Global variation of ULF geomagnetic fields and detection of anomalous changes at a certain observatory using reference data

By using multiple reference stations, we have developed a method to get reliable ULF global geomagnetic variations. This background is extremely useful for detecting local anomalous behaviors. In this paper we report on variable tools developed to identify the anomalies in two frequency ranges: daily variations and variations of 10‐ to 1000‐second span. For estimating background daily variations, the periodical model has been applied for data observed at three reference stations and a study station. Comparison between the first principal component of the periodical data from the reference stations and the periodical data derived from the target station generally provides high correlation. For data with 100‐second periods after wavelet filtering, the nighttime energy variations have been investigated among three reference stations and a study station. Similar principal component analysis as the diurnal variation has been performed and results also show high correlation between the variation at the target and the global background. These tendencies suggest that the two proposed methods are effective in automatically identifying the anomalous patterns. Examining the original data, we can obtain details of waveforms and distinguish whether the anomalies are related to underground activities or simply to some artificial noises. © 2012 Wiley Periodicals, Inc. Electr Eng Jpn, 182(3): 9–18, 2013; Published online in Wiley Online Library ( wileyonlinelibrary.com ). DOI 10.1002/eej.22299