Study On The Mechanism Of Well Water-level Tidal Anomaly

In this paper, we combine groundwater dynamics theory with poroelastic theory, to establish the governing equation of the aquifer pore pressure. Then according to the water exchange balance between the aquifer and the wells, under the undrained boundary conditions of the infinity, analytical solution of well water level to aquifer average stress response is obtained, and the model of well water level to aquifer tide response is presented. Based on the model, a simple method to judge whether the response of well water level to aquifer tide are under the undrained condition is proposed, that is whether there are change of well water level tide partial wave amplitude and initial phase between before and after far field from large earthquakes. A method is put forward by which aquifer parameters, such as Skempton coefficient B and the transmissivity T, are deviated from well water level data. On the ground of aquifer parameters' change, the well water level tidal abnormal can be identified and abnormal mechanisms can be analyzed. Use these methods to analyze the tide response by the well water level and earth tide data of chuan-o6 well and chuan-18 well, deduce the change of aquifer Skempton coefficient B and the transmissivity T by the theory model.This tidal response model is conducive to carry out quantitative analysis of water level tidal response. Meanwhile the estimating method of Skempton coefficient and hydraulic conductivity of aquifer water level provide an effective way to estimating hydrogeological parameters of aquifer; and help to better identify abnormal tidal water level before the earthquake, also tidal anomaly mechanism of water level exception.Through theory analysis and the actual analysis, there are some important conclusions:1,The amplitude of well water level tidal response, not only controlled by the Skempton coefficients which comprehensive reflected the aquifer elastic and poroelastic properties, but also by aquifer hydraulic parameters such as transmissivity, storage coefficient, and the borehole radius and the tidal signal frequency; phase response controlled by aquifer hydraulic parameters, as well as the borehole radius and the tidal signal frequency. When discussing the frequency of a well water level respond to particular sub-tidal wave stress, its mainly affected by aquifer Skempton coefficient and hydraulic conductivity (permeability) coefficient, but the respond to the aquifer storage coefficient is not sensitive.2,When well water level response to tidal met the undrainage conditions, the well water level is mainly affected by tidal changes in poroelastic response, but the hydraulic response can be ignored. Well water level response is mainly affected by tidal aquifer elastic and poroelastic properties affected; response amplitude equal to the aquifer pore pressure head tidal amplitude, phase difference is numerically equal to the pore pressure and the phase difference between tidal stress. When well water level response to tidal conditions to meet the transition, the response not only affected by the poroelastic effect, but also by the hydraulic response; which the amplitude of groundwater level response by the poroelastic effect, while phase response affected by the water. When the well water level response in the transition state of tide and full drainage status, the main response controled by the hydraulic reponse.3,After the far-field earthquake, if the water level amplitude and initial phase didn't change, the well water level response to tidal meet drainage conditions, then you can directly count the aquifer Skempton coefficient which would have high precision. If the phase change, while the amplitude is almost constant, tidal water level response to meet the transition state. If the response amplitude and phase changes have taken place, the well water level in tidal response to meet the full drainage. When the well water level response in the transition state of tide and full drainage status, not only estimating aquifer Skempton coefficient, but also estimating hydraulic conductivity (permeability) coefficient.4,Either change of aquifer Skempton coefficient or of hydraulic conductivit (permeability) coefficient will result in well water level tide anomaly. The far-field change of amplitude and phase response of groundwater level induced by Seismic waves due to the change of aquifer permeability (hydraulic conductivity) coefficient larger; the aquifer transmissivity become smaller under the compressive stress. Huili Sichuan-18 well's analysis showed that the tidal anomaly well water level located in the far-field and is often induced by aquifer transmissivity changes.