Electrical conductivity anomalies in the earth

Anomalies of short-period geomagnetic variations have been found in various regions over the world. It is known that such anomalies arise from electromagnetic induction within an electrical conductivity anomaly or from local perturbation of induced electric currents by a conductivity anomaly. In order to investigate a regional electric state in the Earth, conductivity anomaly (CA) studies based on anomalous behaviors of geomagnetic variations have been extensively undertaken, as well as studies based on magnetotelluries in which induced currents are directly used.Some of the geomagnetic variation anomalies, however, turned out to be caused by surface conductors, such as sea water and sediments. Anomalies of this sort have been intensively studied and classified into coast, island, peninsula, and strait effects in the case of sea effects. Three-dimensional conduction or channelling of induced electric currents is sometimes observed in the cases of sediments and some crustal conductivity anomalies. However, anomalies of such surface origins often provide some information of the underground conductivity structure.Electrical conductivity anomalies can be classified into two types: anomalies originating in the crust and in the upper mantle. Many of crustal anomalies are well correlated with metamorphic belts, fracture zones, and hydrated layers, and magnetic and gravity anomalies are also often found over the conductivity anomalies. Most of mantle anomalies have been interpreted mainly in terms of high temperature and partial melting, since conductivity anomalies coincide well with anomalies in heat flow and seismic wave velocities.     

Electrical conductivity structure in the lower crust

The subject is reviewed, notwithstanding the existence of a number of disagreeing and/or controversial results found in the literature. First, a brief critical reexamination of the methodology is presented. Second, it is attempted to put the results, or partial conclusions by different authors, using different methods and referring to different geographical regions, into a working scheme. This is done by investigating, as far as possible, the relationships between the electrical conductivity information and other types of geophysical and geological information for each geographical area investigated. It appears almost impossible to draw general conclusions that hold for the entire Earth. Conclusions are given for those areas with some very well-defined geomorphological characters. Unfortunately, the available investigations still appear to give a poor coverage of several types of geographic areas with specific tectonic characteristics, and certainly the scientific coverage by electromagnetic methods of investigation cannot be compared with those available today from seismological methods. Investigating the electrical conductivity structure of the lower crust certainly opens relevant heuristic possibilities, but there appears to be a great need both for a refinement in the basic methodology, and for a better coverage of the investigated areas.     

Electrical conductivity of some rocks from the Indian subcontinent

Summary Results on electrical conductivity, chemical analysis and age of the rock samples from the Indian Subcontinent are presented. The old Precambrian Indian rocks tend to be less conductive than characteristic rocks of crustal and upper mantle structures.

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Electrical conductivity of the earth's crust and upper mantle

This review summarizes recent results of electrical resistivity studies of the earth's crust and upper mantle. Where available, the data are discussed in the context of further regional geophysical information. Electrical resistivity is very sensitive to a wide range of petrological and physical parameters, e.g., to carbon, fluids, volatiles and enhanced temperatures, making electrical resistivity methods a powerful tool in crust and upper mantle investigations. Yet, the general increase in resistivity data of the crust and mantle has not ended the battle of explanations for anomalous crustal conductivities.     

Electrical conductivity anomaly in the Lanzhou-Xi’an-Zhengzhou zone

It has been found, on the basis of SSC and geomagnetic bays variations, that there is zone of a nearly vanishingZ component in the east-west trend along the Lanzhou-Xi’an-Zhengzhou zone, and it is called the Lanzhou-Xi’an-Zhengzhou anomalous zone of electrical conductivity. There are some relationships between the goelogical and geophysical characteristics along this zone. It is deduced that the zone may be a zone of weakened crust or a boundary between two sub-plates and it is also deduced that this zone is probably related to channelling effects of induced currents in a large region.     

Electrical conductivity of rocks in the heating and cooling cycle

Summary The values of the electrical conductivity, recorded during the heating and cooling cycle, of eclogites and basalts are compared. The observed difference in the values is explained by reversible and irreversible changes which take place in the samples.     

