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Table of Content
20 June 2001, Volume 7 Issue 2
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Article
Volcanic Petrotectonic Assemblages in Sanjiang Orogenic Belt,SW China and Implication for Tectonics
MO Xuan-xue, DENG Jin-fu, DONG Fang-liu, YU Xue-hui, WANG Yong, ZHOU Su, YANG Wei-guang
2001, 7(2): 121-138.
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Sanjiang Orogenic Belt is located geographically in the area of Jinshajiang, Lancangjiang and Nujiang(abbreviated from the“three rivers area”),and tectonically at the junction between the Himalaya-Tethyan tectonic domain an d the Pacific tectonic domain. It is one of the key areas to understand the Tethyan evolution,Indian-Eurasia collision and the uplift of Tibet Plateau and its eastern extension. Various volcanic rocks Proterozoic to Cenozoic age occur in Sanjiang Orogenic Belt The majority of volcanic rocks,however,formed during the Tethyan and post-Tethyan stages, i.e.,from early Carboniferous 1o the Cenozoic Volcanic petrotectonic assemblages as geological records and a lithoprobe play an important role in understanding tectonic evolution and corresponding deep processes of the Sanjiang area. Five types of volcanic petrotectonic assemblages in Sanjiang Orogenic Belt have been recognized as follows: Oceanic assemblages including M0RB/Pare-MORB(or MORB-LIKE) assemblage and OIB assemblage,island arc and continental marginal arc assemblage,collision-related assemblage,post-collisional assemblage and intracontinental assemblage Fig 1 shows a flame of their spatial and temporal distribution. Sanjiang MORB and para-MORB assemblages occur in four major suture zones,being closely associated with peridotite(or serpentinite),gabbro,diabase,radiolarian chert or deep water limestone,and sometimes accumulates in ophiolites. Geochemically,Sanjiang MORB are mainly E-type MORB,although they have the lowest K20 content and(La/Yb)N ratio among all types of volcanic rocks in Sanjiang Orogenic Belt. Para-MORB is mainly tholeiites and geochemically more enriched in K20 ,LREE and incompatible elements than E-MORB but less enriched than 0IB. Sanjiang OIB occurs so far only in Changling-Menglian belt and is associated with overlying limestone and sometimes underlying MORB/para-MORB They ale mainly alkaline olivine basalts and the most enriched type in Na20 +K2O ,LREE and incompatible elements among all types of oceanic volcanic rocks in Sanjiang Orogenic Belt. Island arc and continental marginal arc volcanic petrotectonic assemblage in Sanjiang orogenic belt is of high variety, although they ale mainly composed of basalts. basaltic andesites, andesites, dacites and rhyolites,belonging to calc-alkaline or island-arc tholeiitic series. Arc volcanic reeks in Sanjiang Orogenic Belt formed in various environments, such as oceanic island arc,continental island arc, continental marginal arc,fore-arc. main arc,back-arc and so on. Among them, it was particularly important to recognize tensional arcs in Sanjiang Orogenic Belt. A typical example of tensional arcs is Yidun arc formed in Late Triassic, which is characterized by an intra-arc rifting with a bimodal volcanic suit. A large Kuroko-type Pb-Zn-Ag ore field was developed within the intra-arc-rifting basin(see Fig. 7 in text) The southern end of Yidun Arc, however,became a compressive one associated with many porphyry-type copper deposits. It is also amazing to see a special type of volcanic rocks,which contain strong subduction zone components (SZC) but formed in post-collisional intra-continental environments. We therefore called it “ post-collisiona arc volcanic rocks”or “delayed arc volcanic rocks” Condie et al(1989),Aldrich et al(1986)reported similar cases in Kaapvaal of South Africa and in Rio Grande rift,respectively A possible explanation for the delay of arc type volcanism and for the formation of post-collisional arc volcanic rocks was presented (see Fig. 8 in text) It is apparently that geotherm in the source region during the subduction was too low to reach the solidus of source rock. The subducted or collapsed slabs, however, might react with surrounding mantle and become a new source rock for magma generation afterwards In post-co llisional stage sOme kind of geological events such as upwelling,convection,intra-continental convergence/subduction might cause generation of magmas,which contained undoubtedly subduction zone components(SZC). Other post-collisional volcanic assemblages in Sanjiang Orogenic Belt are those formed either in overlapping rifting basins within collision gone or in intra. continental rifting basins. The former is potentially valuable for prospecting of VMS-type base metal deposits. Volcanic assemblages developed in relatively stable blocks within Sanjiang Orogenic Belt are helpful to understand the subeontinental mantle as a lithoprobe They ale composed of either tholeiites or alkali basalts with/without acidic volcanic rocks, formed in the period time from the Carbonifero us to the Triassic. In addition,several aspects regarding to the application of petrotectonic assemblage approach to orogenic belts ale also discussed, for instance,reconstruction of tectonic configurati0n from petrotectonic slices ,comments on geochemical discrimination diagrams, and tectonic implication of coupled ophiolite zone and volcanic arc, of delayed arc volcanic rocks ,and so on.
