非冻层的电性结构特征.在此基础上采用电阻率测深法和瞬变电磁法进行了实际探测,根据实测资料,对工作区永久冻层或与永久冻层相关电性层的空间分布做出了推断解释,利用点探槽对不同海拔高度的测点进行了验证.     

FREQUENCY ELECTROMAGNETIC SOUNDING USING A VERTICAL MAGNETIC DIPOLE*

A horizontal transmitter loop (vertical magnetic dipole) is used for frequency electromagnetic (FEM) soundings. The frequency ranges from approximately 6 Hz to about 4000 Hz. The vertical and radial magnetic field components are measured for 20 frequencies per decade several hundred meters from the transmitter loop. A very small bandwidth is selected for amplification using a reference signal. An Apple computer is used for data acquisition. A computer program for Marquardt inversion optimizes the parameters for the n-layer case: the resistivities and thicknesses of individual beds and a correction factor for the primary magnetic field. Interpretation of each component individually yields practically the same parameters. Examples from the field are given with interpretation; comparison with dc resistivity measurements is provided. The ratio of vertical/radial magnetic field components vs. frequency can be transformed simply into apparent resistivity vs. apparent depth. This can be done in the field to obtain an estimation of the depth of the layer boundaries. FEM results are compared with Schlumberger d.c. sounding obtained at the same site.     

含激电效应的CSAMT一维正演研究

地电体对频率域电磁波激发源的响应为电磁感应和激电效应的综合响应.传统CSAMT法进行数据正反演时认为大地介质电阻率是与频率无关的实数,而实际上因为激电效应,地下可极化体的电阻率是一个与频率相关的复数.为推进二者总体响应研究,并扩展激电法的应用范围,同时提高电磁法勘探的精度,本文基于Dias模型,以复电阻率代替不考虑地电体极化效应的直流电阻率,对CSAMT场源一维层状模型进行了正演模拟,为提取CSAMT信号中所含激电信息提供理论基础.结果表明,考虑激电参数后,视电阻率及相位响应曲线出现明显异常（包括远场、过渡场、近场）;极化前后振幅比值异常峰值、相位差值异常峰值可直观体现激电异常;异常峰值与极化层层厚、埋深以及电阻率变化有连续的对应关系.认为从频率域电磁法信号中提取激电信息有乐观的前景.     

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.     

Energy-4 Generator for Monitoring Seismically Active Regions and Electromagnetic Sounding of the Earth’s Crust. Experience of Application in the Kovdor-2015 Experiment

The first section of the paper describes the developed self-powered, chassis mounted mobile Energy-4 generator, which has a power of 29 kW and a maximum output voltage of up to 1200 V. The generator operates in the audio frequency range (2–2000 Hz) and is designed for electromagnetic sounding of the Earth’s upper crust in the search for minerals and monitoring of earthquake source zones in seismically active regions. The main power units of the generator are a PWM inverter and a step-up transformer. The inverter is powered by two DC generators mounted on the driveshaft of a truck, in the body of truck of which the generator is mounted. The circuit diagram and operation of the generator are considered, as well as individual design solutions that made it possible to increase the amplitude of the output voltage and, consequently, the current in grounded power lines. The second section is devoted to full-scale tests of the Energy-4 generator in the Kovdor-2015 experiment, during which multipath frequency soundings with 25 and 50 km spacings were carried out on the territory of the Enskii–Kovdor granite-gneiss complex composed of rocks of the Archaean basement of the Baltic Shield. As a result of the experiment, a ubiquitous intermediate conducting dilatancy-diffusion layer (DD layer) was found at depths from 2–3 to 5–9 km in an area of 100 × 200 km. The parameters of this layer have been investigated. The layer is considered a seismically active element of the brittle Earth’s upper crust with a thickness of 10–15 km.     

基于双通道参考技术的磁共振地下水探测电磁噪声分布规律研究

磁共振地下水探测(MRS)技术能直接测量水中氢质子的拉莫尔进动,可表征地下水含水量和介质孔隙度等信息,因而获得了广泛应用.然而,MRS信号微弱,实际探测中电磁噪声强度大,若噪声未被有效去除,会导致后续水文参数解释不准确,采用"8"字形线圈或运用带参考线圈的噪声抵消等方法可抑制噪声干扰,但这些方法消噪效果的好坏取决于电磁...     

电磁感应研究的进展与展望

最近几年国内外电磁感应研究在以下几方面取得迅速发展:国际和区域合作大尺度电磁感应研究;海洋电磁测深;能源、资源勘探和环境监测等应用研究;理论分析和实验研究;数据采集系统.     

