The groundwater monitoring network in the reservoir area of cascade hydroelectric stations in the lower reaches of Jinshajiang(abbreviation: Jinshajiang groundwater observation network)is the second groundwater observation network in China constructed by enterprise exclusively for earthquake precursor monitoring and study in reservoir areas.In this paper,distribution of the observation network,observational technologic system,basic characteristics of observation well and aquifer,and basic result of trial observation are presented,and the monitoring capability of earthquake precursor are discussed with respect to the conditions of observation well and aquifer and characteristics of multi-monthly,monthly,daily behaviors of water level and temperature in well.

The temperature and its gradient is measured carefully in 6 wells of Jinshajiang groundwater observation network.The result shows that the influence depth of sun-radiation heat is 50～125m.The average water temperature gradient is different in deferent wells,the minimum value is 0.11℃/hm,the maximum is 2.81℃/hm,and the general is 1～2℃/hm.The temperature gradient diversity in deferent depth sections in one well is very obvious,with values being in decimal fraction of ℃/hm,and the larger ones being 4～5℃/hm.Lithology of stratum and completeness of rockmass,particularly the high-angle fault crush zone,play a major role in affecting the influence depth of sun-radiation heat and temperature gradient of each well.The micro-behavior characteristics of water temperature in well,e.g.co-seismic effect,tide effect and anomalies before an earthquake etc,depend largely on the temperature gradient of the depth section where the temperature sensor is fixed.

Violent blowouts occurred in Chagan Well when the M_S 8.7 Sumatra earthquake happened on Dec.26,2004,with the eruptive fountain up to 60m high.The result of investigation and analysis of Chagan well indicates that blowout occurred in the condition of special hydrogeology.The water of monitoring layer was rich of CO₂.When the aquifer of monitoring was disturbed by the earthquake wave,a large number of absorbed gases were released.On the other hand,free bubble was broken by the earthquake wave and the groundwater was gasified,leading to the rapid increase of the pore pressure.

Based on the analysis of fluid anomaly data and investigation of macro precursor of the M_S 8.0 Wenchuan earthquake,and having taken careful consideration of the difficulty and ability of predicting earthquake,we found that the failed prediction of the M_S 8.0 Wenchuan earthquake doesn't mean that earthquakes can't be predicted.There were,though not much in quantity,a certain amount of underground fluid anomalies and remarkable macro anomalies occurring before the earthquake.Though it is difficult to predict the earthquake basing on them,it is possible to be aware of the impending earthquake.To improve the ability of earthquake prediction,the paper proposes to innovate the present work and administrative systems,in which,earthquake monitoring,prediction and research are separate each other,and professional and local forces are separate each other,to change the present work state of computer-replacing-human brain and the work mode of the too early use of formality and standardization,and to attach greater importance to investigation and confirmation of precursory anomalies and to monitoring and studying macro anomalies.

This paper presents the results of analysis of observed data of water levels from 8 observation wells, water temperature from 4 wells and soil radon from 4 well sites obtained during the year of 2001. Based on the work, the normal behaviors of each observation item are established and their responsibility to earthquake event are tested. The results indicate that all the groundwater levels from 8 wells are able to reflect the solid tide. The sensitivity of the response of groundwater to volumetric strain of the earth crust in eight wells ranges from 10^-9 to 10^-10 volumetric strain/mm. Groundwater levels in most wells within the well network have barometric effects and are influenced to some extent by the rainfall. The water temperature in 4 wells is relatively stable, the daily change of which is ranging within several degrees a thousandth. The soil radon in 4 well sites fluctuates dramatically, and the annual change ranges from ten to several tens Bq/L. The anomalies of water levels in 6 wells, water temperature in one well, and soil radon in one well site were observed before the M_L3.6 earthquake occurred at Meijiahe, Zigui County on October 16, 2001. This may indicate that the Three Gorge observation well network has a relatively high capability to respond earthquake event.

On the basis of systematically checking up nearly ten years observation data of 11 water temperature observation wells in the Capital Circle, the behaviors of well water temperature are divided into several types, such as perennial tendency, annual, monthly and daily behaviors. The mechanisms and characteristics of these behaviors are then analyzed. The perennial tendency can be sub divided into smooth, ascending, descending, fluctuate and composite subtypes; the annual behaviors can be sub divided into smooth-stepwise, ascending, descending and fluctuate subtypes; the monthly behaviors can be sub divided into smooth, smooth-stepwise, smooth-fluctuate, descending-fluctuate, ascending fluctuate and composite subtypes; the daily behaviors are mostly fluctuate types, but 5 wells have tidal effects and most wells posses stepwise or pulsed changes. Up to now, the main factors that have been found to have influence on the behaviors of water temperature include the infiltration recharge of atmospheric water, lateral recharge of groundwater, groundwater exploration in adjacent area, and water turbulence in the observation wells, etc. In addition, the instability of observation instruments has great impact on water temperature behaviors. After the normal behaviors are recognized and all kinds of interference factors are eliminated, water temperatures are found to have good reflection on seismic event. Relatively distinct short imminent term anomalies have been found before some moderate-strong earthquakes, and remarkable co seismic anomalies can be recognized while distant strong earthquakes occur. Moreover, some pre seismic anomalies are also identified before the occurrence of a few distant strong earthquakes. Therefore, the further study on the behaviors of water temperature, and the elimination of all kinds of interference factors including stepwise or pulsed variation caused by the instability of observation instruments, will remarkably enhance the capacity and effectiveness of the reflection of water temperature behavior on seismic event. This will also help to improve the capability of short imminent term earthquake predication in our country.

The Three-Gorges well network is the first professional network in China established for the purpose of monitoring the groundwater precursors of reservoir-induced earthquake. It is composed of 8 observation wells. In addition to observe the change of water level, 4 wells are enabled to measure water temperature and soil Radon, 3 wells are enabled to measure rainfall, atmospheric pressure and temperature, and 1 well can be used to observe the water level of the reservoir. The observation items are totaled 26. The present paper introduces in detail the technical system of the observations used in the Three-Gorges well network, including the technical system of the whole network and each well, the technical index, installation and adjustment of every sort of instruments and equipments, as well as the corresponding supporting facilities such as the power supply, communication and lighting arrester. In addition, the running of the technical system of these observations and its preliminary results are also introduced briefly.

The Three-Gorges well network is the first well network in China established specially for the purpose of monitoring the groundwater precursors of reservoir-induced earthquake. The basic consideration of the establishment of this well network is based on the fact that the seismic event of certain magnitude induced by the storage of reservoir may cause the changes of stress and strain states of the aquifer, as well as the corresponding pore water pressures. The number and spacing of the wells are determined according to the magnitude of earthquake(M_S), the length of seismogenic fault(L)and the variation range of stress strain field(R). The selection of well locations is based on the groundwater behaviors sensitive to earthquakes. Such peculiar tectonic sites as the termination, inflection and junction of fault zones are selected for well location. The depth and structure of the wells are decided according to the requirement that the well may reveal well enclosed confined aquifer and minimize the influence of surface water and hydro atmospheric factors on the premise of limit funds. According to the afore mentioned considerations, 4 wells are drilled at the dam site and 4 wells at the head of the Three-Gorges Reservoir. The depths of the wells are between 100～ 200m, and all the wells expose well enclosed confined aquifer. Primary observations show that the arrangement of the Three-Gorges well network and the establishment of the observation wells are relatively successful. They form a good base for monitoring the precursor of reservoir-induced earthquake.