Hosting over 10% of the world's population, low elevation (<10m above mean sea-level) coastal zones are susceptible to saline groundwater intrusion - making fresh groundwater an often scarce and threatened commodity. To inform suitable coastal groundwater management strategies, regional-scale mapping of fresh and salt groundwater occurrence is extremely beneficial. This mapping is usually based on conventional ground-based methods. However, these are not only slow and expensive, but result in localized and disconnected information which is uneconomical and impractical on the large scales required. Airborne electromagnetic (AEM) surveys have been proven a frugal and rapid way to overcome these shortcomings. Consequently, AEM methods are increasingly being used globally. Little is known about the effects of flightline spacing and additional ground-based data on the quality of mapping results, and in general the accuracy of AEM, other than validation against often sparse ground measurements. Understanding this is therefore invaluable as input to groundwater management strategies, survey planning and decision making. Here, we use a regional scale (900km2), high-resolution (50m x 50m x 0.5m) 3D synthetic model of electrical conductivity and geological properties, to investigate the effects of data availability on the accuracy of regional-scale groundwater salinity mapping. This was undertaken by simulating commonly used AEM parameters and realistic data acquisition methods. Two key data components are considered: (1) the AEM survey itself, and (2) geological information used to convert the AEM results into groundwater salinity. Spatially, different data-densities of these two components are quantitatively compared to highlight ideal geometrical configurations for given accuracy requirements. Our results indicate that in terms of optimising costs versus benefits, the value of additional lithological information is dependent on how well the initial distribution of electrical conductivity is resolved by the acquisition and inversion process.

中文翻译：

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区域尺度空中地下水盐度测绘中误差源的实际量化。

拥有世界 10% 以上人口的低海拔（平均海平面以上 <10m）沿海地区容易受到咸水地下水入侵 - 使淡水成为一种经常稀缺和受到威胁的商品。为了告知合适的沿海地下水管理策略，淡水和咸水地下水分布的区域尺度制图是非常有益的。这种映射通常基于传统的基于地面的方法。然而，这些不仅缓慢且昂贵，而且会导致本地化和断开连接的信息，这在所需的大规模上是不经济且不切实际的。机载电磁 (AEM) 勘测已被证明是克服这些缺点的一种节俭且快速的方法。因此，AEM 方法越来越多地在全球范围内使用。除了针对通常稀疏的地面测量进行验证之外，人们对飞行路线间距和额外的地面数据对绘图结果质量的影响以及一般的 AEM 准确性的影响知之甚少。因此，理解这一点对于地下水管理战略、调查规划和决策制定是非常宝贵的。在这里，我们使用区域尺度 (900km2)、高分辨率 (50m x 50m x 0.5m) 电导率和地质特性的 3D 合成模型，研究数据可用性对区域尺度地下水盐度绘图准确性的影响。这是通过模拟常用的 AEM 参数和现实的数据采集方法来进行的。考虑了两个关键数据组成部分：(1) AEM 调查本身，(2) 用于将 AEM 结果转换为地下水盐度的地质信息。在空间上，这两个组件的不同数据密度进行了定量比较，以突出给定精度要求的理想几何配置。我们的结果表明，就优化成本与收益而言，额外岩性信息的价值取决于采集和反演过程对电导率初始分布的解析程度。