Assessing the effectiveness of geoelectrical methods in characterizing geotechnical conditions of earthen dykes in a megatidal environment

Abstract

The flood dykes surrounding the Bay of Fundy are facing increasing risk from rising sea levels and extreme tidal events. In light of these risks, this research utilized Electromagnetic apparent conductivity mapping (EM) and Electrical Resistivity Imaging (ERI), to characterize the internal structure of the Shepody dykes located in south New Brunswick, set in the upper Bay of Fundy. The goal was to evaluate the capability and effectiveness of geoelectrical methods in inferring geotechnical property variability and potential vulnerabilities within the dyke and its foundation.

The first phase of the study established that an integrated approach of EM and ERI is effective for identifying electrically conductive anomalies. Soil sampling and laboratory analysis established that pore water salinity exerts the dominant influence. A prominent conductive anomaly was interpreted as a zone of increased saline water content and higher hydraulic conductivity, representing a potential preferential seepage pathway.

The second phase evaluated the influence of extreme tidal fluctuations through timelapse 3D ERI. The study measured a maximum 15% resistivity decrease at peak high tide (~2.25 m below dyke crest), which was concentrated at depths of 7.5 m and greater. In contrast, resistivities within the upper 5 m, including the 2.5 m high dyke, showed negligible change. The 3D results robustly corroborated the 2D assessment, confirming that important spatial variations in conductivity can be captured by more efficient 2D surveying regardless of the tidal stage.