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...     

Effect of the ionosphere on electromagnetic waves from ground-based emitter in the frequency band 1–10 Hz

The results of the first experimental reception of electromagnetic signals, emitted by the ground antenna at frequencies of 1–10 Hz in the transition zone in the case when the distance from the transmitter is comparable with the equivalent waveguide height, are presented. The works were performed episodically in 2006, in different seasons and at different time of day, under quiet geomagnetic conditions. A pronounced effect of the state of the ionosphere on the received signal value was found out at distances about 80 km from the transmitter; in this case the seasonal manifestations were more substantial than the daily ones. The obtained results indicate that it is necessary to take into account the effect of the ionosphere on the wave amplitude in the transition zone, when the electromagnetic sounding of the Earth’s structure is performed at frequencies of 1–10 Hz, and that it is reasonable to use artificial waves in this band to perform quasi-vertical sounding of the ionosphere.     

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.     

Possibilities of electric field measurements in the audio frequency range using ungrounded electric sensors

Possibilities of electric field measurements in the audio frequency range using underground (capacitive) electric sensors are considered. Features of parameters of electric sensors and input circuits of the measurement equipment, robust methods, and software tools for processing obtained data are analyzed. Examples of experimental studies by the audiomagnetotelluric sounding (AMTS) method, demonstrating possibilities of implementing ungrounded electric field measurements in a frequency range of 7?C300 Hz, are presented.     

ELECTROMAGNETIC SOUNDING NEAR A LARGE SQUARE LOOP SOURCE OF CURRENT*

An electromagnetic sounding experiment with a large square loop as source was carried out on the dried-up bed of a water reservoir near the town of Dharmavaram in Andhra Pradesh. The sounding was performed in both geometric and parametric modes, and involved measurements of phase as well as amplitude of the vertical magnetic field inside and outside the loop. The six-frequency EM system used for the experiment was found to be a workable system for electromagnetic sounding. The results of the experiment more or less confirm the conclusions from theoretical modeling. They also show that even though the earth is not always electrically horizontally layered over a sufficiently large lateral extent for the one-dimensional model to be strictly valid, it is still possible to apply such modeling to sounding curves taken one part at a time and obtain layer parameters which check qualitatively with the layer parameters obtained from direct current resistivity sounding.     

无线电磁法(WEM)在地球物理勘探中的初步应用

无线电磁法(WEM)是上世纪90年代发展起来的一种新的人工源电磁探测技术,它通过一个固定大功率发射台发射0.1~300 Hz超低频/极低频电磁信号来探测地下地质结构.国内对WEM法的应用已做了大量的理论和实际试验工作,例如,大功率发射台辐射的电磁场计算,不同距离电磁信号发射接收试验,油田深部结构探测试验等.从试验的结果看,WEM法在提高电磁测深数据质量上具有很大的优势.     

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。     

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.     

A new approach to gradient geomagnetic sounding

A spherical model of the Earth including a heterogeneous upper mantle and excited by the magnetic field of a magnetospheric ring current is constructed. The obtained synthetic data are used for testing a new approach to gradient geomagnetic sounding ensuring the immunity of the sought impedance to distortions caused by lateral heterogeneity of the Earth. It is shown that this approach significantly increases the informativeness of deep electromagnetic sounding.     

EQUIVALENCE IN ELECTROMAGNETIC (FREQUENCY) SOUNDING*

An analysis of the equivalence phenomenon, analogous to that encountered in resistivity sounding, has been made for electromagnetic (frequency) sounding with the following systems: horizontal coplanar coils, perpendicular coils, vertical coplanar coils, and vertical coaxial coils. Cases analyzed are three-layer H and K type resistivity distributions. The theoretical responses for the EM sounding systems have been computed by the digital linear filter method using short filters. An analysis has been made of the resolution by the four EM systems considered f the equivalence effect. It is concluded that from the equivalence point of view electromagnetic (frequency) sounding has relatively better resolution compared to resistivity sounding for the H type three-layer cases. For the K type three-layer cases electromagnetic (frequency) sounding provides a serious problem in resolution similar to that encountered in resistivity sounding. Empirical relationships have been established for the H and K type cases respectively in order to numerically quantify the equivalence involved in electromagnetic (frequency) sounding.     

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.     

Induction Sounding of the Earth’s Mantle at a New Russian Geophysical Observatory

Deep magnetotelluric (MT) sounding data were collected and processed in the western part of the East European Craton (EEC). The MT sounding results correspond well with impedances obtained by magnetovariation (MV) sounding on the new geophysical observatory situated not far from the western border of Russia. Inversion based on combined data of both induction soundings let us evaluate geoelectrical structure of the Earth’s crust and upper and mid-mantle at depths up to 2000 km, taking into account the harmonics of 11-year variations. Results obtained by different authors and methods are compared with similar investigations on the EEC such as international projects CEMES in Central Europe and BEAR in Fennoscandia.     

从瞬变电磁测深数据到平面电磁波场数据的等效转换

中心回线源装置的瞬变电磁场是一种涡流场,在地下主要以扩散形式传播.平面电磁波在地下介质中传播时具有反射、折射等电磁学特性,为了借鉴平面电磁波场测深成熟的解释方法,文中开展了从瞬变电磁涡流场测深数据到平面波场数据的快速等效转换的研究.通过大量的理论模型的正演计算、曲线对比,误差统计,对瞬变电磁场、平面波场在地下传播机制特性的分析,建立了一种由瞬变电磁测深视电阻率数据向平面波场测深视电阻率数据转换的时间-频率等效对应关系.利用这一关系式,把扩散场数据快速转换成等效平面波场测深数据. 为进一步进行瞬变电磁拟平面波成像解释打下理论基础.     

