The inductive response of the electrically conducting Earth's interior to geomagnetic variations can be inferred from the magnetic Z-H or the magneto-telluric E-H relations. In the absence of lateral non-uniformities of the electrical resistivity within a distance range which is comparable to the depth of penetration these relations are expressible by a single transfer function in in the frequency-wave number domain or by convolutions in the frequency-space domain. Lateral non -uniformities within the mentioned range produce anomalous Z-H and E-H relations. The resulting induction anomaly is dependent on the direction of source-field polarization with respect to subterranean resistivity gradients. The interpretation proceeds from a known or assumed normal inductive response for a layered Earth which is defined on a regional scale and serves as normal reference for lateral non-uniformities to be investigated. -The uniform internal part of D_st-variations shows that the mantle resistivity below 600km depth is of spherical symmetry. The internal part of S_q-variations appears to be affected by lateral non-uniformities in the upper 400km. For fast variations (e. g. bays) the inductive shielding by oceans and interspersed sedimentary basins on land becomes significant. In addition to this surface effect there exist regional and local variation in the depth of penetration of fast variations which originate from non-uniformities in the uppermost mantle.

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This paper reviews the interpretive theory of the electrical conductivity distribution in the mantle as inferred from geomagnetic variations. The laboratory measurements of the electrical conductivity in relevant materials are discussed and their implications for estimates of the average radial temperature distribution are explored.
New developments in geothermal calculations using convection theory are described since they appear to be the most promising means of interpreting the regions of anomalous electrical conductivity well within the upper mantle. The same convection theory applied to the moon clarifies the recent observations of a very short Cowling time for this body.

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The latest results concerning the depth variation of Q, P- and S-wave velocities obtained by various seismological methods are reviewed with special emphasis on the regional variation. The body-wave travel time and the slope, dT/dΔ, have been inverted to obtain P- and S-wave velocity distributions within the mantle. The P velocity is now believed to be known with an accuracy of ±1% in the mantle below 1000km. In the upper mantle, a considerable difference has been found among different regions. The long-period surface waves have been used to reveal the regionality of the upper mantle. Based upon the surface wave and body-wave results, it is now agreed that the mantle beneath shields has much higher seismic velocities than those beneath ocean and tectonically active regions. At such tectonically active regions as Japan, basin and range province of the western U. S., and the Pacific ocean side of South America, the upper mantle velocities have been found to be significantly lower than the average. Teleseismic P waves are delayed by 1.0 to 1.5sec at tectonically active regions as compared with shields. The existence of the low-velocity zone for S waves in the sub-oceanic mantle is almost certain. The shape of the low-velocity zone varies markedly beneath continents. The distribution of Q with depth is not known in detail. However, it is almost certain that the value of Q is higher by more than one order of magnitude in the mantle below 1000km than in the mantle above. The average values of Q_α (Q for P-waves) and Q_β (Q for S-waves) for the upper mantle range from 100 to 300. Horizontal heterogeneities as large as a few per cent in velocity and one order of magnitude in Q have been found beneath mid-oceanic ridges and island arcs. However, the uniqueness of the structure derived is still questionable.

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The results and the possible interpretation of a survey are described. A comprehensive study of the temporal variations of the three components of the geomagnetic field when measured from place to place has been undertaken using eight magnetometers. A total of forty six temporary magnetic observatories were established covering the majority of Wales, Ireland, Southern Scotland & England.
The temporal variations of the horizontal components of the geomagnetic field are very similar over the wole area. Variations of periods in the range of four hours to ten minutes were observed.
The temporal variations of the vertical component differ when measured at stations only 40-50km apart. On translating the data into the frequency domain, it was observed that variations with a frequency of about 36 cycles per day are most affected.
At nearly all stations, there is a strong correlation between the variations of the vertical field and the variations of some component of the horizontal field. It is possible to interpret this behaviour as indicating the presence of local current concentrations in the vicinity of the British Isles. At first sight, the shallow seas and the Atlantic Ocean appear to be the major conductors. A possible interpretation, consistent with the observations, is that the current concentrations are a direct result of electromagnetic induction in the Atlantic Ocean by the variations of an external magnetic field.
Another current concentration, of similar magnitude to the currents in the seas, is observed to be flowing in a NE-SW direction through Southern Scotland. It is possible that this current is driven by electric potentials set up by the current concentrations in the Irish and North Seas. It is suggested that it may not be necessary to postulate the existence of an upper mantle or crustal conductivity anomaly in this region.

