吉林省东部地区地磁短周期变化的初步分析

本文利用吉林省长春、盘石、通化等地磁台的急始型、湾扰型短周期事件的资料进行统计分析,用最小二乘法计算上述台站的转换函数、感应矢量、方位角以及反映地下导体界面的倾角等参数,分析了短周期变化的频率特性,并与有关地区做了相应的对比。结果表明,本省上述地区不存在地磁短周期变化的明显异常现象。此外,根据南从北京、北至满州里等8个地磁台的济料推断:沈阳—大连之间可能存在一条近东西向高导带。     

兰州-西安-郑州电导率异常带

根据磁暴急始和湾扰变化,发现沿兰州——西安——郑州有一条垂直分量接近于零的过渡带,反映其下存在一条电导率异常带或高导带,本文称之为兰州——西安——郑州电导率异常带.考察沿线的地质和地球物理特征表明,异常带处于特定的构造、地震和地热环境中,推测沿异常带是一地壳软弱带或次板块边界,还推测地磁异常可能与电流通道效应有关.      

地震前后地磁短周期异常变化的再研究

一、引言我们曾研究过短期地磁扰动△Z/△H在唐山地震和隆尧地震前后的异常变化。现在进一步研究包括菏泽5.9级地震在内的华北三次地震前后地磁短周期转换函数随时间变化情况,以便更广泛地考察不同地区和不同类型地震之前异常变化特征。地磁短周期是指磁暴急始、瞬时扰动及湾扰等,这些磁扰的幅度在三个分量Z、H、D之间有如下经验关系:     

地震前后短期地磁扰动_(△H)~(△Z)的异常变化

一、引言短期地磁扰动是指磁暴急始、湾扰与地磁脉动等扰动时间较短的地磁变化。磁扰的源场来自磁层和电离层的电流体系,因而它们在较大范围内可看作均匀分布。但是,由于它们在地球内部所产生的感应磁场与地下电导率分布密切相关,所以,相距很近的两地往往观测到磁扰不一致。这就是说短期地磁扰动可以反映地下电导率的横向不均匀性,目前国内外均已发现了一些电导率异常区。一些作者还研究了短期地磁     

云南省的地磁短周期变化异常

云南省处于我国南北地震带的南端.该省新构造运动剧烈,地震活动频繁.同时,这里与西藏高原接壤,地下构造颇为复杂.因此,我们收集了该省的地磁短周期变化资料,并进行了初步分析.所得结果表明,云南省地磁短周期变化存在着异常.其表现为:西北自丽江向东南经下关、楚雄到通海,有一条西北-东南的分界线.此线的东北与西南,地磁湾扰、磁暴急始等短周期事件垂直分量的变化△z 呈现反号.由此推断,在该省地下存在着自西北到东南走向的电导率异常带.最后,简单讨论了与高导带存在有关的其他地球物理现象.      

甘肃省东部地区短周期地磁变化异常及其与地震的关系

本文研究甘肃省东部地区的短周期地磁变化的特点,由此推测地下存在东西方向的一条电导率异常带.结合地热和地震活动现象,对高电导率异常的成因提出了初步解释,最后,试图探索电导率异常与地磁发生的关系.     

山西大地电场观测概述

对山西5个地震台站记录到的大地电场观测数据从日变化特征、数据相关性、地电湾扰等方面进行分析,结果表明,在观测系统、环境正常的情况下,代县、临汾、夏县台观测曲线有明显的日变化,其形态与地磁日变相似;各台站同测向数据均有较高的相关系数;代县、临汾、夏县能记录到地电湾扰变化,且临汾、夏县台记录的湾扰变化形态呈明显相似性特征.     

伴随宫城县近海地震的地磁变化

1978年日本宫城县近海发生7.4级强震前约1个月,东北大学女川地磁观测所观测到地磁明显的短周期变化。 1.序言 在地面观测点观测到的地磁短周期变化(如湾扰、地磁脉动等周期小于1小时的变化),是地球外部原因产生的变化和它在地球内部感应电流所引起的变化的总和,因此地磁短周期     

华北区域性地磁低点位移异常特征

利用河北及邻省共17个台站的地磁Z分量资料,统计分析了地磁低点位移的异常特征。研究结果表明: ①异常并不与静、扰日存在明显的依赖关系;②异常具有季节性变化,冬、夏季最多,秋季次之,春季最少;③异常偏离中午的最大时间大多在下午及偏西方位;④有震异常占总异常35%,异常后1个月内发震的占有震异常47%。并认为一次较强地震前,日变低点位移可能有多次异常出现,利用文中提出的地磁交汇法进行追踪预报效果较好。     