Ore-Forming Material Sources for Xiangshan Uranium 0re-Field in Jiangxi Province: Evidence from Nd-Sr-Pb Isotopes
FAN Hong-hai, LING Hong-fei, WANG De-zi, SHEN Wei-zhou,LIU Chang-shi, JIANG Yao-hui
2001, 7(2): 139-145.
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The Xiangshan volcanic rock related uranium ore-field is the 1argest uranium ore-field in China The Zoujiashan uranium deposit(fluorite-hyromica-uraninite association)and the Shazhou uranium deposit ( alkali feldspar alteration-hemitite-uraninite association) are the two representatives of the ore-field This paper presents Nd,Sr and Pb isotope compositions of these two uranium deposits and their wall rocks-the Xiangshan volcanic rocks εNd(t)values(-6.7~ -8.3)and initial 87Sr/86Sr ratios(0.7145~0.7207)of fluorite formed by the ore-forming hydrothermal fluid are similar to those of the wall rocks(εNd(t)=-6.2~-9.4,initial 87Sr/86Sr Sr=0.7121~O.7192). In the εNd(t) vs .t diagram,the plot area of data points for fluorite of ore-forming stage is generally identical with that of the host rocks. Both of them fall within the evolution domain of Proterosoic basement in Xiangshan area Pyrites from the ores have a linear correlation between 206Pb/204Pb and 207Pb/204Pb, while the wall rocks plot at the 1owe/value end of this corelation 1ine. From this 1ine a reference age of 144 Ma can be obtained, which is consistent with the age of the volcanic rocks(135~ 140Ma) This Nd-Sr-Pb isotope evidence suggests that the ore forming materials were derived mainly from the Xiangshan volcanic rocks The volcanic rocks were derived mainly from the crust which was similar to the exposed basement. Therefore,the main ore-forming materials were derived ultimately from the crust
Characteristics of Proterozoic Metamorphic Basement in Hainan Island and Its Implications for Crustal Growth: Nd and Pb Isotope Constraints
XU De-ru, FAN Wei-ming, LIANG Xin-quan, TANG Hong-feng
2001, 7(2): 146-157.
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The metamorphic basement in the Hainan Island is mainly composed of Baoban and Shilu Groups. The Baoban Group is dominated by am phibolite-facies gneisses,metasedimentary rocks(quartz muscovie biotite schists),whereas the Shilu Group consists mainly of metamorphic volcano-sedimentary associations. There also occur voluminous granitic plutons of different ages from the Mid-Proterozoic to the Late Mesozoic indicating multi-stage modification and complicated accretion history of the crust Nd and Pb isotope data of 63 samples in different lithological associations in Hainan Island demonstrate that the major crustal accretion age ranged from the Early Proterozoic to the Late Proterozoic, with three peak stages of 2.0 Ga,1.7 Ga and 1 2Ga. The regional crust is characterized by low degree of maturation, and the mafic metamorphic basement was mainly derived from a chronically depleted mantle source The εNd(t) values for the Baoban Group vary from 2.2 to 2.8,with the TDM mainly about 2 00Ga and εNd(t) values for Proterozoie metabasic rocks vary from 4.7 to 8.1 The granitic plutons of different ages generally show higherεNd(t)values( -19.1~-35 for the Proterozoic granitic plutons and -6.3~ -O.1 for the Phanerozoic granitic plutons)and lower TDM values(1.68~2.02Ga for the Proterozoic granitic plutons and 0.99~ 2.34Ga for the Phaneromic granitic plutons)than those in southeast China, suggesting their origin from a younger lower/middle crust with involvement of depleted mantle-derived materials The Pb and Nd isotope data of the metamorphic basement in Hainan Island are~mewhat different from those in the Cathaysia Block and Yangtz Block, indicating that the metamorphic basement of these three blocks might be different in formation ages and evolution histories The metamorphic basement beneath the Hainan Island should not be regarded as the southern extended segment of the Cathaysia Block.