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There are two types in anomalies of short period geomagnetic variations: one (High cut ΔZ type) is characterised by high attenuation of high frequency components of ΔZ and the other (Directional variation type) by unusual directionality in the variation vector. “High-cut ΔZ type” anomaly is expected to be found in areas where the electrically conducting layer is raised and “Directional variation type” is expected to exist at boundaries between areas having different electrical properties. Reported anomalies of geomagnetic variations in various parts of the world are classified into the above two types and compared with the observations of terrestrial heat flow. Most of “High-cut ΔZ type” anomalies are associated with high heat flow, whereas most of “Directional variation type” anomalies are associated with the boundaries of high and low heat flow areas. Such correlations are interpreted in terms of the temperature distribution in the upper mantle. Typical of “Directional variation type” anomalies are found at continental margins, where the effect of sea water should not be overlooked. It is further shown that, from the view-point of both heat flow and the conductivity anomaly, there are two types in continental margins: normal (Atlantic type) and island arc type (Pacific type) continental margins. Finally, a model that appears reasonable is presented for Central Japan anomaly of which complex nature so far rejected theoretical interpretations: Central Japan anomaly is caused by the superposition of peninsula effect and undulation of high conductive layer under the Philippine Sea coastal area of southwestern Honshu where heat flow is higher in the sea than in the land. Unlike northeastern Honshu, southwestern Honshu is not a typical island arc.

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Geoelectric studies have shown that a pronounced high conductivity region exists in the upper mantle of the Southwestern United States. This anomaly is well correlated with other geophysical anomalies. Tne electrical measurements can be interpreted to give upper mantle temperatures very close to the melting point. Preliminary measurements near the California ocean-continent boundary indicate even higher conductivities exist in the mantle under the ocean.

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Time-varying magnetic fields have been observed in the summers of 1967 and 1968 by means of two overlapping arrays of three-component variometers, each containing 42 instruments, between 32° and 43°N and longitudes 100° and 116°W. Maps of Fourier spectral component amplitudes and phases sharply define lateral variations in upper mantle conductivity structure. A ridge of high conductivity runs under the Southern Rocky Mounains between the Great Plains and the Colorado Plateau, which marks a low-conductivity region within the Cordillera. The Basin and Range Province both west and south of the Colorado Plateau shows high mantle conductivity, with an additional up-welling of conductive material at its boundaries with the Colorado Plateau and Great Plains. Close correlation with heat-flow data supports the view that temperature variations are the cause of the conductivity structures. These structures modify the daily variation fields and thus extend to great depths. A deep basin of conductive sediments in southwest Oklahoma causes a pronounced anomaly. Such crustal anomalies are distinguishable from upper mantle anomalies through their frequency dependence and phase lead.

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Recent work on prominent magnetic variation anomalies in Canada is reviewed. The first of these is on Ellesmere Island in the Arctic Archipelago, and results from magnetic and magnetotelluric data collected in 1967 are presented. Anomalous effects-i. e., an abnormally high level of magnetic activity coupled with a persistent tendency for the horizontal variation vector to be restricted to a single direction-appear to be confined to a narrow zone nearly 500km long stretching between Alert on the north coast and Eureka on the west. Recent data indicate that the strength of the anomaly is not uniform along its strike, but is somewhat diminished in the central and southern portions. The main features of the anomaly have been explained by postulating the presence of a long narrow conducting body located in the lower part of the crust. The existence of such a conductor would provide a natural channel for currents induced over a much broader area. It may also imply an abrupt upheaval of isotherms underneath Ellesmere Island. Available surface wave dispersion and heat flow information in the Ellesmere Island area will be discussed: the evidence supporting a thermal explanation for the geomagnetic anomaly is unconvincing, but still ambiguous.
The Mould Bay anomaly is located in the eastern part of the Arctic Archipelago and is known to extend over large portions of Prince Patrick and Melville Islands. Over this area shorter period fluctuations in the vertical component are very strongly attenuated. The presence of a massive conducting layer deep in the crust is postulated to explain the effect. Seismic, heat flow and gravity data are available in this area, but no clear relation to the geomagnetic anomaly has been found.

