A New Downhole, Real Time and Near Field Hydrogeophysical Observatory of Groundwater Quality

The downhole imaGeau observatory provides an opportunity to deploy a vertical string of sensors probing outward into the reservoir in order to study changes over time of pore fluid electrical conductivity (hence salinity) and/or saturation. This in-situ set-up is based on near-field and high resolution (in space and time) measurements of formation electrical resistivity in aquifers. This paper presents results related to the salinization of a coastal aquifer exploited for the city of Hossegor (SW France). The subsurface measuring device (SMD) observatory has been located in the near vicinity of the city pumping station for domestic use, where increasing water salinity have been measured over the past 10 years. More than 9 months of daily probing show not only seasonal changes changes but also an heterogeneous profile of pore fluid salinity, far from the first order model expected from the Ghyben-Herzberg gravity model. These more detailed data will lead to more adequate aquifer management strategy in Hossegor. As a consequence, the technology deployed by imaGeau demonstrates to provide an answer to a series of hydrogeological issues such as salt water intrusion in coastal aquifers, pollution studies whether in a petrochemical or dump site context, or else pollution prevention or remediation.