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.     

海底表面磁源瞬变响应建模及海水影响分析

根据电磁场理论,推导了磁偶源和接收点均位于海水中时层状海底模型的频域电磁场响应一般表达式,并通过此式,得到了海水为均匀半空间和有限海水深度两种情况下,垂直磁偶极装置、中心回线和重叠回线分别置于均匀半空间海底表面时的瞬变电磁响应（磁场和感应电压）表达式. 这些表达式将瞬变响应和海底的电导率等参数有机联系在一起,为海底瞬变电磁法的正演计算和反演解释提供了理论基础. 仿真计算表明,海水的存在不仅使得瞬变响应曲线形态发生变化,而且影响其对海底电导率的分辨能力.     

基于异常线圈的时间域AEM系统测试和标定方法研究

为了检验和测试时间域航空电磁系统的测量精度和有效性,采用地面铺设闭合的异常线圈模拟地下有限导体的方法,将异常线圈的电磁响应理论值与系统实测数据进行拟合分析,来确定系统误差和飞行几何参数误差.在计算异常线圈电磁响应的基础上,研究了衰减曲线、剖面曲线与线圈的电性、几何参数关系,设计了野外测试实验方案.在长春市大鹅岛附近,采用吊车进行了系统测试,测试结果表明:单点实测数据的平均绝对误差为0.48 mV,系统相对误差小于1%,飞行高度误差为0.4 m、水平偏移误差为0.2 m.基于异常线圈进行时间域航空电磁系统的测试和标定,是一种准确、快速、经济可行的方法,具有野外施工便捷、参数调整灵活等特点,适用于任何时间域电磁测量系统的检测.     

电阻率随位置线性变化时的三维大地电磁模拟

采用积分方程法实现了对三维体电磁散射的数值模拟研究.在分析电张量格林函数的基础上,针对大地电磁情形，进行了数值模拟研究.分析了异常体的横向走向长度对电磁响应的影响,特别对电阻率作线性变化的异常体进行了数值分析,并得到了相应测深曲线.对三维电磁测深进行了分析,考察了地层参数的影响,得到了一些重要结果.     

Deep electromagnetic sounding of the lithosphere in the eastern Baltic (fennoscandian) shield with high-power controlled sources and industrial power transmission lines (FENICS experiment)

The paper addresses the technique and the first results of a unique experiment on the deep tensor frequency electromagnetic sounding, the Fennoscandian Electrical conductivity from results of sounding with Natural and Controlled Sources (FENICS). In the experiment, Energy-1 and Energy-2 generators with power of up to 200 kW and two mutually orthogonal industrial 109- and 120-km-long power transmission lines were used. The sounding frequency range was 0.1–200 Hz. The signals were measured in the Kola-Karelian region, in Finland, on Svalbard, and in Ukraine at distances up to 2150 km from the source. The parameters of electric conductivity in the lithosphere are studied down to depths on the order of 50–70 km. A strong lateral homogeneity (the one-dimensionality) of a geoelectric section of the Earth’s crust is revealed below depths of 10–15 km. At the same time, a region with reduced transverse crustal resistivity spread over about 80 000 square kilometers is identified within the depth interval from 20 to 40 km. On the southeast the contour of the anomaly borders the zone of deepening of the Moho boundary down to 60 km in Central Finland. The results are compared with the AMT-MT sounding data and a geodynamic interpretation of the obtained information is carried out.     

电偶源频率电磁测深激发极化效应研究

本文研究含激电效应(IP)电偶源频率电磁测深问题.首先,通过对电偶源频率电磁测深电磁场分量分辨率的分析,提出利用电场Ex分量提取IP信息的合理性;然后,给出了同时存在激电和电磁效应时的理论计算公式和算法,对一些典型地电断面进行理论计算并对计算结果进行分析;最后,提出提取IP信息的几种可能方案.这些工作不仅为野外实测资料...     

高频电磁测深在地下热水勘探中的应用

高频电磁测深法是一种利用天然高频电磁场作为场源的电磁勘探方法,具有高分辨率和相对直流电法较深的勘探深度的特点,特别适合中浅层地热和地下水勘探。本文介绍了一个应用高频电磁测深评价地下热水资源的一个成功实例。野外数据采集采用了高频电磁测深系统MT-U5A,采用远参考技术,以保证数据采集质量。高频电磁测深资料二维反演成像处理结果较好地描述了地下热水的空间分布范围,为查明该区的地下热水的来源提供了较有利的证据。     

The results of marine electromagnetic sounding with a high-power remote source in the Kola Bay in the Barents Sea

