Transfer functions of the well‐aquifer systems response to atmospheric loading and Earth tide from low to high‐frequency band

The transfer functions of the well‐aquifer systems response to atmospheric loading and Earth tide can be used to calculate the well‐aquifer properties. Due to the low signal‐to‐noise ratio, the study on barometric response at frequencies higher than 8 cycles per day (cpd for short) is almost blank. Using the recorded water level and barometric pressure as well as the corresponding theoretical tidal volumetric strain at 17 well stations in China, we obtained continuous barometric and tidal responses of the well‐aquifer systems by cross‐spectra estimation. It shows that the barometric responses are stable at low frequencies (0.1–0.5 cpd), while fluctuate at tidal frequencies (0.5–8 cpd). In the high‐frequency band (8–30 cpd), by stacking the transfer functions to suppress the noise, we obtained stable barometric responses for the first time. According to the low‐ and high‐frequency barometric responses, we can better judge whether the aquifers are confined in the timescale that we focused and whether the wellbore storage effect can be ignored, and expect to determine the fluid flow properties of the aquifers more reliably. For the three aquifers whose water table drainage and wellbore storage effects are ignored, we used the barometric and tidal responses to estimate their formation properties, which are consistent with previous results. The tidal strain sensitivities are related to the aquifer lithology, which are mainly controlled by the compressibility of the porous matrixes with different porosities and different aspect ratio fractures.