The continental middle to lower crust (CLC) remains one of the most enigmatic parts of the Earth about which comparatively little is known. Remote-sensing geophysical and geochemical data illustrate our gaps in complete knowledge of the state and composition of the CLC. Generally, it is thought that the CLC, when compared to the upper crust, is somewhat more uniform in its properties. However, studies undertaken in the last decade have revealed that the CLC can be as heterogeneous as the upper crust, but that some regions of surprising homogeneity exist. In this chapter, I will describe attempts to image one particular physical parameter of the CLC, namely its electrical conductivity a. Electromagnetic (EM) sounding methods, as with most geophysical methods, are sensitive to the structure that exists today, and as such are dependent on the current state and composition of the CLC, rather than its state at formation or as a consequence of tectonic or metamorphic activity as are petrological studies on exhumed samples from the CLC. All EM methods give responses that are volumetric averages of the Earth's conductivities sensed by the diffusive EM fields, and as such are in the same class of geophysical techniques as seismic surface wave studies, potential field methods, and geothermal investigations. These are distinct from seismic reflection and refraction methods which deal with non-diffusive waves. However, with EM methods the governing field equations (§2) and wide range of the physical parameter being sensed (electrical conductivity, §1.1) ensure a far greater resolving power to anomalies than with the other diffusive geophysical techniques. EM and seismic methods are the only geophysical techniques for which probing of the deep crust is assured; with all others there are inherent screening effects and depth ambiguities. Imaging the electrical conductivity structure beneath the surface has a number of uses, classified mainly by the depth of investigation: (1) Identifying zones of mineralization, which are of economic importance. (2) Detecting fluids in the deep crust, such as those above downgoing slabs, e.g., the Juan de Fuca plate (Kurtz et al., 1986a, 1990; EMSLAB, 1989). (3) Resolving one physical property of crustal zones. As expressed by Dohr et al. (1989): " ... magnetotellurics works like a broad paint brush, colouring the layers bounded by the seismic reflections" .

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Electrical conductivity of the continental lower crust

As a contribution to the International Magnetospheric Study (IMS, 1976–1979) a two-dimensional array of 42 temporary magnetometer stations was run in Scandinavia, supplementary to the permanent observatories and concentrated in the northern part of the region. This effort aimed at the time-dependent (periods above about 100 s) determination of the two-dimensional structure of substorm-related magnetic fields at the Earth's surface with highest reasonable spatial resolution (about 100 km, corresponding to the height of the ionosphere) near the footpoints of field-aligned electric currents that couple the disturbed magnetosphere to the ionosphere at auroral latitudes. It has been of particular advantage for cooperative studies that not only simultaneous data were available from all-sky cameras, riometers, balloons, rockets, and satellites, but also from the STARE radar facility yielding colocated two-dimensional ionospheric electric field distributions. In many cases it therefore was possible to infer the three-dimensional regional structure of substorm-related ionospheric current systems. The first part of this review outlines the basic relationships and methods that have been used or have been developed for such studies. The second short part presents typical equivalent current patterns observed by the magnetometer array in the course of substorms. Finally we review main results of studies that have been based on the magnetometer array observations and on additional data, omitting studies on geomagnetic pulsations. These studies contributed to a clarification of the nature of auroral electrojets including the Harang discontinuity and of ionospheric current systems related to auroral features such as the break-up at midnight, the westward traveling surge, eastward drifting omega bands, and spirals.

Studies of polar current systems using the IMS Scandinavian magnetometer array

SUMMARY The C-response, which connects the magnetic vertical component and the horizontal gradient of the horizontal components of electromagnetic fluctuations, forms the basis for estimating the conductivity‐depth profile of the Earth. This paper describes new estimates of the C-response obtained from observatory hourly mean values. The Z: Y-method is applied, which means that the vertical component is used locally whereas the horizontal components are used globally by expansion into terms of spherical harmonics. A special eVort is made to obtain unbiased estimates of C. When applied to 90 months of hourly mean values from about 90 observatories, the method yields consistent results for European observatories in the entire period range from 3 to 720 hr, and for two diVerent source mechanisms (S and D st ). A good description of the source structure for individual time segments is essential. This was achieved by a separate spherical harmonic analysis of the data for each month (for D st ) or each day (for S), and by estimating a large number (120) of expansion coeYcients. The results are interpreted by means of 1-D conductivity models, which show that the upper mantle has remarkably little structure, with a monotonic decrease of resistivity from 100 V m near z=200 km to 0.7 V m below z=1000 km.