陕西及其邻近地区地磁短周期异常和上地幔高导层的初步研究

近年来,随着对地震孕育背景和上地幔地球物理特性的研究,人们日益注意用地磁短周期异常来研究地壳和上地幔的电性结构。在我国继陈伯舫提出渤海地区西部地磁短周期异常并推测其下存在电导率异常后,徐文耀等人分析了甘肃东部的地磁短周期异常,也提出该区存在一东西向高导带。最近,祁贵仲等人利用渤海地区25个台站的地磁资料,进一     

Analysis and interpretation of anomalous conductivity and magnetic permeability effects in time domain electromagnetic data: Part I: Numerical modeling

Time domain electromagnetic (TDEM) response is usually associated with eddy currents in conductive bodies, since this is the dominant effect. However, other effects, such as displacement currents from dielectric processes and magnetic fields associated with rock magnetization, can contribute to TDEM response. In this paper we analyze the effect of magnetization on TDEM data. We use a 3-D code based on finite-difference method, developed by Wang and Hohmann [Geophysics 58 (1993) 797], to study transient electromagnetic field propagation through a medium containing bodies with both anomalous conductivity and anomalous magnetic permeability. The remarkable result is that the combination of anomalous conductivity and permeability within the same body could increase significantly the anomalous TDEM response in comparison with purely conductive or purely magnetic anomalies. This effect has to be taken into account in interpretation of TDEM data over electrical inhomogeneous structures with potentially anomalous magnetic permeability.     

Interference effects of aircraft on earth's electromagnetic response at very low frequency and low frequency

Over the last few decades, very low frequency electromagnetics has been widely and successfully applied in mineral exploration and groundwater exploration. Many radio transmitters with strong signal‐to‐noise ratios are scattered in the very low frequency band and low frequency band. Based on experiences gained from ground measurements with the radio‐magnetotelluric technique operating in the frequency interval 1–250 kHz, broadband magnetometers have been used to cover both very low frequency (3–30 kHz) and low frequency (30–300 kHz) bands to increase the resolution of the near‐surface structure. The metallic aircraft as a conductive body will distort the magnetic signal to some extent, and thus it is important to investigate aircraft interference on the electromagnetic signal. We studied noise caused by rotation of an aircraft and the aircraft itself as a metallic conductive body with three methods: 3D wave polarization, determination of transmitter direction and full tipper estimation. Both very low frequency and low frequency bands were investigated. The results show that the magnetic field is independent of the aircraft at low frequencies in the very low frequency band and part of the low frequency band (below 100 kHz). At high frequencies (above 100 kHz), the signals are more greatly influenced by the aircraft, and the wave polarization directions are more scattered, as observed when the aircraft turned. Some aircraft generated noise mixed with radio transmitter signals, detected as ‘dummy’ signals by the 3D wave polarization method. The estimated scalar magnetic transfer functions are dependent on the aircraft flight directions at high frequencies, because of aircraft interference. The aircraft eigenresponse in the transfer functions (tippers) between vertical and horizontal magnetic field components was compensated for in the real part of the estimated tippers, but some unknown effect was still observed in the imaginary parts.     

导电薄层下三维岩脉的CSAMT响应分析

为了研究探测3D导电岩脉状目标体的可控源音频电磁法（CSAMT）的最佳观测装置,我们用压缩积分方程法数值模拟了赋存于导电薄层下的3D导电岩脉的电磁场响应.计算结果表明,最有效的源的位置是把发射源沿岩脉的走向放置.相对于其他几种装置,这种观测装置得到的异常场与主场的比值最大.利用这种最佳的观测装置,通过将异常场E_ya 和 H_xa与噪音作对比,研究了当岩脉的埋深和高度发生变化时岩脉的可探测性问题.当岩脉的顶部埋深增加时,导电岩脉的可探测性降低.岩脉高度的变化对异常场幅值的影响要小于岩脉顶部埋深所产生的影响.所以通常情况下,即便应用了最有效的CSAMT装置,也很难探测到岩脉的高度,应用钻孔中的探测可能会解决这个问题.     

