Electromagnetic perturbations associated with earthquakes and volcano eruptions have been intensively documented in the past decades, from both ground and satellite observations. However, the magnetic signals associated with crustal activity are usually very weak and mixed with global geomagnetic signals originating from external sources. Thus, one of the key issues in seismo-electromagnetic study is to identify local geomagnetic signals from global magnetic pulsations. The interstation transfer function can recover the global magnetic pulsations at the observatory by using the data of reference station, providing an effective way for signal discrimination. To further improve the accuracy of global magnetic signal estimation, in this study, we develop a new method for interstation transfer function calculation based on multivariate wavelet coherence using adaptive selection of time window lengths for different periods. Test on real data demonstrates that the global signals of the external source are accurately estimated, including the horizontal X and Y components and the vertical Z component of the observatory. The results of using different reference stations confirm the robustness of the proposed method. It is proved that the method can be used to eliminate the external ionospheric source signals effectively and identify local magnetic field signals lying in the background at the observatory. The proposed method can be useful in seismo-electromagnetic signal identification and extraction.

中文翻译：

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超低频地磁信号估计：一种基于多元小波相干的站间传递函数方法


在过去的几十年里，通过地面和卫星观测，与地震和火山喷发相关的电磁扰动已被大量记录。然而，与地壳活动相关的磁信号通常非常微弱，并且与源自外部源的全球地磁信号混合在一起。因此，地震电磁研究的关键问题之一是从全球磁脉动中识别局部地磁信号。站间传递函数可以利用参考站数据恢复观测站全局磁脉动，为信号判别提供有效途径。为了进一步提高全球磁信号估计的精度，在本研究中，我们开发了一种基于多元小波相干性的自适应选择不同周期时间窗长度的站间传递函数计算新方法。真实数据测试表明，外部源的全局信号被准确估计，包括天文台水平X和Y分量以及垂直Z分量。使用不同参考站的结果证实了所提出方法的鲁棒性。实践证明，该方法可以有效消除外部电离层源信号，识别天文台背景中的局部磁场信号。该方法可用于地震电磁信号识别和提取。