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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.

Numerical techniques for the model approximating the Earth by conducting and resistive thin sheets that are underlain by a perfect conductor are used to study the on-shore amplification of the electric field induced by the Norwegian Coastal Current. The electric field is induced by sea water movement in the Earth's magnetic field and decreases away from the sea current. However, numerical modelling suggests that the electric field is increased significantly on shore due to the conductivity contrast between the sea and the land. Results of the modelling indicate the significant in-land decrease of the electric field anomaly controlled by the adjustment distance depending on the conductance of the first layer and the transverse resistance of the second one. The coastal electric field anomaly ranges up to 10 mV km −1 which is large enough to be measured by 5–10 km long receiving antenna.

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.

Magnetotelluric measurements show the lower continental crust to be electrically conductive globally, typically 100–1000 times more conductive than dry rocks measured in the laboratory and 10–100 times more conductive than the middle to upper crust. The relatively uniform low resistivity is a fundamental regional characteristic of the lower continental crust upon which local anomalies are superimposed. Conduction mechanisms must exist that are not active in the upper crust. This paper documents geophysical observations that provide important constraints on the mechanisms. They include the following: (1) the resistivity and the depth to the conductive layer show little lateral change across many large-scale geological terrane and structural boundaries; (2) the lower crust on average is more conductive in Phanerozoic areas than in Precambrian areas (10–30 μ m vs. 100–300 μ m, for a layer of about 10 km thickness); (3) the depth to the top of the conductive layer is generally shallower in young areas than in Precambrian regions (10–20 km vs. 20–35 km); the depth appears to be associated with heat flow and commonly corresponds to a present temperature of 350–400°C; (4) in some areas there appear to be correlations between the resistivity of the lower crust and seismic reflectivity, seismic velocity, and seismic attenuation. The most probable candidates for the conduction mechanism are small amounts of interconnected saline pore fluids and interconnected thin films of graphite. An explanation involving about 1% interconnected saline pore fluids is consistent with these constraints, but there are difficulties reconciling the pore interconnection necessary to give low resistivity with the low permeability required to keep the lower-crustal fluid from escaping upward, and with the inference from metamorphic geology that the lower crust consists of dry granulite rocks. The explanation involving thin graphite films less readily explains some of the geophysical constraints such as the difference between Phanerozoic and Precambrian regions and the depth control by temperature. However, this mechanism allows a lower crust with a dry granulite mineralogy as otherwise inferred especially in shield areas.

In the course of seafloor EM survey the topographical changes of the seafloor can have influence on the EM components. These topographical inhomogeneities are 3-D ones. However, they are frequently elongated in one direction and can be treated as 2-D inhornogeneties. The aim of this model study is to present a numerical method for frequency domain marine controlled-source electromagnetic measurements using horizontal electric dipole sources on the seafloor and illustrate how the EM responses are affected by a 2-D trench. The horizontal electric dipole source is parallel and perpendicular to the struetural strike and the EM field components are computed along an in-line horizontal seafloor profile perpendicular to the axis of the trench at two frequencies.

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.

The distortions in the low-frequency plane wave electric field due to variations of the conductance values of the sedimentary cover are considered for the Rheingraben area. The results suggest that the main part of the observed low-frequency magnetotelluric anomaly can be explained by these distortions.

Three groups of electromagnetic data have been considered in order to construct a preliminary geoelectrical depth model of the old basement of the Baltic shield: (i) audiomagnetotelluric data in the period range 1 3700 − 1 8 s; (ii) magnetotelluric data in the period range 25–1000 s; and (iii) global magnetovariation-sounding data in the period range >6 h . These data sets fit together on a common apparent-resistivity curve, indicating the existence of a unified geoelectrical model. This model does not support the existence of conducting crustal or asthenospheric layers.

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.

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 simplified model for the interaction of the cold solar wind with lunar magnetic anomalies is considered. Since on the illuminated side of the Moon the dynamic pressure of the solar wind significantly exceeds the magnetic pressure of the anomalies, upward propagation of the lunar field is possible only by means of diffusion. This process does not depend on the velocity but only on the concentration of the solar wind and the characteristic size of anomalies. Theoretical calculations are compared with the data of Apollo 12 and Explorer 35.

Distortion effects of surface geoelectrical inhomogeneities on deep MHD soundings must be taken into account in the interpretation of the measurements. Computer programs which calculate these effects for the steady-state part of the MHD pulses using the thin sheet approximation and which present the results graphically are used to study the distribution of apparent resistivity for various kinds of models in which the MHD generator is located off the coast of the Kola Peninsula.

