The Response Characteristics And Mechanism Of Groundwater Level To Barometric Pressure And Earth Tides

Groundwater level in a well is a sensitive strainmeter. Groundwater level data are important for precursor observation, which are expected to be used to detect changes in stresses and underground media. At present, a dense network of groundwater observation wells has been built in China, and a large number of waveform records have been accumulated, which provide a basic database for establishing models of groundwater level change, for extracting changes in stress and underground media, and for carrying out research in earthquake prediction. However, since the water levels are affected by many factors, the signal-to-noise ratio of water level change is low; further studies are needed on the model, method and characteristics of extracting aquifer properties and stress changes from water levels.In this paper, we did some fundamental research in the response characteristics and mechanisms of groundwater level to two natural loadings (barometric pressure and Earth tides). We picked put some stations from the network of groundwater observation wells on the Chinese mainland that have continuous and stable water level and barometric pressure data to improve the analysis method for weak signals in water levels; we used the well water level data in the regions of the Wenchuan earthquake and the Lushan earthquake and analyzed the coseismic water level pattern in order to separate out the dynamic and static effects; we analyzed the confinement of aquifers by barometric responses from low-to high-frequency band and tried to determine aquifer properties; we picked out the stations that have sensitive tidal responses and used the phase shift of water level relative to Earth tides to analyze the variations in aquifer permeability. Some conclusions were obtained as following:1. The signal-to-noise ratio of water level can be largely improved by stacking the transfer functions of water level many times, and we obtained continuous barometric responses of the well-aquifer systems from low-to high-frequency band at some stations for the first time. 2. We improved the method for aquifer property inversion by fitting the barometric responses from low-to high-frequency band instead of just using the low-frequency and several tidal frequency responses, as it is of high possibility that the barometric responses at the tidal frequencies are contaminated by the strong energy form Earth tides. We analyzed the confinement and wellbore storage effect of well-aquifers, calculated and obtained the porous matrix compressibility, the porosity, the Skempton’s coefficient, and the specific storage coefficient of confined aquifers.3. The coseismic water level pattern for observed steps coincident with the Wenchuan main shock mainly tracks the expected static stress field. However, the coseismic water level pattern for observed steps within400km of the Lushan main shock is not related to the static stress field. This difference could be due to different seismic energy and geological settings and so on.4. Most of the wells that have resolvable tidal responses show permeability enhancement both after the Wenchuan earthquake and after the Lushan earthquake regardless of whether the coseismic response for the well water level is increasing or decreasing, indicating permeability enhancement is a distinct process from static poroelastic strain, which may be mainly related to dynamic stress.5. The permeability enhancement basically recovers to pre-seismic level within one year for the Lushan earthquake. However, the significant permeability enhancement at well Luguhu during the Wenchuan earthquake has not recovered even after six years, indicating new fractures may be generated in the region during the Wenchuan earthquake.