Conductivity anomalies in the Baltic Shield in Finland

Abstract

In 1981–84 six arrays of 30 or 31 magnetometers were operated on the Baitic Shield in central and southern Finland to deduce information about the electrical conductivity within the Earth's crust. The magnetometer sites cover the area with 20–60 km spacing allowing the identification of large-scale electrical structures within the crust. Each array recorded the three orthogonal magnetic field components on analogue film for about two months simultaneously at all the stations. Altogether 17 magnetic variation events of 2–6hr length have been digitized and analysed. This paper includes results of the last array in south-western Finland, a comparison of two methods of induction vector estimation and combination of results from all the arrays. Four zones of anomalously high electrical conductivity have been observed. They are all interpreted as lying at depths greater than 5 km. The ‘Oulu Anomaly’ is wider than the others and rather short, about 100 km. The Southern Finland Anomaly' is probably a 500 km-long narrow zone. This and the ‘Ostrobothnian Anomaly’ surround three sides of the large Central Finland Granite area. The ‘Outokumpu Anomaly’ is probably deeper lying than the others. Blocks separated by these anomalies are presented. The blocks have different average resistivities.