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An integrated approach using both geomagnetic depth-sounding (GDS) and magneto-tellurics (MT) is applied to the determination of the electrical conductivity structure in western Canada; 42 GDS stations and 6 broad-band MT stations are used. These data are combined with other geophysical information to provide self-consistent petrological models. Excluding geochemically improbable solutions, it can be shown that: a) temperatures at depth 35km must be at least 750°C in the entire western region (eastwards extent undefined); b) in a smaller, sharply defined, region (eastern boundary within about ±50km of the Rocky Mountain Trench) the lower crust and possibly uppermost 10-25km of the mantle are hydrated, probably with partial melting.

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The so-called North German conductivity anomaly has been mapped extensively in recent years from Western Germany to Poland. The revealed spatial structure of geomagnetic rapid variations, the results from different magnetotelluric traverses, and earth current observations with electrode spacings up to 120km together with theoretical calculations show the overwhelming importance of the extraordinarily well conducting sedimentary layers within this region. On the other hand there still seem to be indications in favour of an additional deep-seated conductivity anomaly. Pronounced earth current and magnetic variation anomalies have been found in connection with graben structures in Western Germany. Over the Molasse basin of Bavaria magnetic variations are normal even though there is strong concentration of earth currents. The situation within the Carpathian basin of Hungary seems to bear some resemblance to that one found in Northern Germany and in Poland. Observations on Sardinia and Corsica point towards a strong concentration of induced currents within the Bonofacio Street between these islands. Magnetic investigations of various kinds seem to indicate that large conductivity anomalies are present also in France and in Spain.

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In terms of current knowledge of crustal genesis in the Atlantic Ocean, several unique or highly anomalous features exist in the vicinity of the Azores Islands: the seismically active East Azores Fracture Zone extending from Gibralter to the Mid-Atlantic Ridge; the seismically inactive West Azores Fracture Zone which is offset northwards from the trend of the East Azores Fracture Zone; the transverse island chain of the Azores islands which trend southeast-northwest across the Mid-Atlantic Ridge; the marked change in direction of the Mid-Atlantic Ridge from northeast-southwest to north-south; the broadening of the Mid-Atlantic Ridge to the east; and a proposed equatorward decrease of crustal spreading rate across the transverse fracture zones.
Bathymetric and magnetic data from surveys of the Mid-Atlantic Ridge in the Azores area by R. V. Trident, and the U. S. Naval Oceanographic Office have been compiled. These, together with the previously published data are compatible with a crustal genesis model consisting of a Mid-Atlantic Ridge migrating eastwards at the local crustal spreading rate, which is greater to the north than to south of an east-west transverse fracture system. Superimposed on this is the development of a northwest-southeast trending secondary spreading center or triple junction, within a ‘leaky transform’ system (Menard and Atwater, 1968) which developed as the result of a change in local crustal spreading direction south of the east-west transverse fracture system, from east-west to northwest-southeast. Simple geometrical considerations when combined with the length of a newly proposed ‘Azores Rift’ and an independently determined crustal spreading rate, suggest that the proposed local change in crustal spreading direction and triple junction development began more than 40m. y. ago.

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Seismicity maps are used to demarcate the African plate and regions where the plate maybe breaking into smaller plates. One of these is the Gregory rift through East Africa. In this region, complete crustal separation has not taken place as in the Red Sea and Gulf of Aden. Instead, extension seems to have caused attenuation of the lithosphere with accompanying rifting and volcanism. The ages of rift faults and volcanoes tend to increase with distance from the axis of the rift. Three sets of geophysical data can be used to support plate attenuation. These are (i) the long wavelength negative Bouguer anomaly which is found over a large region of East Africa; (ii) the attenuation of S_n for paths crossing the rift north of 10°S. and (iii) microseismicity studies of the rift floor. The long wavelength negative Bouguer anomaly has been interpreted as being due to the low-Q asthenosphere engulfing the lower part of the lithospheric plate. It is suggested that the areal extent of the regional negative Bouguer anomaly might be used to map the region of plate attenuation.

