In a previous paper (Nordic Hydrology, Vol. 12, 1981), the authors have discussed the inadequacies in the technique of resistivity method for location of sites for constructing deep water walls in hard rock areas. It was pointed out that the water bearing fracture zones in the bed rock could not be identified by merely considering geoelectrical parameters such as layer resistivity. An empirical study based on the correlation of minor irregularities or deviations – “BREAKS” – in the normally smooth sounding curves with the actual driller's logs reveals that under normal geo-electric conditions these water bearing zones (hard rock aquifers) are indicated in the curve by a perceptible lowering of apparent resistivity and hence could be used as a guide for locating well sites. As such breaks may also be caused by other conditions such as lateral inhomogeneities, certain methods for distinguishing them are discussed.

Large scale exploratory work for groundwater in hard rock region of South India has been carried out by the authors during the last ten years, using electrical resistivity method. The standard methods of data interpretation were found to be inadequate in locating water well sites and determining the existence and depth of occurrence of water bearing fractures and joints. Wenner and Schlumberger type vertical electrical sounding (VES) curves were interpreted by matching against theoretical model curves and also by inverse slope method. The interpreted results were correlated with the actual driller's logs. No satisfactory correlation could be observed either between the geoelectrical interpreted layer boundaries and the layer boundaries from the wells or between the resistivity in the hard rock and the water yield. A more pragmatic approach to the problem is desirable to minimize the failures of wells in hard rock terrain.