ABSTRACT
The Very Low Frequency (VLF) EMmethod measures the response of the earth to EM waves transmitted from a distant source. The receiver located at a large distance from transmitter is in the control of the operator. It can be considered as a passive method (Paterson and Ronka, 1971). VLF surveys can be easily conducted in areas where the surface layer is highly resistive and an electrical resistivity survey is difficult due to poor conduction of electrolytic current. It was developed in the 1960s using established military radio communication stations (Paal, 1965; Djeddi et al., 1998). A brief history of its early development is given by Paterson and Ronka (1971). The method was developed for time-efficient, cost-effective shallow subsurface mapping of conductive targets, primarily the ore bodies (Paal, 1968; Paterson and Ronka, 1971; Bayrak, 2002; Shivaji and Gnaneshwar, 1999; Okonkwo and Ezeh, 2012). It can be used for near surface resistivity mapping (Tabbagh et al., 1991), environmental studies (Jeng et al., 2004), groundwater prospecting at shallow levels in hard rock (Palacky et al., 1981; Bernard and Valla, 1991; Chandra et al., 1994; Gomes and Chambel, 2000; Castelo Branco et al., 2004; Sharma and Baranwal, 2005), detecting coal mininginduced fractures (Hutchinson and Barta, 2002), groundwater contamination studies (Stewart and Bretnall, 1986; Benson et al., 1997) and landfill investigation studies (Roberts et al., 1989). McNew and Arav (1995) use VLF measurements to delineate fresh water-salt water interfaces and fresh water aquifer lenses. It has been used to identify steeply dipping structures such as faults and fractured zones (Phillips and Richards, 1965; Gurer et al., 2009). As such, for groundwater exploration in hard rock, the method is mostly used for fast, economic, reconnaissance survey along with magnetic to identify anomalous zones. It is followed by detailed surveys through other geophysical methods to delineate and confirm the location of saturated fractured zones.
