Western North America between the coastal ranges and the Great Plains, and from the Mexican-United States border to the Trans-Canada Highway has been studied by means of three large two-dimensional arrays of recording magnetometers. The paper discusses the conductive structures found by these arrays in the upper mantle of the region. Highly conductive mantle material at least 100km thick for conductivity 0.2S/m underlies the Basin and Range Province and ridges of still higher conductivity (or greater thickness for a given conductivity) underlie the Wasatch Fault Belt and Southern Rockies. Under the Great Plains the upper mantle is more resistive and is of sub-shield type. North of the Basin and Range the Cordillera have a thin layer (10-20km at 0.2S/m) of conductive material in the upper mantle and the conductivity indicates a lower level of tectonic activity than in the midlatitudes of the U. S. A. Under the Colorado Plateau the conductivity is of Great Plains type or intermediate between this and that beneath the Basin and Range. Alternative conductive models satisfying variation-field anomalies are discussed with special reference to the poor depth and excellent lateral resolution of magnetometer arrays. The distribution of heat-flow is compared with that of electrical conductivity and it is shown that there is generally close correspondence and full support for association of high conductivity with high temperature. Seismological velocity-depth profiles, station terms in P and S teleseismic times and other parameters are considered and are again consistent with the conductivity and geothermal information. The case for partial melting is considered. It is strongly supported by seismological parameters in the low-velocity layer and gives the most plausible hypothesis for the contrasts of order 100 in conductivity required by the magnetometer array results. Three tentative suggestions are made concerning tectonics of the region.

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Magnetic variation measurements made at 21 stations along the St. Lawrence River in eastern Canada have shown the existence of current concentrations, which appear to be coupled to oceanic currents and controlled by tectonic features. In particular, the line of Logan's fault, a thrust which marks the northern limit of the Appalachian region against the Precambrian Shield, is the locus of a current concentration for part of its length. However, in the region of present-day seismic activity on this fault, the current becomes much less defined. Possible explanations, for which models are being constructed, are that in this active region the current escapes vertically to the conducting mantle, or simply spreads out laterally. The active area is marked also by present-day decrease in elevation and by a positive gravity anomaly and may be a vestigial hot spot.

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As a result of intensive observations of geomagnetic variations over many years, the overall distribution of anomalous Z fields has become clear in Japan. One of the anomalies, the central Japan anomaly, has been accounted for by a depression of a highly conducting layer in the mantle beneath Japan, although the effect of the sea on the anomaly has not been brought out quite clearly. The period dependence of the anomalies along lines across central and northeastern Japan is examined by making use of the transfer function technique. Anomalous Z fields can be traced on a few islands in the Pacific Ocean south of central Japan, although they are strongly contaminated by island effects there. It is found out that the central Japan anomaly has a strong period dependence. The anomaly along the line across northeastern Japan also has a strong period dependence which is slightly different in its characteristics from that for the central Japan anomaly. Numerical calculations of electromagnetic induction based on the method given by JONES and PASCOE (1971), have been made for two-dimensional models, and the calculated transfer functions are compared with the observed ones. As a result, it is found out that the sea surrounding the Japan Islands plays an important role on both the anomalies. The anomaly in the central part of Japan, however, cannot be accounted for by the sea only. On the basis of these calculations, possible electrical conductivity models beneath the central and northeastern parts of Japan are put forward. It is concluded that a highly conducting layer seems likely to lie at a depth of 30km beneath the Philippine Sea and the Japan Sea. Anomalous Z fields associated with ssc's, geomagnetic bays and similar changes have been found on Oshima, Miyake-jima and Hachijo-jima Islands. Such an island effect is certainly caused by the induced electric currents which are distorted by the low conductivity of the island. However, the calculated effect does not agree with the observed one when electromagnetic coupling between the sea and the conducting layer in the mantle is ignored. A detailed study of electromagnetic coupling suggests that a highly conducting layer lies close to the earth's surface beneath the northern part of the Izu-Bonin arc.

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A chain of geomagnetic observatories was established along the west coast of Greenland during the summer of 1972. Very large variations of the horizontal component of the geomagnetic field vector, with an amplitude of more than twice the normal field variations, were recorded at the Igdlorssuit observatory in the basalt province of northern West Greenland. The geomagnetic variation anomaly was studied by means of seven field stations in addition to the observatory chain. The anomalous variations have been interpreted as due to a concentration of currents in a highly conducting body in the earth's crust. Model calculations indicate that the body is very narrow with a scale width of the order of 30km. Across Nugssuaq the strike of the anomaly coincides well with the major fault of Itivdleq and at Igdlorssuit the anomaly follows the fault-bounded belt of basalt comprising the Ubekendt Ejland. The volcanic origin of the West Greenland basalt province has by various authors been related to the opening of the Baffin Bay by continental drift.

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In low latitude the source field distribution functions and source dimensions constitute outstanding problems in the study of the subsurface electrical conductivity structure. A mathematical structure that takes these factors into account is discussed and the theoretical response curves for various n-layered earth models in regional studies are presented. The results of the first practical test of the theory proposed by ONI (1972) show that a four layer model is able to resolve the basement complex, the continental crust, the upper mantle and the top of a conducting half space at a depth of about 75-80km at Ibadan.

