An analogue model study of electromagnetic induction in the Queen Charlotte Islands region
Date
1981
Authors
Chan, Edward Su Wah
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Abstract
In this thesis, electromagnetic variations in the Queen Charlotte lslands region are studied using scaled analogue model measurements and field station measurements. Contour diagrams and three-dimensional diagrams of the in-phase and quadrature parts of the model magnetic and electric field components are used to show in detail the field response to the complex coastlines and ocean channels.
The results indicate that conductive channelling of induced currents is important in Hecate Strait, the shallow ocean between the Queen Charlotte Islands and the British Columbia mainland, for both source field polarizations studied. Current deflection is observed at Rose Point for E-Polarization and at the northern and southern tips of the Queen Charlotte Islands for H-Polarization.
Model results for the simulated 4, 40, and 120 min. period variations indicate that the behaviour of the fields over the Queen Charlotte Islands are highly frequency dependent. At the eastern island coastline, for E-Polarization, Hz shows fairly large anomalies for the 4 min. period, but Hz is almost zero at this region for 40 min. period. With increasing period, current channelled into Hecate Strait decreases sharply, and proportionately the field responses for the various components shift from the in-phase to the quadrature part. Hz, Hₓ, Eₓ, and Ey show response to capes and bays. These components also respond to channelled currents in the ocean inlets. Large Ey enhancements at the end of ocean inlets indicate diffusion of current into land.
The model results show that in decreasing the depth of the conducting layer in the mantle from 200 km to 100 km, the Hz, Hₓ, Eₓ, and Ey anomalies are attenuated by 20%, and the Hy anomalies by roughly 8% over the deep ocean and by 12% over Hecate Strait for 4 min. period variations. Beyond 200 km, the change in the depth of the conducting layer has very little effect on the various field components.
The behaviour of the model and field station Parkinson arrows are found to be in reasonably good agreement, showing that the magnetic variations in this area are mainly due to the coast effects. The difference between the analogue model results and the field station results will provide information on the tectonic structure of the region.
The results of the present work will help in the determination of reference station locations for the reduction of cross-over error in marine magnetic surveys.