Conductivity Structure Of Earth Research Articles

Comparison of different geoelectric field methods to calculate geomagnetically induced currents in North China

Geomagnetically induced currents (GICs) are caused by the interaction between the geomagnetic storm and the Earth's conductivity structure, which can enter the power grid and cause blackouts in extreme cases. To avoid significant economic losses, it is essential to correctly estimate the intensity of GICs first and then take corresponding protection measures to the power grid. There are many methods to carry out this estimation, of which the results can be very different. To evaluate these differences, four estimates of GICs for the 1000-kV grid in North China were compared during the geomagnetic storms that occurred from November 7 to 9, 2004, based on the conductivity models and magnetotelluric (MT) data from previous studies. These four estimations include using field MT data (data-space method), a one-dimensional (1D) conductivity model (1D model-space method), and two 3D conductivity models (3D model-space method) in the same area from different research teams. In consideration of the complex topology of the power system and the interactions between different power lines, the estimation using field MT data obtains the highest accuracy. Those using the 3D conductivity models rank the second and the one using the 1D conductivity model ranks the third. Among all, the data-space method is sensitive to the so-called corner effect in some transformers, which infers that this method provides the most correct ground current path information. The crustal structures of the two 3D conductivity models are slightly different. This slight difference does not bring any troubles to the final geophysical purpose but leads to a butterfly effect in the estimation of GICs, causing the difference between the estimation results based on the two models to an extreme value of 267.2 A. Based on the statistical analysis of the maximum intensity and duration of GICs in 20 substations, we believe that the GICs of the North China 1000-kV power grid have reached a very high-risk level, and the assessment results are 2 ∼ 3 times higher than other Kp = 8 geomagnetic storm events. Previous studies have shown that there are four conductors in the North China region, namely C1, C2, C3, and C4. After analyzing these conductors, we have concluded that C3 and C4 have a greater impact on the geoelectric field and GICs. Ultimately, it has furnished advantageous guidance for the installation of GICs monitoring apparatus and the discernment of optimal substation siting.

The geoelectric field coast effect: Results from two dimensional finite element method and analytic solutions

The geoelectric fields induced during geomagnetic disturbances (GMD) may affect the normal operation of technological systems at the Earth’s surface. The non-uniform Earth conductivity structure, especially large lateral conductivity contrasts, can lead to the enhancement of geoelectric fields near boundaries such as a coastline. In this paper, we give an overview of the applicability of a two-dimensional finite element method (2D-FEM) for studying lateral variations of the Earth’s conductivity, which can be considered as one of magnetotelluric sounding forward modelling methods, including both the E and the H polarization cases. Then the spatial distribution of the geoelectric field near a continent-ocean boundary is calculated with different models. This is compared with analytic solutions for simple models to verify the accuracy of the FEM modelling, which is calculated by using the ANSOFT software. More realistic scenarios are then developed to emphasize the advantages of FEM. The factors influencing the coast effect are discussed. Results show that frequency, the shape of seafloor boundaries, and the depth of ocean have a significant impact on the geoelectric fields. Therefore, these factors should attract more attention when assessing risk for man-made conducting systems located within the coastal influence range.

Telluric Field Variations as Drivers of Variations in Cathodic Protection Potential on a Natural Gas Pipeline in New Zealand

AbstractA study of variations in cathodic protection potential on a natural gas pipeline in the North Island of New Zealand is reported. Both the measured pipe‐to‐soil potential (PSP) and the current measured through an installed defect in the pipeline coating show strong correlations with variations in measured magnetic and telluric fields. Contrary to predictions from distributed‐source transmission line theory, analysis shows that the closest correlations of PSP and current are with the component of telluric field perpendicular to the pipeline. This orientation is close to perpendicular to major electrical conductivity contrasts associated with both the local coastline and with on‐shore Earth conductivity structure. It is therefore inferred the variations in PSP which drive the variations in defect current are the result of changes in the local Earth potential. While reporting on specific observations from New Zealand, these results should be applicable across a wide range of middle and high‐latitude global locations. Estimates of possible corrosion rates for the pipeline in question suggest that such enhancement of telluric field variations by conductivity structure may pose significant risk to pipelines in the event of defects in the pipeline coating.

