Electromagnetic induction in the San Juan Bay region of Vancouver Island

The response of the electric and magnetic field variations over the San Juan Bay region of Vancouver Island is studied using a scaled laboratory analogue model. The laboratory frequencies simulate periods of 10 and 100 s in the geophysical problem. The model results indicate that, for both E- and H-polarization of the source field, induced current in the ocean is deflected around Cape Flattery and channelled into the Juan de Fuca Strait. With increasing period, the proportion of current channelled into the Strait decreases sharply. Induced current in the strait is also funnelled into San Juan Bay, a finger-shaped bay ca. 4 km wide and 7.5 km long, for both polarizations of the source field. The effect of the Bay on the field response is confined to the local region, within approximately 6 km of the centre of the Bay. Good agreement between field station and analogue model H_z results was obtained for the San Juan Bay. The behaviour of the Parkinson arrow for these two stations is examined with the aid of the analogue model results.     

A laboratory electromagnetic model study of the Juan de Fuca Plate region

The behaviour of the magnetic field variations over the Juan de Fuca Plate region is studied using a scaled laboratory analogue model. The model includes a simulation of the complex Juan de Fuca Plate subducting the Vancouver Island region. The subducting plate is modelled with a profile of increasing inclination from east to west; horizontal offshore, dipping at 10° under Vancouver Island, and bending further under Georgia Strait to subduct the continent at 30° for the B.C. region and 45° for the Washington-Oregon region. The strike of the bending plate follows the general strike of the continental coastline with an abrupt change in direction (42°) in the Puget Sound area. The model substructure simulates a subducting plate, overplated by a sediment layer several kilometres thick, and underlain by a 30 km thick highly conducting upper asthenosphere. The model source frequencies used simulate periods 5–120 min in the geophysical scale. In-phase and quadrature H_x, H_y, and H_z magnetic field measurements for the modelled region are presented for an approximately uniform overhead horizontal source field for E- and H-polarizations (electric field of the source approximately parallel and perpendicular, respectively, to the west coast of Vancouver Island). The fields for three regions of the model; over Vancouver Island, over the Olympic Peninsula and over a linear portion of the U.S. coastline, are examined in detail. The general conclusion is that the effect of the dipping subducting plates is to significantly attenuate, at short periods, the maxima in the anomalies at the coastlines underlain by the 10° dipping plate, while leading to anomalous vertical and horizontal fields over ranges as large as 500 km inland over a wide period range. Anomalous fields are observed over the offshore and inland knee-bends of the subducting plates at all periods for both E- and H-polarizations. For locations on land, the in-phase induction arrows point seaward and perpendicular to the strikes of the dipping plates for all periods, while the quadrature arrows at short periods point landward and rotate to point seaward for periods greater than 20 min.     

An analogue model study of electromagnetic induction for cape and bay coastlines

The electric and magnetic field variations across bay and cape coastlines are determined from scaled laboratory analogue models. The effect on the field response of such features along an otherwise straight coastline is limited to a distance approximately equal to the dimensions (diameter of the bay or cape) of the irregularity. For the E-polarization of the source field the perturbations in H_z and H_y are more confined, mainly to the immediate region of the bay or cape. The coast effect H_z/H_y for this polarization, and variations with a 4.2 min. period, is approximately a factor of two larger for the cape than for the bay, while the straight coastline H_z/H_y is roughly mid-way between the cape and bay values. For H-polarization the coast effect is again approximately a factor of two larger for the cape than for the bay, but both values are larger than the value for a straight coastline. The response of the various field components to the bay and cape features is decreased at longer periods, and as a measure, the coast effect is approximately a factor of 1.2 greater for the cape than for the bay for a 42 min. period as compared with the factor of 2 for a 4.2 min. period. The bay and cape deflect the induced currents flowing in the ocean and modify the currents diffusing into the land.This study demonstrates that data from coastal sites may be unreliable for correcting magnetic surveys or for analyses of short period geomagnetic source-field variations.     

