Global electrical conductivity of the earth

The Banks (1969, 1972) and Parker (1970) models of the electrical conductivity distribution are critically reviewed along with classical models by Chapman (1919), Lahiri and Price (1939), Rikitake (1950a, b, c) and others. The modern models do not seem to account for the geomagnetic variations having a continuum spectrum and S_q at the same time. A large difference in response between the 1-day and 0.5-day period components of S_q is suspected to be caused by a resonance-like induction in the superficial layer of the earth. Dufficulties in determining the conductivity of the earth's top layer are also emphasized.An overall distribution of conductivity within the earth which seems to be the most reliable at present, is drawn mostly on the basis of Banks' model.     

The character of non-uniqueness in the conductivity modelling problem for the earth

Summary In this paper, we examine the character of non-uniqueness in the conductivity modelling problem for a spherically symmetric earth in order to show that methods, for determining upper and lower bounds on the possible range of values for electrical conductivity as a function of depth, must be developed before a better picture of conductivity within the earth can be obtained.     

Introduction to deep earth electrical conductivity

Pure and Applied Geophysics -     

Temperature profiles in the earth of importance to deep electrical conductivity models

Deep in the Earth, the electrical conductivity of geological material is extremely dependent on temperature. The knowledge of temperature is thus essential for any interpretation of magnetotelluric data in projecting lithospheric structural models. The measured values of the terrestrial heat flow, radiogenic heat production and thermal conductivity of rocks allow the extrapolation of surface observations to a greater depth and the calculation of the temperature field within the lithosphere. Various methods of deep temperature calculations are presented and discussed. Characteristic geotherms are proposed for major tectonic provinces of Europe and it is shown that the existing temperatures on the crust-upper mantle boundary may vary in a broad interval of 350–1,000°C. The present work is completed with a survey of the temperature dependence of electrical conductivity for selected crustal and upper mantle rocks within the interval 200–1,000°C. It is shown how the knowledge of the temperature field can be used in the evaluation of the deep electrical conductivity pattern by converting the conductivity-versustemperature data into the conductivity-versus-depth data.     

The magnetospheric disturbance ring current as a source for probing the deep earth electrical conductivity

Two current rings have been observed in the equatorial plane of the earth at times of high geomagnetic activity. An eastward current exists between about 2 and 3.5 earth radii (Re) distant, and a larger, more variable companion current exists between about 4 and 9 Re. These current regions are loaded during geomagnetic substorms. They decay, almost exponentially, after the cessation of the particle influx that attends the solar wind disturbance. This review focuses upon characteristics needed for intelligent use of the ring current as a source for induction probing of the earth's mantle. Considerable difficulties are found with the assumption thatDst is a ring-current index.     

The electrical conductivity of the Moon

Investigations of the lunar electrical conductivity were described in excellent reviews by Sonett (1974) and Dyalet al. (1976). In this paper we will try to consider some new aspects of this problem in comparison with the Earth's data.     

The global conductivity distribution

We still do not have a global conductivity distribution that can be shown to be sufficiently close to a world average that it can be used as a standard. Continental magneto-telluric results show a wide divergence of conductivity distributions with depth, according to the local geology. The recent marine magneto-telluric investigations have also increased the variability of sub-oceanic conductivity. They tend to show that the overall average conductivity of the oceanic mantle differs little from its continental counterpart. Recent work based on diurnal and storm-time variations have generally confirmed earlier estimates. The conductivity at a depth of 1000 km appears to be close to 1 S/m. The secular variation jerk in 1969 has afforded data for a reassessment of lower mantle conductivity. The consensus of opinion puts the conductivity at the base of the mantle close to 100 S/m.     

The electrical conductivity of clouds

Summary A modified Gerdien cell was designed, evaluated, and built for measurement of the polar conductivities in clouds. This conductivity dropsonde was attached to a U.S. Weather Bureau, 1680 mHz, radiosonde for telemetry and to measure pressure, temperature, and relative humidity profiles. The combined instruments were ejected from aircraft, and others were released from balloons into the region of interest.Eight flights were made during the 1967 thunderstorm season. Three of these drops were successful in measuring conductivity inside of electrically active clouds. Two fair-weather profiles were measured for comparison purposes, and three of the drops were faulty.These very preliminary results tend to indicate considerable electrical conductivity in thunderclouds. The data are too few to support a strong statement in favor of increased conductivity, but the instruments were sufficiently reliable to prove that the conductivity was not reduced, as is normally assumed, in the clouds investigated.This research was supported by the Atmospheric Sciences Section, National Science Foundation under Grant GA-701.     

Annual variations in the electromagnetic field of the earth and electrical conductivity of the geological environment

Synchronous annual variations in the geoelectric and geomagnetic field are studied on the basis of long-term electromagnetic monitoring. It is shown that the annual geoelectric variations have intraterrestrial origin and are not related to the annual geomagnetic variations. Temporal variations in the magnetotelluric impedance and magnetic tipper, which characterize the electrical conductivity of the geological environment, are analyzed. It is established that annual variations in the magnetotelluric impedance mainly describe the variations in the electrical conductivity of surface crustal layers and are less sensitive to the deep electrical conductivity of the Earth. The annual variations in the imaginary magnetic tipper at the periods of 1000–3000 s probably reflect the changes in conductivity of a deep transversal low-resistive zone (the fault). It is suggested that annual variations in the geoelectrical and geomagnetic fields, as well as in the electrical conductivity of the geological environment, arise as a response to the changes in the geodynamical processes caused by the revolution of the Earth around the Sun.