Elongated horizontal and vertical receivers in three-dimensional electromagnetic modelling and inversion

Electromagnetic geophysical methods often rely on measurements of naturally occurring or artificially impressed electric fields. It is technically impossible, however, to measure the electric field directly. Instead, the electric field is approximated by recording the voltage difference between two electrodes and dividing the obtained voltage by the distance between the electrodes. Typically, modelling and inversion algorithms assume that the electric fields are obtained over infinitely short point-dipoles and thus measured fields are assigned to a single point between the electrodes. Such procedures imply several assumptions: (1) The electric field between the two electrodes is regarded as constant or being a potential field and (2) the receiver dimensions are negligible compared to the dimensions of the underlying modelling grid. While these conditions are often fulfilled for horizontal electric fields, borehole sensors for recordings of the vertical electric field have dimensions in the order of ≈100 m and span several modelling grid cells. Observations from such elongated borehole sensors can therefore only be interpreted properly if true receiver dimensions and variations of electrical conductivity along the receiver are considered. Here, we introduce a numerical solution to include the true receiver geometry into electromagnetic modelling schemes, which does not rely on such simplifying assumptions. The algorithm is flexible, independent of the chosen numerical method to solve Maxwell's equations and can easily be implemented in other electromagnetic modelling and inversion codes. We present conceptual modelling results for land-based controlled source electromagnetic scenarios and discuss consideration of true receiver geometries for a series of examples of horizontal and vertical electric field measurements. Comparison with Ez data measured in an observation borehole in a producing oil field shows the importance of both considering the true length of the receiver and also its orientation. We show that misalignment from the vertical axis as small as 0.1° may seriously distort the measured signal, as horizontal electric field components are mapped into the desired vertical component. Adequate inclusion of elongated receivers in modelling and inversion can also help reducing effects of static shift when interpreting (natural source) magnetotelluric data.     

Method for continuous monitoring of electrical rock resistivity

This paper presents the main results of developing a method for continuous monitoring of rock resistivity. The method uses continuous electromagnetic radiation of man-made or natural origin as a sounding signal. Underground electrical antennas are used as sensors. The physical basis of this method is the dependence of rock resistivity on the degree of fluid saturation, which changes under mechanical stress variations in geological media.     

庐枞矿集区大地电磁测深强噪声的影响规律

天然大地电磁场信号微弱,极化方向随机,极易受电磁噪声污染.张量阻抗分析、远参考技术、Robust估计等对随机噪声和不相关噪声有比较好的压制效果,但对强的相关噪声目前还没有有效的压制方法.庐枞矿集区人烟稠密、工业发达,是我国著名的铁、铜、硫等矿产基地之一,区内强烈的工业、通讯、矿山、民用等电磁干扰严重污染了大地电磁测深数据.本文首先根据实测的电磁场时间域波形和卡尼亚电阻率测深曲线形态,挑选出基本未受噪声污染的测点(Y1650).然后利用数学形态学从受严重污染的电磁场时间序列中提取出类方波、三角波、阶跃、脉冲和充放电5种典型噪声的波形,并以不同的方式将这些噪声波形与Y1650的电磁场波形叠加,对比分析加噪后Y1650点电阻率和相位测深曲线的变化,进而归纳出典型强噪声对庐枞大地电磁测深资料的影响规律,为进一步压制强噪声和资料处理提供依据.     

天然电磁辐射测深技术的应用

天然电磁辐射测深技术是一项利用天然电磁场探测地球介质相对电阻率与介质埋深的地球物理探测技术．此项技术主要探测在天然电磁场作用下在近地表形成的综合电场的垂直分量,根据其频谱的局部变化构制岩层相对电阻率与埋藏深度的直方图,进而对地下岩层的性质和分布特征作出合理的分析和判断．本文主要以探测实例说明本项技术的应用效果及其适用的范围．     

电磁辐射"EMOLS"仪观测结果原理及震例

“九五”期间研制的“EMAOS”电磁辐射仪,其接收原理是按大地电磁测深的原理改进而研制的。选择接收频段为0．01～10Hz,该频段探测深度在0～20km,是浅源地震发生的层位。记录的电磁扰动是在应力场作用下,动电效应产生的电磁场。电场和磁场同时记录增强了信息可信度的判断能力,记录震例表明,异常规律与已有模拟结果基本相同。因此,已总结的模拟记录预报指标等可在数字记录结果中使用。     

Electromagnetic response function for the semi-annual variations estimated by the single-station method using the night-time values of the geomagnetic fields

The geomagnetic night-time values were used to estimate the electromagnetic response function Q1 for half-year period. If the spatial structure of the source field can be described by the approximation, one can estimate the Q1 value using the single-station Z/H method. This technique enables us to carry out regional deep geomagnetic sounding by the method. The data used for analyses are geomagnetic night-time values for about, typically, 26 years from 5 good-quality stations and for several years from 34 stations distributed over the globe. The results indicate that the night-time values yield more reliable response estimates for half-year period compared to the usual estimates obtained from daily means. It implies that the approximation for the night-time fields holds good for the half-year period, but the daily means are not suitable for estimating the response function of the semi-annual variations by using the single-station method. Source field analyses for daily means data and night-time means data have also been carried out in this paper.     

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.