Tensor VLF measurements: our first experiences

Abstract

VLF and magnetic maps over crystalline areas constitute major tools for delineating lateral changes in electrical conductivity and magnetisation, respectively, in the upper few hundred meters of the Earth's crust. Aeromagnetic data in particular have been utilised on a variety of scales for basement mapping, crustal geology mapping, tectonic mapping, mineral resource exploration, geothermal exploration and many other applications. VLF data have been used in particular to delineate zones of low resistivity in the upper few hundred meters of the crystalline crust. Such zones are often related to fluid filled fracture zones or to sedimentary infill in valleys, which in turn may be underlain by fractured zones. Mineralised zones and black schists are also excellent targets because of their low resistivity, but in general they are less frequent than fluid filled fractured zones and valleys. Tensor VLF measurements using several VLF transmitters simultaneously defines at each measuring site a unique tipper vector. From that a number of rotational invariants are derived, one of which, the so-called peaker, defined as the horizontal divergence of the tipper vector, is particularly useful as an interpretational aid. A simple model is developed to understand the interrelationship between VLF and magnetic anomaly maps. If common sources exist the model predicts that the peaker and the magnetic field reduced to the pole are linearly related. The first results from the flat Stockholm area, obtained with the airborne system of the Geological Survey of Sweden, indicates that most anomalies follow valleys with sedimentary infill. Coincident magnetic highs are interpreted as due to underlying greenstones, that are highly magnetic.