首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   1404篇
  免费   253篇
  国内免费   360篇
测绘学   50篇
大气科学   219篇
地球物理   344篇
地质学   859篇
海洋学   170篇
天文学   59篇
综合类   174篇
自然地理   142篇
  2024年   7篇
  2023年   29篇
  2022年   60篇
  2021年   56篇
  2020年   50篇
  2019年   81篇
  2018年   78篇
  2017年   65篇
  2016年   74篇
  2015年   62篇
  2014年   68篇
  2013年   67篇
  2012年   68篇
  2011年   94篇
  2010年   77篇
  2009年   85篇
  2008年   71篇
  2007年   70篇
  2006年   61篇
  2005年   50篇
  2004年   43篇
  2003年   32篇
  2002年   41篇
  2001年   41篇
  2000年   61篇
  1999年   84篇
  1998年   62篇
  1997年   64篇
  1996年   50篇
  1995年   55篇
  1994年   46篇
  1993年   46篇
  1992年   18篇
  1991年   23篇
  1990年   21篇
  1989年   10篇
  1988年   16篇
  1987年   6篇
  1986年   3篇
  1985年   5篇
  1984年   3篇
  1983年   3篇
  1982年   3篇
  1981年   3篇
  1979年   1篇
  1978年   1篇
  1977年   1篇
  1965年   1篇
  1958年   1篇
排序方式: 共有2017条查询结果,搜索用时 15 毫秒
61.
The key components of S. triqueter root exudates involved 4-oxo-pentanoic acid, succinic acid, glutaric acid, phthalate acid, citric acid, vanillic acid, myristic acid, pentadecanoic acid, decanoic acid, 14-methyl-pentadecanoic acid, hexadecanoic acid, octadecanoic acid and oleic acid, and the content of the water-soluble organic acids (citric acid, succinic acid and glutaric acid) significantly increased in pyrene and lead co-contaminated rhizosphere soil. These three water-soluble organic acids including citric acid, succinic acid and glutaric acid were detected as the specific root exudates of S. triqueter under stress of pollutants for pyrene and lead, so they were chosen as the research objects, and they were added into the bioremediation systems of pyrene and lead co-contaminated wetland soils. Compared with the control, the treatments added the three organic acids always improved the quantity of the bioavailable fraction of pyrene and lead in wetland soils and greatly influenced other chemical states of pyrene and lead fractions in the test concentration range. Under the 50 g kg?1 of organic acids concentration, the amount of the bioavailable fraction of pyrene and lead increased 41.0 and 872.7 % by citric acid, respectively. The enhancement of bioavailability of pyrene and lead in the wetland soil by adding organic acids generally decreased in the following order: citric acid > succinic acid > glutaric acid. Enhancing effects of organic acids on the bioavailability improvement of pyrene and lead is remarkable.  相似文献   
62.
63.
Fracture-cavity reservoirs can be described in terms of storage space, filling materials and distribution, however, characterizing these variables remains a challenge. In this study, a method was developed to characterize complex reservoirs by integrating outcrop, core, well log, and seismic data from the Tahe oilfield. Outcrop analogs were used to identify and qualitatively analyze the genesis of different reservoir spaces. Cores, well logs, and seismic data were used to identify different types of reservoir space in the subsurface, to describe the spatial distribution, and to establish the distribution pattern of fracture-cavity reservoirs. Clastic sediments, collapse breccia, and chemical fillings were described from the cores and used to assess porosity and permeability. On the basis of the results, a three-dimensional geological model was constructed incorporating a structural model, a reservoir space model, and a property model. Caves and fractures were the main storage spaces and seepage passages. The cave model was built using multi-point geostatistical simulation, while the fracture model was established using deterministic modeling combined with manual interpretations and ant-tracking technology.  相似文献   
64.
