At present, the most reliable information for inferring storm-time ground electric fields along electrical transmission lines comes from coarsely sampled, national-scale magnetotelluric (MT) data sets, such as that provided by the EarthScope USArray program. An underlying assumption in the use of such data is that they adequately sample the spatial heterogeneity of the surface relationship between geomagnetic and geoelectric fields. Here, we assess the degree to which the density of MT data sampling affects geoelectric hazard assessments. For electrical transmission networks in each of four focus regions across the contiguous United States, we perform two parallel band-limited (10¹–10³ s) hazard analyses: one using only USArray-style (∼70-km station spacing) MT data, and one incorporating denser (≪70-km station spacing) MT data. We find that the use of USArray-style MT sampling alone provides a useful first-order estimate of integrated geoelectric fields along electrical transmission lines. However, we also find that the use of higher density MT data can in some areas lead to order-of-magnitude differences in line-averaged electric field estimates at the level of individual transmission lines and can also yield significant differences in subregional hazard patterns. As we demonstrate using variogram plots, these differences reflect short-spatial-scale variability in Earth conductivity, which in turn reflects regional lithotectonic structure and history. We also provide a cautionary example in the use of electrical conductivity models to predict dense MT data; although valuable for hazard applications, models may only be able to reproduce surface geoelectric fields as captured by the MT data from which they were derived.

中文翻译：

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大地电磁采样和地电灾害估算：全国范围的调查是否足够？

目前，推断沿输电线路风暴时间地面电场的最可靠信息来自粗采样的国家级大地电磁 (MT) 数据集，例如 EarthScope USArray 程序提供的数据集。使用此类数据的一个基本假设是，它们对地磁场和地电场之间的表面关系的空间异质性进行了充分采样。在这里，我们评估了 MT 数据采样的密度影响地电灾害评估的程度。对于横跨美国本土的四个重点区域中的每一个的电力传输网络，我们执行两个并行的带限 (10 ¹ –10 ³s) 危害分析：一种仅使用 USArray 式（～70 公里站距）MT 数据，另一种结合更密集（≪ 70 公里站距）MT 数据。我们发现，仅使用 USArray 式 MT 采样就可以对沿输电线路的综合地电场提供有用的一阶估计。然而，我们还发现，在某些地区使用更高密度的 MT 数据可能会导致单个输电线路水平线平均电场估计的数量级差异，并且还会产生次区域灾害模式的显着差异。正如我们使用变异函数图所证明的那样，这些差异反映了地球电导率的短空间尺度变化，而这反过来又反映了区域岩石构造结构和历史。我们还提供了一个使用电导率模型预测密集 MT 数据的警示示例；尽管对灾害应用很有价值，但模型可能只能再现由它们所源自的 MT 数据捕获的表面地电场。