Extracting Hydrogeology from Heliborne Dual Moment Transient Electromagnetic Investigations in Geologically Divergent Terrenes

Abstract:

Comprehensive knowledge of aquifer system is an important requisite for its effective management in India. Geological formations are complex and variable, punctual and scarce information are not adequate to understand, asses and manage them. Continuous data acquisition, their interpretation and integration with available geological/geophysical information is the solution.

Heliborne dual moment transient electromagnetic (HeliTEM) and magnetic (HeliMAG) measurements have been carried out in divergent geological terrenes in India comprising Gangetic alluvium, Tertiary sediments underlying the Thar desert, Deccan basalts and Gondwana sediments, weathered and fractured granite gneisses and schists and the coastal alluvium with Tertiary sediments. The survey was carried out using state of the art equipment SkyTEM.

The paper presents a synopsis of the results of the HeliTEM surveys that have helped in obtaining continuous information on the geoelectrical nature of sub-surface. HeliTEM data were supported by a number of ground geophysical surveys. The results provide the 3D subsurface structures controlling the groundwater conditions, the regional continuity of probable aquifers, the variations in lithological character and the quality of water in terms of salinity. Specialized features pertaining to hydrogeological characteristics obtained from this study are as follows:

• A clear delineation of clay beds and their spatial distribution providing the multi-layered aquifer setup in the Gangetic plains.
• Delineation of low resistivity zones in the quartzite below the over exploited aquifers indicating the possibility of new aquifers.
• Presence of freshwater zones underneath the saline water aquifers in the thick and dry sands in deserts.
• Clear demarcation of different lava flows, mapping the structural controls and highly porous zones in the contact of basalts and Gondwanas.
• A complete and continuous mapping of weathered zone in crystalline hard rock areas providing information on the recharge zones.
• The setting of multi-layered aquifer and different zones of salt water intrusion in the coastal sedimentary formations.

The study has helped in establishing an appropriate cost-effective strategy for 3D mapping of aquifers on a regional scale providing valuable inputs to perform aquifer modeling.