Efforts to make more precise and reliable measurements of the electromagnetic response of the Earth at periods less than a few days are baffled by the contamination of the magnetic variation data by currents which flow in the heterogeneous outer shell of the Earth. In this paper, quantitative estimates of the potential contamination are made by analysing induction in an Earth model which, in addition to the usual steep rise in conductivity at a depth of 500km, also involves a superficial conducting layer of integrated conductance 4, 000S. It is concluded that (a) it may be possible, by careful selection of suitable stretches of data from observatories far from conductivity anomalies, to extend global electromagnetic response estimates to periods as short as 8 hours, but that (b) present response estimates in the period range 1-10 days may be contaminated by the currents in the outer shell.
These hypotheses are tested by an investigation of the spatial and temporal variations in geomagnetic transfer functions observed at stations in Europe during a three-month period. Two events recorded during that time show distinctly different responses in the period range 1-3 days. The first yields temporally and spatially consistent values of the transfer function linking vertical and horizontal north components. The excitation is most probably of ring current origin, and the bulk of the induced current flows in the deep, spherically symmetric part of the Earth. The transfer functions generated by the second event are also consistent with time, but differ substantially from one obervatory to another. The source of this event is a current system which is spatially non-uniform. The transfer function is partly determined by the configuration of the localised external current system, and partly by the geometry of induced currents in the surface sheet. Both factors give rise to persistent relationships between the magnetic field components, which contaminate the global response estimates.

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In order to ease the interpretation of geomagnetic data for resource charting and tectonic studies of the Queen Charlotte Islands region, the behaviour of the electromagnetic variations over the region is studied using a laboratory analogue model. The results indicate that conductive channelling is important in Hecate Strait for both E and H source field polarizations. 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 simulated variations of 4, 40, and 120min show a wide range of field values over the Queen Charlotte Islands, for example, H_z varies by a factor of 4 or more between coastal and inland locations for E-Polarization at short periods and by as much as a factor of 5 between certain coastal locations at long periods. The model results show that in decreasing the depth of the conducting layer in the mantle from 200 to 100km, the H_z, H_x, E_x, and E_y anomalies are attenuated by 20%, and the H_y anomalies by roughly 8% over the deep ocean and by 12% over Hecate Strait for 4min period variations.

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