Quantitative influence of coast effect on geomagnetically induced currents in power grids: a case study

In recent years, several magnetic storms have disrupted the normal operation of power grids in the mid-low latitudes. Data obtained from the monitoring of geomagnetically induced currents (GIC) indicate that GIC tend to be elevated at nodes near the ocean-land interface. This paper discusses the influence of the geomagnetic coast effect on GIC in power grids based on geomagnetic data from a coastal power station on November 9, 2004. We used a three-dimensional (3D) Earth conductivity model to calculate the induced electric field using the finite element method (FEM), and compared it to a one-dimensional (1D) layered model, which could not incorporate a coastal effect. In this manner, the GIC in the Ling’ao power plant was predicted while taking the coast effect into consideration in one case and ignoring it in the other. We found that the GIC predicted by the 3D model, which took the coastal effect into consideration, showed only a 2.9% discrepancy with the recorded value, while the 1D model underestimated the GIC by 23%. Our results demonstrate that the abrupt lateral variations of Earth conductivity structures significantly influence GIC in the power grid. We can infer that high GIC may appear even at mid-low latitude areas that are subjected to the coast effect. Therefore, this effect should be taken into consideration while assessing GIC risk when power networks are located in areas with lateral shifts in Earth conductivity structures, such as the shoreline and the interfaces of different geological structures.

A Comparison of Peak Electric Fields and GICs in the Pacific Northwest Using 1‐D and 3‐D Conductivity

AbstractGeomagnetically induced currents (GICs) are a result of the changing magnetic fields during a geomagnetic disturbance interacting with the deep conductivity structures of the Earth. When assessing GIC hazard, it is a common practice to use layer‐cake or one‐dimensional conductivity models to approximate deep Earth conductivity. In this paper, we calculate the electric field and estimate GICs induced in the long lines of a realistic system model of the Pacific Northwest, using the traditional 1‐D models, as well as 3‐D models represented by Earthscope's Electromagnetic transfer functions. The results show that the peak electric field during a given event has considerable variation across the analysis region in the Pacific Northwest, but the 1‐D physiographic approximations may accurately represent the average response of an area, although corrections are needed. Rotations caused by real deep Earth conductivity structures greatly affect the direction of the induced electric field. This effect may be just as, or more, important than peak intensity when estimating GICs induced in long bulk power system lines.

Modeling geomagnetically induced currents

AbstractUnderstanding the geomagnetic hazard to power systems requires the ability to model the geomagnetically induced currents (GIC) produced in a power network. This paper presents the developments in GIC modeling starting with an examination of fundamental questions about where the driving force for GIC is located. Then we outline the two main network modeling approaches that are mathematically equivalent and show an example for a simple circuit. Accurate modeling of the GIC produced during real space weather events requires including the appropriate system characteristics, magnetic source fields, and Earth conductivity structure. It is shown how multiple voltage levels can be included in GIC modeling and how the network configuration affects the GIC values. Magnetic source fields can be included by using “plane wave” or line current models or by using geomagnetic observatory data with an appropriate interpolation scheme. Earth conductivity structure can be represented by 1‐D, 2‐D, or 3‐D models that are used to calculate the transfer function between electric and magnetic fields at the Earth's surface. For 2‐D and 3‐D structures this will involve a tensor impedance function and electric fields that are not necessarily orthogonal to the magnetic field variations. It is now technically possible to include all these features in the modeling of GIC, and various software implementations are being developed to make these features more accessible for use in risk studies.

Mapping geoelectric fields during magnetic storms: Synthetic analysis of empirical United States impedances

AbstractEmpirical impedance tensors obtained from EarthScope magnetotelluric data at sites distributed across the midwestern United States are used to examine the feasibility of mapping magnetic storm induction of geoelectric fields. With these tensors, in order to isolate the effects of Earth conductivity structure, we perform a synthetic analysis—calculating geoelectric field variations induced by a geomagnetic field that is geographically uniform but varying sinusoidally with a chosen set of oscillation frequencies that are characteristic of magnetic storm variations. For north‐south oriented geomagnetic oscillations at a period of T0=100 s, induced geoelectric field vectors show substantial geographically distributed differences in amplitude (approximately a factor of 100), direction (up to 130∘), and phase (over a quarter wavelength). These differences are the result of three‐dimensional Earth conductivity structure, and they highlight a shortcoming of one‐dimensional conductivity models (and other synthetic models not derived from direct geophysical measurement) that are used in the evaluation of storm time geoelectric hazards for the electric power grid industry. A hypothetical extremely intense magnetic storm having 500 nT amplitude at T0=100 s would induce geoelectric fields with an average amplitude across the midwestern United States of about 2.71 V/km, but with a representative site‐to‐site range of 0.15 V/km to 16.77 V/km. Significant improvement in the evaluation of such hazards will require detailed knowledge of the Earth's interior three‐dimensional conductivity structure.

