INTERPRETATION OF THE ALERT ANOMALY IN GEOMAGNETIC VARIATIONS

Abstract

The theoretical basis for the interpretation of the Alert anomaly in geomagnetic variations has been reexamined. Under the uniform horizontal inducing field approximation, the electromagnetic response of an infinite elliptical cylinder of infinite conductivity has been derived, and from this the response for the upheaval of a half-elliptical conductor from a highly conducting mantle. The results have been extended to an asymmetric upheaval of the conducting mantle. A theory has also been developed of the electromagnetic response of an elliptic cylinder which is useful for application to the finite conductivity case.The electromagnetic coupling between an infinite circular cylinder and an underlying conducting layer has been investigated as far as the second-order approximation. An exact solution has then been obtained for the infinite conductivity approximation, and useful examples calculated.The situation at Alert has been reexamined following this work which reduces one of the difficulties of the earlier interpretation. An upheaval of about 100 km of the 1400–1500 °C isotherm to within 25–30 km of the surface appears capable of accounting for the gross characteristics of the response but the solution is certainly not unique and may be in error by as much as 60%. The asymmetry in the response is difficult to explain quantitatively. The response parameters thought to be most useful have been clearly defined and the ambiguity in the electromagnetic response solutions illustrated. Suggestions for further progress are made. Brief mention is made of preliminary magnetotelluric and gravitational studies at Alert.