Continuous gradient VLF measurements: a new possibility for high resolution mapping of karst structures

The Centre of Hydrogeology Neuch‚tel (CHYN) investigates and monitors karst aquifers, which are highly vulnerable to contamination. One of our particular research tasks within this scope is the characterization and localization of the 'epikarstic zone' (0-20 m depth) (Mangin 1975; Kiraly 1995). Density, size and aperture of fractures that connect the water reservoir with the surface, are features which characterize the vulnerability of karst aquifers. Very local scale mapping of these types of high-permeability fracture zone over extensive areas is therefore of interest for karst hydrogeology. In order to investigate this zone the very low frequency (VLF, 15-30 kHz) method has been adapted at CHYN for investigation at shallow depths with high lateral resolution. Geophysical surveying using VLF radio signals began in the 1960s as a tool for prospecting conductive minerals (McNeill & Labson 1991; Paal 1965). The CHYN equipment uses VLF and low frequency (LF, 30-300 kHz) radio transmitters, which are located at several positions around the world and are used principally for navigation and submarine communication. Consequently the CHYN has developed a radio frequency-resistivity device (RF-R), also called radio magnetotelluric (RMT, 12-240 kHz), and a continuously measuring radio frequency-electromagnetic (RF-EM, 15-300 kHz) device (Turberg et al. 1994; Turberg & Barker 1996; Stiefelhagen 1998). In fissured limestone with a resistivity of several hundreds of Wm, the penetration depth for these frequencies is about 10-100 m. However, when investigating small structures at very shallow depths (0-10 m) in the epikarstic zone, these devices have limited use due to their excessive penetration depths and their poor horizontal resolution. In this paper we introduce a method that is based on measuring the vertical gradient of the horizontal magnetic field component of VLF radio signals (VLF-EM-vGRAD(Hy)). According to our knowledge, such a technique has not been previously published.