Shallow subsurface geoelectric structure of the central Ganga Basin by magnetotellurics: A proxy for basin‐scale changes in the sedimentary environment

Abstract A thick succession of alluvial sediments and the rocks of the Shiwalik Group in the Ganga Basin hold clues to the tectonic and climatic factors that controlled the interaction between the Himalayan orogen and the foreland basin. These alluvial tracts are also a repository of groundwater resources. Electromagnetic methods are useful to map these electrically conductive sedimentary successions. However, the depth of investigation of artificial source electromagnetic methods is limited to a few hundred metres due to logistic constraints. In the present study, we demonstrate the efficacy of the magnetotelluric (MT) method in delineating broad sedimentary stratigraphy and deep aquifers of the Ganga Basin. We have inverted high frequency (10 kHz–1 Hz) MT data of four profiles and delineated the subsurface structure to about 2 km depth. A significant result is delineation of a prominent high conductivity (<10 Ω m) clay/silt dominated 80–100 m thick layer in the southern part of the interfluve region between the Yamuna and the Ganga Rivers and its thickening to about 300 m in the northern part. This layer gets buried beneath a gradually thickening surface aquifer layer north of the Ganga River. The presence of moderate resistivity zone beneath this layer could be linked to deep sandy aquifers. Another significant result is from the northern part of the basin where upward coarsening of the Shiwalik Group is obtained in terms of high conductivity clay layer at >1300 m depth, changing to moderate resistivity clay‐sand lithology between 250 m and 1300 m depth, and relatively large resistivity sand and gravel/pebble lithology at <250 m depth. These variations in the subsurface resistivity can be used as a proxy to estimate the lithological changes, which will be useful for both mapping of deep groundwater resources and understanding climate‐tectonic interaction since the Neogene period.