Electrical conductivity of ultramafic rocks to 1820 kelvin

Electrical conductivity, σ, of six ultramafic rocks (garnet-bearing peridotites and an eclogite) has been investigated in the temperature range 670–1820 K under known environment. Between 670 and ～1320 K σ increases with 1/T monotonically, first slowly, by 1–1.5 orders of magnitude. Above 1320 K σ increases sharply; an increase of 3–4 orders of magnitude is observed between 1320 and 1670 K. In the same temperature range the σ values for all the six rocks fall within 1.5 orders of magnitude, the lowest conductivity being for a spinel lherzolite. The range of σ values is narrowed at higher temperatures. The differences in σ values above 1470 K may be explained on the basis of varying degrees of partial melting in the rocks. Over the entire range of temperature, the σ values for the ultramafic rocks are lower than those reported for basalts but higher (by 1–2 orders of magnitude) than those for single-crystal olivines.     

Kinematics of the Danakil microplate

A refinement and extrapolation of recent motion estimates for the Danakil microplate, based on ancient kinematic indicators in the Afar region, describes the evolution of a microplate in the continental realm. The Danakil horst is an elevated part of this microplate, exposing a Precambrian basement within the Afar depression, the site of the Nubia-Somalia-Arabia triple junction. We compare evidence for strike- or oblique-slip faults in data from the Afar depression and southern Red Sea to small circles about published poles of rotation for the Danakil microplate with respect to Nubia. A reconstruction about the preferred pole reunites lengths of a Precambrian shear zone on the Nubia and Danakil sides and preserves a uniform basement fabric strike through Nubia, Danakil and Yemen. Since at least magnetic chron C5 (∼11 Ma) Danakil rotated about a different pole with respect to Nubia than either Somalia or Arabia, but between chrons C5 and C2A Nubia-Danakil motion was a close approximation to Nubia-Somalia motion. Since C2A relative motions of the Danakil microplate have been independent of movements on any of the neighbouring plate boundaries. We relate this to the onset of oceanic-type accretion within Afar. The resulting eastwards acceleration of Danakil was accommodated by westwards propagation of the Gulf of Aden rift that became the new, discrete, plate boundary between the Danakil microplate and the Somalia plate. Present-day activity suggests that the Red Sea and Aden rifts will link through Afar, thereby isolating the Danakil horst as a microcontinent on the Arabian margin.     

Electrical conductivity of some Indian basalts under various temperature conditions

Electrical conductivity of nineteen samples of the Deccan Traps, collected from the Pawagarh area and two boreholes drilled at Koyna, has been measured in the temperature range from 475 K to about 1100 K. The results indicate the change of the conduction mechanism from impurity to electronic/ionic conduction as the temperature increases. Few samples show the normal behaviour, i.e. the increase of conductivity with temperature, while some samples show an anomalous behaviour between the temperatures 600 and 1000 K. This anomalous behavior may be due to either the dehydration of OH^? ions or/and the oxidation of Fe²⁺ to Fe³⁺.     

Electrical conductivity of igneous rocks: Composition and temperature relations

The conductivity of four igneous rocks with, 49, 65, 77, and 84% SiO₂ was measured as a function of temperature in the interval from 20° to 1280°C; measurements were made in a vacuum of 10^?3 torr. No simple relationships were found between conductivity and SiO₂ content or versus major element groupings such as Na₂O=K₂O=CaO and TiO₂=Cr₂O₃=Al₂O₃=Fe₂O₃=FeO. An analytical expression was obtained between conductivity and the albite-quartz ratio, valid for temperatures between 300° and 1200°C. It was necessary to compute the CIPW norm in order to obtain the albite and quartz percentages. The onset of melting apparently occurred between 600° and 700°C. Petrography performed on two samples after cooling showed 70 and 85% partial melting. Three conduction regions were identified: 1) below 300°C, 2) between 300°C and 600°C, and 3) above 600°C. Different activation energies obtained for the heating and cooling intervals confirm that the sample undergoes textural changes in the heating-cooling cycle. Activation energy increments of 0.1 and 0.2 eV per decade of albite-quartz ratio were obtained.     

Electrical conductivity and potential gradient measurements in the free atmosphere over India

Summary Results of measurements of atmospheric electrical potential gradient and conductivity in the free atmosphere using balloon borne radiosondes made at a few stations in India are presented. The potential gradient generally decreases exponentially with height above the exchange layer and the electrical conductivity profiles show corresponding mirror image symmetry. Large variations of electrical potential gradient and conductivity are, however, observed in the troposphere closely associated with clouds, dust and haze and other local meteorological phenomena and space charge effects. Significant though small variations are also observed in the stratosphere, which appear to be associated with the existence of aerosol layers.From an analysis of integrated potential profiles the total atmospheric potential has been calculated and found to have a mean value of 300 kV; the ionospheric potential varies diurnally and seasonally about the mean.

