Identifying subsurface weak zones in Riyadh, Saudi Arabia, using multichannel analysis of surface wave technique

Subsurface heterogeneities, particularly in karst-prone regions like Riyadh, Saudi Arabia, pose significant risks to infrastructure due to the potential for sinkhole formation and ground instability. This study employs the Multichannel Analysis of Surface Waves (MASW) technique across 12 profiles, integrated with borehole data, to delineate shear wave velocity (Vs) distributions and identify weak zones in the near-surface geology. The results reveal three main subsurface layers: an upper fractured and weathered limestone layer (Vs < 1000 m/s), an intermediate layer of moderately competent limestone (Vs 1000–1800 m/s), and a lower intact limestone layer (Vs ≥ 1800 m/s). Sixteen distinct weak zones characterized by Vs < 650 m/s were identified at depths of 2–10 m, with lateral extents ranging from 6 to 50 m, impacting approximately 28% of the surveyed area. These anomalies correspond with borehole-observed fill material and fractured rock and may coincide with observed or inferred sinkhole features. While MASW proved effective for detecting shallow, laterally extensive weak zones, its reliability decreases for deeper or small-scale anomalies, highlighting the need for integration with complementary methods. Engineering implications include the recommendation for deep foundations or ground improvement in areas with weak zones, and policy directives should mandate geophysical site assessments in urban planning. The study presents a robust framework for mitigating sinkhole risk and promoting sustainable development through non-invasive subsurface characterization. Future work is suggested to expand geophysical coverage, monitor cavity evolution, and integrate findings into regulatory land-use planning.