Transfer Functions of Borehole Dilatometer Responses to Atmospheric Loading from Low- to High-Frequency Bands: A Systematic Study in Mainland China.

A very challenging but meaningful task in geodynamic research is determining how to clearly disentangle and quantify the frequency-dependent barometric responses of borehole dilatometers. However, we still lack a fundamental understanding of how the Chinese TJ-2 borehole dilatometer network has responded to barometric pressure at different frequencies since the late 1990s. For this reason, we systematically analyze the transfer functions of volumetric strain changes associated with barometric pressure at 21 stations. The results demonstrate that the barometric responses for most sites are generally flat and near-flat in the low- (0.1–0.5 cpd) and high-frequency (8–30 cpd) bands, respectively, while drastically fluctuating in the intermediate-frequency band (0.5–8 cpd) due to the tidal influences. Overall, many sites exhibit strongly frequency-dependent behaviors over the whole frequency range (0.1–30 cpd), which are likely controlled mainly by the complex properties of sensor-grout-formation systems. In light of both the low- and high-frequency barometric responses of dilatometers, whether the surrounding formations are linear elastic medium can be verified. The mechanical properties of three stations, with surrounding formations that are thought to be linear elastic structures, can therefore be roughly but quantitatively estimated by utilizing barometric responses, which are shown to be consistent with the parameters tested by rock mechanics. Altogether, this work not only provides a useful method for frequency-dependent pressure correction, but also offers profound insights to advance sensor-grout-formation modeling. [ABSTRACT FROM AUTHOR]