Electrical geophysical techniques pin-pointing the bedrock fractures for groundwater exploration in granitic hard rocks of Southern India

Abstract

The growing dependency on groundwater resources in semi-arid hard rock terrains has steadily emerged into desaturation of shallow aquifers limiting them to the sporadically distributed bedrock fractures. Additionally, the heterogeneity in weathering and fracturing distribution elevates the ambiguity for geophysical mapping. This study demonstrates at three distinct lithological sites using integrated gradient resistivity profiling (GRP), vertical electrical sounding (VES), and electrical resistivity tomography (ERT) followed by exploratory borehole drilling and electrical logging to pin point the bedrock fractures in granitic hard rocks of Southern India. We observe, the VES and ERT efficacious in characterizing the major hydro-stratigraphy that consists top soil, followed by underneath unsaturated weathered zone, and bedrock with almost no noticeable signatures of the saturated bedrock fractures. But, the anomalies detected in GRP profiles signify the occurrence of deep bedrock fractures which corroborates with the first derivatives of resistivity contours derived from the ERT data. The qualitative delineation of fractures with low resistivity peaks in GRP complements the borehole lithologs where the bedrock fractures encountered at 70–200 m depth with 1.49–3.84 litres per second (lps) of groundwater yield. The study further points forward that greater the resistivity contrast in GRP anomaly, higher is the possibility of saturated fractures in bedrock. It concludes, the GRP as precursor and key approach for pin pointing the deep fractures which can be further characterized using VES and ERT to appraise the groundwater potentiality, though, at local scale in granitic hard rock terrain.