Array analysis of magnetic and electric field observatories in China: estimation of magnetotelluric impedances at very long periods

Ten years (2008–2017) of continuous measurements from 40 electric and 36 magnetic sites collected in China for earthquake prediction research represent a unique EM array data set, which can be used to explore the challenging problem of very long-period MT data acquisition, to study source characteristics, and ultimately to learn about electric conductivity of Earth's mantle beneath East Asia. In this study, we focus on basic noise and signal characteristics in this data set, and on estimation of the MT impedances. We report a novel method to fix the numerous timing errors in the electric data caused by limitations in instrumentation and data acquisition. Then, we use multivariate array analysis to study signal and noise characteristics for periods from 250 s to 3.5 × 10
⁵
s (4 d). Signal-to-noise ratios (SNR) are above 30 dB in magnetic fields for the first two dominant modes, which correspond roughly to N–S and E–W quasi-uniform sources. SNRs for electric fields are lower, especially at very long periods, and especially for N–S electric components. There are clear peaks in signal strength at the daily variation (DV) periods, but source structure becomes more complex, and significant biases in MT impedance tensors are more often seen at these periods. The MT quasi-impedance, computed using the closest magnetic site for each electric site, is estimated by robust remote reference techniques (RR) and by using linear combinations of PCA (principal component analysis) modes that best approximate a uniform or plane-wave source (PW). For almost all sites, smooth impedances are obtained for periods up to 10
⁴
s using either approach. This result, and a more detailed analysis of impedances estimates obtained with shorter-wavelength (gradient) sources extracted from the array, suggests that source effects in MT impedances are minimal for periods below 10
⁴
s, at least at the latitude of China. At many sites curves can be extended a decade further, to 10
⁵
s, but here results are improved by carefully omitting DV bands and (at a few sites) with the PW approach. For longer periods (>10
⁵
s) SNR is very low in electric field channels at most sites, making estimation challenging. However, at a few sites, even some near big cities (e.g. including a site within 40 km of Beijing) smooth impedance components related to N–S magnetic sources (Z
xx
and Z
yx
) are obtained to periods to 3.5 × 10
⁵
s (4 d). This result suggests that cultural noise may not be the main impediment to collecting very long-period MT data.