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Analytical and analogue methods of studying electromagnetic variations at the earth's surface Dosso, Harry William


This thesis deals with both mathematical and analogue models for studying electromagnetic variations at the earth's surface. The field components are studied for frequencies in the range 10ˉ⁴ to 10³ cycles/sec and for earth conductivities in the range 10ˉ¹⁶ to 10ˉ¹º emu. Expressions are developed for the electric and magnetic field components at the surface and within the upper layer of a horizontally stratified flat conducting earth in the field of incident plane waves. Extensive results of amplitudes and phase angles are obtained for various frequencies, angles of incidence, layer thicknesses, depths, and conductivities. As an extension of this problem, expressions for a multilayer earth (n layers) are developed and evaluated. Each of several thick layers is divided into a sufficient number of sublayers, with changing conductivity, to represent to a good approximation a continuous change in conductivity. The conductivity distributions used are of interest in geophysics. The results for the plane wave model indicate that the amplitudes and phase angles are strongly affected by the conductivity structure. The electric and magnetic fields at the surface of a flat homogeneous conducting earth in the near field of an oscillating line current are studied. The equations for the amplitudes and phase angles developed by Law and Fannin (1961) are used for the calculations. Extensive results of amplitudes and phase angles are obtained for various frequencies, conductivities, source heights, and locations with respect to the overhead current. The results indicate that the vertical to horizontal magnetic field ratios are in the range of experimentally observed values. An analogue model suitable for studying the behavior of the natural geomagnetic and telluric field variations for various geological structures was constructed. The two types of field sources used were an oscillating sheet current and an oscillating line current. Extensive measurements of amplitudes and phase angles for the horizontal electric, the horizontal magnetic, and the vertical magnetic field components are obtained and discussed for various geological structures including a flat layered earth, cylindrical bodies embedded in the surface layer, vertical faults and dykes, sea mounts and conducting domes, coastline structures (sea-land interface and an upwelling in a high-conductivity zone within the mantle), and islands in an ocean channel. The results obtained for the coastline structures and islands in an ocean channel tend to support the proposed structures suggested by various workers (Schmucker 1964, Lambert and Caner 1965, Lokken and Maclure 1966) in describing the experimentally observed coastal magnetic field anomalies. The analogue model constructed and used for this work readily lends itself to studying a wide range of geological structures for a variety of source fields in addition to the ones used here.

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