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Conductivity models were compiled in two geological provinces with thinsediments. The first province is the margin of the Precambrian East EuropeanPlatform. The second one is the Phanerozoic Rhenish Massif as a part of CentralGermany Hercinicum. In both provinces, a conducting layer was revealed at thebase of the upper crust by the magnetotelluric soundings. Its depth is around10 km in the East European Platform and around 15 km in the Rhenish Massif.The conductance of the layer reaches a few tens of Siemens in the first provinceand is almost an order of magnitude greater in the second one. A good correlation between the conductor and a seismic wave-guide (low-velocityzone) exists at the base of the upper crust. Simultaneous decrease of both electricalresistivity and seismic velocity, suggests an increase of porosity and permeability inpresence of saline water. The depth of rheological weak layer in the PhanerozoicRhenish Massif corresponds to the commonly accepted depth of the thermallyinduced brittle/ductile transition. Contrary similar layer in the Precambrian EasternEuropean Platform is much shallower than the thermally induced transition. Somenew mechanism should be considered.

According to recent estimates, the continental mid-crust contains 35–40 per cent amphibolites. Heating of the crust by an underlying mantle plume, for example beneath continental rifts, high plateaus, and areas of intraplate volcanic activity, releases water. Dehydration of amphibole-bearing rocks at depths of 20–40 km occurs mainly in the temperature range 650–700 °C, and this releases about 0.4 wt per cent of water. Seismic tomography studies of the crust in the Kirgyz Tien Shan Range, where the age of the tectonic activity is less than 30 Ma, revealed a low-velocity zone in the mid-crust. The velocity of P waves was 0.4 km s−1 lower than in normal crust. MT sounding data in the region show the existence of a low-resistivity layer with an average resistivity of about 25 Ω m at the depth of the low-velocity layer. The spatial correlation of the observed anomalous layers and calculated effect of fluid phase on seismic and electric parameters of rocks suggests the presence of aqueous fluids released by the heating of the mid-crust.

The Japan Sea Cable (JASC) was retired from telecommunications services and its ownership transferred to the scientific community in February, 1996. For the first stage of its scientific reuse program, a Japan-Russia joint project has been initiated to monitor electrical potential differences across the Japan Sea to study the electrical conductivity distribution in the Earth and the water transport across the cable. This paper describes preliminary investigations by forward modelling study to explore possible results from the voltage observation, before analyzing real data. On the resistivity structure, modelling has suggested that voltage data is most sensitive to the conductance of resistive lithosphere, especially at longer periods. Water transport modelling has shown that the heterogeneity of sub-bottom resistivity does not greatly influence the cable voltage, and that water transport of 5–6 Sv in the Japan Sea can generate voltage differences of 70–80 mV across the JASC. A preliminary observation was found to be roughly consistent with this estimation.

The Fenno-Skan high voltage direct current (HVDC) power line links Finland and Sweden across the Bothnian Sea. A 180-km long cable has been installed at the sea bottom. A return current up to 1280 A flows through water and sea-bottom rocks. This source gives a unique possibility for large-scale DC sounding. First test measurements were done by the Department of Geophysics, University of Oulu in May, 1991. In June, 1992 a bilateral experiment was carried out by Finnish and Russian geophysicists. On the base of 42 sites measured during two field seasons apparent resistivity values were calculated. For the interpretation of the measured data a thin-sheet approximation was used. The model consisted of two thin sheets. The uppermost sheet had a conductance S due to the sea water and sediments according to a priori information, while the second one had a depth-integrated resistivity R due to the Earth's upper crust. A good conductor was located beneath. Comparisons of the experimental and theoretical apparent resistivities show that the R value for the Earth's upper crust in southwestern Finland is about 3 × 108 Ωm2, which is in good agreement with the value calculated from the magnetotelluric (MT) data in the Proterozoic Central Finland Granitoid Complex. However, it is more than a factor of 10 lower than the value calculated similarly from the MT data in the Kuhmo region in the Archaean part of the Fennoscandian Shield in eastern Finland and the reported value based on the magnetohydrodynamic (MHD) data in the Kola Peninsula. The results do not reveal any distinct crustal conductors within the research area. This experimental indicates that deep geoelectrical resistivity studies can be carried out successfully at rather large distances, at least 50 km, in the surroundings of a high voltage, DC power link.