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The geomagnetic variation anomaly on Ellesmere Island has been attributed to an elongated conductor located deep in the crust or in the upper mantle and striking northeast across the northern portion of the island. Recently observations have been made on the ice of the Lincoln Sea lying to the north of Ellesmere and Greenland, and the anomaly has been found to extend beyond the coast for a distance of 100km or more beneath the continental shelf. Fitting the data with a uniform current model suggests that the anomalous conductor is about 100km wide, 10km thick, and located in the crust at depths between 10 and 20km.
The anomaly lies at the northeastern end of the Innuitian Province. It follows approximately the axis of the Franklinian Geosyncline (Ordovician to mid-Silurian) and parallels the structural trend of the lower paleozoic strata which were folded during the Ellesmerian Orogeny of Devonian-Mississippian times. There is no geophysical evidence suggesting a contemporary or recent origin for the anomaly. The high conductivity zone seems more likely to be caused by hydration and structural changes deep in the crust; the geological evidence suggests that it may be a relic of ancient tectonic activity.

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In 1963, 1964, and 1970 geomagnetic variations in 3 components were measured at 14 sites in the Western Arctic Islands of the Canadian Archipelago. The suppression of Z amplitudes, first observed at Mould Bay, is now confirmed to extend from Mould Bay south to Holman Island, a distance of 620km, and from McCormick Inlet to Houghton Head, an east-west distance of 292km.
Spectral decomposition of many component time series gave mean Z/X energy density ratios which fall into two groups. One group (anomalous) is characterized by slopes of 2, with attenuations of 10-50 between the 60 (or 100) and 10min energy ratio estimates. The other group (non-anomalous) do not display such behaviour in the component ratio plots. At periods near 10min the energy ratios of the two groups differ by factors of 5 to 10. Distributions of Parkinson vectors for various period intervals support in part a deep conducting body and in part electric currents in salt water channels or deep ocean.
Coherency analysis of horizontal components between stations supports the assumption that external source dimensions are proportional to the 1/3 power of period of disturbance. However, the electromagnetic theory of induction for horizontal layered models of infinite extent has failed to explain the observed reduction of Z amplitudes near 10min periods. Long period (100-300min) energy ratio estimates are explained as the effect of a conducting (1 S/m) layer or half-space in the upper mantle underlying most of the Western Arctic. Love-wave model studies have detected a low-velocity zone in the upper mantle between Mould Bay and Coppermine.

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Special magnetic observations carried out at twenty temporary stations close to the latitude of Hyderabad in peninsular India, along the Alibag-Hyderabad and the Hyderabad-Bhadrachalam-Kalingapatnam profiles including two in the Koyna region, are discussed and the salient features in the seismically active areas of Koyna and Bhadrachalam are brought out.
It is found that while the Z-amplitudes of both short-period and long-period geomagnetic variations show the normal coastal effect at Koyna, they get enhanced at Malharpeth, 35km east of Koyna. The amplitudes of H-variations, however, remain unaffected. At Bhadrachalam, the ranges of both H and Z long-period daily variations are observed to be enhanced.
The tectonics and the deeper conducting structures of the Koyna and the Bhadrachalam regions as inferred from the anomalous geomagnetic variations, appear to be similar except for the directions of the conductors.

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Two dimensional numerical models of the California coast anomaly have been constructed to compare their predictions with observations, with a view to infer the electrical conductivity structure of crust and upper mantle within this area. The third axis of these models is directed along the coast. Only induction processes with magnetic fluctuations normal to it are considered (Transverse Electric Mode). The ocean is represented by a this sheet with a conductivity equal to the vertically integrated oceanic conductivity. Crust and upper mantle are simulated by free and finite conductivity distributions extending downward several skin depths before termination by an infinitely conductting horizontal plane. The upper boundary, which contains the source field, is transparent to the upward reflected energy, thus simulating the high impedance of the ionosphere. The horizontal wave length of the source fields is long, though arbitrary, and these fields can simulate horizontal perturbations, stationary or progressive. The predicted fields are obtained by solving for the stream function φ of the magnetic fluctuation vector B over a rectangular area centered around the coast. φ has to satisfy the diffusion equation Δ²φ+iμμ₀ωσφ=0 subject to the aforementioned boundary and interface conditions. Application of these techniques to interpret observations of electromagnetic fluctuations in the range 2^-2 to 2¹ cph across the central California coast suggests a substantial increase of conductivity of the basement, seaward of the continental slope.

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Oceanic electric fields originate from (1) induction by magnetic pulsations in the ionosphere, (2) water motions across the earth's field and (3) electrochemical processes associated with sea floor materials and with tissues of marine organisms. Precise recording of these signals is difficult because of (1) their small size, and (2) a large and unavoidable noise occurs at the contact points between sea water and measuring devices.
The spectrum P of average natural electric activity at the sea floor in the band 10^-3 to 10⁺¹ cph roughly follows the trend P=kf^-1, f=frequency, k centered at 10^-2 μV² m^-2. The noise spectrum of typical Ag-AgCl electrode pairs P′ on the sea floor environment approximates k′= 10^-1 μV² with tenfold variations in both directions. Thus, achieving a 10¹ signal to noise ratio requires an electrode separation of 10 meters. Even so, the unavoidable electrode voltage mismatch, typically 10² to 10³ μV, fatally eradicates the signal datum unless considerably longer lines area used.
Rejection of electrode bias and noise to allow short electrode separations can be done by physically inverting the electrodes positions, which is most practically achieved with rotating instruments, or (2) by switching back and forth the sea water connections between electrodes and the salt bridge pipes which perform electrical contact with the ocean. A method to achieve this “water chopping” is described.
Examples of ionospheric and barotropic velocity signals recorded on the sea floor with 5m span instruments and presented.

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Global conductivity models can be based on the complex ratio of external to internal diurnal variation fields. If this ratio is derived from the five quiet days of each month, a great variation from month to month is found. This indicates that the results are very sensitive to the group of observatories from which hourly values are derived.

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