3D ERT comparative assessment of Wenner, Wenner-Schlumberger, and Dipole-Dipole arrays for geotechnical characterisation in carbonate environments at Shakrook, Erbil Governorate, Iraqi Kurdistan Region

Abstract

This study presents a comparative geoelectrical survey utilising Wenner, Wenner-Schlumberger, and Dipole-Dipole arrays to evaluate the subsurface of the Shakrook anticline in the Erbil Governorate. 3D electrical resistivity tomography (ERT) images are obtained using IRIS Syscal Junior with 72 electrodes and three arrays operating concurrently. To assess the engineering behaviour at the site, a 2D parallel array of seven profiles measuring 2D  is established. The data inversion is performed using ZondRes3D software to generate a 3D resistivity model. The Wenner array emphasis on vertical resistivity differences facilitated the identification of three separate electro-lithological units in the region: a shallow conductive soil layer (5 – 30 Ω·m) with variable thickness (2.5 – 12.5 m), marly limestone (30 – 136 Ω·m) with (10 – 23 m) thickness, and deeper, competent dolomitic limestone (136 – 398 Ω·m) from the depth 10 m to the depth of the block. The Wenner-Schlumberger array facilitated a balance between vertical and lateral sensitivity, hence enhancing the resolution of fracture zones. The enhanced lateral resolution of the Dipole-Dipole array delineated the precise limits of narrower conductive anomalies, notably a prominent vertical low-resistivity corridor between x = 95 – 125 m, consistently characterised as a fault-controlled karst or seepage conduit. Geotechnical interpretation indicates that high-resistivity benches at depths of 13 – 36 m serve as optimal foundation layers for supporting substantial loads, but conductive seams are associated with weak, damp, or clay-rich zones that pose risks of settlement, seepage, and instability. The integration of all three arrays demonstrates that no one array can fully resolve complex carbonate environments; rather, their collective use establishes a versatile framework for site characterisation, foundation planning, and hazard assessment in the absence of boreholes. The results illustrate the importance of multi-array ERT for geotechnical design in the structurally complex carbonate terrains of northern Iraq.