A laboratory analogue model study of magnetic variations induced by ocean waves
Date
1978
Authors
Miles, Timothy John
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Abstract
This thesis deals with a laboratory analogue model study of the horizontal component of the magnetic field induced by ocean waves. Following some of the techniques proposed by Ng and Dosso (1970), an analogue model, employing mercury to simulate the ocean, was constructed. The validity of this model was tested by comparing the observed model magnetic field with the magnetic field calculated using Podney's (1975) expression for a finite uniform depth ocean. Excellent agreement was obtained.
The model was used to study cases of non-uniform ocean depths and sea-land interfaces. Measurements of the induced magnetic field were carried out for traverses over a (i) uniform depth model (to provide a reference), followed by various ocean floor and coastline structures, that is, the (ii) step and shelf model, the (iii) wedge and shelf model, the (iv) dyke model, the (v) sea mount model, the (vi) sloping bay and shelf model, and the (vii) reef and shelf model. For shallow ocean depths, over a submerged obstacle (or structure) of large horizontal extent, the attenuation of the induced horizontal magnetic field over the leading edge of the structure is strongly dependent on the depth. The shape of the leading edge of the structure can also have an important effect (especially for the very shallow fluid depths), this being brought out for the models having vertical and sloping sea-land interfaces.
The contour of the coastline affects the induced field through changes in the fluid waves or changes in the electric currents induced by the waves. The observed fields for the bay model are attributed to the focussing of the fluid waves by the geometry of the bay, while the observed fields for the reef model are in agreement with channelling of induced currents around the reef, since the reef was parallel to the direction of wave propagation. The horizontal extent of the structure is important, since fluid wave interference effects would be expected. For the case of the dyke model, and also for the case of the sea mount model, the horizontal extent is approximately half a wavelength in the direction of wave propagation, and constructive interference occurs resulting in large enhancements of the induced field over the trailing edges of the horizontal section for both the dyke and sea mount models. In general, it is apparent that an irregular ocean bottom, or a large submerged structure, affects the induced fields at the surface only for rather shallow ocean depths (less than 40 m for a wavelength of approximately 360 m). Thus it appears that it may be feasible to use aerial measurements of ocean wave induced magnetic fields to detect shallow structures such as reefs and sea mounts.