Comparing the effect of different distances of the remote reference stations on the audio-magnetotelluric responses

The electromagnetic (EM) fields of the audio-magnetotelluric (AMT) method generates from the atmospheric sources, in which the natural EM fields inject the energy sources without large electric currents to the ground. It would not impact the environment and provide deeper sounding depths, whereas the responses of the natural EM fields greatly affected by culture noises. Therefore the remote reference (RR) technique has usually used to enhance data quality, which the noises must be uncorrelated with the RR site. In the study, we carried out an AMT survey in southern Taiwan and set 3 RR stations in different distances of 35 km, 217 km, and 263 km, respectively. Comparing the coherence of the electric and magnetic fields in the survey area with the RR stations. We used the S/N ratios and the responses processed with the RR stations to judge the effectiveness. The results show that the AMT responses have been improved by the RR stations. The responses have been obviously improved at low frequencies of 10¹ - 10⁰ Hz, whereas in part of high frequencies of 10⁴ - 10² Hz become more scattered. It could be related to characteristics of the atmospheric sources at high-frequency bands.