Laboratory analogue modelling of the Schumann Resonance source field

An oscillating vertical line current source was used in the laboratory to simulate the naturally occurring 8 Hz Schumann Resonance field due to lightning strokes. Vertical to horizontal magnetic field ratios (coast effect), and magnetotelluric ratios, for traverses over a laboratory model of the Assistance Bay region of Cornwallis Island (N.W.T. Canada) for the vertical line current source field are compared with those for an overhead uniform source field. The E- and H-polarizations of the source fields used are defined as the cases of the horizontal magnetic field of the source roughly perpendicular and parallel to the general coastline, respectively.The magnetic field ratios are essentially identical for the two field sources for E-polarization, but differ by a factor of approximately two for H-polarization, with the ratios greater for the overhead source than for the vertical line current source. The magnetotelluric ratios are found to be identical for the two field sources of each of the E- and H-polarizations.     

Analogue model studies of induction effects at auroral latitudes

In addition to field observations and numerical models, geomagnetic induction effects can be studied by scaled analogue model experiments. We present here results of analogue model studies of the auroral electrojet with an Earth model simulating the Arctic Ocean and inland conductivity structures in northern Fennoscandia. The main elements of the analogue model used were salt water simulating the host rock, an aluminium plate corresponding to the ocean and graphite pieces producing the inland highly conducting anomalies. The electrojet was a time-harmonic line current flowing at a (simulated) height of 100 km above northern Fennoscandia. The period simulated was 9 min.The analogue model results confirmed the well-known rapid increase of the vertical field when the coast is approached from the continent. The increase of the horizontal field due to induced ocean currents was demonstrated above the ocean, as well as the essentially negligible effect of these currents on the horizontal field on the continent.The behaviour of the magnetic field is explained with a simple two-dimensional thin-sheet model. The range, or the adjustment distance, of the ocean effect inland was found to be some hundreds of kilometers, which also agrees with earlier results of the Siebert-Kertz separation of IMAGE magnetometer data. The modelled inland anomalies evidently had too large conductivities, but on the other hand, their influence decayed on scales of only some tens of kilometers.Analogue model results, thin-sheet calculations, and field observations show that the induction effect on the horizontal magnetic field B_x near the electrojet is negligible. On the other hand, the vertical component B_z is clearly affected by induced currents in the ocean. Evidence of this is the shift of the zero point of B_z 0-1° southwards from the maximum of B_x. The importance of these results are discussed, emphasizing the determination of ionospheric currents.     

Electromagnetic induction in the British Isles region: analogue model and field station results

The results of a laboratory electromagnetic analogue model study, which employs a horizontal inducing field over a simple model of the British Isles region, delineate the location and frequency dependence of the major coast effect induction anomalies of the Scotland region. Contours of amplitudes, amplitude ratios, and in-phase and quadrature parts of the model field measurements are presented. The model vertical magnetic fields for two orthogonal source field polarizations and field station values for two hypothetical events for corresponding polarizations are compared.While major discrepancies occur between model and field H_z amplitudes, the H_z gradients across Scotland, which can be attributed to the coast-effect, are comparable in value, although sometimes reversed in sign. Superimposed on this coast-effect, the field data indicate the existence of current concentrations associated with the Great Glen and the Southern Uplands faults and possibly also of currents within the Scottish mainland near the east and west coasts.     

An analogue model study of electromagnetic induction in the eastern coastal region of North America

