Tidal Analysis Of Groundwater Flow In Well-aquifer System And Its Applicationon

As a power source that can be accurately calculated, tidal force causes the water level tidal phenomenon that reveals the coupling relationship of aquifer pore pressure and rock deformation. It has attracted wide attention of hydro logical and seismological scholars and researchers. Earth tide is periodic deformation of the solid earth’s interior and surface caused by the gravity of the Moon and Sun. By studying the response mechanism and characteristics that different aquifer fluid pressure and water level to earth tide and analyzing tidal phenomenon, it can not only get aquifer characteristics and hydraulics parameters and these parameters changing with time, but also is an effective means to explore the tectonic movement.For homogeneous and isotropic media, the thesis studies isotropic water saturated rock volumetric strain under undrained condition caused by tidal forces applying linear poro-elastic stress equilibrium equation, and derives the linear responding equation of water saturated rock pore pressure to tide generating potential. Physical interpretation of pore pressure response factor (E) is defined, and its range is given. For fractured media, based on Bower’s (1983) research results of the relationship fluid pressure in single fracture to tidal potential amplitude ratio and phase shift, the responding models reflecting amplitude ratio and phase shift of fluid pressure in single fracture and multi-fracture to tidal potential are established. The effects of elastic parameters of rock to amplitude ratio and phase shift are analyzed. The relation curve between fluid pressure tidal components amplitude ratio, phase shift in single fracture and fracture orientation is plotted as curves. Fracture aquifer tidal amplitude ratio and phase shift are influenced by fracture orientation. Amplitude ratio increases increases with the fracture angle (DIP), and phase shift is hardly influenced by rock elastic parameters.Acting on the wells and aquifers, tidal forces result in radial water exchange between well hole and aquifer. Hsieh (1987) derive the formula of amplitude ratio and phase shift between well water-level and tide height under radial flow conditions. Based on Hsieh’s (1987) formula, the thesis further deduced amplitude ratio and phase shift formula between well water level and tide height. The physical meaning of amplitude ratio and phase shift is defined the. Amplitude ratio and phase shift caused by radial flow are influenced by aquifer transmissivity. According to the characteristics of amplitude and phase shift vary with transmissivity, the well-aquifer coupling relationship is divided into three sections, the coupling section, transition section and the non-coupling section. If the aquifer’s transmissivity is higher, well water level can respond to pore pressure promptly when pore pressure changes and In this case, radial amplitude ratio close to 1 and phase lag is 0 or less. Well water level can be fully coupled with pore pressure, So it is called coupled section. If the radial aquifer’s transmissivity is very small, water exchange is difficult with a large hydraulic gradient between the wells and aquifers. Phase shifts are more and radial flow amplitude ratios (A) are smaller in this section, and well water levels tidal effects are not obvious. Well water levels can not fully couple with aquifer pore pressure, and it is called non-coupled section; Transition section is between the coupled and non-coupled section. In transition section, radial flow amplitude ratio and phase lag varies with transmissivity significantly. Aquifer transmissivity before and after earthquakes is be estimated applying the derived formula describing tidal amplitude ratios and phase shift between well water levels and tide height, combined with coupling sections classification, and the reasons of transmissivity changes are discussed.Acting on the vertical heterogeneous aquifer, tidal forces cause vertical water exchange in the aquifer. In this thesis, corresponding model of tidal amplitude ratio and phase shift between well water levels and tidal height is established applying groundwater dynamics and the earth tide theory. The relationship between tidal amplitude ratio, phase shift and hydraulic characteristics, hydrodynamic parameters is studied. Tidal amplitude and phase shift produced by vertical flow in aquifer can explain that the tidal phase shift are positive. Based on the above research result, the formula expressing tidal amplitude ratios and phase shift between well water levels and tide height are further deduced under the condition of radial and vertical flow. A method that identify radial flow, vertical flow and mixed flow is proposed, according the relation amplitude ratios and phase shift of the two tidal components (M2 and Oi) and change characteristic of the two variables.These studies can further improve the cognition level of formation mechanism of tidal amplitude and phase shift.Earthquakes which bring about changes of water levels, can change the permeability of the aquifer, even aquifer poro-elastic parameters through multiple forms, and tidal amplitudes and phase shift would be affected. Static stress can drive the aquifer poro-elastic response, and then aquifer permeability changes; Seismic waves make the water exchange between the aquifer and the wells increases. Aquifer permeability can be improved through the mobilization of colloidal particles, non-wetting phase bubbles or liquid in aquifer pores (cracks). Amplitudes and phase shift step-like increase, and then return to background value gradually.The long-term trend of amplitudes and phase shift represent the information of aquifer deformation caused by tectonic stress. Transmissivity deceases when the fault zone is squeezed and the cracks would be closed. Water levels tidal amplitudes decrease and phase shift decline accordingly. Transmissivity increases when the aquifer cracks expand, then tidal amplitudes increase and phase shift go up. This thesis applies well water level amplitude ratio and phase shift theoretical research achievements to extract deformation information of each segment of Xiaojiang fault zone, and analyses their deformation characteristics. The reason that water levels in the central and south of Yunnan province continued to decline is interpreted. Crustal stress change in the region of Jiangyou well is discussed and the reason of water level drastically changed after Wenchuan earthquake is studied.