The Evolution of Québec Earth Models Used to Model Geomagnetically Induced Currents

Geomagnetically induced currents (GIC) in power systems can interfere with the system operation and, in extreme cases, can damage transformers and cause power blackouts. A notable example is the magnetic disturbance of March 13, 1989 that caused a blackout of the Hydro-Quebec system. The size of the GIC depends on the inducing magnetic-field variations, earth conductivity structure, and system characteristics. This paper examines the earth models developed for Quebec and shows how the changes in earth models have changed the electric-field values calculated for the March 13, 1989 storm. These calculations are made using data from two magnetic observatories: one at the southern end of Quebec and one at the northern end. It is shown how the choice of magnetic-field data and uncertainty in the earth model influence the calculated electric-field values.

VLF waves at satellite altitude to investigate Earth electrical conductivity

AbstractAt and near the Earth surface, electromagnetic (EM) fields radiated from VLF transmitters are commonly used in geological exploration to determine the shallow Earth conductivity structure. Onboard satellites such as DEMETER, the electric and magnetic sensors detect the VLF signal in altitude. While we know for surface measurement that the VLF EM field recorded at some distance from the transmitter is a function of the ground conductivity, we do not know how this relationship changes when the field is measured at satellite altitude. Here we study the electromagnetic field radiated by a vertical electric dipole located on the Earth surface in the VLF range and measured at satellite altitude in a free space. We investigate the EM field as function of distance from the source, the height above the Earth surface, and the electrical conductivity of the Earth. The mathematical solution in altitude has more severe numerical complications than the well‐known solutions at or near the Earth surface. We test most of the solutions developed for the latter case and found that direct summation was best at several hundred kilometers above the Earth. The numerical modeling of the EM field in altitude shows that the field remains a function of Earth conductivity. The dependence weakens with altitude and distance from the transmitter. It remains more important for the electric radial component.

Proximity Effect on the Induced Geoelectric Field at the Lateral Interface of Different Conductivity Structures During Geomagnetic Storms

AbstractThe induced geoelectric fields and telluric currents generated during geomagnetic storms will be distorted by lateral variations of the Earth conductivity. The Galerkin finite element method (FEM) is a useful tool to analyze complicated electromagnetic field problems. In this paper, a uniform thin sheet current with infinite width located at 100 km above the Earth's surface is assumed to represent the source of geomagnetic variations. It is harmonic with the amplitude of 1 A/m at different frequencies. Three Earth conductivity structures are modelled where the basic model is horizontally uniform for comparisons. The second and third structures have the same exact thicknesses and conductivity values with the basic one on one side and five times greater or lower values on the other side to represent conductivity variation from low to high and from high to low. The computational results show that the abrupt changes of conductivities have influences on the induced telluric currents, which can be classified as the proximity effect and the skin effect. For GIC research in power systems, especially in systems located at coastal areas, the geoelectric field and GIC should be determined taking the directions of the transmission lines and of the coast into consideration. Both the proximity effect and the skin effect should be considered along with the coast effect in research of GIC impacts during geomagnetic disturbances. Boundary conditions discussed in this paper can be varied depending on different modelling techniques and on the scales of conductivity structure models.

An Approach to Model Earth Conductivity Structures with Lateral Changes for Calculating Induced Currents and Geoelectric Fields during Geomagnetic Disturbances

During geomagnetic disturbances, the telluric currents which are driven by the induced electric fields will flow in conductive Earth. An approach to model the Earth conductivity structures with lateral conductivity changes for calculating geoelectric fields is presented in this paper. Numerical results, which are obtained by the Finite Element Method (FEM) with a planar grid in two-dimensional modelling and a solid grid in three-dimensional modelling, are compared, and the flow of induced telluric currents in different conductivity regions is demonstrated. Then a three-dimensional conductivity structure is modelled and the induced currents in different depths and the geoelectric field at the Earth’s surface are shown. The geovoltages by integrating the geoelectric field along specific paths can be obtained, which are very important regarding calculations of geomagnetically induced currents (GIC) in ground-based technical networks, such as power systems.

Evaluating the applicability of the finite element method for modelling of geoelectric fields

Abstract. Geomagnetically induced currents in power systems are due to space weather events which create geomagnetic disturbances accompanied by electric fields at the surface of the Earth. The purpose of this paper is to evaluate the use of the finite element method (FEM) to calculate the magnetic and electric fields to which long transmission lines of power systems on the Earth are exposed. The well-known technique of FEM is used for the first time to simulate magnetic and electric fields applicable to power systems. Several test cases are modelled and compared with known solutions. It is shown that FEM is an effective modelling technique that can be applied to determine the electric fields which affect power systems. FEM enables an increased capability beyond the traditional methods for modelling electric and magnetic fields with layered earth conductivity structures, as spatially more complex structures can be considered using FEM. As an example results are presented for induction, due to a line current source, in adjacent regions with different layered conductivity structures. The results show the electric field away from the interface is the same as calculated for a single region; however near the interface the electric field is influenced by both regions.

