Introduction to Electrical Properties of the Earth’s Mantle

Campbell, Wilbur H.

doi:10.1007/978-3-0348-7373-4_1

Cited by 8 publications

(10 citation statements)

References 44 publications

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“…However, a rise in conductivity to approximately 1 Sm-' averaged over the interval 400-670 km is a pervasive feature of all global conductivity profiles (Banks 1969(Banks , 1972Campbell 1987). This conductivity, while not shown in Fig.…”

Section: )mentioning

confidence: 83%

The electrical conductivity of the oceanic upper mantle

Heinson

Constable

1992

Geophysical Journal International

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S U M M A R Y Previous inversions of sea-floor magnetotelluric (MT) sounding data have predicted upper mantle electrical conductivities which are more than an order of magnitude higher than laboratory measurements of the conductivity of olivine would suggest, and controlled source-field electromagnetic (CSEM) soundings require a lithospheric mantle conductivity of 3 x lop5 Sm-', which is so low that the electromagnetic (EM) coast effect would produce more anisotropy in MT soundings than is observed. We address these issues by constructing an olivine mantle model for conductivity and examining the inversion of MT data from three sea-floor sites, and show that the incompatibilities can be largely resolved if the effects of oceans and coastlines are considered. Our mantle model is based on recent measurements of olivine conductivity, the conductivity of tholeiite melt, a thermal model for the upper mantle based on lithospheric cooling and the temperature of a+ a + / 3 olivine transition, the pyrolite model of mantle petrology, and conductivities derived from CSEM sounding. We propose Archie's Law with exponent 2 and interconnected tubes as realistic lower and upper bounds for the effect of partial melt on rock conductivity, and Archie's Law with exponent 1.5 as the preferred estimate. The 1-D forward response of this model is not compatible with observed sea-floor MT data. Three data sets presented by Oldenburg (1981) pass a test for one-dimensionality based on the size of the residuals when fit with Parker's D+ algorithm, but two of the three soundings fail a test for independence of residuals. We also find that the presence of the upper mantle 'high-conductivity zone' previously inferred from these data is highly dependent on data misfit and not required when the misfit criterion is relaxed a reasonable amount. Re-inversions of the MT data produce models which are incompatible with our petrological model of mantle conductivity. However, by adding an ocean with various coastlines of simple geometry to our petrological model and solving the forward 3-D problem using thin-sheet analysis we predict MT responses which are distorted in a manner that is remarkably similar to the observed data.

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Section: )mentioning

confidence: 83%

The electrical conductivity of the oceanic upper mantle

Heinson

Constable

1992

Geophysical Journal International

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“…When a uniform Earth conductivity is assumed, the Earth can be simplified as an infinitely conducting sphere covered by a perfect insulator layer that is at least 250 km thick [Chapman, 1919;Kisabeth and Rostoker, 1977;Campbell, 1987]. In our model the magnetic effect of the Earth induction current is calculated from image currents of the TCVs' ionospheric current system, which flows in the Earth conducting sphere.…”

Section: Formulationmentioning

confidence: 99%

Model study of ground magnetic signatures of traveling convection vortices

Zhu¹,

Gifford²,

Sojka³

et al. 1997

J. Geophys. Res.

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Abstract. We conducted a model study of ground magnetic signatures of traveling convection vortices (TCVs) that included both the ionospheric conductivity enhancement associated with the TCVs and the ground induction effect. We found that the localized conductivity enhancement can cause a significant distortion of the TCV current system and lead to a distortion of the ground magnetic disturbance patterns. The patterns of all three magnetic components are asymmetric, mainly in the E-W direction, and the patterns of the Z component show the strongest asymmetry (20-30%). We also found that the effect of induction currents on ground magnetic signatures of the TCVs is insignificant (less than 5%). The results show that because of the presence of localized conductivity enhancements the polarity and speed of the TCVs can significantly influence the distortion features of ground magnetic patterns. The upward and downward current filaments of a TCV with a clockwise leading convection cell can wrap with each other, resulting in a rotation of the whole ground magnetic disturbance pattern. This rotation feature is most significant when the speed of the TCVs is high.

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“…Magnetotelluric (MT) measurements revealed that the Earth's continental crust contains zones of high electrical conductivity (Haak and Hutton, 1986) and provided evidence that the structure of the crust is not simply layered but varies laterally and with depth (Campbell, 1987). Various models and hypotheses were suggested to explain such electrical inhomogenities.…”

Section: Introductionmentioning

confidence: 99%

Electrical Properties of Crustal and Mantle Rocks – A Review of Laboratory Measurements and their Explanation

Nover¹

2005

Surv Geophys

112

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The electrical properties of rocks and minerals are controlled by thermodynamic parameters like pressure and temperature and by the chemistry of the medium in which the charge carriers move. Four different charge transport processes can be distinguished. Electrolytic conduction in fluid saturated porous rocks depends on petrophysical properties, such as porosity, permeability and connectivity of the pore system, and on chemical parameters of the pore fluid like ion species, its concentration in the pore fluid and temperature. Additionally, electrochemical interactions between water dipoles or ions and the negatively charged mineral surface must be considered. In special geological settings electronic conduction can increase rock conductivities by several orders of magnitude if the highly conducting phases (graphite or ores) form an interconnected network. Electronic and electrolytic conduction depend moderately on pressure and temperature changes, while semiconduction in mineral phases forming the Earth's mantle strongly depends on temperature and responds less significantly to pressure changes. Olivine exhibits thermally induced semiconduction under upper mantle conditions; if pressure and temperature exceed $ 14 GPa and 1400°C, the phase transition olivine into spinel will further enhance the conductivity due to structural changes from orthorhombic into cubic symmetry. The thermodynamic parameters (temperature, pressure) and oxygen fugacity control the formation, number and mobility of charge carriers. The conductivity temperature relation follows an Arrhenius behaviour, while oxygen fugacity controls the oxidation state of iron and thus the number of electrons acting as additional charge carriers. In volcanic areas rock conductivities may be enhanced by the formation of partial melts under the restriction that the molten phase is interconnected. These four charge transport mechanisms must be considered for the interpretation of geophysical field and borehole data. Laboratory data provide a reproducible and reliable database of electrical properties of homogenous mineral phases and heterogenous rock samples. The outcome of geoelectric models can thus be enhanced significantly. This review focuses on a compilation of fairly new advances in experimental laboratory work together with their explanation.

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Introduction to Electrical Properties of the Earth’s Mantle

Cited by 8 publications

References 44 publications

The electrical conductivity of the oceanic upper mantle

The electrical conductivity of the oceanic upper mantle

Model study of ground magnetic signatures of traveling convection vortices

Electrical Properties of Crustal and Mantle Rocks – A Review of Laboratory Measurements and their Explanation

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