An analogue model study of electromagnetic induction in the eastern coastal region of North America

The behavior of electric and magnetic field variations over the eastern coastal region of North America is studied using a scaled laboratory electromagnetic analogue model. The model source frequency used simulates a period of 1 h in the geophysical scale. The results indicate that deflection and conductive channelling of induced electric current is important for both the E-polarization (northeast-southwest direction of the electric field of the source) and the H-polarization (northwest-southeast) of the source field. In the model, conductive channelling occurs through the Strait of Belle Isle, Cabot Strait, and in the St. Lawrence River. Current deflection is particularly prevalent around the southeast coast of Newfoundland for both E- and H-polarization, and around the northeast coastline of Nova Scotia for E-polarization. The model results also show current deflection by cape and bay coastal features, as well as by ocean depth contours.A comparison of model measurements for the cases of a uniform source field and a line current source indicate that the nature of the source field has a measurable but surprisingly small effect on the vertical to horizontal magnetic field ratio for both E- and H-polarizations, and negligible effect on the magnetotelluric ratio for coastal regions.The model fields in coastal regions were found to be strongly influenced by induced currents, deflected and channelled by the coastline and ocean bathymetry, and were dependent on the nature and particularly the polarization of the source field. Thus, along the complex coastline of eastern North America, a wide range of electric and magnetic field values should be expected. In some regions the coast effect, measured by the vertical to horizontal magnetic field ratio at the coast, could be expected to be extremely small or absent, while in other regions the ratio could approach a value as large as unity for variations of 1 h period.     

用积分方程法研究源与勘探区之间的三维体对CSAMT观测曲线的影响

在可控源音频大地电磁法野外作业中,源和勘探区间的距离可达几公里,为了了解源和勘探区间的异常体对勘探区内异常响应的影响,我们用三维压缩积分方程法做了数值模拟研究.首先对勘探区目标体进行了数值模拟,发现在高频时,观测到的异常中心位于目标体的正上方,随着频率降低,出现异常中心向远离源的方向略有移动的现象,所以对三维异常体最好用3D软件来解释.然后,对源和勘探区间存在三维异常体的情况进行了数值模拟与分析讨论.模拟结果表明只有当三维异常体达到较大的规模时,才会对目标体上方的观测曲线造成影响,否则其电阻率的变化及埋深的变化对观测曲线的影响较小,可以忽略.当异常体在源方向有延伸时,观测曲线受到的干扰最大,沿垂直源布设的方向延伸时引起的干扰中等,垂直地面向下延伸引起的干扰最小.     

矫顽力组分定量分析揭示下蜀黄土磁化率异常降低的原因

镇江大港下蜀黄土剖面多个层位的磁化率出现异常降低,导致与北方黄土的磁化率记录难以对比.为探讨磁化率异常降低的原因,我们在大港钻探ZK孔获取了岩芯.在10个具代表性的层位采样并测量了其等温剩磁获得曲线.通过基于期望最大化算法的计算程序（Irmunmix V2.2）,定量分析了样品的磁性矫顽力组分（magnetic coercivity component）.结果显示这10个样品可大致分成3类,第一类不含中磁组分,第二类含有较少的中磁组分,第三类则含较多的中磁组分.矫顽力组分的含量与样品中铁锰结核的含量密切相关.铁锰结核含量高的样品,中磁组分、硬磁组分含量也高,而软磁组分含量低,与此对应的是这类样品的磁化率低.表明在铁锰结核的形成过程中,原始的软磁组分被溶解,而产生新的中磁组分、硬磁组分,这个次生变化过程导致样品磁化率异常降低.铁锰结核含量高的样品,经历过较强的还原作用,因此ZK剖面磁化率异常降低是还原作用的结果,可能是某时期降水量增大所致.     

Large Magnetic Storm on September 7–8, 2017: High-Latitude Geomagnetic Variations and Geomagnetic <Emphasis Type="Italic">Pc</Emphasis>5 Pulsations

The features of daytime high-latitude geomagnetic variations and geomagnetic pulsations in the Рс5 range during the recent, large, two-stage magnetic storm of September 7–8, 2017 are studied. The discussed disturbances were observed at the recovery phase of the first stage of the storm after the interplanetary magnetic field (IMF) turned northward. It is shown that the large sign-alternating variations in Ву and Bz components of the IMF caused intense geomagnetic disturbances up to 300–400 nT with a quasi-period of ~20 min in the daytime sector of polar latitudes, probably in the region of the daytime polar cusp. These disturbances may have reflected quasi-period motions of the daytime magnetopause and may have resulted from nonlinear transformation of the variations in the interplanaterary magnetic field in the magnetosheath or in the magnetospheric entry layers. The appearance of high-latitude long-period variations was accompanied by the excitation of bursts (wave packets) of geomagnetic Pc5 pulsations. The onset of Pc5 pulsation bursts often coincided with a sudden northward turn of the IMF. It was discovered for the first time that the development of a “daytime polar substorm,” i.e., a negative magnetic bay in the daytime sector of polar latitudes, led to a sudden termination of the generation of geomagnetic Pc5 pulsations over the entire latitude range in which these oscillations were recorded before the appearance of the daytime bay.     

