More than buried valleys: Do tunnel valleys serve as conduits for preferential freshened groundwater flow within the North Sea?

我们社会不断增长的人口导致对淡水的需求不断增加，而地下水的开采是其最重要的来源之一。由于地下水供应短缺，人们开始在近海寻找地下水，一些研究表明，海底下存在新鲜的地下水。然而，就地下水流动而言，更新世冰川时期产生的大型地下地貌的作用仍然知之甚少。特别是，在冰盖下形成的所谓的隧道谷（TVs）充当了冰川融水的排水系统。它们的大小（宽达5公里，深400米，长100公里）反映了大量的融水冲刷着冰盖下的地下。由于其形成机制，埋地电视充填体通常由下部高渗透性砂和砾石组成，顶部为细粒沉积。这种结构促进了它们作为从全新世咸水中封闭的地下水的优先流动通道的作用。为了了解电视作为北海东南部海上新鲜地下水（OFG）优先流动通道的潜力，该项目有以下具体目标：(1)结合电磁和地震数据，我们的目标是圈定几个电视的结构约束电阻率模型；（O2）我们的目标是估计不同结构和深度的电视相应的盐度值；在前两个目标的基础上，我们的目标是在详细的3D岩性模型中推断整个工作区域的发现。由此得出的地下盐度分布将有助于划定OFG体积的上限和下限，并为详细的地下水模型提供基础。为了实现我们的目标，我们将采取以下步骤：（S1）利用现有地震数据绘制和表征研究区域电视的三维空间异质性；（S2）利用现有的多道地震剖面，建立Amrum - Heligoland之间0 ~ 400 m深度的地下岩性模型；（S3）确定有潜力的地点，并进行可控源电磁（CSEM）测量，以研究地下电阻率的分布；（S4）结合电阻率测量和MCS计算孔隙流体盐度；（S5）对全岩性模型区的结果进行推断和讨论。隧道谷存在于世界上大多数以前的冰川地区。在这些地下地形中证明近海淡水可能对迄今未知的淡水分布和水文系统具有重大意义。广泛而密集的MCS数据为我们提供了一个独特的机会，可以在有限的额外努力下整合CSEM和地震测量。该项目的研究结果将为研究近海冰川地貌及其在更新世水循环中的作用提供新的视角。

Our society’s steadily growing population leads to increasing demands of freshwater and the abstraction of groundwater is one of its most important sources. Shortages of groundwater supply have led to the search for groundwater offshore and several studies have shown freshened groundwater to exist beneath the seafloor. However, the role of large subsurface landforms produced during glaciations of the Pleistocene is still poorly understood with respect to groundwater flow. In particular, so-called tunnel valleys (TVs) formed beneath ice sheets acted as drainage systems of glacial meltwater. Their dimensions (up to 5 km width, 400 m depth, 100s of km length) reflect the massive amount of meltwater that flushed the subsurface beneath ice sheets. Due to their mechanism of formation, the fill of buried TVs often consists of highly permeable sands and gravels in their lower part and fine-grained deposits at their top. Such a configuration promotes their role as preferential flow pathways for groundwater sealed from the Holocene saltwater above. To understand the potential of TVs as preferential flow pathways of offshore freshened groundwater (OFG) in the southeastern North Sea, this project has the following specific objectives: (O1) Combining electromagnetic and seismic data, we aim to delineate structurally constrained resistivity model for several TVs; (O2) We aim to estimate the corresponding salinity values for different architectures and depths of TVs; (O3) Building on the first two objectives, we aim to extrapolate the findings for the full working area in a detailed 3D lithological model. The resulting salinity distribution in the subsurface will aid to delineate upper and lower limits of OFG volumes and provide the basis for a detailed groundwater model. To fulfil our objectives, we will take the following steps: (S1) Map and characterize the 3D spatial heterogeneity of TVs in the study area using existing seismic data; (S2) Build a lithological model for the subsurface between Amrum and Heligoland from 0 to 400 m depth using existing multi-channel seismic profiles; (S3) Identify promising sites and conduct controlled source electromagnetic (CSEM) measurements to investigate the distribution of electrical resistivities in the subsurface (TVs); (S4) Combine resistivity measurements with MCS to derive pore fluid salinity estimates; (S5) Extrapolate and discuss the results for the full lithological model area. Tunnel valleys exist in most formerly glaciated regions worldwide. Proving offshore freshened groundwater in these subsurface landforms may have substantial implications for as of yet unknown freshwater distributions and hydrological systems. The extensive and dense MCS data available to us provide a unique opportunity for the integration of CSEM and seismic measurements with limited additional effort. The results of the project will provide a new view on offshore glacial landforms and their role in Pleistocene water cycles.