Global Variation of ULF Geomagnetic Fields and Detection of Anomalous Changes at a Certain Observatory Using Reference Data KATSUMI HATTORI, 1 PENG HAN, 1 and QINGHUA HUANG 2 1 Chiba University , Japan 2 Peking University , Japan SUMMARY By using multiple reference stations, we have devel- oped a method to get reliable ULF global geomagnetic variations. This background is extremely useful for detect- ing 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 vari- ations, 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 peri- odical data derived from the target station generally pro- vides 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 Librar y (wile y onlinelibrar y.com). DOI 10.1002/eej.22299 Key words: ULF geomagnetic data; global vari- ation; daily variation; geomagnetic pulsation; wavelet fil- ter; principal component analysis. 1. Introduction Various electromagnetic phenomena, from DC to the microwave band, related to lithospheric activities such as earthquakes and volcano eruptions have been reported in many studies [1–3]. Due to deep skin depth in lower fre- quencies, ULF (f < 10 Hz) waves have an advantage for sensing in seismogenic areas under the ground. Therefore, these observations can become a promising method of monitoring anomalous behavior in the lithosphere. However, the electromagnetic fields in the DC–ULF range include geomagnetic perturbations caused by inter- action of the Earth’s magnetosphere and solar wind, artifi- cial noises produced by DC trains, industrial equipments, and various interferences. Generally , anomalies of the Earth’s electromagnetic field associated with seismic or volcanic activities are much weaker than the above-men- tioned signals. Therefore, discrimination of these signals is a very important issue. Especially strong fluctuations are produced by electromagnetic disturbances in the iono- sphere. It is known that the interstation method based on wavelet transform (method of estimation of the correlation coefficient of two observation stations, of which one is with a low level of man-made noise as the remote reference point) makes it possible to substantially eliminate the influ- ence of geomagnetic variations originated in the ionosphere from the ULF range geomagnetic data (period from several seconds to 1000 seconds) and to detect signals embedded in these data [9, 10]. The interstation method generally makes it possible to eliminate global variations of the horizontal components of the magnetic field but the accu- racy with respect to the vertical components is rather insuf- ficient. Other shortcomings of the interstation methods © 2012 Wiley Periodicals, Inc. Electrical Engineering in Japan, Vol. 182, No. 3, 2013 Translated from Denki Gakkai Ronbunshi, Vol. 131-A, No. 9, September 2011, pp. 698–704 Contract grant sponsor: Grant-in-Aid for Scientific Research of the Japan Society for Promotion of Science (B) (Subject No. 194030002, Repre- sentative: Katsumi Hattori), the International Joint Research Fund of the National Institute of Information and Communications Technology (2008 to 2010) (Representative: Katsumi Hattori), and the Joint Research Col- laboration Program by the Ministry of Science and Technology of China (2010DFA21570, Representative: Qinghua Huang). 9