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Zusammenfassung Messungen der luftlelektrischen Leitfähigkeit und des atmosphärischen Potentialgradienten werden berichtet, welche mit ballongetragenen Radiosonden in der freien Atmosphäre über Indien ausgeführt worden sind. Oberhalb der Austauschschicht vermindert sich das Potentialgefälle exponentiell mit der Höhe, während die Leitfähigkeitsprofile das dazu spiegelbildliche Bild zeigen. In der Troposphäre werden jedoch grosse Änderungen des Potentialgefälles und der Leitfähigkeit im Zusammenhang mit Wolken, Dunst, Staub und mit anderen meteorologischen Erscheinungen und mit Raumladungseffekten beobachtet. Auch in der Stratosphäre werden signifikante, wenn auch kleine Änderungen beobachtet, welche mit der Existenz von Aerosolschichten zusammen zu hängen scheinen.Das gesamte atmosphärische Potential wurde aus den integrierten Potential-gefällekurven berechnet zu 300 kV als Mittel gefunden; dieses Ionosphärenpotential zeigt Veränderungen um diesen Mittel in Form von Tagesgängen und Jahresgängen.

     

云南南部地区深部电性结构特征研究

在云南南部地区布设了一条孟连—罗平的北东向大地电磁测深剖面,以开展该地区的深部电性结构探测和孕震环境探查.沿该剖面进行了114个大地电磁测深点的观测,经过对观测资料的远参考Robust处理、定性分析和二维反演,得到了沿该剖面地壳、上地幔电性结构模型,从模型的电性结构特征进一步探讨了剖面穿过的3个地震区的深部地震孕育环境.研究结果表明：沿剖面的地壳上地幔电性结构反映出与区域地质构造资料基本一致的构造特征;该区的三个强震带地球深部都存在壳内低阻体,地震发生在电阻率梯度带上;断裂带的两侧块体介质的电阻率差异是强震活动带重要的深部背景.     

Electrical conductivity from Australian magnetometer arrays using spatial gradient data

Quiet daily magnetic variations recorded by magnetometer arrays in Australia are analysed to obtain electromagnetic response parameters for two parts of the Australian continent remote from electrical conductivity anomalies. The parameters are based on measurements of vertical-field and horizontal-field spatial gradient, and three different methods are followed in their computation. The response parameters are checked for consistency with a compilation of globally-determined Earth apparent resistivities, and are then interpreted for one-dimensional conductivity structure in the two different parts of the continent. There is evidence that the rise in electrical conductivity from 10^?1 S m^?1 to 10⁰ S m^?1 which occurs at a depth of order 500 km beneath central Australia may occur at a substantially shallower depth of order 230 km beneath southeast Australia.     

Numerical investigation of the anisotropic hydraulic conductivity behavior in heterogeneous porous media

The behavior of the mean equivalent hydraulic conductivity normal and parallel to stratification (K₁, and K₂, respectively) is studied here through Monte Carlo simulations of three-dimensional, steady-state flow in statistically anisotropic, bounded, and heterogeneous media. For water flow normal to stratification in strongly heterogeneous porous media (²_Y=3) the value of K₁ is not unique; it ranges from an arithmetic to a geometric, and finally, to a harmonic mean behavior depending on field dimensions, and medium anisotropy. For a fixed anisotropy ratio and variance of Y = ln K, the larger the distance, in the direction perpendicular to stratification, over which water flow takes place, the faster the rate at which, K_H, behavior is approached. However, even for large anisotropy ratios, harmonic mean behavior appears to be a good approximation only for aquifer thickness L₁ that is large enough to allow stratified flow to occur. For small aquifer thickness (L₁/₁<8, where ₁ is the integral scale normal to stratification) the limiting behavior, for large anisotropy ratios, appears to be, instead, that of two-dimensional flow, i.e., water flows primarily parallel to the planes of stratification. When the aquifer thickness is very small compared to the horizontal dimensions (and with relative similar integral scales in the three directions) a behavior resembling arithmetic mean conditions is exhibited, i.e., water flow takes place through heterogeneous, vertical, soil volumes. The geostatistical expressions of Desbarats (1992a) for upscaling hydraulic conductivity values were utilized and closed form empirical relations were developed for the main components of the upscaled hydraulic conductivity tensor.     

