Bimodal electromagnetic induction in non-uniform thin sheets with an application to the northern Pyrenean induction anomaly

Vasseur, G.; Weidelt, Peter

doi:10.1111/j.1365-246x.1977.tb04213.x

Cited by 217 publications

(187 citation statements)

References 19 publications

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“…The "normal" earth section is given in Fig. 2(b) and WEIDELT (1977) for an electrojet source similar to that presented by MARESCHAL et at. (1987) to simulate the daytime induction arrows observed in the region of southern India directly beneath the equatorial electrojet.…”

mentioning

confidence: 77%

The contamination of substorm fields by induction effects: Extension to the heterogeneous surface layer.

Mareschal

Vasseur²

1987

J. geomagn. geoelec

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In the past decade, several arrays or lines of magnetometers have been used in order to help monitor the behaviour of external current systems. In most cases, the basic approach consists in fitting model currents with adjustable parameters to the data, a technique which has the severe disadvantage of restricting the number of parameters allowed to vary. Consequently, it is common practice to approximate the induced fields, which always contaminate ground-based magnetic measurements, by those of a perfect conductor located at some depth below the surface of the earth, this type of representation having the advantage of being solved extremely easily (even in a spherical earth) by image theory. Many careful semi-quantitative analyses of the region in terms of its electrical properties are usually carried out before the data recorded are investigated for their external contribution (e.g. KuPPERs et al., 1979), but it may happen that the depth to the equivalent perfect conductor is arbitrarily fixed without consideration of the source field. This latter procedure is never justifiable since the depth is dependent on the frequency and-to a lesser extent-on the wavenumber content of the source as well as on the average electrical properties of the region. It is also worth remembering that the definition of a single perfect conductor, fixed at a given depth during the model fitting procedure, implies that the source consists of a single dominant periodicity. A random event, such as a substorm, contains a broad spectrum of energy and may have to be analysed by initially passing the data through a narrow band filter before the calculation is done.The determination of the depth at which to locate the perfect conductor so that it will generate field amplitudes similar to those observed at the surface of the real earth is always easy to accomplish once the periodicity content of the source is determined (e.g. see Fig. 1, after MARESCHAL,1981). However, it is essential to remember that, since the image of a real source field by a perfect conductor is real, this representation loses all phase information. In order to evaluate the severity of the simplification, MARESCHAL (1976) compared synthetic electrojet fields induced in a flat earth either of finite conductivity varying with depth alone or of perfect conductivity. She showed that for an electrojet oriented along the E-W (Y) axis, phases (or time lags as she put it) over a typical section were totally negligible for the X -component but more important for the Z-component. Depending on the location of the observer, the

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mentioning

confidence: 77%

The contamination of substorm fields by induction effects: Extension to the heterogeneous surface layer.

Mareschal

Vasseur²

1987

J. geomagn. geoelec

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“…The charges collected at conductivity discontinuities affect the local electric field (Price, 1973). Examples of the formulations approximating 3-dimensional structures by thin sheets are those by Vasseur and Weidelt (1977), and Dawson and Weaver (1979). Such algorithms have been applied to a variety of real and synthetic problems.…”

Section: Numerical Modellingmentioning

confidence: 99%

“…Such algorithms have been applied to a variety of real and synthetic problems. We used the thin sheet algorithm of Vasseur and Weidelt (1977) for estimating the electric field components (E1 and Ev) and the magnetic field components (By. and By) in a model of Bay of Bengal (Fig.…”

Section: Numerical Modellingmentioning

confidence: 99%

Seafloor Geomagnetic Sounding near the 85.DEG.E Ridge in the Bay of Bengal.

Joseph

Iyengar

D'Cruz

et al. 1995

J. geomagn. geoelec

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“…The subject has recently been extensively reviewed by JONES (1982). Current channelling has been invoked to explain the observed behaviour of difference magnetic fields (the difference between the measured field and the field at a reference station) in various parts of the world, particularly in the Rhinegraben MOSNIER, 1979, 1980) and the Pyrenees (BABOUR et al, 1976;VASSEUR et al, 1977). However, SUMMERS (1981SUMMERS ( , 1982 has argued that much of the evidence from differential sounding (as the analysis of difference fields is sometimes called) can be explained just as well by local induction in a 2-dimensional model.…”

mentioning

confidence: 99%

“…Algorithms for solving induction and "disturbed skin-effect" problems in three dimensions have been available for several years (VASSEUR and WEIDELT, 1977;DAWSON and WEAVER, 1979b). Both methods are based on the thin sheet approximation in which it is assumed that the conductivity anomalies are confined to a thin layer at the surface of the Earth.…”

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confidence: 99%

A numerical study of the channelling of induced currents between two oceans.

McKirdy

Weaver

1983

J. geomagn. geoelec

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The channelling effect is studied with the aid of a numerical model in which two oceans are represented by conductive thin sheets, one occupying a half-plane and the other a quarter-plane, situated on the surface of and in electrical contact with a conducting half-space. The oceans are connected by a narrow conductive channel through which induced currents can be channelled. The electromagnetic response of this model to a regional magnetic field which is periodic in time and uniform in space, oriented either perpendicular or parallel to the channel, is calculated using the algorithm developed by Dawson and Weaver. The limitations of the numerical method restrict the study to periods between 1.5min and 10min, and only the latter period is allowed if the thin conductive sheets are to be interpreted as oceans. It is found that for a field of period 10min both horizontal deflection (into the channel) and vertical leakage (into the underlying medium) of the induced currents are significant near the channel entrances. The actual paths of the anomalous currents in the surface plane are displayed by plots of the anomalous surface current vectors. These show that the quadrature currents are very definitely channelled from one ocean to the other even when the regional flow is perpendicular to the channel. The effect is less marked with the in-phase currents but channelling is still present. At a period of 1.5 min, however, the channelling effect has all but disappeared.Local induction appears to predominate when the period is small. Difference magnetic field vectors and induction vectors in the region of the channel are also shown, and comparisons are made with the corresponding vectors calculated for the same model but with the channel removed. It is concluded that the presence of the channel can be clearly recognized from the behaviour of these vectors.

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Bimodal electromagnetic induction in non-uniform thin sheets with an application to the northern Pyrenean induction anomaly

Cited by 217 publications

References 19 publications

The contamination of substorm fields by induction effects: Extension to the heterogeneous surface layer.

The contamination of substorm fields by induction effects: Extension to the heterogeneous surface layer.

Seafloor Geomagnetic Sounding near the 85.DEG.E Ridge in the Bay of Bengal.

A numerical study of the channelling of induced currents between two oceans.

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