Surface and subsurface sinkholes and karstic cavities in the Uppermost Jurassic–Lower Cretaceous Sulaiy Formation in An Narjis District, Riyadh, Central Saudi Arabia: Field and geophysical investigation

Abstract

Ground-penetrating radar (GPR) techniques have gradually improved for imaging different types of subsurface sedimentary structures, particularly in the top zone of carbonate rocks. The majority of typical underlying sedimentary heterogeneities, such as karst in An Narjis district north of Riyadh, Saudi Arabia, were created by natural processes. Geologically, the investigated area is dominated by limestone formations of the Arab and Sulaiy groups, allowing sinkholes, fractures, and collapse structures to emerge. This study employs field investigations and GPR to identify and detect the karst feature crucial for the construction of new urban areas to withstand the predicted socioeconomic pressures caused by the dense population. The GPR profiles were measured with an antenna of nominal central frequency 400 MHz, which was selected based on clutter limitations, depth of investigation, and desired resolution. The survey parameters, including time window and spatial and temporal sampling intervals, were estimated based on the relative dielectric permittivity of the encountered geologic materials and the frequency of the utilized antenna. A dewow filter, automatic gain control (AGC), background removal, predictive deconvolution, muting, time cut, and time-depth conversion were successively applied to all GPR profiles. The GPR radargrams were carefully analyzed by picking reflectors using phase follower and continuous pick, which were then combined and shown in layer show models. The GPR cross-sections and the derived layer show models highlight distinct reflection patterns (departure or broken parts of low amplitude reflections, diffraction hyperbolas, steeply dipping reflectors, resonant oscillations with no diffracted edges, chaotic background with no lateral continuity, and quasi-hyperbolic form), indicating karst features with different dimensions and depths while suggesting potential risks of collapse in certain areas. On the other hand, visually inspecting the outcrops of the Sulaiy Formation carbonates revealed NE-oriented widening karst fractures filled with red clay-rich residual deposits (karstic residue). GPR radargrams were compared to direct observation of geological features of karst structures found in exposures of vertical road cutting and exhibited a strong correlation. The study underscores the significance of proactive measures to mitigate these hazards in urban planning and construction. Mapping the risk areas, identifying early signs of sinkholes, and implementing preventive measures, such as reinforcing infrastructure, emerge as essential strategies to avert potential disasters. Overall, the integrated approach of field study and GPR presents a valuable method for understanding and addressing karst-related hazards in urban expansions, offering insights to safeguard against future risks in Riyadh's development and other sites with similar conditions.