A Magnetometer Array Study in Southern Africa

Abstract

Summary An array of 24 three-component magnetometers were operated in central South Africa during September and October, 1971. Results from three substorm sequences are presented. Magnetograms and maps of Fourier transform amplitudes and phases show a large anomaly in the vertical and northward horizontal variation fields over the period range 24–293 min. The vertical field has a maximum near Beaufort West and the horizontal field increases southward to a presumed maximum just south of the array. The Karroo sedimentary basin can contribute only a minor part of the anomaly and the principal result is the discovery of a major conductive structure under the Cape Folded Belt and southernmost Karroo. Transfer functions between mean horizontal field components and the vertical component show that induction is principally in phase with the normal field. The effect of the Karroo sediments is seen in the quadrature phase induction arrows and is small compared with the in-phase induction anomaly. For this and other reasons the conductive body under the Cape Folded Belt and southernmost Karroo is provisionally placed in the upper mantle. Quantitative interpretation will follow further observations south of the 1971 array.