流量异常的统计特征及其映震效能的综合评价

本文通过对国内外２９个地下水流量震例资料的整理和总结，分析了震前流量动态的时、空分布和形态特征，客观评价了流量动态的映震效能及对应力应变的反映能力。文中还对流量动态映震敏感的原因进行了分析讨论。并分别从岩土力学和地下水动力学的角度探讨了流量异常形成的机制。     

地下水诱发的浅层前兆异常及其机理与有关的地震预报问题(1)

依照“源、场、外”相结合的地震科学研究思路,提出地下水诱发浅层前兆异常机理。认为降水等因素造成的地下水动态变化,可产生一种作用于地壳岩石的附加流体力;地震前,通过多种流体力作用,引起已经积累较高应力的地壳应力－－应变场调整变化,并使岩石强度改变,从而促进与诱发浅层地壳的构造变动;其结果是派生或伴生出地形变、地应力、水化学及地电阻率等多种浅层前兆异常。在降水、地下水的多种周期变化成分中,只有那些能与地壳应力－应变过程产生力学耦合的周期变化,才能调制与诱发出异常,这一部分变化可做为广义的地震前兆。孔隙压力、动水压力以及化学腐蚀等原理,是该异常机理的理论基础。列举出六方面的事实做为该机理直接的或间接的证据。     

地震地下水动态研究中的若干问题

综合分析了近年来国内外地震地下水研究中的一些热点问题, 包括地下水动态影响因素及其相关的数据处理方法、水位同震响应及机理解释、水温同震响应及其机理、地下水反映的断层活动低频信息几个方面, 探讨了地震地下水位变动与应力应变的关系, 展望了地震地下水研究的发展方向.     

饱和黄土动态液化和静态液化机理的差异性研究

饱和黄土在不同外荷载作用下其液化机理具有显著差异.为研究饱和黄土动态液化和静态液化机理的差异性,基于室内动三轴试验和静三轴试验,研究岷县永光饱和黄土动态液化后的动应力与轴向动应变关系、动孔隙水压力比与轴向动应变关系,分析其静态液化后的偏应力与轴向应变关系、孔隙水压力比与轴向应变关系,并结合液化前、后的SEM试验结果,研...     

地下水兔出气动态特征形成机制的实验研究

通过研究管径和结构不同的井孔－含水气层物理模拟以及地下水、气补给量与逸出气动态的关系实验,得出地下水逸出气动态的最基本特征是突发、阵发,其形成机制的于逸出气运移、排放过程中存在积累－释放的往复过程。在地震等地质事件孕育、发生过程中,地壳应力、应变积累和释放可能引起的地下含水、气层的气体储存和多的场所空间及通道的变形、贯通或阻塞,地下水 流速、流量的增减及气体发生、补给量等的变化,都能在逸出气动态中     

地下水微动态形成过程的水动力学模拟试验研究

地下水微动态形成过程中的水动力学机制是很重要的。笔者利用模拟试验装置,在三种不同的渗透介质与二种不同温度的水的条件下,进行了井孔水位或流量对含水层内各种孔隙压力变化的响应方面的一系列试验研究。本文叙述了这一试验的装置、方法与某些结果。     

氡前兆的气载机理

关于水氡前兆机理的研究,王吉易等曾做了全面详细的评述。其主要的假说是基于应力—应变学说建立起来的破裂扩散、破裂混合以及超声振动等机理;应力应变机制还处于讨论研究阶段,水氡的前兆机理不是唯一的。在应力应变作用下引起的水氡前兆,一般都认为是以地下水做为氡气的载体的。本文试图提出另一种可能:即在应力应变作用下,高压地下水气体上涌,并做为氡气的载体,引起水氡和土氡的前兆异常;而氡气异常量的大小则由地下气流强度所控制。地下气体做为水氡载体的现象,是存在的。例如新疆乌苏一口温泉热水井,水温     

地下水动态映震机制的试验与观测研究

本文根据一系列室内外试验与观测研究结果,论述了震前地下水位动态异常信息形成的四个基本环节:①地壳应力活动与含水层应力状态的变化,②含水层变形破坏与孔隙压力的变化;③孔压变化信息在井—含水层系统中的传递;④孔压变化信息在井孔地下水动态中的表现。在此基础上,提出了地下水动态映震特性的若干观点。     

