5:15 PM - 6:45 PM
[MIS04-P08] A development of noise reduction method for ULF band electromagnetic data using Multi-channel Singular Spectrum Analysis (MSSA) (3)
Keywords:MT method, Multi-channel Singular Spectrum Analysis (MSSA), Signal discrimination, Boso peninsula, ULF band
The electromagnetic phenomena suspected to be related to earthquakes in the ULF band (f<10 Hz) are reported because the skin depth (more than a few km) corresponds to the interior of the crust of the plate boundary. However, the observed signals include not only seismo-electromagnetic signals but also natural electromagnetic field variations caused by solar-terrestrial coupling with a scale of several thousand kilometers, leak currents from DC-driven railways, and artificial noise caused by the transmission lines. Since the amplitude of seismo-electromagnetic signals is smaller than that of natural magnetic field fluctuations and artificial noise, and especially similar to that of train noise, analysis has focused on nighttime (e.g., LT 2:30~4:00) when human activities are calm. Therefore, conventional analysis methods may miss seismic EM signals during most of the day. Therefore, a signal discrimination method that can discriminate seismic signals sufficiently, even if the signal-to-noise ratio is poor, is indispensable to extracting seismic signals buried in other signal components. Therefore, we are developing a signal discrimination method based on Multi-channel Singular Spectrum Analysis (MSSA), which is a time series signal separation method.
In the MSSA process, first, create Hankel matrixes (window length M × (data length N -M+1)) in each channel and then perform singular value decomposition (SVD) of the covariance matrix. After the SVD, each channel is decomposed to the C channel × M principal components (PC). We can reconstruct the time series by selecting the PC based on its characteristics, such as the correlation and contribution rate.
The abstract of the method follows: 7ch data (target site 5 components (horizontal magnetic field 2ch (Hx and Hy), vertical magnetic field 1ch (Hz), and horizontal electric field 2ch (Ex and Ey)) and reference site 2ch (magnetic field 2ch (Rx and Ry))) are used. First, apply MSSA to 7ch, then eliminate the trend. Second, apply MSSA to 4ch (Hx, Hy, Rx, and Ry) to extract solar-origin magnetic field fluctuation. Finally, apply MSSA to 7ch (extracted magnetic field by step 2, Hz, Ex, and Ey) to obtain the MT signal in Hz, Ex, and Ey.
The results of applying this method to actual data acquired on the Boso Peninsula showed that the global magnetic field corresponding to the reference magnetic field and the electric field corresponding to the global magnetic field could be separated even during daytime when there is the influence of artificial noise. The separated local signals are about 1 nT for the magnetic field and 1.5 mV/km for the electric field. The relation to earthquakes will be discussed in the future.
In the MSSA process, first, create Hankel matrixes (window length M × (data length N -M+1)) in each channel and then perform singular value decomposition (SVD) of the covariance matrix. After the SVD, each channel is decomposed to the C channel × M principal components (PC). We can reconstruct the time series by selecting the PC based on its characteristics, such as the correlation and contribution rate.
The abstract of the method follows: 7ch data (target site 5 components (horizontal magnetic field 2ch (Hx and Hy), vertical magnetic field 1ch (Hz), and horizontal electric field 2ch (Ex and Ey)) and reference site 2ch (magnetic field 2ch (Rx and Ry))) are used. First, apply MSSA to 7ch, then eliminate the trend. Second, apply MSSA to 4ch (Hx, Hy, Rx, and Ry) to extract solar-origin magnetic field fluctuation. Finally, apply MSSA to 7ch (extracted magnetic field by step 2, Hz, Ex, and Ey) to obtain the MT signal in Hz, Ex, and Ey.
The results of applying this method to actual data acquired on the Boso Peninsula showed that the global magnetic field corresponding to the reference magnetic field and the electric field corresponding to the global magnetic field could be separated even during daytime when there is the influence of artificial noise. The separated local signals are about 1 nT for the magnetic field and 1.5 mV/km for the electric field. The relation to earthquakes will be discussed in the future.