Application of electromagnetic method to seaward slope: Case study from Saemangeum Seawall in South Korea

ABSTRACT As the world’s dams and seawalls age, there has been increasing attention toward the safety of these structures and monitoring of their condition using various geophysical methods. However, assessing seaward slopes, which are directly impacted by waves and tides and a seawater-filled riprap stone deposit structure, has been challenging due to the highly conductive environment and the difficulty in installing contact equipment. This study examines the feasibility of using electromagnetic (EM) surveys to monitor seawall slopes by analyzing and interpreting multifrequency EM data measured across a two-year period along the seawall slope of the Saemangeum Seawall in South Korea, which is recognized as the world’s longest seawall. The repeatability of data measured at the same survey line across the two years substantiates the reliability of EM survey data. The field data are inverted using a 2.5D regularized Gauss-Newton method. The EM responses calculated from the inverted resistivity model closely mirror the field data. The inversion results consistently indicate the presence of an isolated conductive anomaly with seawater resistivity below the survey line on the seaward slope. This suggests scouring erosion by seawater and affirms the applicability of the EM surveys to seaward slope monitoring. In addition, the EM responses are affected significantly by sea level changes during the survey; these variations are quantified in this study. Thus, our results indicate the effectiveness of the EM method in monitoring the seaward slope to assess the safety of seawalls and provide a comprehensive overview of the entire data analysis and inversion processes, as a valuable reference for those using EM technology in their work.