Since magnetic signals recordable on the deep sea floor are very small and since records of long duration must be obtained to extend the frequency range abruptly limited on the high end by ocean shielding, sea floor magnetic variographs depend critically on long term stability. Temperature effects are not limiting since temperature variations at great oceanic depth are minimal, but stability of position in situ at landing depends on little known as well as little controllable conditions.
Magnetic variations on the sea floor are presently carried out almost exclusively by means of self contained magnetic variographs (often simply called magnetometers) freely traveling between ocean surface and bottom. The sensors are flux gates, thinfilm single domain transducers and suspended magnets with optical readout. In the first two cases, the achieved stability is dependent upon the stability of the cancelling field and associated electronic circuitry. In suspended magnet types, initial torsion of the suspension fibers can relax the requirements upon electronics considerably.
Electric field recording on the sea floor by means of long lines requires a sufficiently large separation to minimize effectively electrode drift. Azimuth and electrode separation must be established with adequate accuracy, a non-trivial requirement. Electrode drift can be eliminated within the frequency range of interest —below a few cycles per minute— by electrode switching. This technique leads to more compact instruments. Its implementation is illustrated in detail.
Meaningful electromagnetic observations related to oceanic signals are also made at the periphery of continents or on islands by means of wires specially laid locally or by means of abandoned transoceanic cables.

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We have measured electromagnetic fluctuations at a number of stations on islands and on the sea floor of the Atlantic. Magnetic fields H were measured on the Bahamas, on Bermuda and at two sea floor stations. Horizontal components E of the electric field were measured on Bermuda and at three deep sea stations. The latter form a triangular array with 100km legs on the Hatteras Plain. The age of the underlying lithosphere is estimated at 130 million years.
E at the sea floor is dominated by the Lorentz force U×B below 0.1cpd. These fields have a typical horizontal scale less than 100km and are unsuitable for deep electromagnetic soundings but provide limits to shallow conductivity. At higher frequencies, up to the oceanic cut off of several cycles per hour, the fields are of ionospheric origin.
The impedance tensor has been estimated at Bermuda and two sea floor stations for the ionospheric fields. On Bermuda the local influence of the island has been removed by Larsen's method for comparison with the sea floor impedances. At both sea floor stations where impedances were measured, only the component generated by eastward E and northward H is well determined. Models of lithospheric conductivity which are consistent with all measurements show diminishing conductivity below a crustal layer, then sharp rises at 70 and 250km.

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Electromagnetic signals from two deep sea floor magnetotelluric stations located far from coastal influence in the North Central and North Eastern Pacific are evaluated.
The electric field was recorded by means of salt bridge chopper type instruments with a resolution of .01μV/m. Independent magnetic variographs using suspended magnet sensors recorded magnetic variations accurate to .2γ. Both electric and magnetic recorders are characterized by very small long term drift. They are self contained, store data on magnetic cassettes and travel free, between surface and bottom.
Magnetotelluric interpretation of records from a first station, located 450 nautical miles to the NNE of Hawaii, at a position which corresponds to magnetic reversal anomaly 31 and to a plate age of 72 m. y. suggests a moderate to low lithospheric conductivity with an average value less than 3×10^-3 (ohm m)^-1 over the upper 60km, increasing sharply around 100km. A high conductivity tongue, with an average value around 6×10^-2 (ohm m)^-1 occurs over the interval 180±40km. Meaningful information ceases for a depth beyond 400km.
A second and more recent magnetotelluric sounding 700km off central California suggests that a very high and quite shallow conductivity feature implied from earlier work was probably overemphasized. Nevertheless, according to the new data a relatively higher conductivity at a shallower depth seems to differentiate this second station from the first. This result is in harmony with the younger crustal age at the second station (32 m. y.; magnetic reversal 12-13).

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Electromagnetic induction in the Vancouver Island region of British Columbia, Canada, was studied using scaled analogue model measurements and field measurements from two pairs of stations spanning the channel between Vancouver Island and the continent. Good agreement between model measurements and field measurements for the vertical magnetic field component was obtained. The model results show that conductive telluric current-channeling in the shallow seawater channel is important for the frequencies studied. The results also show that the geometry of the channel affects the current channeling and that there is some diffusion of current into the continent where the ocean channel direction is not parallel to the electric field of the inducing field.

