Unravelling heterogeneities in the coastal basaltic aquifer in Saurashtra, India by airborne electromagnetics

Summary The Deccan flood basalt in Saurashtra, India, primarily comprises multiple lava flows with a few interbedded sediments in hiatus, underlain by Mesozoic formations. Unlike conventional seismic methods, airborne electromagnetics (AEM) offer the possibility of reliably mapping heterogeneities within basalts and sedimentary strata due to the large variabilities of electrical properties. The dual‐moment heliborne transient electromagnetics data were collected over ∼32,000 sq. km of the Saurashtra region, comprising coastal sediments, basaltic heterogeneities and sedimentary formations. With limited calibration and validation from borewells, AEM modelling results demarcate some electrical heterogeneities, explaining the highly resistive (>100 Ωm) massive basalts, resistive (50–100 Ωm) fractured, amygdaloidal basalts, moderately resistive‐to‐conductive (10–50 Ωm) weathered and fractured basalts/sandstone and shale formations, conductive (3–10 Ωm) limestone and clay and highly conductive (<3 Ωm) saline‐enriched coastal and glacial sediments. The weathered and/or fractured amygdaloidal basalts, dykes and sandstone formations provide the fresh aquifer settings with moderate‐ to high yield. Shallow aquifers (<30 m) in moderately conductive‐weathered and fractured basalts depict maximum yield and brackish‐fresh water quality near the river flow and reservoirs. In the coastal aquifers, most of the EM signal attenuates in the top formations rich in salinity, demarcating the extent of freshwater saline water interface and/or ingress of saline water up to 40–60 km from the coastline towards north, west and east. The depth to the top of the saline layer below the basalts is relatively deep in the south (>100 m). The results suggest that the east–west aligned water divide for surface flow rivers and/or drainages directly associated with resistive basaltic uplands controls the subsurface conditions for groundwater flow and/or accumulation. This study provides a new dimension of less‐studied coastal basaltic aquifers and the influence of the extent of saline zones/saltwater ingress in them and recommended for limited groundwater extraction and prior assessment of recharge potential for long‐term aquifer sustainability, especially in arid and semi‐arid regions like Saurashtra.