The behavior of electric and magnetic field variations over the eastern coastal region of North America is studied using a scaled laboratory electromagnetic analogue model. The model source frequency used simulates a period of 1 h in the geophysical scale. The results indicate that deflection and conductive channelling of induced electric current is important for both the E-polarization (northeast-southwest direction of the electric field of the source) and the H-polarization (northwest-southeast) of the source field. In the model, conductive channelling occurs through the Strait of Belle Isle, Cabot Strait, and in the St. Lawrence River. Current deflection is particularly prevalent around the southeast coast of Newfoundland for both E- and H-polarization, and around the northeast coastline of Nova Scotia for E-polarization. The model results also show current deflection by cape and bay coastal features, as well as by ocean depth contours.A comparison of model measurements for the cases of a uniform source field and a line current source indicate that the nature of the source field has a measurable but surprisingly small effect on the vertical to horizontal magnetic field ratio for both E- and H-polarizations, and negligible effect on the magnetotelluric ratio for coastal regions.The model fields in coastal regions were found to be strongly influenced by induced currents, deflected and channelled by the coastline and ocean bathymetry, and were dependent on the nature and particularly the polarization of the source field. Thus, along the complex coastline of eastern North America, a wide range of electric and magnetic field values should be expected. In some regions the coast effect, measured by the vertical to horizontal magnetic field ratio at the coast, could be expected to be extremely small or absent, while in other regions the ratio could approach a value as large as unity for variations of 1 h period.     

Effect of magnetic storms in variations in the atmospheric electric field at midlatitudes

The observations of the variations in the vertical component of the atmospheric electric field (E _z ) at Swider midlatitude Poland observatory (geomagnetic latitude 47.8°) under the conditions of fair weather during 14 magnetic storms have been analyzed. The effect of the magnetic storm main phase in the daytime midlatitude variations in E _z in the absence of local geomagnetic disturbances has been detected for the first time. Considerable (～100–300 V m^?1) decreases in the electric field strength (E _z ) at Swider observatory were observed in daytime simultaneously with the substorm onset in the nighttime sector of auroral latitudes (College observatory). The detected effects indicate that an intensification of the interplanetary electric field during the magnetic storm main phase, the development of magnetospheric substorms, and precipitation of energetic electrons into the nighttime auroral ionosphere can result in considerable disturbances in the midlatitude atmospheric electric field.     

Empirical and Numerical Modeling of T-phase Propagation from Ocean to Land

—?T-phase propagation from ocean onto land is investigated by comparing data from hydrophones in the water column with data from the same events recorded on island and coastal seismometers. Several events located on Hawaii and the emerging seamount Loihi generated very large amplitude T phases that were recorded at both the preliminary IMS hydrophone station at Point Sur and land-based stations along the northern California coast. We use data from seismic stations operated by U. C. Berkeley along the coast of California, and from the PG&;E coastal California seismic network, to estimate the T-phase transfer functions. The transfer function and predicted signal from the Loihi events are modeled with a composite technique, using normal mode-based numerical propagation codes to calculate the hydroacoustic pressure field and an elastic finite difference code to calculate the seismic propagation to la nd-based stations. The modal code is used to calculate the acoustic pressure and particle velocity fields in the ocean off the California coast, which is used as input to the finite difference code TRES to model propagation onto land. We find both empirically and in the calculations that T phases observed near the conversion point consist primarily of surface waves, although the T phases propagate as P waves after the surface waves attenuate. Surface wave conversion occurs farther offshore and over a longer region than body wave conversion, which has the effect that surface waves may arrive at coastal stations before body waves. We also look at the nature of T phases after conversion from ocean to land by examining far inland T phases. We find that T phases propagate primarily as P waves once they are well inland from the coast, and can be observed in some cases hundreds of kilometers inland. T-phase conversion at tenuates higher frequencies, however we find that high frequency energy from underwater explosion sources can still be observed at T-phase stations.     

