An analogue model study of electromagnetic induction for cape and bay coastlines

Abstract

The electric and magnetic field variations across bay and cape coastlines are determined from scaled laboratory analogue models. The effect on the field response of such features along an otherwise straight coastline is limited to a distance approximately equal to the dimensions (diameter of the bay or cape) of the irregularity. For the E-polarization of the source field the perturbations in H z and H y are more confined, mainly to the immediate region of the bay or cape. The coast effect H z / H y for this polarization, and variations with a 4.2 min. period, is approximately a factor of two larger for the cape than for the bay, while the straight coastline H z / H y is roughly mid-way between the cape and bay values. For H-polarization the coast effect is again approximately a factor of two larger for the cape than for the bay, but both values are larger than the value for a straight coastline. The response of the various field components to the bay and cape features is decreased at longer periods, and as a measure, the coast effect is approximately a factor of 1.2 greater for the cape than for the bay for a 42 min. period as compared with the factor of 2 for a 4.2 min. period. The bay and cape deflect the induced currents flowing in the ocean and modify the currents diffusing into the land. This study demonstrates that data from coastal sites may be unreliable for correcting magnetic surveys or for analyses of short period geomagnetic source-field variations.