Investigations During 1962 of the Alert Anomaly in Geomagnetic Variations

Abstract

Magnetic variations measured a t three stations across the strike of the anomaly in electrical conductivitv of the earth's mantle suerested bv Whitham and Andersen (1962) near Alert, Elles~nere Island, havelbeen analyzed by simple potential theory. T h e anomalous internal contributions to the magnetic variation 1-ector show striking confinement consistent with an underground current in a northeast-southwest direction a t a depth between 50 and 70 km, a few kilometers southeast of Alert. If the anomalous c o n d~~c t o r is approxin~ated by a uniform infinite cylinder, the horizontal field response as a function of frequency can then be explained with a conductivity of ~3 X 1 0 -'~ e.m.u. and a cylindrical radius of about 50 km. T h e 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 T~l a n r lPreliminary earth potential measurements show that, a s expected, the electric field variations are abnormally low a t Alert.The gravitational consequences of the cylindrical model are discussed and compared with two profiles from the same region. A Bougucr anomaly is found in the predicted region if certain regional gradients are assumed, but its magnitude is snlaller than that predicted from the cylindrical model in hydrostatic equilibriunl. I t is clear that approximate agreement can be obtained with crustal thinning of some 20 km.The significance of the thermal ano~naly thought t o be responsible for the electrical conductivity anomaly is discussed briefly.

Geomagnetic Variation Anomalies in Canada

“…On northern Ellesmere Island, the magnitude, directional characteristics and frequency dependence of the anomaly have been reported to be in general agreement with the hypothesis of a large anomalous conducting body striking approximately northeast-southwest with its centre a short distance to the southeast of Alert (Whitham and Andersen, 1962;Law et al, 1963). Potential analyses, spectral analyses, and gravity and aromagnetic evidence support this hypothesis, the latter two being consistent with the suggestion of a thermal rather than compositional origin.…”

“…Basham (1964) in an attempt to relate certain unusual features noted on these, with the suggestion made earlier by Whitham (1963) for the vertical field suppression effect. In parti-cular, P travel-time residuals, unified magnitudes from Mould Bay seismograms and spectral analyses of the initial P-wave trains have been studied.…”

“…An exact solution, using the infinite conductivity approximation, has been obtained for the problem of the electromagnetic coupling between an infinite circular cylinder and an underlying conducting layer. A number of two dimensional models have then been tested against the admittedly sparse experimental data obtained on one profile perpendicular to the strike of the proposed Alert conductor, and it was found that certain difficulties in the earlier interpretation of Law et al (1963) could be reduced.…”

“…The spatial extent of the striking suppression of vertical field magnetic variations reported at Mould Bay observatory by Whitham (1963Whitham ( , 1964 The results from these seven stations have been visually examined on an event basis…”

A Geographical Relation Between Geomagnetic Variation Anomalies and Tectonics

“…Two areas in which hture geomagnetic studies may be expected to provide a test of this implication, are the eastern United States and Cariada where the North American craton is bounded by the Appalachian Mountain belt, and, as suggested by Law et al (1963), across the East African Rift System.…”