Towards quantitative assessment of the hazard from space weather. Global 3-D modellings of the electric field induced by a realistic geomagnetic storm

In order to estimate the hazard to technological systems due to geomagnetically induced currents (GIC), it is crucial to understand the response of the geoelectric field to a geomagnetic disturbance and to provide quantitative estimates of this field. Most previous studies on GIC and the geoelectric field generated during a geomagnetic storm assume a 1-D conductivity structure of Earth. This assumption however is invalid in coastal regions, where the lateral conductivity contrast is large. In this paper, we investigate the global spatio-temporal pattern of the surface geoelectric field induced by a typical major geomagnetic storm in a conductivity model of Earth with realistic laterally-heterogeneous oceans and continents. Exploiting this model makes the problem fully 3-D. Data from worldwide distributed magnetic observatories are used to construct a realistic model of the magnetospheric source. The results of our numerical studies show large amplification of the geoelectric field in many coastal regions. Peak amplitudes obtained with 3-D modelling exceed the amplitudes obtained in a 1-D model by at least a factor 2, even if the latter makes use of the local vertical conductivity structure. Lithosphere resistivity is a critical parameter, which governs both amplitude and penetration width of the anomalous electric field inland.

Calculation and Analysis of Geo-Magnetically Induced Current in Earth Based on Plane Wave Method

Geo-magnetically Induced Current (GIC) is not only flowing in conductors such as power grid and pipeline, but also flowing in earth and GIC in earth will affect GIC in conductors. Thus studying the relationship between GIC in earth and earth conductivity is significant to understand the impacts of conductivity on GIC in earth and conductors. This paper uses Maxwell equations and plane wave method to analyze GIC density lever and distribution characteristics in different earth depth of uniform/layered earth conductivity model and gives the relationship between conductivity and GIC in earth. The results indicate that, the smaller the earth conductivity, the deeper earth GIC distributes, and earth conductivity structure has great impact on GIC. At last, this paper proposes that to calculate GIC in power grid accurately, we need to establish more accurate earth conductivity models.

Effects of Geophysical Parameters on GIC Illustrated by Benchmark Network Modeling

Geomagnetically induced currents (GICs) are identified as a potential hazard to power grids. Significant progress in understanding the physical processes leading to GIC production and its effects on power grid components have been made in recent years. With the development of a GIC benchmark network, researchers are now equipped with a test model that can be used to separate the effects of different environmental conditions from effects of network configuration on the distribution of GIC in a system. This paper describes the effects of the environmental conditions (i.e., geomagnetic variations and earth conductivity structures) on GIC in the benchmark network. Geomagnetic variations during the October 29-31, 2003, space weather events are used, together with two realistic models of the earth conductivity (i.e., more resistive and more conductive). The results of the modeling of different geophysical scenarios expressed in terms of GIC distributions through the system and as two GIC-related indices show the crucial importance of the geophysical conditions in assessing the GIC risk to power systems.

Practical model applicable to investigating the coast effect on the geoelectric field in connection with studies of geomagnetically induced currents

The horizontal geoelectric field induced at the Earth’s surface during temporal variations of the geomagnetic field drives Geomagnetically Induced Currents (GIC) in groundbased conductor networks, such as electric power transmission grids. Usually studies of the geoelectric field in connection with GIC research employ the assumption of a onedimensional (1D) Earth conductivity structure. At ocean-land interfaces the conductivity contrast in the horizontal direction is large making the 1D assumption invalid in coastal areas. Geoelectromagnetic induction literature contains many publications dealing with complicated 2D or 3D Earth models. However, these studies mainly aim at the investigation of the Earth’s structure and often do not provide an easy theoretical discussion and clear numbers to be used in practical power engineering investigations of GIC in networks located at coasts. Regarding GIC applications, James Gilbert introduces a two-dimensional (2D) Earth conductivity model for studying the “coast effect” near an

Earth conductivity structures and their effects on geomagnetic induction in pipelines

Abstract. Anomalous, large pipe-to-soil potentials (PSP) have been observed along a natural gas pipeline in eastern Ontario, Canada, where there is a major geological contact between the highly resistive rocks of the Precambrian Shield to the west and the more conductive Paleozoic sediments to the east. This study tested the hypothesis that large variations of PSP are related to lateral changes of Earth conductivity under the pipeline. Concurrent and co-located PSP and magnetotelluric (MT) geophysical data were acquired in the study area. Results from the MT survey were used to model PSP variations based on distributed-source transmission line theory, using a spatially-variant surface geoelectric field. Different models were built to investigate the impact of different subsurface features. Good agreement between modelled and observed PSP was reached when impedance peaks related to major changes of subsurface geological conditions were included. The large PSP could therefore be attributed to the presence of resistive intrusive bodies in the upper crust and/or boundaries between tectonic terranes. This study demonstrated that combined PSP-MT investigations are a useful tool in the identification of potential hazards caused by geomagnetically induced currents in pipelines.

