Challenges of 2D electrical resistivity tomography for detecting thin clay layers under levees: A case study of Melvin price reach of the wood river levee on the Mississippi river

Abstract Levees provide critical flood protection, but are vulnerable to failure from internal erosion and piping. According to the risk assessment study performed by the US Army Corps of Engineers’, more than 20% of the levees in the United States are considered to have a very high risk of failure. Therefore, rapid noninvasive methods that can assess subsurface conditions over long distances are needed to identify the critical zones along the levees. This study evaluates electrical resistivity tomography (ERT) to characterize levee composition and detect anomalies indicative of defects arising from thin clay layers. ERT surveys were conducted on the Melvin Price reach of the Wood River Levee and compared to previously published multichannel analysis of surface wave (MASW) measurements and historical data. Longitudinal and transverse two‐dimensional (2D) ERT imaging revealed large‐scale stratigraphy related to old river channel deposits but lacked the resolution to discern fine‐scale layering details, including a discontinuous clay layer resolved in the MASW shear wave velocity profiles. This limitation is challenging because such features are critical for understanding internal erosion at the levee. Additionally, the 2D nature of ERT inversion is susceptible to bias caused by the complex three‐dimensional (3D) geometry of levee structures, which introduces artefacts that can affect the interpretation of resistivity data. These 3D effects are particularly pronounced for longitudinal lines with air on both sides of the levee or when resistivity contrasts are low. Forward modelling confirmed limitations in the ability of 2D ERT sections to resolve certain thin clay layers during inversion. However, ERT provided a rapid overview of subsurface stratification and moisture patterns; integrating with MASW, drilling data and advanced inversion methods that correct for 3D effects is recommended to improve characterization levee condition and defects. This study highlights both the potential and inherent challenges of geophysical methods for assessing critical flood protection infrastructure.