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Anomaly in geomagnetic variations on the west coast of British Columbia Lambert, Anthony

Abstract

Four portable magnetometer stations were set up at intervals of 80 - 100 kilometers along an east-west profile running from Tofino on the west coast of Vancouver Island to Abbotsford on the mainland in order to study the spatial dependence of the coastal anomaly. These were supplemented by records from the permanent Victoria Magnetic Observatory. The Tofino-Abbotsford chain extends and partly overlaps an earlier chain of stations set up to search for geomagnetic anomalies, along an east-west profile from Lethbridge, Alberta to Vancouver, British Columbia, The coastal anomaly recorded at Tofino is observed exclusively in the vertical component, diminishing rapidly inland and reaching its maximum value when the inducing field changes in approximately an east-west direction with a frequency between one and two cycles per hour. The horizontal and vertical variations are in a ratio of two to one at the coast which is in agreement with induction ratios calculated at coastlines in Australia and California. The directional dependence and limited spatial extent of the anomaly indicate a rather shallow conductivity discontinuity, at most 100 kilometers deep, running approximately parallel to the continental shelfline. Since at the maximum response frequency the upper mantle beneath the ocean is largely shielded by the overlying wedge of sea water, the anomaly is thought to be mostly due to the conductivity contrast between the deep ocean and the continent. The diurnal geomagnetic variations which pass through the surface layers virtually unattenuated show at least a twenty five percent enhancement in the vertical component from Abbotsford to Tofino. This anomaly perhaps reflects a change in upper mantle conductivity more accurately than does the higher frequency Tofino anomaly. At a still higher frequency of three cycles per hour where the Tofino anomaly is already reduced, there is a small anomaly in the vertical component at Westham Island on the east side of Georgia Strait which is completely absent at lower frequencies. The influence of a shallow body of sea water such as Georgia Strait is expected to be small. Hence this anomaly is probably due to a conductivity structure beneath the Strait in the crust or upper mantle.

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