Analysis Of Barometric Effect On The Water Level In A Well Tipped In A Confined Aquifer

Barometric effect on confined aquifer is one of the most important groundwater micro-behaviors.Nowadays,there are two application areas for the study about the barometric effect. First, in the forecast of the earthquake, in order for more precise information about the groundwater fluctuation originated from the earthquake to be identified,other disturbances(including the barometric effect) which also cause the groundwater level changes should be estimated quantitatively. Secondly,after knowing the physical mechanisms about the barometric effect on the undergroundwater head variations, hydrologists started to modeland estimate aquifer parameters based on analytical solutions of barometric effect.In recent years, mathematical model about the effect of atmosphere pressure variation on head variation in an aquifer is improved by revising the well-wall boundary conditions, taking the water exchange between the aquifer and thewell-wall into account.Song Jinying(2012)studied the water level attenuation and time lag in response to the atmosphere pressure changes in the well caused by barometric effect with the wellbore storage. An analytical solution is given to quantify the relationship between water level and barometric pressure fluctuations. When the analytical solution was applied to explain the observed water level fluctuations in a well in Caofeidian, there existed a great difference between the analytical prediction and observations.In this thesis, a case study is conducted using the observed data from a borehole in Xitun Village,Yanqing Town.Two monitoring wells were installed with pipes of diameter of 127 mm. Two mini-divers were installed in the wells for the measurement of the underground water head and temperature in the shallow and deep confined aquifer, respectively. The monitoring frequency is once per hour, and the monitoring period is from 21/1/2010 to 26/12/2011.A baro-diver was used to measure the atmosphere pressure at the same time.Here only the deep confined aquifer was studied. In the selected time period, barometric pressure variation displays an obvious annual and daily periodicity, low in summer, high in winter, and with two peaks and with two valleys in a day. Fourier Spectrum Analysis was conducted to eliminate the random disturbing terms contained in the data, taking advantage of the different frequency band that the barometric effect itself owned. The results demonstrated that when the frequency equals 2π/12 and 2π/24 rad/h, the frequency spectrum reaches the maximum. Then moving average method was used to detrend the data with the moving interval equals 12 h and 24 h, respectively and obtain the periodic water level and atmosphere pressure. The detrended data with moving interval of 24 h is better than those of 12 h.Finally, data for two days with similar shapes of barometric pressure and water level were chosen and expanded into Fourier series to analyze the detrended water head and atmosphere pressure fluctuations.The corresponding amplitudes and phase shifts of the water level and atmosphere pressure fluctuations with different frequencies are obtained. According to the analytical solution solved by Song(2012), the amplitude ratio and phase shift of the main frequency component, the value range of the three important parameters(the aquifer’s barometric efficiency Be, storativity S and transmissivity T) are obtained.