地下水逸出气动态特征形成机制的实验研究

通过研究管径和结构不同的井孔-含水气层物理模拟实验以及地下水、气补给量与逸出气动态的关系实验,得出地下水逸出气动态的最基本特征是突发、阵发,其形成机制在于逸出气运移、排放过程中存在积累-释放的往复过程。在地震等地质事件孕育、发生过程中,地壳应力、应变积累和释放可能引起的地下含水、气层的气体储存和迁移的场所空间及通道的变形、贯通或阻塞,地下水流速、流量的增减及气体发生、补给量等的变化,都能在逸出气动态中以爆发式地显示出来。由气体的性质致使逸出气在地震前兆观测中具有更灵敏、更显著的异常显示能力。     

应力加载作用引起地下水微温度场变化的研究综述

在地震地下流体动态观测中,水温对地震活动的响应非常敏感,井水温度微动态观测越来越受到有关学者的关注。本文收集了相关资料,以引起水温变化的水动力学机制为基础,从应力加载作用下渗流场的变化、渗流场变化引起地下水温度变化以及裂隙介质热量运移三个方面分别阐述了前人研究成果,旨在理清“含水层受力变形→井-含水层系统内水流变化→井水温度变化”的理论模式,探讨应力加载作用引起地下水微温度场变化的理论基础和技术途径。对这一领域研究成果的综合分析将有助于我们深入地研究地震孕育过程中的前兆成因机理。     

Monitoring of drawdown pattern during pumping in an unconfined physical aquifer model

In this study, we attempted to analyse a drawdown pattern around a pumping well in an unconfined sandy gravelly aquifer constructed in a laboratory tank by means of both experimental and numerical modelling of groundwater flow. The physical model consisted of recharge, aquifer and discharge zones. Permeability and specific yield of the aquifer material were determined by Dupuit approximation under steady‐state flow and stepwise gravitational drainage of groundwater, respectively. The drawdown of water table in pumping and neighbouring observation wells was monitored to investigate the effect of no‐flow boundary on the drawdown pattern during pumping for three different boundary conditions: (i) no recharge and no discharge with four no‐flow boundaries (Case 1); (ii) no recharge and reservoir with three no‐flow boundaries (Case 2); (iii) recharge and discharge with two no‐flow boundaries (Case 3). Based on the aquifer parameters, numerical modelling was also performed to compare the simulated drawdown with that observed. Results showed that a large difference existed between the simulated drawdown and that observed in wells for all cases. The reason for the difference could be explained by the formation of a curvilinear type water table between wells rather than a linear one due to a delayed response of water table in the capillary fringe. This phenomenon was also investigated from a mass balance study on the pumping volume. The curvilinear type of water table was further evidenced by measurement of water contents at several positions in the aquifer between wells using time domain reflectometry (TDR). This indicates that the existing groundwater flow model applicable to an unconfined aquifer lacks the capacity to describe a slow response of water table in the aquifer and care should be taken in the interpretation of water table formation in the aquifer during pumping. Copyright © 2001 John Wiley & Sons, Ltd.     

Predicting average annual groundwater levels from climatic variables: an empirical model

On the basis of one-dimensional theoretical water flow model, we demonstrate that the groundwater level variation follows a pattern similar to recharge fluctuation, with a time delay that depends on the characteristics of aquifer, recharge pattern as well as the distance between the recharge and observation locations. On the basis of a water budget model and the groundwater flow model, we propose an empirical model that links climatic variables to groundwater level. The empirical model is tested using a partial data set from historical records of water levels from more than 80 wells in a monitoring network for the carbonate rock aquifer, southern Manitoba, Canada. The testing results show that the predicted groundwater levels are very close to the observed ones in most cases. The overall average correlation coefficient between the predicted and observed water levels is 0.92. This proposed empirical statistical model could be used to predict variations in groundwater level in response to different climate scenarios in a climate change impact assessment.     