Electrical conductivity as a proxy for groundwater density in coastal aquifers

Groundwater density is an important parameter in the interpretation of flow patterns. This paper investigates the relationship between electrical conductivity (EC) and groundwater density in coastal aquifers and evaluates the suitability of the UNESCO 1980 equation of state, developed for the world's oceans, for determining the density of groundwater based on its EC. To achieve this aim, a dataset of groundwater samples from four different types of coastal aquifers was collected. It is found that the density of a sample can be estimated to a good approximation on the basis of its EC using the UNESCO 1980 equation of state. Deviations from the equation of state were found to be due to the changes in EC and the density caused by geochemical reactions, such as the dissolution of carbonates, degradation of organic carbon, cation exchange, and sulfate loss. Owing to these deviations, the UNESCO 1980 equation of state may underestimate the density by up to 1.5 kg/m(3). The effect of this uncertainty on the correction terms applied to the hydraulic head required for a proper interpretation of groundwater flow patterns and rates is quantified. It was found that the fresh water head may be wrong by centimeters to a few decimeters. From this it is concluded that, unless the purpose of a groundwater investigation requires great accuracy, the equation of state provides an efficient and inexpensive way to estimate density from EC.     

基于Tikhonov正则化的双频电磁波电导率成像反演

本文将Tikhonov正则化方法与active-set算法相结合,利用双频电磁波电导率成像原理,求解其反演成像方程.不仅对现有算法进行了改进,也促进了算法的实际应用.本文研究了在双频电磁波电导率成像方程建立后,如何根据其严重病态性质,选择合适的算法求解矩阵成像方程.针对电导率非负的特性,引入正则化参数,将问题转化为一个非负最小二乘问题,并用active-set算法求解.采用改进后的迭代算法对理论模型进行了数值模拟计算,验证了该方法的有效性.应用到实际电导率成像反演,与常规的LSQR、SP-LSQR、Tikhonov正则化等算法进行比较,取得了满意的结果.     

Electrical conductivity of hydrous silicate melts and aqueous fluids: Measurement and applications

The combination of magnetotelluric survey and laboratory measurements of electrical conductivity is a powerful approach for exploring the conditions of Earth’s deep interior. Electrical conductivity of hydrous silicate melts and aqueous fluids is sensitive to composition, temperature, and pressure, making it useful for understanding partial melting and fluid activity at great depths. This study presents a review on the experimental studies of electrical conductivity of silicate melts and aqueous fluids, and introduces some important applications of experimental results. For silicate melts, electrical conductivity increases with increasing temperature but decreases with pressure. With a similar Na⁺ concentration, along the calc-alkaline series electrical conductivity generally increases from basaltic to rhyolitic melt, accompanied by a decreasing activation enthalpy. Electrical conductivity of silicate melts is strongly enhanced with the incorporation of water due to promoted cation mobility. For aqueous fluids, research is focused on dilute electrolyte solutions. Electrical conductivity typically first increases and then decreases with increasing temperature, and increases with pressure before approaching a plateau value. The dissociation constant of electrolyte can be derived from conductivity data. To develop generally applicable quantitative models of electrical conductivity of melt/fluid addressing the dependences on temperature, pressure, and composition, it requires more electrical conductivity measurements of representative systems to be implemented in an extensive P-T range using up-to-date methods.     

Electrical conductivity of molten FeNiSC core mix

The electrical conductivity of liquid (Fe₉₀Ni₁₀)₃S₂ saturated with 2.6 weight percent carbon averages 2.7·10⁵ mho/m at 1000°C and zero pressure. This may imply a slightly lower electrical conductivity for the earth's core than that obtained by extrapolating the properties of pure liquid iron and solid iron alloys to core pressures and temperatures. Although a sulphur-rich core would have a smaller proportion of sulphur, the effect of lowering the sulphur content of the FeNiSC liquid to about 15 weight percent would be unlikely to increase the conductivity above 5·10⁵ mho/m.     

上地幔橄榄石电导率影响因素及高压相变初探

人们对上地幔标志性矿物橄榄石的认识，基本上是经历一个从直流到交流，又从交流到阻抗谱的认识过程．本文主要通过对橄榄石电导率的影响因素出发，包括温度、压力、铁镁比值、氧逸度等因素的影响和制约．用目前国际高温高压学术界公认的阻抗谱技术探讨了橄榄石在高压下的相变，并在此基础上，进一步研究地球物理学家非常关心的热点问题-冷地幔的实验室的模拟，并作了充分的论证．