Geophysical investigation at Sainte‐Marguerite‐sur‐Mer (Normandy, France): From a supposed landfill to a forgotten military network?

Abstract

This study focuses on characterising a strong and unexpected anomaly observed during an electromagnetic induction (EMI) survey conducted on the chalk cliff plateau at Sainte‐Marguerite‐sur‐Mer, in the Normandy region, France. The site was selected for a long‐term study of chalk cliff coastal erosion that began in 2017 and has been studied for stability risk assessment through hydrogeophysical and thermal approaches since 2022. Sainte‐Marguerite‐sur‐Mer also has significant historical relevance due to its occupation during World War II and the construction of the Atlantic Wall. A bunker from this period is located on the shore platform at the study site, and other bunkers remain visible south of the survey area, near the village. According to local reports, the 340‐ anomaly was initially suspected to overlie an old municipal landfill. Coarse geophysical imaging at 10 m spacing was initially conducted, followed by finer geophysical surveys at 1 m spacing and continuous monitoring, using, for instance, multi‐depth TDR sensors (time‐domain reflectometry) to follow near‐surface soil properties. Various methods were employed to characterise the properties of this anomaly with greater precision, including electrical resistivity tomography (ERT), EMI and ground‐penetrating radar (GPR). Recent high‐resolution geophysical conductivity maps have revealed a linear anomaly of less than 50 extending from the cliff inland over 50 m, suggesting an anthropogenic origin. The typical geometry of a buried pipe, rather than a landfill site, led our research to further investigations through simulations to estimate its diameter, conductivity and magnetic permeability. Physical properties were simulated for GPR, ERT and EMI methods. This pipe‐shaped buried network could therefore be related to the WWII military defence systems identified at the site. The conductive linear anomaly may correspond to old cable trenches carrying telephone or electrical wires that once connected the bunkers before being abandoned.