Characteristics of electromagnetic Taiwanese-Railway signals and the signals’ influences on magnetotelluric measurements

  • Train noises exhibit “wideband” spectra and cannot be filtered by notch filter
  • The leaking currents cause significant ULF noise for sites farther than 1 km
  • The train noises are different from the natural signals on the diffusion pattern

We measured electromagnetic responses near tracks of Taiwan’s railway system in order to identify the possible influence of these artificial signals, which derive from several types of electric trains. Our power-spectrum analysis shows that the noise frequencies of the “broadband”train signals extend from 10-2 to over 10 Hzafter applying the 60-Hz notch filter. We also discovered that different types of trains exhibit unique electromagnetic-signal characteristics. Hence we were able to pinpoint the train-based sources of the signals. If we exclude the train signals from the spectrum measurements, we obtain a consistent estimation of apparent resistivity in relation to our data that we collected in the quiet early morning hours after spectrum normalization for sites that are over 1 km from train tracks. We also observed that leakage current may induce ULF noise and therefore influence the estimation of apparent resistivity. Sites near tracks suffer from both leakage current and magnetic dipole effects, resulting in inconsistencies between our apparent resistivity estimations and the estimations from quiet periods. Moreover, the train noises exhibit a different diffusion behavior from the natural signals. As a result, one should avoid using measurements that contain train signals. In the study, we did not examine the phase components of the train-contaminated and the noise-free magnetotelluric measurements. Our future work will focus on the phase components of signals as they relate to the influences of electric trains and high-speed railroads for subsurface exploration.

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