Fluorine Distribution in Plants of Xuzhou Epidemic Fluorosis Area and Its Environmental Significance
ZHU Fa-hua, ZHANG Jing-rong, YAO Su-ping
2001, 7(2): 158-163.
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Fluorine contents in grains of Xuzhou epidemic fluorosis area range between 0.70 and 0.85 mg/kg,and don’t exceed the national hygienic standard values. Fluorine contents in various main vegetables are from 1.88 to 12.25 mg/kg,among which green vegetable has the highest fluorine content of 12.25 mg/kg They are much higher than the national hygienic standard values Fluorine in vegetable leaves is higher than that in tubers and seeds of the same vegetables. Fluorine contents in different tree leaves are up to 24.56- 34.29 mg/kg The correlation analysis demonstrates that fluorine in plants mainly comes from water-soluble fluorine in soil Leaf vegetables(green vegetable and cabbage)and tree leaves have much higher water-soluble fluorine content than that in soil. It shows that fluorine has a trend to be accumulated in leaves and is transported in aqueous solution The main food,grains,is not harmful to human health in Xuzhou epidemic fluorosis area High fluorine content in main vegetables is one of the key factors to cause epidemic fluorosis
Trace Element Geochemistry and Tectonic Setting for Proterozoic Metavolcanics of the Mamianshan Group in the Central Part of Fujian Province
ZHOU Li-ya, WANG He-nian, SUN Cheng-yuan
2001, 7(2): 164-174.
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The Proterceoic metavolcanies of th Mamianshan Group in the central part of Fujian province are distributed along the Zhenghe--Dapu fault belt. Based on major element geochemical signatures, the volcanic rocks in this area are “bi-modal”. The chondrite-normalized REE pattern s of greenschists and leptynite are enriched in LREE with no or small Eu anomaly. Their MORB normalized spiderdiagrams and trace element tectonic discrimination diagrams have the WPB (within-plate basalt) features, showing continental rifting character Plagioclastic amphibolites have no obvious Eu negative anomaly. Their MORB normalized spiderdiagrams have a Rh-Ba enrichment and a Nb-Ta depletion These are typical of transitional calc-alkaline basalts and T-type MORB. Their primary mantle normalized and Nb-normalized spiderdiagrams indicate their derivation from a depleted mantle or contamination by the continental crusts. It is considered that this area was a failed rift There was an evolution from a continental rifting background to an initial oceanic basin environment, Soon after,convergence and subduction took place
Analysis of Geologic and Seismologic Conditions of the Taiwan Strait Tunnel
PENG Fu-nan1, YE Yin-can1, SHI Bin2, YANG Shu-feng3, QIAN Xiang-lin4, LI Pin5
2001, 7(2): 175-188.
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The Taiwan Strait Tunnel(T. S.T),as a great project,has been proposed for a long time and met many challenges from the beginning of the feasibility study that has attracted attentions among scientists in both sides of the Taiwan Strait The key question is whether this undersea tunnel could he constructed with present technologies under such complex geologic and seismologic conditions. Results of current research indicate that sea floor in the north of the Strait along Coast of Taoyuan county of Taiwan is covered by the late Tertiary to Quaternary sandy shale and shale with a thickness of more than 300m without large fault-belt,which could serve as a good bearing and impermeable layer Along the coast of Ping tan Island,Fujian in the west of the Strait,there is an area with vast distribution of granite。which is ideaI for the construction of the tunnel termina1. We therefore suggest that the shortest line with a distance of 126km from Pingtan Island ,Fujian to Hsinchu,Taiwan should he considered as the undersea tunnel1ine for its simple and stable geological condition.