The experimental MT data and theoretical data calculated by the thin sheet technique for a low-frequency telluric field are analysed and compared for the Kamchatka Peninsula. The comparison shows a good agreement between the experimental and theoretical data and results in a conclusion that the DC-type distortions of the telluric field play the most important role in the structure of the MT field. The component of the telluric field which is parallel to the general direction of the Kamchatka Peninsula is less distorted and should be used for the estimation of deep geoelectric structures.

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.

The results of geoelectrical studies in the Baikal region have been reconsidered. A new geoelectrical model of the Baikal rift zone has been constructed using a simplified statistical approach, with a normal cross-section as the background. The interpretation proved a success due to several long-period reference magnetotelluric soundings. The most trustworthy element of the model is the highly-conducting layer which is supposed to consist of two horizons. The first horizon belongs to the Earth's crust and is likely to be of hydrothermal nature; the second one, which has been found in Trans-Baikal, overlaps the upper mantle and is connected with partial melting of rocks. The asthenosphere of the Baikal region is believed to be of moderate type.

Block forward modeling and smooth Backus-Gilbert linearized inversion have independently been used to two-dimensionally model two east-west magnetotelluric profiles in the central Rio Grande rift. A crustal conductive zone of at least 1500 S has been detected at about 10 km depth along the northern line. However, along a line 40 km to the south where shallow, active magma is inferred and microseismicity is highest, no high conductivity zone is found. The conductive zone beneath the northern line is hypothesized to occur where water is trapped beneath an undisturbed ductile cap. Under the southern line, magma injection through the cap may release the water, thereby resulting in a relatively more resistive crust with no conductive, midcrustal layer. The resulting paradox is that where there is active magma injection into the upper crust of the Rio Grande rift, the crust is more resistive rather than more conductive.

A comparative analysis of the anomalous magnetic field of the Moon, information about which was obtained by the Apollo 15 subsatellite, and the anomalous magnetic field of the Earth, involving data provided from surveys at various altitudes (up to 500 km) is given. As a result of spectral analysis of these fields it is shown that the main difference of the spectra is in the lower intensity of long period lunar anomalies and the increased rate of their damping with height, which is probably connected with the absence of any kind of magnetization by induction.

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.

On the basis of numerical modelling and statistical investigations of the horizontal electrical inhomogeneities (narrow fractures, change of conductance of the surface sediments) ρ min magnetotelluric (MT)-sounding curves were found to be less distorted and, therefore, they can be used for the determination of deep conducting formations in the crust and upper mantle. Carefully selected MT curves indicate the crustal conductive layer at depths of 7–13 km and the conductive asthenosphere between 50 and 80 km. The relatively shallow position of the asthenosphere corresponds to the high heat flow ( q > 80 mW m −1 ) in the Pannonian basin according to the empirical relation derived by Adam (1978).

Station “Khibiny”, equipped with a powerful magnetohydrodynamic (MHD) generator and/or diesel generator, has been successfully used since 1976 to study the electrical conductivity of the Earth's crust in the northern part of the Baltic Shield. The present paper describes the techniques of measurement, data processing and interpretation of the five-component electromagnetic fields created by this source. A longitudinal conductance map for the upper 10 km of the Earth's crust has been constructed. Several blocks, with conductances varying from 0.1 to some thousands of Siemens, have been revealed on the Kola Peninsula, in northern Karelia and in northern Finland. The blocks of high conductance are connected with relatively young complexes of Early Proterozoic and Late Archean ages. In some places, they create thick and extensive conductive belts such as the Imandra-Varzuga and the Pechenga zones. More often, they appear in the shape of vast regions with enhanced conductivity (e.g., the granulite belts and the Allarechen region). The geoelectric cross-section of the Imandra-Varzuga ore-critical structure has been studied in detail, using the method of electromagnetic field migration. Its depth extent is approximately 10 km. Highly resistant blocks are associated with the most ancient geological units, of early Archean age. Resistant regions have been found in the Murmansk and Central Kola regions, as well as in the Kovdor massif and in the Central Finland granite area. These regions are the most promising ones for deep electromagnetic sounding of the lower crust and the upper mantle of the Earth.

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 theoretical curves presented here for use with the variable-frequency electromagnetic sounding method for a four-layer sequence were compiled by L. L. Vanyan and G. M. Morozova at the Computer Center of the Siberian Department of the Academy of Sciences of the USSR, and prepared for publication by E. I. Terekhin and A.I. Shtimmer at the All-Union Research Institute for Applied Petroleum Geophysics.

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

A generalized curve of the apparent resistivity constructed from global and regional data leads us to suggest that the ancient shields of the East European Platform have no well-developed asthenosphere