Analysis of magnetotelluric data near the southern boundary of the Indo-gangetic basin reveals a clear two-dimensional structure. Two-dimensional numerical modelling of the data has been carried out; the E-polarization response is found to be very similar to the local one-dimensional response. A study has been carried out to remove the screening effect on the magnetotelluric data caused by the top layer sediments. After the removal of the top layer sediments, a best fit model of the averaged magnetotelluric field data has been found, which has a very thick resistive upper crust and a conducting lower crust. The conductance of the lower crust is found to be much less than the values of lower crustal conductance obtained in Eastern Siberia. In contrast, the upper mantle below the Indo-gangetic plains has a resistivity significantly greater than that of the upper mantle below Eastern Siberia.

Far from relatively narrow conducting anomalies (graphite shist belts, fracture zones, etc.) the background electrical resistivity of the upper part of crystaline Earth crost reaches 105 -106 Ohm.m. It coincides with laboratory measurements if taken into account porosity value and water in pores. One could expect an increase of the resistivity with depth due to closing of pores. But unexpectedly resistivity is decreasing at dep th of 10-20 km practically for all investigated areas of Europe. Magnetotelluric soundings of the last decade showed that the conductivity value of the lower crost is similar to that of the sediment cover. This fact is in a sharp disagreement with high resistivity of the dry lower crost rocks derived from laboratory measurements.

An important result of recent years is the normal resistivity profile. It was obtained by interpretation of the combined apparent resistivity curve (magnetotelluric sounding and geomagnetic deep sounding) for the East European platform. This profile has no highly conducting layer and resistivity is greater than 100 ohm-m at asthenospheric depths. It corresponds well with geothermal indications of the absence of partial melting beneath the Precambrian plates. Nearly the same profiles have been obtained for the Canadian shield, and the Siberian and Australian platforms. Investigations carried out in many “hot” regions confirm the existence of a well-developed low-resistivity asthenosphere. Partially molten zones have conductances of about several thousand Siemens in the Eastern Pacific, Iceland and in the North American rift zone. Magnetotelluric soundings show that in many continental areas the lower part of the crust has low resistivity, in the range 10–20 ohm-m. Usually this crustal conductive layer is observed in regions of recent activity. Its total conductivity changes from several hundred to several thousand S. Many investigators propose that the most natural explanation of this conductivity is water solutions. It is necessary to note the distorting role of near-surface inhomogeneities. Local distortions can be eliminated by simple averaging of the experimental data. These average apparent resistivity curves are the starting point for the construction of deep geoelectrical models.

Electromagnetic induction in a stratified Moon with a trailing cavity is discussed. The influence of the Moon wake is studied by using a two-layer lunar model with a perfectly conductive core. The magnetic field is shown to be independent of the wake length when that quantity is greater than 3 lunar radii. Regions on the sunlit and dark sides where the magnetic field may be described in terms of its first spatial harmonic have been distinguished, together with the corresponding errors admitted. It is in these regions that the electrical conductivity of the Moon can be found with very high accuracy, by simultaneous observations on the lunar surface and in the undisturbed solar wind. Results of these observations can be conveniently related to values of the apparent resistivity.

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.

Magnetic fluctuations measured by the Lunokhod 2 magnetometer in the Bay Le Monnier are distinctly anisotropic when compared to simultaneous Apollo 16 magnetometer data measured 1100 km away in the Descartes highlands. This anisotropy can be explained by an anomalous electrical conductivity of the upper mantle beneath Mare Serenitatis. A model is presented of anomalously lower electrical conductivity beneath Serenitatis and the simultaneous magnetic data from the Lunokhod 2 site at the mare edge and the Apollo 16 site are compared to the numerically calculated model solutions. This comparison indicates that the anisotropic fluctuations can be modeled by a nonconducting layer in the lunar lithosphere which is 150 km thick beneath the highlands and 300 km thick beneath Mare Serenitatis. A decreased electrical conductivity in the upper mantle beneath the mare may be due to a lower temperature resulting from heat carried out the magma source regions to the surface during mare flooding.