The effect of climate change on extreme waves in front of the Dutch coast

Coastal safety may be influenced by climate change, as changes in wave conditions (height, period, direction) may increase the vulnerability of dunes and other coastal defences. Dune erosion depends on mean water level, storm surge height and wave conditions. In this paper, we investigate the change in wave conditions in the North Sea in a changing climate. Until now, the effect of climate change on annual maximum wave conditions has been investigated, while events with higher return periods are actually most damaging for the coast (e.g. severe dune erosion). Here, we use the 17-member Ensemble SimulationS of Extreme weather under Non-linear Climate changeE (ESSENCE) change of climate change simulations, to analyse A1b-induced changes in the mean wave climate, the annual maxima and wave conditions with return periods of up to 1:10,000?years in front of the Dutch coast. The mean wave climate is not projected to differ between 1961–1990 and 2071–2100, with both wave height (H _s) and wave period (T _m) remaining unaltered. In the annual maximum conditions, a decrease is projected; especially, the annual T _m maximum decreases significantly by 0.3 to 0.6?s over the whole study area. Furthermore, we find that the direction of the annual maximum wave conditions shifts from north and north-west to west and south-west for both H _s and T _m. This is induced by a similar shift in the direction of the extreme wind speeds. Despite the decrease in annual maximum conditions, the return H _s and T _m are not projected to change significantly as a result of climate change in front of the Dutch coast for the period 2071–2100 relative to 1961–1990.     

An analogue model study of electromagnetic induction for island-continent ocean channels for the H polarization

Using an electromagnetic analogue model, the behavior of time-varying electromagnetic fields for an island near a continental coastline is examined for the H-polarization case, in which the electric field of the inducing source is perpendicular to the continental coastline. A study of the effect of the shape of the island, using square- and circular-island models, indicates that over the island, anomalies in the field components are confined to a smaller area for the circular island than for the square island. A study of the effect of ocean-channel width between the island and continent shows that, as the channel width decreases, anomalies in the magnetic field components over the island decrease. The anomaly in the electric field at the continental coastline first increases and then decreases with decreasing channel width.     

Generation of superDreicer electric fields in the solar chromosphere

The electric field generation at the front of the current pulse, which originates in a coronal magnetic loop owing to the development of the Rayleigh–Taylor magnetic instability at loop footpoints, has been considered. During the τ_A ≈ l/V _A ≈ 5?25 s time (where l is the plasma plume height entering a magnetic loop as a result of the Rayleigh–Taylor instability), a disturbance related to the magnetic field tension B _?(r,t), “escapes” the instability region with the Alfvén velocity in this case. As a result, an electric current pulse I_z(z ? V _A t), at the front of which an induction magnetic field E _z, which is directed along the magnetic tube axis and can therefore accelerate particles, starts propagating along a magnetic loop with a characteristic scale of Δξ ≈ l. In the case of sufficiently large currents, when B _? ²/8π > p, an electric current pulse propagates nonlinearly, and a relatively large longitudinal electric field originates E _z ≈ 2I _z ³ V _A/c ⁴a²B_z ²l, which can be larger than the Dreicer field, depending on the electric current value.     

Chaotic behaviour of interplanetary magnetic field under various geomagnetic conditions

In the present study, the deterministic chaotic behaviour of interplanetary magnetic field (IMF) under various geomagnetic conditions of low and high solar active periods was analyzed, using the time series of IMF |B| and B_z, by employing chaotic quantifiers like, Lyapunov exponent, Tsallis entropy, correlation dimension, and non-linear prediction error. We have investigated whether the chaotic behaviour of interplanetary magnetic field would modify, when it produces major geomagnetic storms, and how it depends on the phase of solar activity. The yearly average values of Lyapunov exponent for the time series of IMF |B| and B_z, show solar flux dependence, whereas those values of entropy, correlation dimension and non-linear prediction error had no significant solar flux dependence. The yearly average values of entropy for quiet periods are higher compared to those values for major storm periods belonging to low/high solar active conditions, for both the time series |B| and B_z.     

