INVESTIGATIONS DURING 1962 OF THE ALERT ANOMALY IN GEOMAGNETIC VARIATIONS

Abstract

Magnetic variations measured at three stations across the strike of the anomaly in electrical conductivity of the earth's mantle suggested by Whitham and Andersen (1962) near Alert, Ellesmere Island, have been analyzed by simple potential theory. The anomalous internal contributions to the magnetic variation vector show striking confinement consistent with an underground current in a northeast–southwest direction at a depth between 50 and 70 km, a few kilometers southeast of Alert. If the anomalous conductor is approximated by a uniform infinite cylinder, the horizontal field response as a function of frequency can then be explained with a conductivity of [Formula: see text] e.m.u. and a cylindrical radius of about 50 km. The vertical field response is, however, an unsatisfactory fit to this model. A possible asymmetry is noted based on the hourly range data: this could be explained by the dipping of the real conductor under Ellesmere Island.Preliminary earth potential measurements show that, as expected, the electric field variations are abnormally low at Alert.The gravitational consequences of the cylindrical model are discussed and compared with two profiles from the same region. A Bouguer anomaly is found in the predicted region if certain regional gradients are assumed, but its magnitude is smaller than that predicted from the cylindrical model in hydrostatic equilibrium. It is clear that approximate agreement can be obtained with crustal thinning of some 20 km.The significance of the thermal anomaly thought to be responsible for the electrical conductivity anomaly is discussed briefly.