Seismic Wave Simulation and Imaging of an Inversion Structure Model
DONG Liang-guo, ZHOU Zu-yi
2001, 7(2): 189-195.
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Tectonic inversion could usually result in the complication of structure styles in that either pre-existing faults are reactivated or new reverse faults formed during the inversion stage. Correct seismic interpretation of inversion structures is fundamental for the study of inversion tectonics. The validity of seismic interpretation in inversed basins is related to the origina1 available data and the appropriate processing methods. In this study, we used the pop-upstructure as an inversion structure model for the seismic wave simulation and imaging High-order difference method was applied to reduce the numerical dispersion and a new kind of absorbing boundary condition was adopted in the simulation Mid-point shot was used in the simulation with a total of 185 shots recorded Depth-migration imaging was then applied on the zero-offset profile to achieve a high quality seismic profile that is comparable to the original model The simulation and imaging results demonstrated that high-order difference method is an effective approach for the simulation of seismic wave propagation in complicated structures. With proper imaging methods,such as Fourier finite difference method, most inversed structures can be recognized from seismic profiles.
Determination of Bulk Strain in Rocks from Linear Passive Markers: An Optimum Approach
SHAN Ye-hua, LI San-zhong, XU Shu-mei
2001, 7(2): 196-202.
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The information on bulk strain is recorded by the final distribution in azimuth of nonisometric minerals or mineral aggregates in tectonites. With the increase of strain in rocks, they became more evidently aligned to the direction of the maximum principal strain axis. However,besides the bulk strain,the final distribution in azimuth of particles are dependent upon the distribution prior to the strain in azimuth of particles,difference in viscosity between the particles and the matrix,the deformation within the particles,strain path,temporal relationship between the growth of minerals and the bulk strain,etc. All these factors are likely to produce an extremely complicated or even indeterminable relationship between the final distribution in azimuth of particles and the bulk strain in rocks. Under some assumptions, the antho128 have established a simple quantitative model of orientations of passive strain markers which is deterministical1y associated with the bulk strain in rocks, We use the modern technique of anneal modeling to estimate the bulk strain from observed orientations of passive strain markers. It is robust and has a wide application, especially in the general cases where no or few strain markers are available to other methods for strain analysis. It can make a better estimation of bulk strain in rocks since each observation is equally weighted in calculation In order to validate our method,artificial orientation data of passive strain markers were generated by Monte Carlo sampling under sortie given bulk strains,which then were used to estimate the bulk strains in our method. As we expected, the accuracy of estimated bulk strains tends to increase with the number of observations and seems to increase or reach a relatively stable state when the number is no less than ca.160. This implies that at least 160 observation should be required for an accurate estimation of the bulk strain in rocks. When the number of observation is equally set, large bulk strains estimated seem less accurate than small ones, but the there is no apparent difference in proportional errors between both estimated bulk strains. Estimated axial ratios of axial ratios of strain ellipse in artificial examples have a proportional error of less than 5~ l0% when the number of observations exceeds160, and of less than 20% when less than 80. Furthermore,estimated concentration index(π)in von Mises distribution have a good linear relation to estimated ratios (R) of strain ellipse. Their empirical equation is k=0.402×R-0 087. As an example,deformed graptolite branches in the Ordovician shale from Ramsay and Huber(1984) are studied respectively by our method and maximum-minimum angle method to estimate the bulk strain in rock. There is no apparent difference in estimated axial ratios of strain el1ipse between both methods but a relatively great difference in estimated orientations of maximum principal strain axis.
Refined Evaluation of Reservoir Parameters in Development Stage of Oilfield
YANG Shao-chun1, REN Huai-qiang1, ZHOU Jian-lin2
2001, 7(2): 203-211.
