Integration of Electrical Resistivity Tomography and Seismic Refraction Tomography to Investigate Subsiding Sinkholes in Karst Areas

Operational and safety issues associated with subsiding sinkholes in karst areas start with the definition of fractures and joints, causing ground weakness. Conventional geotechnical boreholes and geological mapping must be complemented with indirect subsurface exploration techniques to detail those structures. This work aims to use electrical resistivity tomography (ERT) and seismic refraction tomography (SRT) near-surface geophysical techniques to infer the 2D and 3D geometry of sediment-infilled sinkholes formed by the conjunction of fractures and joints in karst areas. Geophysical surveys were performed in a sediment-infilled sinkhole area with two sectors of different subsiding and infilling degrees formed by the conjunction of two fault systems in an experimental research area in the Sierra de Gador Mountains in southeastern Spain. The ERT survey delimited the geometry of the sinkhole area, including the main fault-boun4ded limits, other minor faults, buried epikarst forms, and the distribution of coarse and clay-rich infilling. The SRT survey corroborated the structure and disambiguated clay-rich and high-moisture-content structures giving similar velocity fields. The integration of the ERT and SRT techniques provides indirect 2D and 3D visualizations of the ground of interest in predicting weakness-triggering mechanisms associated with the regional karst structure. This trial in an experimental uninhabited area, with the possibility of exploring subsiding karst structures is of special interest for designing operational and safety measures in urban areas, where similar karst structures may go undetected and monitoring capability is often more limited. The technological development of the used techniques enables the periodical geophysical monitoring of karst structures, thus making the identification of structural changes modifying the land safety and hydrological mechanisms feasible over time.