Theoretical calculations using a stationary ionospheric electrojet model are presented for a resistivity model of interest in studies of the Baltic Shield. The Earth model includes the undistorted East European Platform resistivity-versus-depth model, and crustal and asthenospheric conducting layers. The results show that close to the source region, for periods > 15 min, the magnetic field depends only weakly on the Earth's conductivity. Accordingly, the resolution of apparent resistivity curves obtained by the horizontal spatial gradient (HSG) technique becomes poor at longer periods close to the source region. A comparison with experimental HSG apparent resistivity from Kiruna and Kevo in northern Scandinavia supports the existence of high conductivity in the lower crust. The sharp decrease of apparent resistivity at periods > 1 h may—within the limits of experimental error—be a result of the non-uniform source rather than an indication of a good conductor in the upper mantle.

A thin-sheet model is considered which is equivalent to the gradient resistivity structure of stable regions. For this model a numerical algorithm is developed using the FEM-technique. Results of the calculations can be applied to the interpretation of deep DC-soundings.

Four sets of published data are used: frequency dependence of the day-side horizontal magnetic amplification, the same for the dark-side vertical decrease, the day-side transient amplification and the dark-side transient decrease. Transient data are transformed into the frequency domain and the dark-side data are transformed into the corresponding day-side horizontal amplification. Finally, all experimental results are presented in the form of the day-side frequency response. The summarised apparent resistivity curve is obtained by this response. It corresponds to the model with resistivity about several hundreds of Ω × m to the depths of 700–800 km. It suggests the absence of significant amounts of molten material to these depths.

Interpretation of magnetotelluric soundings on the Siberian platform suggests the presence of anomalously high conductivities in the lower crust around a depth of 35 km. The most probable explanation of this phenomenon is the presence of supercritical fluid in pores and microcracks.

A powerful generator supplies 6–8-s current pulses with the amplitude of ∼ 2 × 104 A in a 7-km cable earthed on the Kola Peninsula coast. The electric field was measured at around 100 sites in the Soviet Union and Finland to about a 700-km distance from the source. Additionally, a diesel generator was used for separations between the source and the receiver < 100 km. Observations indicate an increase of the apparent resistivity to 104 ω m at separations up to ∼ 200 km with a further decrease of the resistivity at larger separations owing to the increase of the deep electric conductivity. This gives the possibility of estimating the depth-integrated (transverse) resistivity if the near-surface conductance is known. For conductance values of ∼ 3 S, the depth-integrated resistivity is near 4 × 109 ω m2.

The phase and modulus of the apparent resistivity of the upper mantle beneath the northwest Atlantic have been calculated from available data for the time variation of the geomagnetic field. Using an ultramafic model of the upper mantle with a basalt melt in the asthenosphere the best fit to the inferred resistivity corresponds to a lithospheric thickness of 90 km and integrated conductivity of the asthenosphere of about 1.5 · 104 ω−1

The application of electromagnetic sounding methods is based on differences in the electrical properties of various rocks, such as the electrical resistivity, ρ, and the dielectric constant, e.

In sedimentary basins in a number of regions of the country (as for example, the Volga—Ural platform), which are potentially productive of oil reserves, there are broad areas where evaporite beds and carbonates with high electrical resistivity occur within the section.

Let us conside r in more detail the weakening of the vertical magnetic field h.(t). From the analysis we eliminate values of h.(t) with t < 30 s, which are distorted by the finite length of the forefront of the interplanetary field impulse as well as by 5 s 104 km, i.e., 30 lunar radii. However, for a correct application of this model, the corrections for the lunar cavity must be introduced into experimental data. The calculations [Vanyan and Yegorov, 1973] show that in the antisolar point the effect of the cavity increases the vertical component by 11% when h, = 0.6/0.7, 9% when h, = 0.7/0.8, and 5% when h, = 0.8/0.9. In Figure 1 crosses and dots show the experimental values of h,(t) [Dyal and Parkin, 1971] and the values of h,(t) corrected for cavity effect, respectively. It is seen that the corrected values coincide with the curve calculated for the model with a conducting layer (model 1). So the night side data with 30 < t < 1200 s are in agreement with the conductivity distribution obtained from daytime observations. The same results were obtained by Schubert and Colburn [1971], who used another method. RESULTS