Soil-gas helium and surface-waves detection of fault zones in granitic bedrock

Fracture and fault networks are conduits that facilitate groundwater movement in hard-rock terrains. Soil-gas helium emanometry has been utilized in Wailapally watershed, near Hyderabad in southern India, for the detection of fracture and fault zones in a granite basement terrain having a thin regolith. Based on satellite imagery and geologic mapping, three sites were selected for detailed investigation. High spatial resolution soil-gas samples were collected at every one meter at a depth of >1.5m along 100m long profiles (3 in number). In addition, deep shear-wave images were also obtained using the multichannel analysis of surface waves. The study clearly indicates several soil-gas helium anomalies (above 200 ppb) along the profiles, where the shear-wave velocity images also show many near-surface vertical low velocity zones. We thus interpret that the soil-gas helium anomalous zones and the vertical low-velocity zones are probable traces of fault/fracture zones that could be efficient natural recharge zones and potential groundwater conduits. The result obtained from this study demonstrates the efficacy of an integrated approach of soil-gas helium and the seismic methods for mapping groundwater resource zones in granite/gneiss provinces.