Innovative Contaminant Mass Flux Monitoring in an Aquifer Subject to Tidal Effects

Exposure from groundwater contamination to aquatic receptors residing in receiving surface water is dependent upon the rate of contaminated groundwater discharge. Characterization of groundwater fluxes is challenging, especially in coastal environments where tidal fluctuations result in transient groundwater flows towards these receptors. This can also be further complicated by the high spatial heterogeneity of subsurface deposits enhanced by anthropogenic influences such as the mixing of natural sediments and backfill materials, the presence of subsurface built structures such as sheet pile walls or even occurrence of other sources of contaminant discharge. In this study, the finite volume point dilution method (FVPDM) was successfully used to characterize highly transient groundwater flows and contaminant mass fluxes within a coastal groundwater flow system influenced by marked tides. FVPDM tests were undertaken continuously for more than 48 h at six groundwater monitoring wells, in order to evaluate groundwater flow dynamics during several tide cycles. Contaminant concentrations were measured simultaneously which allowed calculating contaminant mass fluxes. The study highlighted the importance of the aquifer heterogeneity, with groundwater fluxes ranging from 10⁻⁷ to 10⁻³ m/s. Groundwater flux monitoring enabled a significant refinement of the conceptual site model, including the fact that inversion of groundwater fluxes was not observed at high tide. Results indicated that contaminant mass fluxes were particularly higher at a specific monitoring well, by more than three orders of magnitude, than at other wells of the investigated aquifer. This study provided crucial information for optimizing further field investigations and risk mitigation measures.     

井水温度微动态形成的水动力学机制研究

井水温度微动态观测越来越受到有关学者的关注，已成为我国地震地下流体动态观测的主测项之一。观测结果表明，无论是水温的正常动态还是震前的异常动态的形成，用传统的热传导或热对流机制难以给出合理解释。因此笔者根据观测到的事实、异常特征与同震效应等提出了水动力学机制，即含水层变形→含水层内孔隙压力变化→井－含水层系统内水流变化→井水温度的变化。     

Hydrodynamic characterization of a Sahelian coastal aquifer using the ocean tide effect (Dridrate Aquifer,Morocco)

Abstract

In wells tapping coastal aquifers, piezometric fluctuations can be observed in response to the ocean tide. Simultaneous recordings of the ocean tide and of the piezometric variations may provide a basis for characterizing the hydrodynamics of the aquifer. This approach was attempted to characterize the Dridrate aquifer, located on the Atlantic coast of Morocco. This aquifer accounts for most of the regional drinking water resources. However, its hydrodynamic characteristics are very poorly known. The study compares observed and simulated piezometric fluctuations, under various assumptions (confined, semi-confined aquifer). The model, which best explains the hydrodynamic behaviour of this aquifer is a semi-confined and strongly heterogeneous aquifer model (calculated hydraulic diffusivity values vary over several orders of magnitude). This result is new and rather surprising, since to date this aquifer was considered confined in view of its geological setting. Consequently, new questions are raised regarding the protection and management of the groundwater resources of this aquifer.     

Vertical Wellbore Flow Monitoring for Assessing Spatial and Temporal Flow Relationships with a Dynamic River Boundary

A useful tool for identifying the temporal and spatial ambient wellbore flow relationships near a dynamic river boundary is to monitor ambient vertical wellbore flow with an electromagnetic borehole flowmeter. This is important because the presence of the wellbore can result in significant mixing or exchange of groundwater vertically across the aquifer. Mixing or exchanging groundwater within the well-screen section can have significant impacts on the distribution of contaminants within the aquifer and adverse effects on the representativeness of groundwater samples collected from the monitoring well. Ambient monitoring data, collected from long screened wells at Hanford’s 300-Area Integrated Field Research Challenge site, located approximately 260 m from the Columbia River, demonstrate that vertical wellbore flow exhibits both a positive and inverse temporal relationship with periodic river-stage fluctuations that can change over short distances between wells. The spatial distribution of these vertical flows across the well field indicates two general regions of ambient wellbore flow behavior. The western region of the site is characterized by vertical flows that are positively related to river-stage fluctuations. In contrast, the eastern region of the site exhibits vertical flows that are inversely related to river-stage fluctuations. The cause of this opposite relationship is not completely understood; however, the positive relationships appear to be associated with high-energy Hanford formation flood deposits. These flood deposits have a well-defined northwest-southeast trend and are believed to coincide with a local paleochannel. The inverse relationships are attributed to an erosional, subsurface high in the Hanford/Ringold Formation contact between the site and the Columbia River. Under these complex hydrogeologic and hydrodynamic conditions, the behavior of ambient vertical wellbore flow in monitoring wells near a dynamic river boundary can have important implications for collecting groundwater-quality samples, for contributing to contaminant distribution within an aquifer system, and for implementing effective remediation strategies.     

