Workflow for Buried Pipe Detection and Geotechnical Characterization in Conductive Clay–Marl Environments

Accurate detection of buried utilities and reliable characterization of shallow subsurface conditions are critical requirements in civil and industrial engineering projects, particularly in urban areas developed over conductive clay–marl formations. In such environments, commonly used electromagnetic techniques often fail due to severe signal attenuation, increasing uncertainty during excavation and infrastructure planning. This study presents a high-resolution engineering workflow based on Electrical Resistivity Tomography (ERT) for the simultaneous detection of buried stormwater and sewer pipes and the geotechnical characterization of shallow subsurface materials. The methodology was applied in an industrial area southwest of Pamplona (Navarra, Spain), where Eocene marls and clays dominate the geological setting. Three ERT pro-files, each 23.5 m long, were acquired using a pole–dipole array with a dense electrode spacing of 0.5 m, allowing decimetric-scale resolution and investigation depths of up to 7–8 m. Data were processed and inverted using both smooth (L2-norm) and robust (L1-norm) inversion schemes to evaluate their influence on anomaly detection and stratigraphic imaging. The resulting resistivity models clearly identified elongated conductive and resistive anomalies corresponding to known buried sewer and stormwater pipes, despite the highly conductive background. In addition, the ERT sections revealed lateral and vertical variations within the clay–marl sequence, including sandy and compact detrital facies of direct relevance for foundation design and excavation planning. Borehole data available in the study area corroborated the geophysical interpretation. A complementary Ground Penetrating Radar (GPR) survey confirmed the ineffectiveness of electromagnetic methods under the same conditions due to rapid signal attenuation. Rather than focusing solely on utility detection, the proposed approach frames ERT as a dual-purpose engineering tool capable of providing continuous subsurface infor-mation that bridges the gap between sparse borehole data and construction needs. The workflow presented here is transferable and scalable, offering a practical protocol for urban and industrial projects in conductive soils where conventional techniques are limited.