Transfer functions of borehole dilatometers response to barometric loading and solid Earth tides from low to high-frequency band : case study of Shaanxi Province

The quest to find successful methods to distinguish and separate the contributions of the barometric loading and solid Earth tides from the time-series of volumetric strain, which are frequency-dependent, has proven to be very challenging in geodynamic studies. In order to uncover the continuous atmospheric and tidal responses of the volumetric strain in the frequency domain, we attempt to apply the transfer function to explore the signatures of frequency dependence observed at the four borehole dilatometer stations in Shaanxi Province, the adopted method is obviously different from the previous studies in China. The results show that: (1)The barometric responses are strongly stable and valid in the low-frequency band (0.1 similar to 0.5 cpd),while fluctuate in the intermediate-frequency band (0.5 similar to 8 cpd), and in the high-frequency band (8 similar to 30 cpd), the barometric responses are stable, respectively. (2) The averaged phase delays of barometric effect on the volumetric strain are less than 9 in different frequency bands, with the exception of the diurnal and partial frequency bands. (3) The discrepancies are remarkable for the trends of barometric coefficients over the full frequency range. Ignoring the significant fluctuations in the diurnal band, the Ankang station is approximately linear and stationary, while the dominant patterns are evidently nonlinear and nonstationary for the other three stations. (4) The tidal responses are stable and convergent in the semidiurnal frequency band, while scatter in the diurnal frequency band. Meanwhile, the results of tidal analyses for the main tidal constituents using transfer function compared with the least square fit method, are in good agreement. (5) The full signatures for the trend of barometric coefficients cannot be completely simulated by the double bush theory in the special case of a homogeneous and isotropic elastic medium, the usability of this theoretical model is quite limiting. Furthermore, we also discuss the potential driving sources which could contribute to the discrepancies and/or similarities in transfer functions from low to high-frequency band, respectively. In summary, the obtained results are useful for reasonably separating the frequency-dependent barometric and tidal effects from the volumetric strain records, as well as for providing the new insights and empirical evidences aiming to further constrain and refine the related geodynamic models.