The first Russian six-component seafloor electromagnetic (EM) receivers were tested in an experiment carried out in Kola Bay in the Barents Sea. The signals transmitted by a remote high-power ELF source at several frequencies in the decahertz range were recorded by six receivers deployed on the seafloor along the profile crossing the Kola Bay. Although not all the stations successfully recorded all the six components due to technical failures, the quality of the data overall is quite suitable for interpretation. The interpretation was carried out by the three-dimensional (3D) modeling of an electromagnetic field with neural network inversion. The a priori geoelectrical model of Kola Bay, which was reconstructed by generalizing the previous geological and geophysical data, including the data of the ground magnetotelluric sounding and magnetovariational profiling, provided the EM fields that are far from those measured in the experiment. However, by a step-by-step modification of the initial model, we achieved quite a satisfactory fit. The resulting model provides the basis for introducing the corrections into the previous notions concerning the regional geological and geophysical structure of the region and particularly the features associated with fault tectonics.     

编码电磁测深

利用逆重复M序列伪随机信号良好的自相关特性,提出相关检测地电传输特性的编码电磁勘探法.分析了编码电磁测深原理及参数提取方法.同时记录发送电流信号和多收发距电磁场响应,对源信号和场信号进行相关运算,解卷积分离接收系统响应后可得到大地的频率特性或时间特性,以此实现地电断面的精细探测.在时间域,大地冲激响应和阶跃响应含有丰富的地电信息.通过冲激响应的峰值时间或阶跃响应的晚期渐近值可估计地电阻率分布.基于层状模型的大地冲激响应和阶跃响应正演计算结果表明,编码电磁测深法对大埋深薄层目标体有精细的分辨能力.可应用于能源、矿产资源、水资源、环境地质及工程地质勘察,有良好的应用前景.     

Analysing two‐dimensional effects in central loop transient electromagnetic sounding data using a semi‐synthetic tipper approach

We present a simple and feasible approach to analyse and identify two‐dimensional effects in central loop transient electromagnetic sounding data and the correspondingly derived quasi two‐dimensional conductivity models. The proposed strategy is particularly useful in minimising interpretation errors. It is based on the calculation of a semi‐synthetic transient electromagnetic tipper at each sounding and for each observational transient time point. The semi‐synthetic transient electromagnetic tipper is derived from the measured vertical component of the induced voltage and the synthetically calculated horizontal component. The approach is computationally inexpensive and involves one two‐dimensional forward calculation of an obtained quasi two‐dimensional conductivity section. Based on a synthetic example, we demonstrate that the transient electromagnetic tipper approach is applicable in identifying which transient data points and which corresponding zones in a derived quasi two‐dimensional subsurface model are affected by two‐dimensional inhomogeneities. The one‐dimensional inversion of such data leads to false models. An application of the semi‐synthetic transient electromagnetic tipper to field data from the Azraq basin in Jordan reveals that, in total, eight of 80 investigated soundings are affected by two‐dimensional structures although the field data can be fitted optimally using one‐dimensional inversion techniques. The largest semi‐synthetic tipper response occurs in a 300 m‐wide region around a strong lateral resistivity contrast. The approach is useful for analysing structural features in derived quasi two‐dimensional sections and for qualitatively investigating how these features affect the transient response. To avoid misinterpretation, these identified zones corresponding to large tipper values are excluded from the interpretation of a quasi two‐dimensional conductivity model. Based on the semi‐synthetic study, we also demonstrate that a quantitative interpretation of the horizontal voltage response (e.g. by inversion) is usually not feasible as it requires the exact sensor position to be known. Although a tipper derived purely from field data is useful as a qualitative tool for identifying two‐dimensional distortion effects, it is only feasible if the sensor setup is sufficiently accurate. Our proposed semi‐synthetic transient electromagnetic tipper approach is particularly feasible as an a posteriori approach if no horizontal components are recorded or if the sensor setup in the field is not sufficiently accurate.     

A NOTE ON THE LINEAR FILTER METHOD OF COMPUTING ELECTROMAGNETIC SOUNDING CURVES *

The values of the filter coefficients used for the computation of electromagnetic sounding curves are studied in conjunction with the values of the input function to the filter, or the range of values which the input function may assume, and the filters are broken off at such a place that the error in the sum of the products of filter coefficient and input function does not exceed a prescribed value. This analysis is carried out for the horizontal coils system, the perpendicular coils system, and the vertical coplanar coils system. The lengths of the filters so derived depend on the layer parameters, the frequency and the coil spacing. Even in the most unfavourable cases the filters are shorter than the filters published by Koefoed, Ghosh, and Polman (1972).