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Comparison of methods measuring electrical conductivity in coastal aquifers
Journal of Hydrology ( IF 5.9 ) Pub Date : 2024-09-03 , DOI: 10.1016/j.jhydrol.2024.131905
Nico Skibbe , Thomas Günther , Kai Schwalfenberg , Rena Meyer , Anja Reckhardt , Janek Greskowiak , Gudrun Massmann , Mike Müller-Petke

Coastal aquifers, the transition zone between freshwater and saltwater, show large salinity contrasts in the subsurface. Salinity is a key parameter to understand coastal groundwater flow dynamics and consequently also geochemical and microbial processes. For mapping porewater salinity, a variety of methods exists, mainly using electrical conductivity as a proxy. We investigate methods including hydrological/geochemical (well sampling, fluid logger) as well as geophysical method (direct push, geoelectrics) utilizing measurements near the high-water line of a high-energy beach at the North Sea island of Spiekeroog. We compare the methods, discuss their benefits and limitations and assess their spatial and temporal resolution. One key to enable a comparison is the estimation of formation factors transforming bulk conductivity measured by geophysical tools in to fluid conductivities obtained from direct measurements. We derive depth-dependent formation factors derived from time-series measurements of fluid loggers and a vertical electrode installation. Using these formation factors, the vertical electrode chain proves to provide reliable salinities at high spatial and temporal dimension. Direct-push profiling data provide the highest vertical resolution. However, a careful calibration is needed to allow for salinity quantification. On the other hand, electrical resistivity tomography (ERT) exhibits the lowest spatial resolution, but can image two-dimensional salinity distributions. We found ERT to fit very well to all other methods, but the data analysis should be aimed at salinities instead of bulk conductivities, i.e. including formation factors and temperature models into the inversion process.

中文翻译:


测量沿海含水层电导率的方法比较



沿海含水层是淡水和咸水之间的过渡带,在地下显示出很大的盐度对比。盐度是了解沿海地下水流动动力学的关键参数,因此也是了解地球化学和微生物过程的关键参数。对于绘制孔隙水盐度,存在多种方法,主要使用电导率作为代理。我们利用北海 Spiekeroog 岛高能海滩高水位线附近的测量方法,研究包括水文/地球化学(井采样、流体记录器)以及地球物理方法(直接推动、地球电学)在内的方法。我们比较了这些方法,讨论了它们的优点和局限性,并评估了它们的空间和时间分辨率。进行比较的一个关键是估计地层因子,将地球物理工具测量的体电导率转化为通过直接测量获得的流体电导率。我们从流体记录仪和垂直电极安装的时间序列测量中得出深度依赖性的形成因子。利用这些形成因子,垂直电极链被证明可以在高空间和时间维度上提供可靠的盐度。直推性能分析数据提供最高的垂直分辨率。但是,需要仔细校准以允许盐度定量。另一方面,电阻率层析成像 (ERT) 的空间分辨率最低,但可以对二维盐度分布进行成像。我们发现 ERT 非常适合所有其他方法,但数据分析应针对盐度而不是体电导率,即将形成因子和温度模型纳入反演过程。
更新日期:2024-09-03
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