Groundwater exploration in fissured media with electrical and VLF methods

Abstract

Direct current electrical methods are among the most popular tools for groundwater exploration, in both porous and fissured media. In the case of fissured media, the areas most favourable to the presence of water are selected according to the minima of apparent resistivity values measured at the surface, the alteration and fissuration making the resistivity of rocksto decrease. VLF methods, which are less used than the previous ones because their depth of investigation is more limited, permit location of conductive zones through an electromagnetic induction process. In order to improve the groundwater field surveys as much as possible, BRGM has been developing easy-to-use instruments with automatic measurement process, automatic computation of geophysical parameters (apparent resistivity) and digital storage of data. Hence, SYSCAL-R1 and R2 resistivity meters and SYSCAL-VLF and T-VLF electromagnetic instruments have been designed. The newly developed T-VLF receiver has the advantage of not requiring any specific orientation of the operator with respect to the direction of the antenna. Several cases histories of groundwater exploration with electrical and VLF methods in fissured media are presented in various geological backgrounds: • - groundwater exploration in Botswana, in fissured dolomites, with resistivity maps (electrical profiling); • - village groundwater project in Burkina Faso, in crystalline basement, with resistivity profiling and VLF (tilt angle mode); • - village groundwater project in Togo, in crystalline basement, with resistivity mapping (gradient array); • - finally, methodological study in France, in a granitic basement, with the comparison of anomalies obtained by electrical profilings (one station and two station arrays) and by VLF profilings (in the tilt angle and in the resistivity modes). Although the rate of success cannot be guaranteed (which yield can be obtained for a given conductive anomaly?), the DC and VLF classical methods are efficient tools for borehole siting in groundwater exploration: the new measuring instruments should make them still more cost effective.