ABSTRACT
The magnetic method is the oldest geophysical method. It is a passive method. It was developed for mineral exploration and still continues as a primary geophysical exploration method. The historical development and various applications are described by Nabighian et al. (2005) and Hinze and Frese (1990). Near-surface applications are described by Hansen et al. (2005). The method is applied to measure the lateral variations in magnetic field which are magnetic expressions of the subsurface geological conditions. For applying this method it is essential that the subsurface lithology and structural fabric is associated with variations in magnetic property – the magnetic susceptibility. In its diverse applications to groundwater exploration in igneous and metamorphic rocks which, in general, have higher magnetic susceptibilities as compared to sedimentaries, the subsurface geological structures and lithological contacts, such as basic dyke intrusives with high susceptibilities (Roberts and Smith, 1994) and prominent fault and fractured zones with low susceptibilities, can be usefully picked up (Henkel and Guzman, 1977; Cull and Massie, 2002; Chary et al., 2005 and Lopez-Loera et al., 2010). These structures may form the preferential flow path of groundwater, affect groundwater movement and also form groundwater repositories. Gobashy and Al-Garni (2008) use a high resolution survey to identify the location of a subsurface dam to constrain groundwater movement and raise groundwater level in wadis of Makkah Al-Mukarama. It is used to delineate an igneous body concealed under alluvial capping. The depression in unaltered, high susceptibility bed rock leading to thickening of the aquifer in the sedimentary overburden or the thick altered zone of weathering can be detected (Ram Babu et al., 1991). The measurements can also be made to estimate magnetic susceptibility, though the variations in magnetic susceptibility are seldom diagnostic of lithology or rock type because of overlapping ranges. However, the variations can be used as an indicator of lithologic changes (Bordie, 2002). As such, to acquire precise subsurface information magnetic surveys are to be essentially followed by electrical resistivity, electromagnetic or seismic surveys.
