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.     

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.     

Tectonic interpretation of regional conductivity anomalies

This review paper selects key results from electromagnetic induction studies of a variety of distinctive tectonic phenomena in the top 200 km of the Earth. Its main theme is that electromagnetic data are essential for an understanding of tectonism involving partial melting, recycling of large volumes of fluids (CO₂ and H₂O) and underthrusting of metasedimentary rocks. The wide variety of tectonic regimes in which these processes are known to be important is reflected in the choice of case studies. A discussion of conductivity models for young oceanic lithosphere and asthenosphere is followed by results from induction studies across the S.E. Australian passive margin, the North American active margin, the Ryukyu Island-Arc and the Oregon Cascades continental arc. The importance of partial melting and free fluid movement i apparent in these regions. Terrain accretion and/or continent-continent collisions recorded at palaeosuture zones in Ireland, Germany and Scandinavia have left distinctive conductivity structures. These are often associated with grain-boundary graphite either in weakly-metamorphosed black shales in underthrust sedimentary basins or precipitated from CO₂-rich fluids. They are discussed in the context of the evolution of mature continental crust. All of the case studies are based on experiments published since 1989 in which the electromagnetic results have been central to an integrated geophysical and geological interpretation.     

Recent Japanese studies on conductivity anomalies

A research team called the “C.A. (conductivity anomaly) Group” has been active since the early 1960's in Japan. A number of organized observations of geomagnetic variation anomaly have been carried out by the group. As a result, the overall distribution of Parkinson vectors for short-period variation over Japan has become clear. It seems likely that the anomalously large vertical component as observed in central Japan can be accounted for by assuming a depression of the highly conducting mantle layer beneath Japan, although effects of the sea surrounding Japan on geomagnetic variations are also seen. Such a model seems to harmonize with observations of heat flow, seismic wave velocities and attenuation. Use of the transfer function technique has recently become popular and consequently frequency characteristics of geomagnetic variation anomaly will be thoroughly examined in coming years.Intensive observations of island and peninsula effects on geomagnetic variation have been conducted, sometimes in cooperation with the University of Hawaii. Beautiful reversal of the sign of the vertical component at the northern and southern observation points on an island has often been reported.Numerical work on electromagnetic induction in two-dimensional conductors, thin sheets and so on has also been intensively advanced. It is particularly important to estimate the frequency response of an underground model which is believed to account for the geomagnetic variation anomaly in central Japan.     

Introduction to deep earth electrical conductivity

Pure and Applied Geophysics -     

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 investigation of the Corso-Sardinian microplate area

The analysis of magnetovariational data from an array quasi- simultaneously covering Sardinia (Italy) and Corsica (France), from summer 1988 to spring 1991, indicates the existence of two major conducting bodies, one north of Corsica, the other south of Sardinia. Fourier maps also show local conductivity anomalies along the Campidano Graben (Sardinia), across the Bonifacio Straits and in the northernmost part of Corsica. Two-dimensional (2D) modelling and inversion procedures have been carried out in order to find the optimum conductivity and geometrical characteristics of the conductive bodies deduced from Fourier maps, induction arrows and pseudosections studies. As a result of the 2D inversion, two zones with enhanced electrical conductivity are found, respectively, in the Sardinia Channel and in the Bonifacio Straits. The former can be related to the thermal characteristics of the area; the latter, shallower, can be ascribed to an accumulation of sediments. Some short-period magnetovariational anomalies in northern Corsica can be related to recent reheating processes that occurred there, as well as to the crustal thinning in the Ligurian sea.     

Electromagnetic induction of a three-dimensional conductivity inhomogeneity in the two-layered earth

Summary The calculation of the elements of Green's tensor function is presented for solving the problem of the electromagnetic induction by means of a vector integral equation. A two-layered Earth is considered as the medium, the surface layer including a three-dimensional conductivity inhomogeneity. Use is made of the boundary condition requiring the vertical component of the electric current to be zero at the Earth's surface which partly simplifies the theoretical computation. Long-period asymptotics of the individual complicated functions, occurring in Green's tensor function as well as in the tensor function required to calculate the components of the anomalous magnetic field at the surface of the halfspace, were effected. With the aid of these asymptotics one can obtain estimates of the functions occurring in the theoretical analysis of the problem.     

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.     

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.     

