A study of the effect of coastline contours on electromagnetic induction
Date
1979
Authors
Chan, Gee Hung
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Abstract
In this thesis, the effect of coastline contours on electromagnetic induction is investigated with the aid of an analogue model study of various idealized non-rectilinear coastline models. The laboratory modelling system is essentially that first described by Dosso (1966b) and more recently by Nienaber et al. (1967). A laboratory analogue model study of an actual bay, the San Juan Bay on the southwestern coast of Vancouver Island is also carried out and the results are compared with field station measurements.
Two simple models of non-rectilinear coastlines consisting of a semi-circular bay and a cape are first considered. The perturbation in the induced electromagnetic fields due to these structures are obtained from the analogue model measurements of the magnetic and electric field components for the E and H polarizations of a uniform horizontal inducing field. The effect of these irregularities along an otherwise straight sea-land interface is found to be confined to small local regions. The vertical magnetic field changes by as much as a factor of two along the bay- or the cape-shaped coastline. The enhancements in the horizontal magnetic field component which is parallel to the straight coastline, as well as the spatial variations in the two horizontal electric field components, indicate the deflection of the induced currents by the contours of the bay and cape.
The simple cape and bay models are modified by including an elliptical island in the shallow ocean near the continental coastlines. The perturbation of the induced electromagnetic fields over the island due to the bay or cape coastlines are then examined. A more complex model (called the bay-cape model) in which a bay is situated beside a cape is also treated. Results are compared with those of the simple bay-shaped and cape-shaped coastline models. The coast effect along traverses for the bay-cape model and for a simple straight coastline model are compared with Launay's (1970) model results. It is observed that the coast effect is largest for the case of a cape, intermediate for the straight coastline and least for the bay.
In the San Juan Bay model study, the results in both the E and H polarizations of the source field indicate the deflection of induced current for the two frequencies studied. The induction arrows at locations near the bay are found to point towards a direction parallel to the coastline of the bay for a very high frequency. Values of the vertical magnetic field component obtained from measurements at field stations are found to be in good agreement with the laboratory analogue model results.