Application of Seismic Refraction and Electrical Resistivity Cross-Plot Analysis: A Case Study at Francis Levee Site

Abstract

Geophysical methods such as electrical resistivity tomography (ERT) and seismic refraction tomography (SRT) provide a rapid, more economical, and non-invasive option of investigation of dams and levees with better and more complete sub-surface coverage. Several factors, such as water content and porosity, affect both SRT and ERT results although their sensitivity may differ. By combining electrical resistivity and seismic refraction tomography, a unique assessment of the integrity of earthen embankment dams and levees can be obtained. Cross-plot analysis based on seismic and electrical attributes to seepage and piping can be used to achieve this goal. In this study, a method of combining SRT and ERT using cross-plot analysis is discussed. The method is applied to geophysical surveys conducted at the Francis Levee Site, a site affected by sand boil formations during the 2011 Mississippi river flood event. Requiring consistency between seismic velocity and electrical resistivity models to predict feasible porosity values, an anomaly on the waterside that could be associated with the sand boil formations is identified. Using the seismic velocity and electrical resistivity values of the anomaly on the waterside as limiting values, a cross-plot analysis is performed to identify similar anomalies on the landside. The results from the geophysical methods, cross-plot analysis, and with the help of the geomorphology of the site, indicate that preferential flow occurs within the sand layer in an old oxbow. Sand boils at the site outcrop where the overlying clay layer is thin or the weakest.