SIGNAL ET BRUIT EN MAGNETOTELLURIQUE *

Conventional processes of extracting magnetotelluric signals from noisy records are reviewed: instrument noises and noises that are generated close to the detectors can be eliminated by the usual auto- and crosscorrelation processes. Identification of coherent noises, such as pulses due to field sources that are not uniform over at least 100 km in oil exploration or 1000 km in crustal studies, is much more tedious. The 5 components H_x, H_y, H_z, E_x, E_y, of the magnetotelluric field have been recorded in many areas in France at different periods of the year, (a) in non-uniform field sources in the vicinity of electric railways and of 50 cycle power lines, and (b) in areas of strong inhomogeneity at depth on the flanks of steep structures and near the sea shore. Means for detecting non-uniformity are reviewed. Measuring the vertical component of magnetic pulses is a good way of estimating field uniformity: if H vertical/H horizontal <10%, the uniform field assumption is valid, and the classical restitution formulas can be used; if H vertical/H horizontal > 10%, uniformity can not be assumed and there is some difficulty in deciding whether non-uniformity is due to the field source or to anisotropy or inhomogeneities at depth. Several ways to solve this difficulty are described. The reliability of calculation of actual resistivity at various depths is examined as a function of the precision of apparent resistivity measurements.     

地形对大地电磁勘探的影响

本文讨论了地形对大地电磁场的影响。用有限元方法计算了几种地形的大地电磁影响曲线。计算表明,地形对E_X型波视电阻率的影响,主要出现在地形起伏幅度与电磁波在地下介质中的波长可以比较时。H_X型波的地形影响比E_X波严重,随着周期的增长,H_X型波的地形影响变得稳定。地形凸起处的影响比下凹处严重。     

On the effects of a bumpy core-mantle interface

Motivated by the high degree of correlation between the variable parts of the magnetic and gravitational potentials of the Earth discovered by Hide and Malin (using a harmonic analysis approach and utilizing the geomagnetic data) when one field is suitably displaced relative to the other, Moffatt and Dillon (1976) studied a simple planar model in an attempt to find a quantitative explanation for the suggestion that this high degree of correlation may be due to the influences produced by bumps on the core-mantle interface. Moffatt and Dillon assumed that the core-mantle interface was z = η(x) where |?η/?χ| ? 1 and such that in the core [z < η(x)] a uniform flow (U₀, 0, 0) prevails in the presence of a uniform ‘toroidal’ field (B₀, 0, 0); (here z is the vertical coordinate and x is the eastward distance). The whole system rotates uniformly about the vertical with angular velocity Ω. The present work extends the model discussed by Moffatt and Dillon to include a horizontal component of angular velocity Ω_H and a uniform small poloidal field B_p. In addition, the uniform toroidal field is here replaced by one which vanishes everywhere in the mantle and increases linearly, from zero on the interface, with z. It is shown that the presence of Ω_H and B_p, together with the present choice of toroidal magnetic field, has a profound effect both on the correlation between the variable parts of the magnetic and gravitational fields of the Earth, and on how far the disturbances caused by the topography of the interface [which is necessarily three-dimensional i.e. z = η(x, y) here] can penetrate into the liquid core. In particular it is found that the highest value of the correlation function is +0.79 which corresponds to a situation in which the magnetic potential is displaced both latitudinally and longitudinally relative to the gravitational potential.     

Two field applications of the rotating current EM method1

The rotating current EM method has been applied to the delineation of two conductive orebodies, Elura near Cobar, NSW, and Thalanga near Charter's Towers, Queensland. The field data were collected in the form of observations of the vertical magnetic field strength ratio and phase difference using a Turam-style receiver with twin vertical coils. By reconstituting this data back to the ring source field and phase, i.e. the observed H_z, phasor, it is possible to present contoured maps of the EM field. Anomaly phasors are obtained by subtracting theoretical phasors from the observed phasors in the complex plane of the H_z phasor. The theoretical phasors for the finite source are based on horizontally layered, half-space earth models, computed at each point of the survey grids, then normalized to selected points of the observed fields. Use is made of the intrinsic circular symmetry of the method in X–Y plots of field versus source-receiver distance to ascertain geoelectric parameters for the earth models. A steel picket fence at Thalanga is modelled by a line source grounded at each end and its H_z, phasor is removed by the same process. A considerable improvement in anomaly delineation is gained over previous Turam-style anomalies and the two survey examples illustrate the limitations of the method in the presence of a conductive overburden (Elura) and its abilities in the absence of a conductive overburden (Thalanga).     

