AN AIRBORNE TENSOR VLF SYSTEM - FROM CONCEPT TO REALIZATION

Radio signals from very low frequency (VLF) transmitters distributed w orld-wide have been used for several decades to study the lateral vari ations of the electrical conductivity in the upper few hundred metres of the earth's crust. Traditionally, in airborne applications, the tot al magnetic fields from one or two transmitters are measured to form t he basis for construction of maps that primarily show those conductive structures that are parallel or subparallel to the direction to the t ransmitters. The tenser VLF technique described in this paper makes us e of all signals available in a predefined frequency band to construct transfer functions relating the vertical magnetic field and the two h orizontal magnetic field components. These transfer functions are uniq uely determined for a particular measuring site and contain informatio n about the lateral conductivity variations in all directions. First e xperiences with real field data, acquired during a test survey in Swed en, show that maps of the so-called peaker, the spatial divergence of the transfer functions, give an image of the conducting structures. Mo st of the structures can be correlated to small valleys filled with co nducting sediments or valleys underlain by conductive fracture zones i n the crystalline rocks.