     

科学家首次绘制地幔电导率全球三维图像

家用电器警示标签中通常会注明,水是很好的导电材料。现在,科学家发现,地幔局部电导率的增大很可能意味着地表下含水量的增大。研究人员首次绘制了地幔电导率全球三维图像,他们的研究结果发表在近期的Nature杂志上。     

The bedrock electrical conductivity map of the UK

Airborne electromagnetic (AEM) surveys, when regionally extensive, may sample a wide-range of geological formations. The majority of AEM surveys can provide estimates of apparent (half-space) conductivity and such derived data provide a mapping capability. Depth discrimination of the geophysical mapping information is controlled by the bandwidth of each particular system. The objective of this study is to assess the geological information contained in accumulated frequency-domain AEM survey data from the UK where existing geological mapping can be considered well-established. The methodology adopted involves a simple GIS-based, spatial join of AEM and geological databases. A lithology-based classification of bedrock is used to provide an inherent association with the petrophysical rock parameters controlling bulk conductivity. At a scale of 1:625k, the UK digital bedrock geological lexicon comprises just 86 lithological classifications compared with 244 standard lithostratigraphic assignments. The lowest common AEM survey frequency of 3 kHz is found to provide an 87% coverage (by area) of the UK formations. The conductivities of the unsampled classes have been assigned on the basis of inherent lithological associations between formations. The statistical analysis conducted uses over 8 M conductivity estimates and provides a new UK national scale digital map of near-surface bedrock conductivity. The new baseline map, formed from central moments of the statistical distributions, allows assessments/interpretations of data exhibiting departures from the norm. The digital conductivity map developed here is believed to be the first such UK geophysical map compilation for over 75 years. The methodology described can also be applied to many existing AEM data sets.     

Models of deep electrical conductivity obtained from data on global magnetic variational sounding

An analysis of published papers containing magnetovariation sounding data (MVS) is carried out. A catalogue of the most trustworthy MVS parameters' values covering periods from 6 hours up to 11 years is compiled. With the help of averaged data of the magnetovariation sounding a planetary geoelectric profile is constructed meeting the requirements of up-to-date understanding of the physical processes in the Earth.     

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.     

西藏南部蛇绿岩套电导率研究

大地电磁(MT)资料显示,青藏高原地壳及地幔中普遍存在着高导层.作为大陆造山带中古洋盆岩石圈残片,蛇绿岩套的电导率测量可为了解古洋盆地区地壳及地幔的电性结构提供极其有用的信息.本研究中,我们在压力为1 GPa或3 GPa下,用交流阻抗谱法测量了采自西藏南部地区的蚀变辉长岩、玄武岩、角闪橄榄岩及方辉橄榄岩四个样品的阻抗谱,并进一步得出样品的电导率,不同样品电导率与温度之间的关系满足Arrhenius关系式.在实验温度范围内,蛇绿岩套电导率的对数logσ位于-6.0~-0.5 S/m之间,且随着温度的增高,不同样品电导率增大约4~5.5个量级.样品在未脱水的情况下,低温段的活化焓变化范围在0.4~0.6 eV之间,高温段的活化焓变化范围为1.7~2.6 eV之间.同时,我们研究了样品中结构水含量及铁含量对实验电导率的影响,验证了样品电导率与铁含量之间呈正比关系.当对样品结构水含量进行归一化后,相同温度下各样品的电导率随铁含量的增加而增大,而对样品铁含量归一化后,相同温度下各样品的电导率随样品中水含量的增加而增大.将实验电导率与藏南地区大地电磁结果进行了对比,发现本研究中各样品高温段实验电导率结果均落在大地电磁结果范围内.     

The effect of aerosols on the electrical conductivity in the atmosphere

Summary The value of the electrical conductivity of the free atmosphere is inversely related to the concentration of nuclei. Measurements made with an airborne conductivity meter illustrates this effect. It was found that for these particular meteorological conditions a change of 47 nuclei/cm³ could be detected. It was also found that in one case nuclei from a smoke source were carried down wind for a distance of 400 miles. The concentration of nuclei varied from 14.6×10³ nuclei/cm³ near the source to 1·3×10³ at the distant location.This research was performed while the author was employed by USAF Cambridge Research Center, Bedford, Mass.     

The effect of pressure on the electrical conductivity of KTB rocks

Complex electrical resistivity and permeability were measured on two gneiss samples and nine amphibolites (originally located at a depth of 4150 m to 5012 m) from the main drilling of the German deep drilling project (KTB). Measurements were performed as a function of hydrostatic pressures up to 240 MPa on core samples (30 mm in diameter and 10–20 mm high). For each measurement, two samples were used, one being parallel, and one perpendicular to the borehole axis. At low pressures and again at maximum pressure the frequency dispersion (1 kHz up to 1 MHz) of the complex resistivity was measured using a two electrode device. An unusual pressure effect was detected on some of the samples and was established to be due to the oriented deposition of good conducting phases in the foliation. Rock fabric and the orientation of ore mineralization was measured on thin sections and polished sections prepared from the same samples.     

The influence of oxidation state on the electrical conductivity of olivine

The electrical conductivity of a single crystal of San Carlos olivine (Fo₉₂, 0.16 wt.% Fe₂O₃) has been measured as a function of temperature and oxygen fugacity (fO₂). After heating to 1338°C at fO₂ = 10^?12 atm., the conductivity at 950°C was 10^?5 (ohm-m)^?1, almost 3 orders of magnitude less than that measured in air. This decrease is due to the reduction of Fe³⁺ to Fe²⁺. Further heating to 1500°C at fO₂ = 10^?14 atm., decreased the electrical conductivity at 950°C to 10^?6 (ohm-m)^?1. When recovered at room temperature, the speciment had Fo₉₆ composition and contained small, opaque blebs distributed throughout the crystal. Derivations of temperature profiles for the earth's mantle from conductivity-depth models must take account of the important role played by iron oxidation state in the electrical conductivity of olivine.