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The refined evaluation of reservoir parameters in development stage of oilfield is an important content of remaining oil study,because accuracy of parameters for calculation directly influences the reservoir calculation, establishment of reservoir model,reservoir evaluation and prediction of remaining oil distribution in development stage,reservoir parameters are obtained from log treatment and interpretation In fact,interpretation evaluation is a process to obtain quantitative data from log curves. But traditional interpretation is mostly limited in individual relationship inside geological body. We recommend treatment and interpretation with multi-well log data and emphasize truthfulness of change relationship between geological data and log data. In this case,there are advantages of abundant data and rich information. Different type reservoirs have different geological characteristics and corresponding log respond. We propose that log interpretation model can be established by characterizing quantitatively its changes to geological characteristics of different reservoirs. Taking Shengtuo oilfield as an example, we studied refined evaluation on reservoir parameters in development stage of oilfield our study consists of four research contents,including pretreatment and standardization of log data, variation analysis of reservoir tetrad property parameters in different development periods,establishment of log interpretation model and calculation of reservoir parameters ,fuzzy evaluation of reservoir parameters. Based on a lot of studies ,we believe that interpretation according to rate can be improved with tetrad property analysis on reservoir parameters,establishment log models using production test data and neural method . Results suggest that use of interpretation results and fuzzy mathematics method could make more accurate characterization and the distribution pattern of reservoir can be revealed. The study results on Shengtuo oilfield have been applied in remaining oil development and obtained good effects.
Identificati0n and Utilization Water-Bearing Reservoir of Fine Structure in High Northern Songliao Basin
LIU Bo1;2, ZHOU Zai-1in2, LI Xin-feng2, WANG Liang-shu1, LIU Shao-wen1
2001, 7(2): 212-221.
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Identification and utilization of fine structure are required in high water-bearing phase of oilfield development using the high-solution seismic technology Fine structures include low amplitude structure and reservoir mini structure,but they differ in formation and require different methods in identification Low amplitude structure can not be determined in seismic profiles by ordinary data processing and interpretation. Utilizing 3-D high-resolution seismic technique,low amplitude structures,which consist of small size and small area structura1 deformation,low fluctuation and tilt,and so on,can be identified in seismic profile. While reservoir mini structure are formed in sedimentation and related to sedimentary environment,different compaction and paleotopography. The top and bottom of oil layers are not even with small local fluctuation and tilt. The late development phase of Daqing oilfield is characterized by high water-bearing reservoir. Based on the 3-D high-solution seismic technology, low amplitude structures are focused on. The identification methods of low amplitude structure include semi-automatic layer tracing,velocity scanning for calculating velocity,structure trend analysis and automatic identification of small fault Especially with the method s of slip correlation,dip angle scanning,relative dimension,the smal1 faults in formation can be identified. The effect factors of identification include seismic processing and interpretation. These methods were applied to Yushulin oilfield of Daqing to predict small fault as well as to determine the plan of exploration and development. According to close spacing well logging data,utilizing mini structure plot and trend surface remaining analysis,the fluctuation extent an d shape of bedding plane of reservoir can be described in detail. P1oting the structure contour is the key to identify reservoir mini structure. The effects of sedimentary micro-facies ,boundary of sedimentary unit,topography and tilt of well are taken into account. Combination of reservoir mini structure and sedimentary micro-facies can be used to determine the relatively abundant areas of remaining oil. At the later high water-bearing development stage of heterogeneous sand body oilfield,study of the identification method of fine structure and presentation effect factors of identification process are used to understand the heterogeneity of reservoir and establish the fine structure mod el,which can be used to predict the distribution of remaining oil and locate effective wells.
Depositional Characteristics of Sandstone and Conglomerate Reservoirs of Fan Delta and Relationship to Reservoirs’ Physical Properties - Taking the Fourth Member of Shahejie Formation,Luojia Oilfield,Zhanhua Depression for An Example
CAO Hui-1an1, HUA Ren-min1, JI You-liang2, HU Wen-xuan1, ZHANG Shan-wen3
2001, 7(2): 222-229.
