Comparative analysis of surface and deep underground seismic ambient noise

Deep underground spaces have a noise environment lower than the surface, providing an excellent platform for high-precision and multi-physical fields observation. By conducting a joint seismic observation of the underground roadway at- 848 meters below sea level and the surface in the Panyidong mining field in Huainan, Anhui Province of China, we analyzed their characteristics of the ambient vibrations. The comparison results reveal that the Power Spectral Density (PSD) value of noise records at the surface above 1. 0 Hz is 20 similar to 40 dB higher than that at the deep underground. Moreover, a day-night pattern variation with respect to human activities is observed at the surface. In contrast, the underground records do not exhibit this time-varying phenomenon, which can be attributed to attenuation of the 870 m thick sedimentation overlying the underground roadway. In the 0. 1 similar to 1. 0 Hz frequency band, the difference between the surface and deep underground is relatively reduced, with a power gap of 10 dB. The difference of low frequency noises below 0. 1 Hz is small, possibly due to the interference of downhole airflow and the instrument background noise. When comparing the Double-Frequency (DF) microseism spectrum, the site amplification effect and the splitting of DF microseism peak are clearly observed. Additionally, the deep underground's low-noise environment highlights several high-frequency time-invariant signals, which can be identified as equipment-induced vibrating sources through polarization analysis in frequency domain. Overall, our results confirm that the deep underground provides an excellent environment for high-precision seismic observation and source location. This study also provides an important reference for the subsequent construction of deep underground laboratory and related researches.