The perturbation of alternating geomagnetic fields by discontinuities with high conductivity contrasts

The perturbation of alternating geomagnetic fields by conductivity discontinuities is considered. A numerical method is used to solve the two-dimensional induction problem. Models in which the conductivity contrast between conductive regions is great, such as between oceanic and continental regions, are considered. The perturbation field distributions for models with higher conductivity contrasts are compared with lower conductivity contrast models using a method recently developed by Jones (1972). Total field solutions are obtained for several models by using a very large grid as well as a folding-in technique. Results from the two methods for the various models are compared. It is found that the folding-in technique offers an alternative method for handling higher conductivity contrasts.     

The calculation of perturbation and induction arrows for a three-dimensional conductivity model and dipole source fields

A numerical method is used to calculate the electromagnetic fields associated with a three-dimensional conductivity anomaly. The source field is due to horizontal magnetic dipoles placed at two different positions with respect to the conductivity anomaly. The transfer functions and related perturbation and induction arrows associated with the fields are calculated and compared with the arrows obtained from a uniform source calculation. The results show the source effect on the induction arrows and indicate that the perturbation arrows provide a method of outlining the spatial extent of the anomaly. The transfer function calculations are made for both exact and approximate normal fields. In the transfer function calculation the anomalous fields are correlated with a normal field as suggested by Schmucker (1970) and Cochrane and Hyndman (1970).     

Irregular coastline and channeling effects in three-dimensional geomagnetic perturbation models

Two models, one with two small islands in a relatively narrow channel between two larger land masses and the other with an island near an irregular coast, are investigated with a view to assessing the importance of small-scale features. The perturbation of a uniform alternating geomagnetic field of various frequencies is studied by a numerical technique in which the inhomogeneous models are represented by cells of different conductivity within a three-dimensional mesh. The results are presented as contour plots of the amplitudes of the various field components over the surface of the conducting region, as well as by selected profiles of the field component amplitudes and phases. The channeling of currents around the higher resistivity island structures is apparent in the results. The effects of the small-scale structure of the coastline are pronounced in the model, and indicate that considerable care needs to be taken in selecting sites for experiments and in interpreting the results.     

地磁场三维曲面Spline模型

利用149个地面实测数据以及12个子午工程测点数据,50个CHAMP卫星高度实测数据,并结合高空180 km处的50个IGRF12数据点,基于这三个高度的数据首次建立了中国地区地磁场三维曲面Spline（3D Spline）模型.在境外添加了39个测点以控制边界效应.通过CM4模型将所有测点的场源进行分离,统一通过主磁场值建模分析.通过将模拟结果与实测值、曲面Spline（2D Spline）以及Taylor（2D Taylor）、三维Taylor（3D Taylor）模型及IGRF12模型相比较,结果显示3D Spline模型的空间分布与其他模型整体趋势一致,但更为曲折,随着高度上升,3D Spline模型的要素Y的强度逐渐减弱.通过比较3D Spline、2D和3D Taylor模型对于不同高度6个缺测点的模拟值,残差和均方根偏差（RMSE）,3D Spline模型的模拟效果最好,要素Y、Z和总强度F的RMSE值要比其他模型低50%以上.3D Spline模型在不同高度处的模拟效果主要取决于该高度附近的实测值数量和精度.

     

天津轨道交通对地磁观测干扰的影响研究

直流牵引城市轨道交通系统在运行过程中产生的磁场对我国大城市周边地磁台站的观测数据产生较为严重的干扰.城市轨道交通运行过程中产生泄露电流是产生干扰的重要原因.本文以真空中的线电流磁场计算模型为基础,通过简化的远场磁场干扰模型对天津地铁轨道交通干扰进行理论建模.该模型计算简洁高效.通过对模型的数值模拟,获得了干扰随距离衰减...     

Upper-mantle conductivity determined by the geomagnetic lunar daily variation

The geomagnetic lunar daily variation, and the solar daily variation, have been recently analysed by Malin. The response measurements that he obtained are used here to investigate the radial distribution of electrical conductivity in the upper mantle. It is found that these observed response measurements require low conductivity in the mantle, at least as far down as 650 km from the surface. This result is compatible with the conductivity models determined by the quiet-day daily variation, and so justifies the use of these variation fields for geomagnetic deep sounding.     