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Based on the sedimentology and reservoir geology, combined with drilling,logging and analytical data,depositiona1 characteristics of sandstone and conglomerate bodies’ reservoirs for the fourth member of the Shahejie Formation in the Luojia oilfield are studied in detail. The results show unacceptability of some previous cognition. As to depositional environment of fan deposits,the conception of fan delta is better than the nearshore subaqueous fan and the subaqueous alluvia1 fan According to the characteristics of sandstone and conglomerate bodies,the fan-deltas can be subdivided into three subfacies and five microfacies. It is elaborated that physical properties of reservoirs are effected by lithological properties ,sedimentary microfacies and other factors. The porosity and permeability of reservoirs of sandstone and conglomerate bodies is improved by dissolution feldspar and carbonate rock-flagments. However,it would be decreased awfully with increase of fine matrix and carbonate contents. A style of parabola is shown in the relationship of the mean grain size with the porosity and permeability of the reservoirs which is greatly different from sandstone reservoirs. The oil-bearing properties of the packsand and the pebbled gritstone with parallel bedding and homogeneous bedding is better than glutinite with wavy bedding and graded bedding. Among all microfacies classified,the best reservoir is developed in subaqueous distributary channels.
Variation of Modern Reservoir Stress Field in Toutai Oilfield,Songliao Basin
ZHANG Shun1, LIN Chun-ming1, GU Lian-xing1, CHEN Li2, WU Chang-zhi1
2001, 7(2): 230-236.
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Reservoir stress is caused by both gravity and tectonic movements The well pattern for oil-and-gas development is considerably constrained by distribution of reservoir stress,particularly in the reservoirs with low permeability. In this paper,we wil1 prensent the results of an investigation on the distribution pa tern of modern reservoirs stress of the Toutai Oilfield based upon in situ measurements,core observations,laboratory work and numerical simulation The maximum principal stress(artificial fracture)in the horizontal plan of the Toutai Oilfield is directed N100°~ 120°E with a mean of Nll0°E,and the natural fracture between NO°~ 50°E. The southern part of the oilfield has smaller reservoir stresses in three directions than the northern part. The direction of structural faults is almost parallel to that of natural fractures. Therefore,a reasonable wel1 pattern cutting the water flood wel1 pattern should be designed to be 300m ×300m and at 45° to artificial and natural fractures. Waterflood pressure should be smaller than the minimum principal stress,and the technique of unstable intermittent waterflooding should be adopted.
Study Oil Exploitation of Groundwater at Headstream Area of Xiaofu River
WANG Hao-ran1, ZHU Guo-rong1, WANG Xiao-qin2
2001, 7(2): 237-244.
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Groundwater control and recharge to the downstream in Nanshentou-Yaoguang area is studied systematically by using numerical simulation and optimal control modeling Nanshentou-Yaoguang area is located at the headstream valley of Xiaofu River in Zibo city,Shandong province,which provides the groundwater resource to the whole valley. Shentou-Xihe fault is selected as the north boundary. The west boundary is Yuwangshan fau1t. The east and south boundaries are both water divide between Xiaofu River and Zi River. The groundwater of the area comes mainly from precipitation and partly from the supply of west mountainous area. There are 36 wells discharging from the Ordovician fracture-karst aquifer. According to the hydrogeo1ogical characteristics of its top aquifer,fracture-karst aquifer is therefore considered as objective aquifer The frequent tectonic movements in geological history conditions the aquifer heterogeneous and anistropic. Because of the difference in hydrogeological conditions, precipitation and discharge,the confined aquifer and the phrcatic aquifer appears alternatively. According to dynamic characteristics of groundwater of the area,the Ordovician fracturekarst aquifer system can he described as a two-dimensional numerical mode1. The wells,rainfall,spring,river recharge and boundary “supply” are all included. There are 9 hydrogeological parameter zones and 8 precipitation zones. The ground water flow is simulated successfully in this model. Then a forecast model is set up to meet the requirement of loca1 development. Thus, recharge to the downstream is calculated using the finite element method. With the operational research and system technology theory, an optimal control model is built up. The objective is to minimize pumping costs over entire planning area. The mod el is subjected to the planned well’s capacities and water demands The recharge to the downstream is studied more scientifically by programming the wells’ discharging. Compared to the non-programming recharge,the recharge to downstream increases by 6,900 m3 per day, while the cone of depression becomes smaller. It is beneficial to the ecological balance and the environment conservation.
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