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Remarkable anomalies in the physical properties of the upper mantle are associated with the seismicity and volcanism of island arcs. The inclined zones of earthquakes, which reach depths of nearly 700km in many regions, are associated with mantle material with anomalously high seismic wave velocities, low seismic wave attenuation, and perhaps high density. Studies of the orientations and magnitudes of the stresses that produce deep and intermediate-depth earthquakes suggest that the anomalous material contains the zones of mantle earthquakes, has a thin plate-like geometry, and is stronger and more dense than the adjacent mantle. In contrast, the uppermost mantle behind the island arc (the wedge-shaped region above the inclined zones of earthquakes) appears to have anomalously low seismic wave velocities, high seismic wave attenuation, and high heat flow. This anomalous region appears to extend several hundred kilometers behind the line of active volcanoes. These features all seem to be explainable by the tectonic model in which a slab of lithosphere descends into the mantle beneath an island arc. The lithosphere would be colder, and hence would have higher seismic velocities and lower attenuation and would be denser and stronger than the adjacent mantle. The wedge-shaped region above the descending slab would be heated up as a result of one or more processes (such as localized convection, frictional heating, or igneous activity) that result from the descent of the lithospheric slab.

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The margins of the continents or transition zones between continents and oceans are divided traditionally into two types: Atlantic and Pacific. There are also some transition types between these two.
From the geophysical point of view the border between a continent and an ocean is determined now as an area where changing the crust from continental type to oceanic one is observed.
Atlantic type is characterized by abruption of continental structure spreading from the shore to the shelf with the border of the bottom on the deep ocean. The area of changing the crustal type is not so wide, about 50-150km.
Pacific type is much more complicated and includes not only structures typical for Atlantic type but also such specific ones as marginal seas, island arcs with active volcanoes and deep oceanic trenches. The trend line of the border of the deep oceanic bottom is usually parallel to the trend line of the geological structure of marginal zones and adjoint lands.
Geophysical data on the deep crustal structure of the transition zones are scattered up to day still. Nevertheless many important regularities were observed last decade. The reverse relationship between the oceanic and sea depths and the thickness of the crust is the most certain regularity. We observed considerable decreasing the crustal thickness in comparison with that on the adjoint land just near the shore where water depth on the shelf was only some hundred meters. But this relationship is quite certain for the large underwater structures and breaks down usually at the transition zones.

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It has recently become possible to study the history of the geomagnetic field from the magnetization of the oceanic crust and independently from the magnetization of the oceanic sediments. Both studies show, essentially, the same history of the geomagnetic field as that derived from the study of volcanic rocks on land. Such comparisons are only possible for the last 4.5 million years.
Extending in time the geomagnetic field history from the study of the oceanic crust permits one to map large scale crustal movement. The possibilities of such movements being related to the generating mechanism of the geomagnetic field and to other global phenomena is discussed.

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As the largest island on the worldwide ocean ridge system, Iceland may offer the possibility to study on land some of the characteristic features of ocean ridges. The paper gives a survey of geophysical research in Iceland related to this problem. It includes measurements of geothermal gradient and electrical conductivity, seismic sounding, observation of earthquakes, crustal displacement, gravity, paleomagnetism and geomagnetic surveys.

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The paper informs about the regional geoelectric characteristics of the upper mantle in the Carpathian Basin based on several deep magnetotelluric soundings carried out mainly in Hungary.
The best determined curves are those of the electromagnetic observatory near Nagycenk.
Theoretical computation for electrically inhomogeneous and anisotropic models (geological structures) plays a part in the complex interpretation of the curves along with petrographycal parameters, geothermal, seismic and gravity data.

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Results of magnetotelluric soundings have been obtained for a series of five field stations located approximately along the international profile of deep seismic sounding No. VI. Multilayer sounding curves at almost all stations show anisotropic and/or horizontally non-homogeneous situation. Methods for estimating the inhomogeneity measures were applied and attempts have been made to locate the homogeneity axis and corresponding two-dimensional structures. Directional properties of the electric conductivity have been discussed and fitted to the local geology.
On respecting the uncertainty and ambiguity of apparent resistivity curves the depths of remarkable discontinuities have been estimated and traced along the profile.

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