Continuously recording radio-frequency electromagnetic (RF-EM) method (15–300 KHZ) without ground contact: A powerful tool for groundwater vulnerability mapping in fissured rocks

Introduction Density and size of open fractures, which connect the water reservoir with the surface, are main features for characterizing the vulnerability of fractured aquifers. The Hydrogeology Centre, University of Neuchâtel has developed a very fast and precise geophysical survey technique to locate and extensively map these types of high-permeability fracture zones: a Radio Frequency Electromagnetic (RF-EM) system. Theoretical background The RF-EM method measures the earth’s electromagnetic response (secondary field) induced by radio signals emitted from terrestrial transmitters (horizontal, primary field). These transmitters are used for navigation and submarine communication in the frequency range of 15-300 kHz. The induced secondary field has components that are oscillating both ‘in phase’ and ‘out of phase’ with the primary field. The intensity of the vertical component of the secondary field depends on the size of the conductive body. Therefore electrically conductive features such as geologic faults and fractures are detectable. In particular using the higher frequencies (up to 300 kHz) enables fracture detection in the epikarstic zone. Our prototype instrumentation continuously measures both fields with two orthogonal induction coils and