基于井水位潮汐响应探讨含水层的水力特征及其变化

Discussion on hydraulic characteristics and changes of aquifer based on tidal response of well water level

为了科学地认识应力荷载对地下水渗流运动规律的影响,揭示井孔-含水层系统与地震活动的响应关系,以华蓥山断裂带附近的大足井和北碚井为例,基于潮汐响应推求观测段含水层水力特征参数,探讨汶川地震对含水层水力特征及地下水流运动状态的影响,并对其变化机制进行解释。结果表明:大足井附近含水层中地下水运动以径向流为主,利用潮汐径向流模型计算出含水层水平渗透系数与以往抽水试验的结果基本一致;北碚井孔-含水层系统中地下水垂向流和径向流并存,引入越流含水层系统模型得到越流系数范围为1.5×10^(-8)~2.5×10^(-8)s^(-1);汶川地震后,北碚和大足井水位均发生了阶降变化且含水层中地下水垂向流运动增强,分析认为原因是该区域侏罗系砂岩含水层在较大范围内存在着统一的水力联系,且局部裂隙发育,倾角较大,地震导致含水层发生膨胀变形和裂隙疏通,在重力作用下地下水顺层或沿裂隙通道流动。研究结果有助于提高对含水层受力变形与渗流相互作用的认识,进一步深化井孔-含水层系统对地震活动响应机制的研究。

The traditional method of calculating an aquifer′s hydraulic characteristic parameters can not deduce the real-time continuous change of the hydraulic characteristic,which makes it difficult to analyze the change of aquifer permeability under external load.The response model of the aquifer′s head to the periodic high-frequency natural force provides a new idea for research.In recent years,it has been shown that the external stress loads may cause changes of the aquifer′s hydraulic characteristic parameters and the corresponding changes in groundwater flow movement.At present,the research mainly focuses on qualitative inference and explanation,and there were relatively few conclusions from quantitative calculation.Based on the systematic analysis and summary of the theoretical relationship between the tidal response of the well water level and the aquifer′s hydraulic characteristics,taking wells Dazu and Beibei in Huayingshan fault zone as examples,the specific storativity of the observation aquifer was deduced by the amplitude of tidal response,and the movement state of groundwater flow near the well hole was analyzed based by the phase shift of tidal analysis.Using the radial flow model and leaky aquifer system model,the permeability and the coefficient of leaky in the aquifer near the well bore were calculated.Taking the Wenchuan earthquake as an example,the changes in well water level and the aquifer′s hydraulic parameters during the earthquake were analyzed,the influence of seismic stress on the hydraulic characteristics and groundwater flow movement was discussed,and its change mechanism was proposed.The phase shift of the water level in well Dazu response to the M_(2) wave was negative,indicating that the groundwater movement in the aquifer near well Dazu is dominated by the radial flow.It was assumed that the specific storativity in the observation section of well Dazu is 1.55×10^(-6) m^(-1),and the horizontal permeability is 0.0419 m/d,which was consistent with the results of the previous pumping test(0.0534 m/d),indicating that it is feasible to calculate the hydraulic parameters of the aquifer by the tidal response of the well water level.Moreover,the locally developed fractures in the aquifer near well Beibei resulted in the coexistence of vertical and horizontal flows near the well.The specific storativity of the aquifer was estimated to be 1.41×10^(-6 )m^(-1) and the coefficient of leakage ranged from 1.5×10^(-8) to 2.5×10^(-8) s^(-1).After the Wenchuan earthquake,the water levels of wells Beibei and Dazu changed step by step,and the vertical flow of groundwater in the aquifer increased.Combined with the regional hydrogeological conditions,it was proposed that the reasons for the water level changes in wells Beibei and Dazu caused by the Wenchuan earthquake were as follows:The Jurassic sandstone aquifer in this region has a unified hydraulic connection over a large range,and local fractures with large dip angle are developed.The occurrence of earthquake led to expansion deformation and fracture dredging of aquifers,under the action of gravity,groundwater flowed along the fracture channel,resulting in the decline of water level in wells Beibei and Dazu,and the enhancement of vertical water exchange near the well bores in the runoff area.The research results are helpful to improve the understanding of the interaction between aquifer stress and deformation and seepage,and further deepen the research on the response mechanism of borehole aquifer systems to seismicity.