Local time stacking of geomagnetic solar daily variations

Regular geomagnetic solar daily variations, the Sq signal, is used in electromagnetic induction studies to investigate the electrical conductivity structure of Earth's upper mantle. Traditionally, Sq induction studies employ observatory magnetic data from a few quiet days in which the overprint on the regular Sq ionospheric current vortices from various irregular disturbances is minimal. Active days are typically discarded, a procedure which results in a gappy time series that is difficult to analyze. Furthermore, the quiet day selection is largely arbitrary since daily geomagnetic activity levels range continuously from very quiet to very active. This fact precludes the establishment of rigorous quiet/active thresholds. The average local time variation on active days resembles typical quiet day variations except for a multiplicative scaling factor. A multiyear stack of yearlong geomagnetic time series aligned with respect to local time incorporates “regular active” days while greatly attenuating the effects of irregular geomagnetic disturbances. External and internal current systems that rise and fall in step with the Sun's transit across the sky above an observatory contribute coherently to the local time stack. All other current systems are incoherent and tend to stack out. An exact 1‐year‐long stack preserves the seasonal variation. The resulting stacked time series contains no gaps, is dominated by regular solar daily variations, and can be used for improved Sq induction studies and comprehensive geomagnetic field modeling. Hourly geomagnetic values from Niemegk, Boulder, Mbour, and Meanook observatories between 1970 and 2002, along with the ap index, are used in this study.

Induction effects on ionospheric electric and magnetic fields

Abstract. Rapid changes in the ionospheric current system give rise to induction currents in the conducting ground that can significantly contribute to magnetic and especially electric fields at the Earth's surface. Previous studies have concentrated on the surface fields, as they are important in, for example, interpreting magnetometer measurements or in the studies of the Earth's conductivity structure. In this paper we investigate the effects of induction fields at the ionospheric altitudes for several realistic ionospheric current models (Westward Travelling Surge, Ω-band, Giant Pulsation). Our main conclusions are: 1) The secondary electric field caused by the Earth's induction is relatively small at the ionospheric altitude, at most 0.4 mV/m or a few percent of the total electric field; 2) The primary induced field due to ionospheric self-induction is locally important, ~ a few mV/m, in some "hot spots", where the ionospheric conductivity is high and the total electric field is low. However, our approximate calculation only gives an upper estimate for the primary induced electric field; 3) The secondary magnetic field caused by the Earth's induction may significantly affect the magnetic measurements of low orbiting satellites. The secondary contribution from the Earth's currents is largest in the vertical component of the magnetic field, where it may be around 50% of the field caused by ionospheric currents. Keywords. Geomagnetism and paleomagnetism (geomagnetic induction) – Ionosphere (electric fields and currents)

The role of 3-D geomagnetic induction in the determination of the ionospheric currents from the ground geomagnetic data

Abstract. The geomagnetic field variations measured at the surface of the Earth are composed of both internal and external parts. The external field arises from the sources in the magnetosphere and ionosphere, whereas the internal field is generated by the currents induced within the Earth. The internal part may in some situations comprise a notable part of the measured total field and thus a blind usage of geomagnetic field recordings potentially produces significant errors to estimated ionospheric currents. In this paper the role of geomagnetic induction in auroral ionospheric studies is investigated by modeling the induction using simultaneously the realistic ionospheric source and a realistic three-dimensional Earth conductivity structure. The modeling results imply that the effects of the lateral ground conductivity anomalies on ionospheric equivalent current patterns are, though clearly detected, less severe than anticipated for fields varying with periods from 5 to 120min. However, the amplification of the determined currents caused by induction is significant, leading to an overestimation of up to 30% of the main current flow intensities, with the overestimation increasing sharply when moving away from the region of the main flow. In addition to the 3-D modeling, a simple method is introduced to help estimate the internal contribution to the measured variations of the IL index (local variant of the AL index). A test with the 26 June 1998 substorm event indicates that the method can help to extract the internal contribution from the IL index.