The North Qilian orogenic belt is a typical plate suture zone, which is studded with numerous different sizes of fragments of micro-continental crust consisting of pre-Sinian metamorphic rocks and the North Qilian orogenic belt provides important study carrier for studying the pre-orogenic tectonic process of the North Qilian. The relatively complete volcanic-clastic-carbonate rocks formation of Zhulongguan Group, Changchengian System, Mesoproterozoic are distributed in the Kawa-Qiqing area, Western North Qilian orogenic belt. Geochemical characteristics of the Aoyougou Group basalts in the North Qilian indicated that the contents of major element were relatively stable, high K2O contents(4.27%~6.07%), TFe2O3(10.49%~13.01%),TiO2(1.96%~2.90%)and middle MgO(5.37%~6.71%,Mg# 48~51), CaO(2.57%~5.51%)content. Thus, the Aoyougou Group basalts belong to potassium alkaline basalt. The contents of Cr and Ni of the basalts decreased with the decreasing of Mg#, and there was a positive correlation between CaO and CaO/Al2O3, Furthermore, the Eu anomaly of samples was slightly negative, high contents of high field strength elements (HFSE, Nb, Ta, Ti), and the ratios of Gd/Yb(2.5~3.0), Zr/Y(8.6~10.2), Ta/Yb(0.96~1.23), Ti/Yb(5 074~6 021), Zr/Yb(81~97), Ce/Nb(1.87), Zr/Nb(6.41)indicated that the Aoyougou Group basalts had obvious OIB-type magmatic affinity. In combination with the regional geological background, the study results showed that Aoyougou Group alkaline basalt generated from deep enrichment mantle similar to the source of OIB, with fractional crystallization of olivine and monoclinic pyroxene and a certain degree of contamination of continental crust in the process of magmatism, formed in the background of continental rift, might response to the Proterozoic Columbia supercontinent rifting, which further provides important petrological evidences for the relationship between North China Craton rifting events and Columbia supercontinent rifting during Proterozoic.  相似文献   
65.
中更新世气候转型(MPT)是第四纪环境变化的一个重要时期,该时期气候模式、气候周期等均发生了明显变化,然而不同区域对中更新世气候转型的响应特征存在明显差异.本研究选取了长治盆地XZK4钻孔中更新世转型前后(1.8~0.1 Ma)共207个样品,通过古地磁定年,运用粒度端元分析和孢粉分析,阐述了长治盆地中更新世转型期的环境变化与湖泊演化特征.结果显示,1.8 Ma以来,孢粉组合中乔木花粉含量多高于60%,但个别时段草本植物花粉占优,显示区域植被多数阶段为森林,但存在明显冷期,并且1.1 Ma和0.55 Ma是研究区环境转变的重要转折点.中更新世转型期前(~1.1 Ma):沉积物粒度总体较细,黏土含量多高于25%,EM1组分(湖相沉积)多高于60%,EM3(河流相组分)在个别时段出现,总体显示该阶段沉积物组成以湖相沉积为主,个别阶段受到河流影响大;中更新世转型时期(1.1~0.55 Ma):沉积物粒度较上一阶段更细,黏土含量(平均为30%)、EM1占比和孢粉浓度均达到研究段最高,沉积物颜色以棕灰色为主,EM3组分占比降至最低,其中1.1~0.95 Ma喜冷的云杉属花粉出现,显示该时期研究区气候总体偏冷湿,湖泊面积较1.1 Ma之前扩大;中更新世转型后(~0.55 Ma):沉积物粒度明显变粗,达到钻孔最高值,沉积物颜色变黄,EM2(风成组分)和EM3占主导,草本植物含量增加,尤其是喜干的蒿属花粉增加明显,表明气候较之前变干,沉积物类型以风力沉积为主,湖泊消亡.综合对比显示,构造运动是影响长治古湖消长的主要驱动力,气候变干加剧了湖泊消亡.  相似文献   
66.
Hou  Jingming  Zhou  Nie  Chen  Guangzhao  Huang  Miansong  Bai  Guangbi 《Natural Hazards》2021,108(2):2335-2356
Natural Hazards - Urban flood inundation is worsening as the number of short-duration rainstorms increases, and it is difficult to accurately predict urban flood inundation over a long lead time;...  相似文献   
67.
