Analogue model measurements of electromagnetic variations in the near field of an oscillating line current

Abstract

An analogue model for studying electromagnetic variations for an overhead oscillating line current is described. Measurements of electric and magnetic fields for model earth–sea interfaces are discussed and compared with results reported earlier for an overhead sheet current. A truncated cone representing a circular sea, a graphite wedge representing an ocean coastline, and a graphite wedge underlain by a graphite block representing an upwelling conducting zone in the earth's mantle near a sea coast are considered. In some cases the results for the line current model differ appreciably from those for the sheet current model, indicating that the nature of the source field can be an important factor in determining the behavior of the field components. The model measurements indicate that land–sea interfaces produce no appreciable anomalous magnetic fields at frequencies below 0.01 cps and hence the presence of the sea alone cannot account for coastal magnetic field anomalies. The results obtained for the model coastline structures tend to support the idea put forth by various workers that experimentally observed coastal magnetic field anomalies can in some cases be explained by assuming an upwelling conducting zone within the earth's mantle.