识别地下水超采区井水位异常性质的理论与方法

长期过量开采地下水,使地下水位持续下降、水质发生变化,动水位观测井断流;地面沉降造成井管上窜,观测管路系统被损坏等,这些现象对地震地下流体观测地震前兆异常的正确判断带来很大困难。应用水文地质理论与方法,分析含水层的水均衡状态、应力-应变状态及其与水位动态的关系,初步探讨了超采区井水位异常性质的理论与方法。结果表明,根据井孔所在区水位下降漏斗的扩散特征,结合以上所提到的理论和方法,依据资料多年变化特征,可以较准确地判断异常的性质。研究结果有助于区分单一集中抽水与长期地下水超采对水位观测的影响,有助于正确识别超采区水位前兆异常,有助于地震分析预报水平的提高     

Hydrologic trade-offs in conjunctive use management

An aquifer, in a stream/aquifer system, acts as a storage reservoir for groundwater. Groundwater pumping creates stream depletion that recharges the aquifer. As wells in the aquifer are moved away from the stream, the aquifer acts to filter out annual fluctuations in pumping; with distance the stream depletion tends to become equal to the total pumping averaged as an annual rate, with only a small fluctuation. This is true for both a single well and an ensemble of wells. A typical growing season in much of the western United States is 3 to 4 months. An ensemble of irrigation wells spread more or less uniformly across an aquifer several miles wide, pumping during the growing season, will deplete the stream by approximately one-third of the total amount of water pumped during the growing season. The remaining two-thirds of stream depletion occurs outside the growing season. Furthermore, it takes more than a decade of pumping for an ensemble of wells to reach a steady-state condition in which the impact on the stream is the same in succeeding years. After a decade or more of pumping, the depletion is nearly constant through the year, with only a small seasonal fluctuation: ±10%. Conversely, stream depletion following shutting down the pumping from an ensemble of wells takes more than a decade to fully recover from the prior pumping. Effectively managing a conjunctive groundwater and surface water system requires integrating the entire system into a single management institution with a long-term outlook.     

Modification of the SWAT model to simulate regional groundwater flow using a multicell aquifer

The soil and water assessment tool (SWAT) has been widely used and thoroughly tested in many places in the world. The application of the SWAT model has pointed out that 2 of the major weaknesses of SWAT are related to the nonspatial reference of the hydrologic response unit concept and to the simplified groundwater concept, which contribute to its low performance in baseflow simulation and its inability to simulate regional groundwater flow. This study modified the groundwater module of SWAT to overcome the above limitations. The modified groundwater module has 2 aquifers. The local aquifer, which is the shallow aquifer in the original SWAT, represents a local groundwater flow system. The regional aquifer, which replaces the deep aquifer of the original SWAT, represents intermediate and regional groundwater flow systems. Groundwater recharge is partitioned into local and regional aquifer recharges. The regional aquifer is represented by a multicell aquifer (MCA) model. The regional aquifer is discretized into cells using the Thiessen polygon method, where centres of the cells are locations of groundwater observation wells. Groundwater flow between cells is modelled using Darcy's law. Return flow from cell to stream is conceptualized using a non‐linear storage–discharge relationship. The SWAT model with the modified aquifer module, the so‐called SWAT‐MCA, was tested in 2 basins (Wipperau and Neetze) with porous aquifers in a lowland area in Lower Saxony, Germany. Results from the Wipperau basin show that the SWAT‐MCA model is able (a) to simulate baseflow in a lowland area (where baseflow is a dominant source of streamflow) better than the original model and (b) to simulate regional groundwater flow, shown by the simulated groundwater levels in cells, quite well.     

Spatial and Temporal Changes of Groundwater Level Induced by Thrust Faulting

Changes of groundwater level, ranging from a fall of 11.10 m to a rise of 7.42 m, induced by thrust faulting during the 1999 M_w 7.6, Chi-Chi earthquake have been recorded in 276 monitoring wells in Taiwan. Most coseismic falls appeared near the seismogenic fault as well as other active faults, while coseismic rises prevailed away from the fault. Coseismic groundwater level rises and falls correlated fairly well with hypocentral distance in the vicinity of the thrust fault. We found a major difference of coseismic changes in wells of different depths at most multiple-well stations. The recovery process of coseismic groundwater level changes is associated with the confining condition of the aquifer. Cross-formational flow is likely to play an important role in groundwater level changes after the earthquake. In the hanging wall of the thrust fault, an abnormal decline of groundwater level was observed immediately before the earthquake. The underlying mechanism of the unique preseismic change warrants further investigation.