High-frequency background noise levels on water-tube tiltmeters in Yunnan Province, SW China

The duration of observation of China digital water-tube tiltmeter network has spanned several decades, raising the scientific question regarding the level of high-frequency background noise and the source of the noise. To address this question, this study is the first to examine 11 water-tube tiltmeter observatories in Yunnan Province (SW China) using the power spectral density and probability density function method to establish the high-frequency background noise levels for the water-tube tiltmeters in this region. The findings reveal that: (1) Within the frequency band of 10⁻⁴–8.3×10⁻³ Hz, the background noise level of each observatory exhibits a nonlinear attenuation pattern as frequency increases; among them, the noise sources in the frequency band of 10⁻⁴–5×10⁻⁴ Hz are mainly from the fluctuation of barometric pressure, while those in the frequency band of 5×10⁻⁴–8.3×10⁻³ Hz are complicated and diverse. (2) The Mengla, Yunxian, Yunlong, Lijiang, Zhaotong, Yongsheng, Midu, and Baoshan stations had the lowest high-frequency background noise levels, followed by the Chuxiong and Kunming observatories, whereas the Eryuan observatory had the highest. (3) Most individual observatories had similar background noise levels between the NS and EW components. These results demonstrate that the above methods and results can provide a reference for the calculation of high-frequency background noise levels of China water-tube tiltmeter network and provide empirical evidence for establishing its noise model.