F020 RADIOMAGNETOTELLURIC MEASUREMENTS FOR DETECTION OF FAULTS AND FRACTURE ZONES IN SWEDEN PASCAL TURBERG' and LENA PERSSON2 ' Neuchste! University, Institute of Geology, Centre for Hydrogeology, 11 rue Emile-Argand , 2007 Neucharel, Swirzerlend 2 Luled University of Technolog y We have used the Radiomagnetotelluric (RMT) method which employs distant radio transmitters in the frequency range from 12 kHz up to 240 kHz for mapping faults and fracture zone in Northern Sweden . The horizontal electric and magnetic fields are measured and interpreted in the same way as with magnetotelluric data . The possibility to measure the electric field gives more detailed information about variations in resistivity in highly resistive rocks like crystalline rocks in Sweden compared with usual VLF-EM techniques were only the magnetic component is measured . RMT is allo a fast method compared with electrical sounding and mapping . The RMT-rnethod has earlier been used mostly for hydrogeological applications in areas with sedimentary rocks (Turberg et . al . 1994) . In the present investigation six RMT-profiles of 1000 m each were measured in Northern Sweden, in an area with Precambrian crystalline bedrock, mainly granites . The resistivity of the bedrock is usually more than 10 000 Ohmm and the skin depth is thus often more than 300 m at the lowest frequency employed . The area of investigation has earlier been mapped by low-altitude aeromagnetic measurements by the Swedish Geological Survey (SGU) . In a regional study made by Nisca (1995) low-magnetic lineaments on these aeromagnetic data have been interpreted as faults and fracture zones since, according to Henkel and Guzmán (1977) fracture zones cause linear magnetic minima due to alteration of magnetite to hematite . One of the objectives with this investigation was to make a comparison between magnetic anomalies and RMT anomalies over fracture zones . Fig . 1 shows the aeromagnetic map over a part of the measured area and the location of four profiles which all cut low-magnetic lineaments . Fig . 2 shows the measured resistivity and phase together with the measured magnetic field from the profile marked S . The magnetic minimum corresponds to the low-magnetic lineament . From the measured resistivity values at three frequencies it is seen that a low-resistive zone coincides with the magnetic minimum . All profiles were measured in two polarisations to detect preferential directions in faulting . This directional effect is specially wel] shown by the phase . 54001) n T 49000 n t Fig 1 . Location of four RMT-profiles . Aeromagnetic map (SGU) . EAGE 59th Conference and Technical Exhibition - Geneva, Switzerland, 26 - 30 May 1997 - Geophysical Division