Imaging Subsurface Cavities in Carbonate Rocks Using Electrical Resistivity Tomography

Urban development in central Saudi Arabia frequently encounters subsurface hazards associated with karstification, weathering, and fracturing of carbonate rocks. These features pose significant geotechnical risks for high-rise structures if not properly identified at the early design stage. This study presents the results of a high-resolution Electrical Resistivity Tomography (ERT) survey conducted at the proposed King Fahad Tower site in Riyadh, Saudi Arabia, aimed at detecting subsurface cavities and weak zones within shallow limestone formations.

The site is located on the Arabian Shelf and is underlain predominantly by sedimentary formations comprising limestone, dolomite, marl, and evaporites of Jurassic to Cenozoic age. A total of 25 ERT profiles were acquired using a multi-channel resistivity imaging system with a dipole-dipole electrode configuration. The survey achieved an investigation depth of approximately 10 m below ground level, providing detailed two-dimensional resistivity images of the shallow subsurface. Inverted resistivity models reveal a wide range of resistivity values, from as low as 2 Ω·m to greater than 600 Ω·m, reflecting strong subsurface heterogeneity. Very low resistivity anomalies (<20 Ω·m) are interpreted as zones of saturated cavities, clay-filled voids, or highly weathered and fractured limestone. Moderate resistivity values (20-150 Ω·m) likely correspond to weathered or partially saturated strata, while higher resistivity zones (>150 Ω·m) are associated with competent limestone bedrock. Several low-resistivity anomalies exhibit vertical continuity and lateral persistence, suggesting potential pathways for infiltration and zones susceptible to collapse.

Based on the integrated interpretation, a subsurface risk map was developed to delineate high-risk zones requiring verification and mitigation. The results demonstrate that ERT is an effective non-invasive tool for mapping shallow karst-related features in carbonate terrains and for optimizing intrusive investigations and ground improvement measures. This study highlights the importance of incorporating geophysical imaging into urban geotechnical site investigations to reduce construction risk and support safe foundation design in karst-prone regions.