Magnetic fields due to sea tides in the english channel

The M₂ (12.42-h period) magnetic field variation present at night in the United Kingdom can be interpreted in terms of sea tides, the main contribution being from the Atlantic Ocean. Osgood et al. (1970) observed a difference between the M₂-variations in the total magnetic field (F) variations at Sidmouth (0.8 km from the coast) and Exeter (16 km from the coast) of amplitude 0.8 γ, which must be due to local effects. Analysis of the phase of this difference between Exeter and Sidmouth has shown that it cannot be due entirely to tidally induced electric current systems confined to the English Channel. Measurements with two three-component magnetometers have shown that the night-time variations in the north-south (H) component of 12.42-h period are significantly different at Sidmouth and Exeter. The difference in H is in phase with the expected electric potential difference across Devon due to the difference in the phases of the sea tides in the English Channel and the Bristol Channel. The results of Donato and Rosser (1973) on micropulsations suggest the existence of a region of high conductivity under Exeter. If this connects the English Channel to the Bristol Channel, a current will flow beneath Exeter giving a difference in the M₂-variations in H at Exeter and Sidmouth. This difference in H dominates the M₂-variations in the total magnetic field (F) variations at Exeter and Sidmouth measured by Osgood et al. (1970).     

Magnetotelluric response of a two-dimensional sloping contact by the finite element method

Summary The finite element method, with triangular elements, is used to study the effect of a two-dimensional sloping contact on the surface electromagnetic fields. It is found in the case ofH-polarization and small slopes that the electric field and the apparent resistivity near the contact, on the conductive side, are higher than their asymptotic values. In the case ofE-polarization the apparent resistivity and phase values on the conductive side fall off less rapidly to their asymptotic values with decreasing slope resulting in higher apparent resistivity and phase values on the conductive side, than those expected for a vertical contact. The peak in the amplitude and phase of the normalized vertical magnetic field shifts from the resistive side for a vertical contact to the conductive side for a sloping contact. Far from the sloping contact, on the conductive side, higher values are observed for the normalized vertical magnetic field than in the case of a vertical contact.     

Electromagnetic scattering by a perfectly conducting half-plane in a conductive half-space

FollowingDmitriev (1960) a rigorous theoretical solution for the problem of scattering by a perfectly conducting inclined half-plane buried in a uniform conductive half-space has been obtained for plane wave excitation. The resultant integral equation for the Laplace transform of scattering current in the half-plane is solved numerically by the method of successive approximation. The scattered fields at the surface of the half-space are found by integrating the half-space Green's function over the transform of the scattering current.The effects of depth of burial and inclination, of the half-plane on the scattered fields are studied in detail. An increase in the depth of burial leads to attenuation of the fields. Inclination introduces asymmetry in the field profiles beside affecting its magnitude. Depth of exploration is greater for quadrature component. An interpretation scheme based on a phasor diagram is presented for the VLF-EM method of exploration for rich vein deposits in a conductive terrain.List of symbols x, y, z Space co-ordinates - Half-space conductivity - ₀ Free-space permeability - Excitation frequency (angular) - T Time - h Depth of the half-plane - a Inclination of the half-plane - E _x x-Directed total electric field - E _x ^p x-Directed primary electric field - E _xo ^p x-Directed primary electric field atz=0 directly over the half-plane - H _y y-Component of total magnetic field - H _y ^p y-Component of primary magnetic field - H _y0 ^p y-Component of primary magnetic field atz=0 directly over the half-plane - H _z z-Component of total magnetic field - H _z ^p z-Component of primary magnetic field - J _x Surface density ofx-directed scattering current - G Green's function - k ₀,K Wave numbers - u,u ₀,u ₁,u ₂ Functions - Space co-ordinate - s Variable in transform domain - Variable of integration - Normalized scattering current - Laplace transform of - N Normalized - , ₀, ₁, ₂ Functions - t Variable of integration - Skin depth - H Total magnetic field - H ^p Primary magnetic field - H ₀ ^p Primary magnetic field atz=0 directly over the half-plane - M,Q,R,S,U,V Functions - N ₁,N ₂ Functions