Airborne geophysics to map water from Earth's hardrocks
doi:10.1038/nindia.2019.138 Published online 15 October 2019
Researchers report a new airborne approach to find groundwater in hitherto untapped hardrock terrains that cover about 20% of the Earth’s surface1. The approach, they say, could be useful in mapping sustainable water sources in hardrocks worldwide.
Groundwater in hardrocks is usually locked in narrow, isolated fractures, joints, faults, and other discontinuities. The challenge has been to locate and delineate these sporadically distributed fracture networks and recharge zones.
Researchers from CSIR-National Geophysical Research Institute (CSIR-NGRI) have used airborne electromagnetic (AEM) surveys in combination with borehole data and geological information. This could be a potential tool to map the groundwater-saturated fracture networks. Their survey in India's southern state of Karnataka resulted in ‘resistivity’ maps derived from the AEM data. These maps show some deep groundwater-bearing zones ‘hydro-geologically connected through fracture networks’.
The survey revealed a threshold groundwater horizon (TGWH), beyond which a strong correlation exists between the depth of a well and its yield. The groundwater potential of a given watershed can be assessed by determining the availability of fracture zones below the TGWH, the report says.
The study also helps in identifying zones suitable for the construction of artificial recharge structures. The results of the study must be confirmed by expanding the analysis to hardrocks of the Precambrian shields that occupy large parts of tropical countries, the researchers say.
1. Chandra, S. et al. Large scale mapping of fractures and groundwater pathways in crystalline hard rock By AEM. Sci. Rep. 9, 398 (2019) Doi: 10.1038/s41598-018-36153-1