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 in 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. First, the sedimentary samples from the drilled wells were classified according to their particle size distribution and analyzed in the lab using spectral IP in controlled salinity conditions to derive their formation factors, surface conductivity, and normalized chargeability. Second, the deduced thresholds are applied on the field to distinguish clay-bearing sediments from brackish sandy sediments. The results are validated with logging data and direct salinity measurements on water samples. We applied the approach along the Luy River catchment and found that the formation factors and surface conductivity of the different unconsolidated sedimentary classifications vary from 4.0 to 8.9 for coarse-grained sand and clay-bearing mixtures, while normalized chargeability above 1.0 mS.m indicates the presence of clay. The clay-bearing sediments are mostly distributed in discontinuous small lenses. The assumption of homogenous geological media is therefore leading to overestimating SI in the heterogeneous clay-bearing aquifers.

中文翻译：

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通过 ERT 和 IP 量化异质沿海含水层的盐度：来自实验室和现场调查的见解

沿海含水层的岩性和地层非均质性对盐水入侵（SI）有很大影响。这使得预测 SI 路径及其持续时间变得困难。在这种背景下，电阻率层析成像（ERT）和激发极化（IP）方法在区分含盐水和粘土沉积物方面受到越来越多的关注。为了简化 ERT 数据的解释，通常假设体电导率主要取决于孔隙填充流体的电导率，而在含水层的空间和时间变化中通常忽略表面电导率，特别是当含水层受到影响时由于盐水的存在。基于简化的岩石物理关系量化盐度可能会导致对富含粘土沉积物构成的含水层的误解。在这项研究中，我们依靠钻孔的同位数据来制定含粘土和无粘土沉积物的体积电导率和流体电导率之间的岩石物理关系。首先，根据粒度分布对钻井中的沉积样品进行分类，并在受控盐度条件下使用光谱 IP 在实验室中进行分析，以获得其形成因子、表面电导率和归一化荷电率。其次，将推导的阈值应用于现场，以区分含粘土沉积物和微咸砂沉积物。结果通过测井数据和水样的直接盐度测量进行了验证。我们沿 Luy 河流域应用该方法，发现对于粗粒砂和含粘土混合物，不同松散沉积分类的形成因子和表面电导率在 4.0 至 8.9 之间变化，而高于 1.0 mS.m 的归一化荷电率表明粘土的存在。含粘土沉积物大多分布在不连续的小透镜体中。因此，同质地质介质的假设会导致高估异质粘土含水层中的 SI。