瞬变电磁法对存在山谷地形时的多个异常体的探测能力研究

以层状地层中的2个三维导电薄板为例,考虑了山谷地形存在时,瞬变电磁法(TEM)分辨地下多个三维异常体的能力。模型计算的结果表明,当有山谷地形存在时,总的异常场由薄板的异常和地形的异常叠加在一起,表明异常场在空间上具有可加性质。在地形平坦时,TEM方法对相同尺寸的近源异常体的分辨能力要优于远源,而且离源越远的异常体异常越小,最后直到无法分辨。但是当存在山谷地形并且把源放在其中时,它们的总异常场叠加后会“放大”远源异常体的异常场,在一定意义上说,这对于分辨远处的异常体更加有利。当异常体在接收器下方埋深不是太大时,不论山谷地形在源处还是接收器处,TEM法对这些异常体的探测能力都是比较好的。2个异常体相距太近时,其异常会叠加在一起,给TEM的分辨增加一些困难。总体表明,在有山谷地形存在时,TEM方法对多个异常体仍具有较好的探测能力     

Exploration of geothermal structure in Puga geothermal field, Ladakh Himalayas, India by magnetotelluric studies

To understand the crustal electric structure of the Puga geothermal field located in the Ladakh Himalayas, wide band (1000 Hz–0.001 Hz) magnetotelluric (MT) study have been carried out in the Puga area. Thirty-five MT sites were occupied with site spacing varying from 0.4 to 1 km. The measurements were carried out along three profiles oriented in east–west direction. After the preliminary analysis, the MT data were subjected to decomposition techniques. The one-dimensional inversion of the effective impedance data and the two-dimensional inversion of the TE (transverse electric) and TM (transverse magnetic) data confirm the presence of low resistive (5–25 Ω m) near surface region of 200–300 m thick in the anomalous geothermal part of the area related to the shallow geothermal reservoir. Additionally, the present study delineated an anomalous conductive zone (resistivity less than 10 Ω m) at a depth of about 2 km which is possibly related to the geothermal source in the area. A highly resistive basement layer separates the surface low resistive region and anomalous conductive part. The estimated minimum temperature at the top of conductive part is about 250 °C. The significance of the deeper conductive zone and its relation to the geothermal anomaly in the area is discussed.     

APPARENT RESISTIVITY TEMPORAL VARIATION CHARAC-TERISTICS AFFECTED BY THE FIXED DISTURBANCE SOURCE ON SURFACE OF MEASURING AREA

Current leakage,metallic conductor,and local anomalous resistivity body are main disturbance sources which affect the successive observation of apparent resistivity in stations,besides the observing system failure.We construct a finite element model using a 3-layered horizontal medium to discuss the dynamic characteristics of disturbances caused by metal conductor and local anomalous resistivity body in the measuring filed.The numerical results show that low resistivity source which is located in areas where the sensitivity coefficient is positive will cause decline on apparent resistivity observation.While low resistivity source will cause increase when it is located in areas where the sensitivity coefficient is negative.Disturbance caused by high resistivity source is opposite to the one from low resistivity source.The general dynamic feature of disturbance is that the disturbance amplitude increases as the resistivity of shallow layer decreases,while the amplitude declines when the shallow layer's resistivity increases.For the measuring direction which has normal annual variation form,low resistivity source which is located in area where the sensitivity coefficient is positive will increase the annual variation amplitude,while it will reduce annual amplitude when it is in a negative sensitivity coefficient area.Annual amplitude changes caused by high resistivity source are opposite to the changes caused by low resistivity source.For the measuring direction which has abnormal annual variation form,dynamic annual feature is opposite to the one in direction of normal annual variation form.If the dynamic feature is opposite to the annual variation and disturbance amplitude is also greater than annual amplitude,the annual variation will change direction.Disturbance amplitude from metallic conductor is affected by the resistivity and cross-section area,the lower of the resistivity and the larger of the cross-section area,the greater of the disturbance amplitude.