Integrated geophysical approach in assessing karst presence and sinkhole susceptibility along flood-protection dykes of the Loire River, Orléans, France

Non-invasive geophysical methods are often used for detecting near-surface defects and monitoring seepage in river dykes or dams. Between 2008 and 2009, a series of geophysical experiments were conducted to detect karst features below the dykes of the Loire River, Orleans, France. Multi-channel analysis of seismic surface waves (MASW) was used to obtain the shear wave velocity (Vs) profile of the subsurface below the dykes. As an effective approach for investigating the structure of the dykes, multi-channel resistivity surveys were also used to evaluate the electrical properties of material inside and under the dykes. This study discusses the exploration strategy and results for several sections of the dyke system. Based on the experimental data, Vs contours are used to geometrically and qualitatively describe the subsurface under the dykes and identify areas of mechanical weakness corresponding to karst features, while resistivity contours allow distinguishing interactions between karst and the dykes through the identification of areas of downward material movement. Known and unknown anomalies are identified. A practical approach that combines seismic and electric resistivity results is proposed to assess karst presence and sinkhole hazard along the investigated dykes using a susceptibility index referring to the likelihood level of the events. As a validation method, areas of strong karst susceptibility index along the dykes are compared to known collapse event (sinkholes, dyke breaches etc.) locations, available in databases.