Cong-Thi Diep, Pham Dieu Linh, David Caterina, Xavier De Pauw, Dang Thi Huyen, Ho Huu Hieu, Frédéric Nguyen, Thomas Hermans

The lithological and stratigraphical heterogeneity of coastal aquifers has a great influence on saltwater intrusion (SI). This makes it difficult to predict SI pathways and their persistence in time. In this context, electrical resistivity tomography (ERT) and induced polarization (IP) methods are receiving increasing attention regarding the discrimination between saltwater-bearing and clayey sediments. To simplify the interpretation of ERT data, it is commonly assumed that the bulk conductivity mostly depends on the conductivity of pore-filling fluids, while surface conductivity is generally disregarded in the spatial and temporal variability of the aquifers, particularly, once the aquifer is affected by the presence of saltwater. Quantifying salinities based on a simplified petrophysical relationship can lead to misinterpretation of aquifers constituted by clay-rich sediments. In this study, we rely on co-located data from drilled boreholes to formulate petrophysical relationships between bulk and fluid conductivity for clay-bearing and clay-free sediments. We investigate both the laboratory scale through spectral induced polarization (SIP) and field-scale through the comparison of ERT and time-domain IP with electromagnetic logging and lithologs. First, the sedimentary samples from the drilled wells were classified according to their particle size distribution and then analyzed in the lab using a SIP system using the four-point measurement method in controlled salinity conditions. Second, field inversion results were compared with logging results and direct salinity measurements on water samples. We find that the formation factors and surface conductivity of the different unconsolidated sedimentary classifications are varying from 4.0 to 8.9 for coarse-grained sand and clay-bearing mixtures, respectively. The clay-bearing sediments are mostly distributed in discontinuous small lenses along the Luy River Catchment. The assumption of homogenous geological media is therefore leading to overestimating SI in the heterogeneous clay-bearing aquifers