小尺度地磁场勒让德多项式建模方法

根据地面实测地磁场数据,提出一种基于勒让德多项式的对小尺度地磁场的模型建立方法,结合数据预处理、最小二乘拟合方法,采用高阶的模型描述典型小尺度区域的地磁场,绘制出地磁图.通过实验验证,模型拟合实验点的数据与真实数据的磁场总强度平均误差小于30 nT,并且反映出小尺度区域中磁场的精细结构,有效地抑制了边界效应,大大提高了地磁场建模能力.另外讨论了勒让德多项式建模方法的优缺点以及使用范围.这为地磁场在空间延拓、探矿、地下结构分析提供了重要的方法和理论基础.     

小尺度地磁场勒让德多项式建模方法

根据地面实测地磁场数据,提出一种基于勒让德多项式的对小尺度地磁场的模型建立方法,结合数据预处理、最小二乘拟合方法,采用高阶的模型描述典型小尺度区域的地磁场,绘制出地磁图.通过实验验证,模型拟合实验点的数据与真实数据的磁场总强度平均误差小于30 nT,并且反映出小尺度区域中磁场的精细结构,有效地抑制了边界效应,大大提高了地磁场建模能力.另外讨论了勒让德多项式建模方法的优缺点以及使用范围.这为地磁场在空间延拓、探矿、地下结构分析提供了重要的方法和理论基础.     

Inversion of two-dimensional conductivity structures

This review shows that the inversion of two-dimensional structures is still an almost uninvestigated area. Only for very restricted anomalous domains exact methods exist: (a) undulated interface between perfect conductor and insulator; (b) thin non-uniform sheet. So far, real conductors must be inverted by linearization. A method for the computation of the pertinent kernels is described and the well-developed method of generalized matrix inversion is applied in a preliminary study both to artificial and real data.     

A numerical study of the topographic effect on electromagnetic fields in a three-dimensional conductivity model

Summary The topographic effect on electromagnetic fields in a three-dimensional conductivity model is investigated using a finite-difference numerical method. The model is that of a conducting block buried in the host Earth beneath a conducting hill. The altitude effect as well as the effect of conductivity of the hill has been studied by comparing the results for the field components at the surface of the hill with those at the surface of a flat half-space. The results indicate that both altitude and the hill conductivity influence the behaviour of the electromagnetic fields. For certain traverses beyond the edge of the block, it is seen that the variations in some field components are mainly due to the topographic effect and that the contribution from the block itself is negligible.     

Comparison of geomagnetic planetary reference fields over Italy

Geomagnetic field measurements on a network at 106 repeat stations (1/3000 km²), and a second order network at 2550 stations (1/100 km²), were completed in 1981. Total intensity and the horizontal and vertical component of the field were measured at all stations, and in addition declination was measured at the first order stations. Normal fields for each element were computed from the first order network measurements (using a second degree polynomial in latitude and longitude), and comparisons were made for total intensity with 3 planetary reference fields. The use of the polynomial normal normal field is suggested to draw anomaly maps.     

Anomalous geomagnetic fields of internal origin in Czechoslovakia

Summary The magnetovariational data of 143 stations distributed over the eastern margin of the Bohemian Massif, over the Brunovistulicum and the West Carpathian sector were analysed to obtain transfer functions of the geomagnetic field. Methods of multivariate coherence analysis (spectral domain approach) and of impulse response (time domain approach) were employed. Complex induction vectors were estimated and contour maps of transfer functions were computer generated. Analysing their spatial distribution, we mapped the zones of anomalous induction and interpreted them in terms of electrical conductivity structure with its tectonic implications.

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Contribution No. 105/90, Geophysical Institute, Czechosl. Acad. Sci., Prague.     

Numerical solution of the two-dimensional inverse geomagnetic induction problem

Summary An effective numerical approach to the solution of the two-dimensional inverse geomagnetic induction problem using the linearization method is presented. The numerical realization of the inversion is based on Marquardt's algorithm, for which the solution of the direct problem and the partial derivatives of this solution with respect to the electrical parameters of the medium are computed by the finite difference method. Theoretical models are studied and numerical results are presented.     

The upper mantle conductivity analysis method using observatory records of the geomagnetic field

The electrical conductivity of the Earth's upper mantle can be inferred from geomagnetic quiet-day,Sq, variations recorded at the world's observatories using the, coefficients of a spherical harmonic analysis (SHA) that separate the external (source) and internal (induced) parts of the surface field. The conductivity profile determined from such an analysis can be sensitive to special characteristics of the quiet field itself as well as the separation techniques employed. This review of the Sq-analysis features critical to a conductivity derivation is pictorially presented along with the equations for application of theSchmucker (1970) technique to theSHA coefficients for a conductivity determination. Three examples illustrate the use of these equations with differentSq models.     

