Large Scale Mapping of Fractures and Groundwater Pathways in Crystalline Hardrock By AEM

Abstract

In hardrocks that cover about 20% of the Earth’s surface, it is difficult to locate steady sources for groundwater due to inadequate understanding of the fracture networks. A comprehensive knowledge of fracture distribution at the regional scale is necessary to delineate sustainable aquifers and manage them efficiently. The resistivity maps derived from the airborne electromagnetic (AEM) survey over the Ankasandra watershed in Karnataka, India, reveal sharp and deep zones of low formation resistivity, which indicate groundwater-bearing zones. It is found that some of these zones are hydrogeologically connected through fracture networks resulting in augmented yield. AEM results in combination with an in-depth understanding of the geological structures successfully map these groundwater-saturated fracture networks (or hydrogeological lineaments) that we term as ‘Hydrolins’. As groundwater occurrence is generally associated with lineaments, we analyzed the drilling and geophysical logs from 21 wells within a 380 sq.km area to study the relationships of various lineaments with ‘Hydrolins’, particularly in respect of their groundwater potential. AEM results, though calibrated and correlated with a limited number of well data, revealed a threshold groundwater horizon (TGWH), found to be at 80 m depth for Ankasandra watershed, beyond which a strong correlation exists between the depth of a well and its yield. While the TGWH may differ for different watersheds, the approach presented here can be readily adopted to map sustainable groundwater sources in hardrocks worldwide.

Hydrogeophysical Characterization in a Volcanic Context From Local to Regional Scales Combining Airborne Electromagnetism and Magnetism

“…However, since it is essential to integrate local heterogeneities into larger groundwater behaviors, mapping groundwater pathways at a watershed level and on a regional scale remains challenging. In order to address this issue, we use airborne time domain electromagnetic (TDEM) and total magnetic field data (Sørensen & Auken, 2004) to image the subsurface electrical resistivity and magnetic properties at a regional scale, up to several thousands of kilometers of flight lines (Chandra et al., 2019; Ley‐Cooper et al., 2019). Airborne electromagnetics (AEM) is an efficient method for tackling several groundwater issues as it is sensitive to lithological contrasts, clay content, groundwater saturation, and salinity (Ball, Bedrosian, et al., 2020; Dumont et al., 2018; Foged et al., 2014; Minsley et al., 2012).…”

Review: Saltwater intrusion in fractured crystalline bedrock

“…Griffiths and Barker 1993;Wyns et al 2004). Some of these methods can be used in airborne specifications to map lineaments as well as to characterize the fractured bedrock properties in terms of hydrogeological characteristics (Chandra et al 2019). Several other methods exist in the characterization of groundwater flow and storage and the general characterization of the bedrock (cf.…”

Assessment of long-term hydrogeological changes and plausible solutions to manage hydrological extremes in the transnational Ganga river basin

“…• Undertaking heliborne geophysical surveys. In recent years these have been shown to provide extensive spatial coverage of the subsurface that can help to identify aquifer structures and properties (Chandra et al 2019). CGWB has initiated a pilot project on aquifer mapping aided by the heliborne geophysical technique in six different hydrogeological terrains of India that includes part of the middle Ganga plains situated around Patna, Bihar state (CGWB 2015).…”