Abstract We present magnetotelluric (MT) data from the northern Fennoscandian shield, which show extreme behavior due to the complex geoelectric setting left behind by the tectonic reworking of the Archaean bedrock during the Palaeoproterozoic. The core of the MT data set from the central Finnish Lapland were acquired in 2014 in the context of the MaSca-project. Extreme data are defined by the condition where the determinant of the real and/or imaginary part of the impedance tensor becomes negative. This is related to, but is a more anomalous condition than out-of-quadrant off-diagonal impedance phases. Physically, these are associated to reversal of e.g. the electric field as compared to its direction for one-dimensional resistivity structure. So far, only few examples of extreme data have been explicitly reported in geophysical literature. Data previously classified as erroneous might belong to the class of extreme data instead and data quality analysis should take this into account. A resistivity contrast of at least ca. 1:1000 is required to observe extreme data, as indicated by synthetic modelling studies. Extreme data are challenging from a modelling point of view and thus also drive the need for new developments in forward and inverse codes for MT. In this study, behavior of the phase tensor parameterization in case of extreme data is clarified. The quadrants of the principal value phases, a piece of information inherently missing from the phase tensor, can be recovered if the parameters are followed as a function of period. The behavior of the determinant (average) of the impedance tensor is investigated, since out-of-quadrant determinant phases are sometimes observed in conjunction with extreme data. A special condition is demonstrated, which links out-of-quadrant determinant phases and extreme data to negative determinant of the galvanic distortion tensor.

中文翻译：

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芬兰拉普兰中部的大地电磁阵列 I：极端数据特征

摘要 我们展示了来自芬诺斯坎迪亚地盾北部的大地电磁 (MT) 数据，由于古元古代时期太古代基岩的构造改造留下的复杂地电环境，这些数据表现出极端的行为。芬兰拉普兰中部 MT 数据集的核心是在 2014 年在 MaSca 项目的背景下获得的。极端数据由阻抗张量的实部和/或虚部的行列式变为负的条件定义。这与象限外非对角阻抗相位有关，但比该相位更异常。在物理上，这些与例如电场与其一维电阻率结构的方向相比的反转相关联。迄今为止，地球物理文献中只有少数极端数据的例子被明确报道。以前归类为错误的数据可能属于极端数据类，数据质量分析应考虑到这一点。至少大约的电阻率对比。如合成建模研究所示，需要 1:1000 来观察极端数据。从建模的角度来看，极端数据具有挑战性，因此也推动了对 MT 正向和逆向代码的新发展的需求。在本研究中，阐明了极端数据情况下相位张量参数化的行为。如果将参数作为周期的函数进行跟踪，则主值相位的象限（相位张量中固有的一条信息缺失）可以恢复。研究了阻抗张量的行列式（平均值）的行为，因为有时会结合极端数据观察到象限外行列式相位。演示了一种特殊条件，它将象限外行列式相位和极端数据与电流畸变张量的负行列式联系起来。