Radio signals from very low frequency (VLF) transmitters distributed world-wide have been used for several decades to study the lateral variations of the electrical conductivity in the upper few hundred metres of the earth's crust. Traditionally, in airborne applications, the total magnetic fields from one or two transmitters are measured to form the basis for construction of maps that primarily show those conductive structures that are parallel or subparallel to the direction to the transmitters. The tensor VLF technique described in this paper makes use of all signals available in a predefined frequency band to construct transfer functions relating the vertical magnetic field and the two horizontal magnetic field components. These transfer functions are uniquely determined for a particular measuring site and contain information about the lateral conductivity variations in all directions. First experiences with real field data, acquired during a test survey in Sweden, show that maps of the so-called peaker, the spatial divergence of the transfer functions, give an image of the conducting structures. Most of the structures can be correlated to small valleys filled with conducting sediments or valleys underlain by conductive fracture zones in the crystalline rocks. lntroduction It has been known for several decades that the electrical properties of the subsurface affect the behaviour of radio waves. Indeed, measurements of the conductivity and dielectric constant of the earth using 'wave-tilt' techniques were first performed in the 1930s (Feldman 1933;Smith-Rose 1933;Barfield 1934). These early measurements were, however, made at relatively high frequencies and hence had shallow depths of penetration. It was not until 1965 that Paal (1965Paal ( ' 1968 observed that radio waves at very low frequencies (15-30 kHz) could be used to prospect for electrically conductive orebodies. By surveying over known shallow I paper presented at the 54th EAEG meeting,
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