Detection of Subsurface Cavities in Limestone Terrain Using 3D Electrical Resistivity Tomography (ERT): A Case Study from Riyadh, Saudi Arabia

Urban development in central Saudi Arabia is frequently challenged by subsurface cavities formed through dissolution and weathering of limestone formations. These hidden features pose significant geotechnical risks, including differential settlement and structural instability. This research presents the results of a high-resolution three-dimensional Electrical Resistivity Tomography (ERT) investigation conducted at a construction site in Riyadh to detect and delineate subsurface cavities and weak zones prior to foundation construction.

The survey covered a 30 m × 40 m site excavated to basement level and was performed using an AGI SuperSting R8 system with a total of 111 electrodes deployed at 1 m spacing along three sides of the plot. A mixed dipole-gradient array configuration was adopted to optimize lateral resolution and depth penetration. Approximately 7,600 data points were acquired and processed using AGI EarthImager 3D software, with rigorous quality control applied prior to inversion. The resulting 3D resistivity model imaged subsurface conditions down to 10 m depth.

The inversion results reveal a heterogeneous limestone subsurface characterized by high-resistivity zones corresponding to competent, massive limestone and distinct low-resistivity anomalies interpreted as cavities, fractured zones, or weathered limestone. Three major weak zones were identified at the southwestern, southeastern, and northeastern portions of the site, extending to depths of 5-7 m. Borehole data confirmed the presence of cavities in two of these zones, validating the ERT interpretation. This research demonstrates the effectiveness of 3D ERT as a non-invasive tool for detecting subsurface cavities in karst-prone limestone environments and highlights its value in guiding targeted ground improvement and foundation design in urban construction projects.