Delineation of groundwater-bearing fracture zones in a hard rock area integrating very low frequency electromagnetic and resistivity data

Integrated electrical and electromagnetic surveys were carried out in hard rock areas of Purulia district (West Bengal), India, for delineation of groundwater-bearing zones that would be suitable for construction of deep tube-wells for large amounts of water. Groundwater movement that occurs through fractures in hard rocks is suitable to be delineated by very low frequency (VLF) electromagnetic surveys. A detailed survey of the area was done using a VLF-WADI instrument and appropriate locations were selected for further study using Schlumberger resistivity sounding. Hence, the entire area was surveyed in a relatively short time by the combined use of resistivity and electromagnetic surveys. Areas showing VLF anomalies may or may not be appropriate for drilling tube-wells. In the northern part of the area, fracture zones are shallow, as exhibited by the small magnitude of VLF anomalies and by shallow conducting structures interpreted from the resistivity data. A VLF survey and subsequent resistivity sounding at suitable locations suggest the existence of deep groundwater sources in the southern part of the area. VLF anomalies have shown larger magnitudes in the southern part of the area than those in the northern part of the area. Self-potential and resistivity profiling data also showed correlation with results obtained using VLF and resistivity sounding. A typical variation in self-potential (SP) anomaly, i.e. positive SP anomaly for low resistivity, was observed near the locations found suitable and could be interpreted as the result of potential developed due to streaming of fluid within the fractured rocks.