Integrated ERT, petrophysics and borehole logging for geotechnical zonation and sustainable coastal development in...

Rapid urban expansion along Egypt's northwestern Mediterranean coast requires reliable subsurface characterization capable of supporting both geotechnical safety and sustainable groundwater management. This study integrates 87 Electrical Resistivity Tomography (ERT) profiles with borehole logging data, including gamma ray (GR), spontaneous potential (SP), and granulometric analyses, to establish a high-resolution geotechnical and hydro-stratigraphic framework for the Ras El-Hekma coastal zone in Egypt. The integrated dataset resolved four principal subsurface units with distinct resistivity signatures, lithological compositions, and engineering characteristics, calibrated through co-located borehole logs and laboratory granulometric analyses. Statistical analyses revealed strong inverse relationships between resistivity and both porosity (r ≈ - 0.52; r ≈ - 0.85 for thickness-corrected values) and clay content (r ≈ - 0.78; 95% CI (- 0.85, - 0.69)), while GR values showed a strong positive correlation with clay percentage (r ≈ 0.95; p < 0.001). The third geoelectrical layer, dominated by clay-rich Pliocene shale, represents the most problematic geotechnical unit, characterized by resistivity values below 12 Ω·m, clay content exceeding 34%, and thicknesses ranging from approximately 1.5 to 3.5 m. Spatial analysis demonstrates marked NE-SW heterogeneity associated with buried oolitic ridges, sabkha environments, and localized saline intrusion. Based on integrated petrophysical signatures, the study area was classified into four competency zones (A-D), each linked to specific foundation and ground-improvement recommendations. The proposed workflow provides a transferable, cost-effective framework for coastal arid regions by combining geophysical imaging, borehole calibration, and statistical interpretation to support sustainable urban planning, infrastructure development, and subsurface risk assessment.