Electromagnetic (EM) geophysical methods are well equipped to distinguish electrical resistivity contrasts between freshwater-saturated and seawater-saturated formations. Beneath the semi-arid, rapidly urbanizing island of Malta, offshore groundwater is an important potential resource but it is not known whether the regional mean sea-level aquifer (MSLA) extends offshore. To address this uncertainty, land-based alongshore and across-shore time-domain electromagnetic (TDEM) responses were acquired with the G-TEM instrument (Geonics Ltd., Mississauga, ON, Canada) and used to map the onshore structure of the aquifer. 1-D inversion results suggest that the onshore freshwater aquifer resides at 4–24 m depth, underlain by seawater-saturated formations. The freshwater aquifer thickens with distance from the coastline. We present 2D and 3D electromagnetic forward modeling based on finite-element (FE) analysis to further constrain the subsurface geometry of the onshore freshwater body. We interpret the high resistivity zones that as brackish water-saturated bodies are associated with the mean sea-level aquifer. Generally, time-domain electromagnetic (TDEM) results provide valuable onshore hydrogeological information, which can be augmented with marine and coastal transition-zone measurements to assess potential hydraulic continuity of terrestrial aquifers extending offshore.

中文翻译：

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时域电磁学对马耳他东南部（地中海）沿海淡水含水层的 3D 表征

电磁 (EM) 地球物理方法可以很好地区分淡水饱和地层和海水饱和地层之间的电阻率对比。在半干旱、快速城市化的马耳他岛下方，近海地下水是一种重要的潜在资源，但尚不清楚区域平均海平面含水层 (MSLA) 是否延伸到近海。为了解决这种不确定性，使用 G-TEM 仪器（Geonics Ltd., Mississauga, ON, Canada）获取陆基沿岸和跨岸时域电磁 (TDEM) 响应，并用于绘制含水层的陆上结构. 一维反演结果表明陆上淡水含水层位于 4-24 m 深度，下方是海水饱和地层。淡水含水层随着远离海岸线而变厚。我们提出了基于有限元 (FE) 分析的 2D 和 3D 电磁正向建模，以进一步限制陆上淡水体的地下几何形状。我们将高电阻率区解释为咸水饱和体与平均海平面含水层相关联。通常，时域电磁 (TDEM) 结果提供了宝贵的陆上水文地质信息，可以通过海洋和沿海过渡区测量来增强这些信息，以评估向外延伸的陆地含水层的潜在水力连续性。