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The electrical conductivity of the mantle beneath Europe derived from C-responses from 3 to 720 hr

Practical Magnetotellurics: References

[1] We present a new procedure to describe the one-dimensional thermodynamical state and mineralogy of any Earth-like planetary mantle, with Mars as an example. The model parameters are directly related to expected results from a geophysical network mission, in this case electromagnetic, geodetic, and seismological processed observations supplemented with laboratory measurements. We describe the internal structure of the planet in terms of a one-dimensional model depending on a set of eight parameters: for the crust, the thickness and the mean density, for the mantle, the bulk volume fraction of iron, the olivine volume fraction, the pressure gradient, and the temperature profile, and for the core, its mass and radius. Currently, available geophysical and geochemical knowledge constrains the range of the parameter values. In the present paper, we develop the forward problem and present the governing equations from which synthetic data are computed using a set of parameter values. Among all Martian models fitting the currently available knowledge, we select eight candidate models for which we compute synthetic network science data sets. The synergy between the three geophysical experiments of electromagnetic sounding, geodesy, and seismology is emphasized. The stochastic inversion of the synthetic data sets will be presented in a companion paper.

https://hal.archives-ouvertes.fr/hal-00155776/document

Interior structure of terrestrial planets : Modeling Mars' mantle and its electromagnetic, geodetic, and seismic properties

Summary It is shown that the ratio of the vertical field to the horizontal field gradient outside a geological body can often be used to measure the electrical conductivity at depth within it. The in-phase part of this ratio, at a given frequency, gives the penetration depth of the magnetic field, the out-of-phase component yields approximately the conductivity at the penetration depth. In this paper it will be shown that the electrical conductivity within an object can often be related to simple properties of fluctuating magnetic fields observed and generated outside of it. The specific question to which we address ourselves is the determination of the electrical conductivity as a function of depth into a geological body by observing the properties of fluctuating magnetic fields outside that body. In the case of the Earth, we are concerned with the fluctuating magnetic fields which are excited by electric currents flowing in the ionosphere, in the case of the Moon by magnetic fields which are convected by the solar wind. We shall mathematically analyse the interaction of an oscillating magnetic field which is slightly non-uniform in the horizontal plane with a laterally uniform body whose electrical conductivity increases with depth. It will be shown that, above and outside the body, the magnetic field has two simply interpretable properties whose importance for interpreting magnetic deep sounding data has not been realized heretofore. The ratio of the ' in phase ' part of the vertical field to the ' gradient ' of the oscillating horizontal field component is equal to an appropriately defined field penetration depth. The ratio of the out-of-phase part of the vertical field and the same horizontal field gradient is approximately equal to the field attenuation length associated with the electrical conductivity of the body evaluated at the field penetration depth. Chapman (1919) pointed out that the fluctuating magnetic fields observed at the surface of the Earth could be resolved into normal modes and that the part of each normal mode which is due to eddy currents flowing inside the body could be separated from that due to currents flowing externally. By noting that the internal currents were coupled to and excited by the external currents, he obtained an approximate value for the Earth's conductivity. Various combinations of the ' complex ' ratio of the internal current and the external current have subsequently been denoted as magnetic response functions. By studying these response functions as a function frequency means have been devised to generate conductivity profiles of the Earth and Moon from experimental data (Rikitake 1966). Most analyses have seriously utilized only the real part of this response function (e.g. Banks 1969), i.e. they have considered only the magnitude of the horizontal

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Relations between Electrical Conductivity of a Mantle and Fluctuating Magnetic Fields

Summary It is shown that a complex penetration depth parameter derived from fluctuating magnetic field experiments bears a simple correspondence to the complex impedance derived from magnetotelluric experiments. The magnetotelluric method used for deducing the electrical conductivity as a function of depth within a geological body has received considerable attention since its conception by Cagniard (1953). The initial difficulties and inconsistencies experienced were partially removed by the suggestion of Cantwell & Madden (1960) that the Earth impedance be analysed as a tensor rather than as a scalar quantity. Recently Kuckes (1973) pointed out that studies of fluctuating geomagnetic magnetic fields in a region whose dimensions are somewhat less than the lateral wavelength associated with the fluctuations, can also be used to quantitatively deduce the conductivity us. depth in the body. It was shown that the experimental data, Fourier analysed in time, could be used to define a physically significant complex field penetration depth which is a function of frequency. It is the purpose of this note to point out that there is a one to one correspondence between the complex penetration depth derived solely from magnetic measurements and the complex impedance measured in magnetotelluric experiments. After showing this correspondence we shall consider the strengths and weaknesses which we can expect of each method in practical applications. The complex penetration depth is derived from the Fourier analysed components of the magnetic field, if we denote the Fourier analysed component of the field perpendicular to the surface as B,(w) and the tangential components as B,(o) and B,(o), the real and imaginary parts of the penetration depth are defined by

A. F. Kuckes

Papers

Relations between Electrical Conductivity of a Mantle and Fluctuating Magnetic Fields

Correspondence Between the Magnetotelluric and Field Penetration Depth Analyses for Measuring Electrical Conductivity