Shallow sub-surface stratigraphy of interfluves inferred from vertical electric soundings in western Ganga plains, India

Abstract

The application of geophysical surveys such as Vertical Electric Sounding (VES) for inferring shallow sub-surface stratigraphy in the vast alluvial tract of India has been under-utilized. This paper is aimed at demonstrating the potential of resistivity methods for mapping alluvial stratigraphy in parts of the Ganga plains where the availability of exposed sections is scarce and bore logs are very limited. Data was generated from 40 vertical electrical resistivity soundings in a stretch of ∼150 km in the interfluve areas between the Ganga and Yamuna rivers and Yamuna and Betwa rivers in the western Ganga plains. Based on the available lithological data from cliff sections along the major and minor rivers draining the study area and a series of 7 drill cores along the transect, the resistivity sounding data was calibrated to interpret the sub-surface stratigraphy. The data shows the vertical extension of the valley fills and interfluve sequences down to ∼100 m. A high-resistivity layer runs quite extensively across the interfluves and below the valley fills, interpreted as a calcrete layer that may mark a major discontinuity. Patches of very low resistivity layers in valley fills as well as interfluves suggest the presence of saline aquifers. This study demonstrates significant sub-surface heterogeneity across the interfluves, which is a manifestation of variable fluvial activity over a period in excess of 100 ka. Lateral continuity of the sandy aquifers is often broken by muddy deposits of the floodplains which form a dominant component of the interfluve sequence. Except for the locations close to major valleys, there are no major sand bodies in the upper ∼100 m of the interfluves which suggests that the modern configuration of the valleys and interfluves in this region is of Late Quaternary antiquity. A few buried sand bodies, however, are comparable to the dimensions of the minor rivers draining the interfluve surfaces.