Geomagnetic sounding of conductivity anomalies in the lower crust and uppermost mantle

Knowledge of the structure of the lower crust and uppermost mantle is of special importance in understanding plate tectonics. Electrical conductivity of this region has been measured recently in various parts of the world. Transfer functions are still the most widely used quantity in data analysis and model fitting. Anomalies in the horizontal magnetic components in combination with anomalies in the vertical component have been found very useful in locating conductivity contrasts. With these, when the cause of the anomaly is a concentrated line current, both the position of the line current and its depth can be directly located. The method of hypothetical event analysis is another new technique and this is highly suited for areas having complex subsurface geology or areas under non-uniform source fields or both. The anomaly is more suitable for modelling geophysical structures when it is separated into regional and local components. Model calculations still are not very satisfactory and the importance of one dimensional calculations must be emphasized for they give direct information on the variation of conductivity with depth, which is the purpose of GDS. We need more results, especially from tectonically active areas, before the underlying physical processes can be completely understood.     

地电阻率的年变畸变异常分析

讨论分析了3种地电阻率年变畸变异常,即年变幅度增大型、年变幅度减小型、年变畸变混合型。并针对不同的异常给出了相关分析方法,为识别和使用这类异常提供了一定的思路和科学依据。     

A new approach to the electrical conductivity anomalies in the Drauzug-Bakony geological unit

A major electrical conductivity anomaly has been detected in Transdanubia (West Hungary) in the eastern part of the Drauzug-Bakony geological unit (DBU), a collision zone of the Alpine orogeny. Assuming the source of the conductivity anomaly to be a characteristic formation of the whole DBU, long period magnetotelluric (MT) measurements were made in two regions of the western part of the DBU, in the Gail valley and in the Karawanken. The general features of this formation were studied also in order to elucidate the cause of the Transdanubian conductivity anomaly. The conductivity anomaly detected by long period MT in the Western DBU lies much deeper (12–17 km) than in Transdanubia (5–9 km). As the anomaly in both regions is associated with a well-known tectonic zone, its origin was initially explained by ionic conduction, arising from the presence of pore fluids. The very low resistivity values, the great anisotropy, the effect of charges on the H-polarized MT curves, and the anomalous magnetic field variations also indicated an increase of electronic conduction, possibly caused by graphitic rocks or ores nearer to the surface.

In the Gail valley, audiomagnetotelluric measurements made in 1986 have led to the discovery of these standing blocks of graphitic shales lifted up gradually to the surface by tectonic forces from below the dolomites of the Carnic Alps. It is suggested that the same plate collision arranged the Paleozoic graphitic shales to narrow dikes in the Gail valley as well as in Transdanubia along the Balaton line and zones parallel to it.     

The qualitative character of the global electrical conductivity of the earth

A spectral stacking and smoothing procedure has been applied to unbroken hourly values of H and Z, for 1964 and 1965, from 17 observatories, in order to estimate the magnitude and phase of the P₁⁰ response of the Earth to long-period geomagnetic fluctuations. Exploratory techniques have been used to gauge when sufficient smoothing has been applied, and to identify the qualitative character of the global electrical conductivity of the Earth.     

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.     

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.

---

nuam mam uu mnm¶rt;mu, uu aaua u n¶rt;u u uu n¶rt; u ¶rt;uu. ma nauu n¶rt; u ¶rt;uu naam mn¶rt;m aamumuu n¶rt; u amuu.

     

Negative cerium anomalies in the rare earth element patterns of oceanic ferromanganese nodules

Ferromanganese nodules from the Bauer Basin of the south equatorial Pacific are unlike virtually all oceanic nodules so far analyzed in showing negative Ce anomalies in their REE abundance patterns. In comparison with similarly Cu-Ni-enriched nodules from the north equatorial Pacific they are depleted in REE by 50–80% and are heavy REE enriched relative to intermediate REE. The REE patterns can be accounted for by the input of hydrothermal iron oxyhydroxides and associated REE to the Bauer Basin and the transfer of the REE to the nodules because of diagenetic reactions in the sediment. The excess iron input also is reflected in lower Cu/Ni ratios in the nodules as compared with nodules from the north equatorial zone, apparently because of the larger proportions of a residual Fe phase in the nodules relative to todorokite. Cerium anomalies of the Bauer Basin nodules range from ?0.17 to ?0.29 as compared with +0.33 to +0.07 in the north equatorial Pacific but show a parallel sensitivity of Ce anomaly to Mn/Fe ratios of the nodules. Nodules with the more positive anomalies within each group have the smallest Mn/Fe ratios and have been subjected to the greatest seawater influence whereas nodules with the more negative anomalies have the largest Mn/Fe ratios and have been subjected to the greatest diagenetic influence.     

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.     

Properties of the lithosphere-asthenosphere system in Europe with a view toward earth conductivity

Over the past 20 years the study ofP- andS-wave velocities in the upper mantle of the Mediterranean area and continental Europe has been the subject of intensive research work. We present a summary of results based on the inversion of available surface-wave dispersion data andP-wave trave time observations. For areas characterized by different tectonic settings and very large lateral variations, a discussion is made about structural models based on seismological, geothermal and electrical conductivity data.