On the spatial pattern of magnetic fluctuations in the Cobar area, NSW

Abstract

Five recording magnetometers have been established in the Cobar area of NSW to examine the spatial uniformity (or otherwise) of natural fluctuations with time of the geo- magnetic field. The Cobar area is a particularly suitable test area for such a study because of its history in the development and use of magnetic survey methods in the search for base metal ore deposits, and because magnetic fluctuations are now a limiting factor in the production of accurate aeromagnetic survey maps. The magnetic fluctuations in the Cobar area are found to be not uniform, but appear affected by a current concentration in the vicinity of the east side of the Cobar basin. Thus changes as great as 25% in magnetic fluctuation amplitude are observed over distances of order 30 km (and may occur over even shorter distances). Some differences are also observed in the amplitude of the east-west component of the magnetic daily variation. The phenomena are interpreted to indicate a geologic unit of good electrical conductivity (possibly the Cobar slate member of the Cobar Trough group) which runs along the eastern edge of the basin, and carries a concentration of the natural electric current flow induced in the earth.