Geotechnical and Geological Engineering - The stability monitoring of surrounding rock in layered roadway is an important method to ensure the safety of deep mining coal mine. In view of the actual...  相似文献   
68.
岩土参数的正态分布是岩土工程可靠分析的基本假定之一。目前,岩土工程参数的正态性检验以KS (Kolmogorov-Smirnov)检验为主,然而诸多研究表明,对于小样本数据,KS检验的正态性检验结果并不可靠。本文以废黄河泛滥沉积相粉土中的孔压静力触探(CPTU)锥尖阻力qt数据为研究对象,首先采用ANOVA (方差分析)检验,从CPTU测试资料中进行总体的样本筛选,然后应用SW (Shapiro-Wilk)检验、KS检验、LF (Lilliefors)检验和AD (Anderson-Darling)检验等4种正态性检验方法,在不同样本容量条件下,对测试数据进行正态性检验评价。当测试数据的样本容量充足时,分析结果表明:SW检验结果最为严格,KS检验结果最不保守,而LF检验和AD检验的严格性介于两者之间;对于常规岩土工程设计,采用KS检验结果即可满足稳定性分析要求,对于复杂岩土工程设计,采用SW检验对设计参数进行正态性检验,可降低设计的不确定性。  相似文献   
69.
冀东地区位于华北板块北缘燕山造山带东部,矿产资源丰富,是中国金矿的主要产地之一,尤其是热液型金矿。峪耳崖金矿床是冀东众多热液型金矿床的典型代表,金矿主体赋存于侏罗纪峪耳崖花岗岩体内,少部分赋存于侵入花岗岩体的侏罗纪闪长岩脉中。闪长岩脉元素地球化学分析显示:w(SiO2)为51.99%~61.88%、Mg#值为0.37~0.64,w(Na2O+K2O)为5.07%~8.89%,A/NK为1.37~2.26,A/CNK为0.73~0.96,属准铝质高钾钙碱性系列;富集轻稀土元素和大离子亲石元素Rb、Ba、K、Sr,亏损重稀土元素和高场强元素Nb、Ta、Ti。锆石原位Hf同位素测试表明,εHft)为负值,介于-16.5~-6.5之间。综合研究认为:峪耳崖金矿区侏罗纪闪长岩浆源于俯冲流体交代的富集岩石圈地幔,岩浆上侵过程中受到华北板块古老下地壳物质的有限混染;成矿期闪长岩浆为金成矿提供了部分物质来源;峪耳崖金矿区闪长岩脉形成于古太平洋板块向欧亚大陆俯冲的动力学背景。  相似文献   
70.
Hou  Michael Z.  Li  Mengting  Gou  Yang  Feng  Wentao 《Acta Geotechnica》2021,16(4):985-1000

Hydraulic fracturing is an essential technology for the development of unconventional resources such as tight gas. The evaluation of the fracture performance and productivity is important for the design of fracturing operations. However, the traditional dimensionless fracture conductivity is too simple to be applied in real fracturing operations. In this work, we proposed a new model of dimensionless fracture conductivity (FCD), which considers the irregular fracture geometry, proppant position and concentration. It was based on the numerical study of the multistage hydraulic fracturing and production in a tight gas horizontal well of the North German Basin. A self-developed full 3D hydraulic fracturing model, FLAC3Dplus, was combined with a sensitive/reliability analysis and robust design optimization tool optiSLang and reservoir simulator TMVOCMP to achieve an automatic history matching as well as simulation of the gas production. With this tool chain, the four fracturing stages were history matched. The simulation results show that all four fractures have different geometry and proppant distribution, which is mainly due to different stress states and injection schedule. The position and concentration of the proppant play important roles for the later production, which is not considered in the traditional dimensionless fracture conductivity FCD. In comparison, the newly proposed formulation of FCD could predict the productivity more accurately and is better for the posttreatment evaluation.

  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号