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Analytic edge corrections are developed suitable for matching with approximate solutions (derived otherwise) for perfectly conducting oceans. By comparison with the solution of a particular problem, it is verified that the matched solutions are good all the way to the edge of the idealized oceans. They reproduce accurately the well known singularity on the coastline, and this property is essential if the solutions are in turn to match those for finite conductivity. Often an appreciable (oscillating) magnetic field passes beneath the “ocean”, and this should perhaps be included in future calculations involving ocean edge effects.

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The problem solved is that of electromagnetic induction in a thin non-uniformly conducting hemispherical shell underlain by, and insulated from, a perfectly conducting sphere. Since the shell is assumed thin, only the component of a varying magnetic field normal to the shell induces electric current in the shell. This mode of induction is termed vertical component induction. The conductance and configuration of the surface shell are simple approximations to those of the Pacific Ocean. Calculations are made of the effect of induction by the Sq variation field and results are presented for typical “land” and “ocean” regions, and for a profile of stations near the ocean edge. The results from this profile of stations are compared to observations made by SCHMUCKER (1970) near the coast of California. The calculated and observed daily variations of the vertical component of the magnetic field are in qualitative agreement.

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The electromagnetic response of an ocean coast model to a vertically incident monochromatic plane wave is studied. The model consists of a perfectly conducting half-plane (the model ocean) resting at the surface of a good conductor (the model earth). Values of electric and magnetic field components at the surfaces of land and ocean, as well as at the ocean floor, are given in tabular form for both E- and H- polarization induction.

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Two-dimensional induction problems are solved by means of an equivalent electrical network, which is then solved by the method of diakoptics. The network is divided into several subnetworks, which are partially solved independently of each other, and then are re-assembled into the full network. The advantages are that the size of arrays handled is reduced, and that it is possible to solve a problem which has most of the subnetworks the same as a previous problem more quickly than by solution without subdivision, since the common subnetworks need not be re-solved. A computer program has been written to use this method, and an example is given with storage and time used.

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The earliest estimates of the conductivity of the earth were based on the analysis of Sq. The picture that emerged from analyses performed up to 1940 was that of a shallow conducting layer at the surface underlain by a few hundred kms in which the conductivity is low followed by a region of very rapid or discontinuous increase of conductivity. The overall pattern now considered most likely is not essentially different, but is now realised that the period range spanned by Sq is too small. Furthermore, when trying to match a radial model of conductivity, Sq data is biased by conductivity anomalies. Longer period variations seem to be less subject to surface conductivity irregularities.
Important progress has been made towards estimating the conductivity structure beneath oceans; it seems to be very different from that under continents. The work of Larsen on Oahu indicates a distribution in which the conductivity is an order of magnitude greater for the top 300km. This, and similar work off the coast of California, have done much to elucidate the behaviour of the geomagnetic field at the edge of oceans.
Sq periods have proved valuable in investigating conductivity anomalies also explored by shorter period variations. Generally anomalies that influence the field at sub-storm periods also appear at diurnal frequencies, but “dead-Z” anomalies of the Tuscon type often do not.

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General methods for constructing reliable electromagnetic response functions from interrupted and noisy recordings of the naturally occurring electromagnetic variations are described. The aim is to be able to effectively utilize the 11 years of hourly mean electric and magnetic data (1932-42) from Tucson, Arizona, despite the many gaps and large extraneous values. This long-time series is extremely useful in helping one to focus on techniques that yield stable and reproducible response functions. The ultimate goal is to obtain accurate response functions in order to construct reliable earth electrical conductivity models and to determine the ocean fluid-induced part of the electromagnetic field. The Tucson electric field data is found to have diurnal and semidiurnal tidal frequencies most likely of oceanographic origin and a 6 d^-1 peak of unknown origin.

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Local magnetotelluric sounding data from the Russian Platform are interpreted jointly with global magnetic data of long periodic variations. In the first stage of interpretation the magnetotelluric data are used to find for each site a layered conductivity model for the sediments and the underlying crust and uppermost mantle, regarding the deeper upper mantle as a uniform conductor. In the second stage conductivity estimates are derived down to 1, 200km depth from global data, using for the top layers the results from stage one.
The interpretation follows the statistical approach of the NEWTON-LeCAME-MARQUARDT method which takes a random component in the data into account. The best fitting model involves a conductivity increase by two orders of magnitude in 340km depth, even though this result should not be regarded as final and unambiguous.

