Comprehensive Geophysical Assessment of Complex Geological Terrain in Kadapa Basin, India

Abstract

Traditional geotechnical investigation consisting of drilling many of boreholes, soil sampling and lab testing becomes uneconomical and time consuming for large and geologically intricate areas. In such cases, non-destructive geophysical surveys offer a time-efficient and economical solution, simultaneously providing an extensive areal coverage of subsurface profiling of geotechnical and geophysical properties. For the estimation of subsurface properties, three geophysical methods were employed in this study, which consisted of Ground Penetration Radar (GPR), Electrical Resistivity Tomography (ERT), and Multi-Channel Analysis of Surface Waves (MASW). Thus, the sub surface’s shear wave velocity profiles and apparent resistivity profiles were obtained. In the shallow depths, discontinuities and fractures were studied using electromagnetic radargrams from GPR. These tests were complemented with borehole exploration at the site. Results obtained from surface-based methods were validated against bore log data and visual observation of surface terrain. Subsequently, bore logs were integrated with the geophysical survey results to construct an integrated subsurface profile. The integrated subsurface profiles presented a three-layer subsurface structure consisting of dense gravelly sand in the top 1m, followed by strong rock formations and, ultimately, very strong fractured rocks. The extent of fracture in the rocks was studied using samples obtained from boreholes and available trenches at the test location. These findings helped compare the three methods and their applicability in delineating different subsurface layers in this study. GPR proves to be effective at shallow depths, while ERT and MASW help investigate deeper layers better. Further, this study offers critical insights for site characterization and engineering decisions in complex geological environments, improving the knowledge base of efficient and reliable subsurface evaluation techniques.

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