Multi-dimensional Electrical Resistivity Tomography (ERT) for the mapping of the zones with (or without) cavities in Hofuf KSA

The Hofuf area in Eastern Saudi Arabia is featured by subsurface sinkholes, solution cavities, voids, karst and caves. These features have resulted in hazards to the human safety and stability of the structures founded in them. The development of the area involved the construction of flyover bridges in Hofuf city. For these structures, several investigations were attempted including conventional geotechnical investigations using boreholes and cavity probing, however, these did not provide necessary information to visualize the subsurface zones with cavities. Finally, Electrical Resistivity Tomography (ERT) survey was selected to map the shallow subsurface strata at proposed construction sites with the aims to detect and map the subsurface cavities. ABEM Lund Imaging System consisting of Terrameter with Automatic Electrode Selector was used to acquire apparent resistivity data during the survey. Cables with 2m to 5m electrode spacing take-outs were used with a total of 84 to 400m surface spreads using Wenner-Schlumberger configuration. Windows based software; RES2DINV and RES3DINV developed by Geotomo Software were used for the inverse modeling of the acquired apparent resistivity data resulting in 2-D and 3-D absolute / true resistivity models of the subsurface conditions. The results revealed several anomalous locations which were subsequently verified by drilling boreholes. This study resulted in most feasible recommendations for the design and construction of safe and economical foundation systems for the proposed structures.