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The deepest electric conductivity increase determined by electromagnetic induction methods can be attributed to the conductivity increase observed in the laboratory during rock phase transition. It can be called the Ultimate Conducting Increase (or UCL), since no further increase of conductivity has yet been detected at greater depths. A positive dP/dT gradient is characteristic of rock phase transition, where P is the pressure in BARS, T the temperature in °C.
The aim of the research reported here was to look for an indication of a positive dP/dT gradient in the data set available, i. e. whether greater depths of the UCL (or greater pressures) correspond to greater temperatures at the depth of the phase transition. The surface heat flow (q in HFU), the characteristic part of which is coming from the upper mantle, can be used as an indicator of the temperature. Since in the upper mantle of the territories with lowest heat flow (q<1HFU) there is no partial melting, it can be assumed that the depth of 250-300km of the conductivity increase determined on platforms and crystalline shields corresponds to the depth of the rock phase transition. Using these data, which are very critical for our interpretation, an empirical formula was calculated by least squares fit between the depth of the conductivity increase H_UCL (km) and the surface heat flow q (HFU): H_UCL=16.3+292.5q. This formula indicates a positive dP/dT gradient. On the basis of the data set, the average H_UCL is 420km and the average heat flow 1.38HFU. From this formula, taking dP/dT=30BARS/°C, it can be concluded that the temperature at a depth of 300-400km below the platform areas is about 1, 000°C less than the average.

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The induction vectors and profile graphs of the anomalous field indicate clearly the existence of narrow elongated electrical conductivity anomalies in Central Europe. The anomaly map and parameters are presented. According to the analysis of synchronous variations in the region under study the horizontal magnetic field hodographs of the bay variations over the central part of the Carpathian anomaly are considerably stretched in the direction normal to the anomaly strike. The majority of permanent geomagnetic observatories of Central Europe are favourably located about the anomalies so that distortions of the horizontal magnetic field of bay variations are relatively small (except Niemegk and Moscow). The vertical component is distorted stronger. The behavior of electric component of bay and S_q variations over the central part of the Carpathian anomaly led us to suggest that this part of the anomaly connects conductors larger than itself. In all probability these are the North German-Polish anomaly and Black Sea.

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A geomagnetic induction anomaly in eastern Newfoundland is identified with the line of closure of the proto-Atlantic Ocean. A model consisting of an electrically conductive fossil lithospheric slab embedded within the continental lithosphere is shown to be quantitatively consistent with experimental observations. The conductivity of the Appalachian central mobile belt in Newfoundland does not greatly exceed that of other crustal sections in Eastern Canada.

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A two-dimensional array of 20 Gough-Reitzel magnetometers was operated over S. Scotland in 1973 and in 1974-5 magnetotelluric and magnetovariational observations in the period range 10-10, 000s were made in the same region. In this paper, the analyses of the magnetic data from both studies are presented in the form of induction vectors and hypothetical event contours. They suggest that the lateral variations in electrical conductivity structure associated with the Eskdalemuir anomaly are more complex than suggested by earlier studies. A marked discontinuity in electrical structure is apparent in a narrow belt parallel to and south of the S. Uplands fault. This belt is associated with a major gravity anomaly and with steep gradients in the seismic profile at crustal depths. Another discontinuity is indicated near the North-umberland Basin. Representative examples of the magnetotelluric analysis and of one-dimensional Monte Carlo inversion of the M-T data are presented for the three regions separated by these discontinuities. They show that the conducting zone associated with the Eskdalemuir anomaly is at a depth greater than 24km, while on either side of this region, there are good conductors within crustal depths.

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A laboratory analogue model, described previously by MILES et al. (1977), was used to study the effect of a sloping ocean floor with a shelf, and a step with a shelf, on the magnetic variations induced by ocean waves moving in a static magnetic field. The model measurements indicate that for a shallow ocean, the amplitudes of the magnetic variations are attenuated as the wave travels over the wedge, reach a minimum at the shelf edge, then again increase over the shelf. For the case of the step with a shelf maximum attenuation occurs at the shelf edge followed by an enhancement over the shelf. For both cases, the depth of the fluid over the shelf is the important factor in determining the behaviour of the induced field.