The correlation between the variation in ionospheric conductivity and that of the geomagnetic Sq field

Effects of the solar activity on the geomagnetic Sq field were studied by examining the correlation of the Sq amplitude in the Y-component with the sunspot number or height integrated Pedersen and Hall conductivities of the ionosphere at Kakioka, Chambon La Foret and Port Moresby for a period of 21 years. It was found that solar activity dependence of the Sq amplitude is almost explained by the effect of the local ionospheric conductivity if the month is fixed. That is, the solar activity dependence in each month is mainly caused by the local conductivity. However, the amplitude is clearly small in winter for the same conductivity value. This is probably due to the seasonal difference of the neutral winds driving ionospheric dynamo currents or to the magnetic effect of the field-aligned currents connecting both hemispheres driven by the asymmetric dynamo action in the ionosphere.     

中国地磁台观测的2014年地磁急变

地磁急变（jerk）是起源于地球外核并在导电地幔过滤效应后在地球表面观测到的一种地磁现象,其反映了地核内部某些动力学过程.Jerks在空间范围上既可以是区域性的,也可以是全球性的.中国地区地磁台能否检测到2014年jerk？针对这一问题,利用中国大陆10个地磁台的磁静日月均值和CHAOS-6全球磁场模型,分析了X、Y和Z分量2008—2018年期间的长期变化,估算了2014年前、后的长期加速度值,确定了2014年地磁jerk的时间和强度.研究表明中国地磁台Y分量的长期变化为"Λ"型,Z分量存在明显的"V"型,具有典型的jerk特点.Y分量jerk出现的时间大约在2014年6月,比非洲大陆的Algeria TAM台和南美洲French Guiana KOU台时间滞后大约4个月.这暗示着产生jerks的地核流体波动的时序特点.中国西部和东北部地磁台的长期变化形态有明显的差别,主要由非偶极磁场引起.CHAOS-6模型与地面台站的长期变化形态并非始终一致.本文结果有助于更好地理解和解释长期变化的时间演变和地理分布,并为深入探讨jerks的地核起源和驱动机制提供新的观测约束.     

中国地磁台观测的2014年地磁急变

地磁急变（jerk）是起源于地球外核并在导电地幔过滤效应后在地球表面观测到的一种地磁现象,其反映了地核内部某些动力学过程.Jerks在空间范围上既可以是区域性的,也可以是全球性的.中国地区地磁台能否检测到2014年jerk？针对这一问题,利用中国大陆10个地磁台的磁静日月均值和CHAOS-6全球磁场模型,分析了X、Y和Z分量2008—2018年期间的长期变化,估算了2014年前、后的长期加速度值,确定了2014年地磁jerk的时间和强度.研究表明中国地磁台Y分量的长期变化为"Λ"型,Z分量存在明显的"V"型,具有典型的jerk特点.Y分量jerk出现的时间大约在2014年6月,比非洲大陆的Algeria TAM台和南美洲French Guiana KOU台时间滞后大约4个月.这暗示着产生jerks的地核流体波动的时序特点.中国西部和东北部地磁台的长期变化形态有明显的差别,主要由非偶极磁场引起.CHAOS-6模型与地面台站的长期变化形态并非始终一致.本文结果有助于更好地理解和解释长期变化的时间演变和地理分布,并为深入探讨jerks的地核起源和驱动机制提供新的观测约束.     

Numerical calculations of the perturbation of normal heat flow by topographic and local conductivity structure

A numerical method is used to investigate the effect of topographic and local thermal conductivity anomalies on near-surface heat flow for two-dimensional models. Heat flow associated with a sloping topographic structure is calculated. Also, the effects of a fault structure associated with the sloping topography are considered. Vertical and horizontal heat flow components are calculated alone; the surface of the earth as well as throughout the whole region of interest. The results indicate that surface heat flow is substantially affected by topographic relief and the horizontal heat flow component associated with topographic features can be large. Also, regional heat flow is greatly perturbed by local thermal conductivity anomalies and the effect of topographic features may be considerably modified by the subsurface structure.     

Morphology of slowly-varying geomagnetic external fields — a review

The morphology of slowly-varying geomagnetic external fields (such as Sq, L, and Dst) and the production mechanisms of electric current systems for those fields are briefly reviewed to provide background knowledge of the present state of research regarding the source fields for scientists concerned with the electromagnetic induction within the earth. It is concluded that both the Sq and Dst fields seem to have sources both in the magnetosphere and the ionosphere, while the classic idea of L-field production solely in the ionosphere by the wind dynamo is still acceptable.