Comment améliorer la connaissance géologique de l'aquifère de la craie à des fins hydrogéologiques ? Approche géophysique multiméthode focalisante appliquée aux champs captants du sud de Lille

Abstract

National audienceCretaceous chalk is a highly heterogeneous formation, characterized by matrix and fracture porosity, which gives it its aquifer properties. However, this significant and strategic water resource is under pressure (reduced recharge vs. increased demand) and is highly vulnerable to the risk of quality deterioration (high concentrations of undesirable elements such as Se, Ni, Co, As, etc.), particularly in the Hauts-de-France region. In this region, the main aquifer is the chalk, which covers 90% of the territory. It is disrupted by major regional faults, the impact of which on the geometry and hydrodynamic properties of the aquifer remains poorly understood, particularly at the scale of the catchment fields south of Lille (CCSL), where assessing the sustainability of the water resource and its exploitation represents a major challenge.The CCSLs have been the focus of a multidisciplinary study aimed at improving knowledge of the structure and functioning of the Chalk aquifer. A hierarchical, multi-method geophysical approach (gravity, seismic and Electrical Resistivity Tomography (ERT)) was implemented to improve geological and structural mapping on a global scale (gravity), characterize certain major tectonic structures (seismic) and the impact of fracturing and faulting on the Chalk (ERT), and finally, to understand the arrangement of Paleogene and Quaternary overburden formations with the Chalk.The integrated analysis of geophysical data suggests that major faults within the basement propagate into the chalk, which is locally affected by intense fracturing. The observations confirm the strong lateral heterogeneity of the chalk and shed light on local hydrogeological functioning. In particular, they provide a better understanding of water geochemistry data. This study has shown that the Seclin Fault, of regional amplitude, which crosses the study area, should no longer be considered as a single linear structure, but rather as a complex horst and graben structure, with all the consequences this has on the functioning of the chalk hydrosystem