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The depth of penetration and the modified apparent resistivity are estimated by using the results of the annual and the solar cycle variation of the geomagnetic field, and then the deep conductivity is discussed.

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In fitting geoelectrical models to sounding curves from certain localities in Czechoslovakia, we applied “the Hedgehog” method. In the first step it uses the Monte Carlo procedure to find a successful model. Then, it goes systematically through the values of both the electrical conductivities and the layer thicknesses varying within given limits in the neighbourhood of the successful model previously found.
The inversion was effected for different types of magnetotelluric sounding curves corresponding to specific geological structures. From a large family of models tested by the Hedgehog procedure the best fitting and the most frequently occurring models were chosen. The direct problem was solved for their parameters and the resulting theoretical sounding curves were compared with the experimental ones.

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Geomagnetic observatory data were analysed to obtain the spectral amplitudes and phases of the 27-day variation and its harmonics over a network of mostly European observatories, but extended to global scale by including some suitably distributed geomagnetic stations on other continents. The spectral amplitudes, estimated by different methods, were subjected to spherical harmonic analysis by expanding the geomagnetic potential function into a series of spherical harmonics up to the 2nd order and by calculating the corresponding coefficients. The contours of the H and Z components were then calculated to analyse their latitude-longitude dependence over European area. The moduli of electromagnetic response functions estimated from global data for individual harmonics were compared with the results obtained previously for European data only. The distribution of deviations of (Z/H) ratios defined by differences between the actual data and the theoretical fits were investigated. Using the theoretical basis of electromagnetic induction in stratified conductors, we estimated the depths of a perfect substitute conductor at those depths from characteristics of the 27-, 13-, and 9-day variations.

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Induction arrows (Wiese vectors) for night-time SSCs and Bays are determined and discussed along with geological and other geophysical data at eight Indian magnetic observatories, from Sabhawala in the Himalayan foothills to Trivandrum on the seacoast in the extreme south.
A medium-sized induction arrow at Sabhawala pointing northwards indicates that the region of higher subsurface conductivity lies to the south of Sabhawala, possibly along the Aravalli Hills, and not beneath the Kashmir Himalaya, where the conductive upper mantle appears to be depressed. East-south-east pointing arrows at Jaipur and Ujjain are again indicative of a high electrical subsurface conductivity on the west along the Aravalli Hills and the Cambay region, where high heatflow and gravity values have also been reported, and the upper mantle appears to be elevated.
At Alibag, a medium-sized induction arrow points eastward, with the conductive seawater lying on the west. Hyderabad is the only station where small induction arrows (small Z variations), characteristic of an inland station, are observed. Negative Z variations for positive H variations along the Alibag-Hyderabad-Kalingapatnam profile, become positive about 100km east of Hyderabad due to ocean effect from the east coast.
Unusually large induction arrows (very large positive Z variations) in the extreme south Peninsular tip (largest one at Trivandrum), so close to the dip equator, are indicative of very strong induced current concentrations near and along this coast. These oceanic induced currents causing the anomalous geomagnetic variations in Peninsular India, appear to concentrate along the coastline and the continental shelf, and flow from north to south along the east coast. The currents concentrate further as they pass through the narrow Palk Strait and the Gulf of Manar between India and Sri Lanka on to Kanyakumari and the Trivandrum coast, and turn northwards along the west coast due to the obstruction provided by the volcanic ridge of Lakshadweep, Minicoy and Amindivi islands lying to the west in the Arabian Sea, about 300km off the Trivandrum coast. The current concentration would turn westwards near Calicut (11°N) and pass north of these islands in a diffuse form. Another current concentration is expected to flow along the Alibag west coast from north to south (so as to explain the reversed coast effect at Alibag), and turn westwards near Calicut to merge with the Bay of Bengal current system. There is no need to postulate a couductor and current channelling beneath the sea near the Trivandrum coast without any supporting geological evidence. There could be a channelling of the induced currents at the interface of the conductive upper mantle beneath the ocean and the less conductive upper mantle beneath the Peninsular India (100-200km), in a step-structure around the coast, which will account for the induction anomalies observed in long-period variations like Sq.

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