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   10195篇
  免费   4079篇
  国内免费   2262篇
测绘学   1402篇
大气科学   1795篇
地球物理   2564篇
地质学   6448篇
海洋学   1712篇
天文学   203篇
综合类   1007篇
自然地理   1405篇
  2025年   24篇
  2024年   295篇
  2023年   383篇
  2022年   637篇
  2021年   753篇
  2020年   643篇
  2019年   694篇
  2018年   688篇
  2017年   630篇
  2016年   671篇
  2015年   670篇
  2014年   652篇
  2013年   772篇
  2012年   741篇
  2011年   715篇
  2010年   747篇
  2009年   684篇
  2008年   613篇
  2007年   607篇
  2006年   524篇
  2005年   484篇
  2004年   394篇
  2003年   321篇
  2002年   410篇
  2001年   410篇
  2000年   363篇
  1999年   311篇
  1998年   254篇
  1997年   210篇
  1996年   187篇
  1995年   165篇
  1994年   161篇
  1993年   133篇
  1992年   118篇
  1991年   86篇
  1990年   68篇
  1989年   59篇
  1988年   48篇
  1987年   41篇
  1986年   24篇
  1985年   24篇
  1984年   19篇
  1983年   23篇
  1982年   12篇
  1981年   13篇
  1980年   8篇
  1979年   7篇
  1978年   11篇
  1958年   8篇
  1954年   6篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
41.
剑门地貌见于四川盆地北部的剑阁县剑门关一带,是一种特定的砾岩地貌,以发育砾岩悬崖绝壁、砾岩关隘、砾岩峡谷、砾岩山峦、砾岩峰丛等为特征,规模宏大,气势如虹。龙门山山前广泛分布,景观独特,与广东丹霞地貌显著不同。  相似文献   
42.
To improve the ecological environment in China, the Chinese government implemented a country-wide ecological protection and reforestation project (namely the “Grain for Green Project”) in 1999 to return cultivated land with slopes of 25° or more to perennial vegetation. Vegetation restoration reduces soil erosion mainly by changing the soil physical properties. Different vegetation restoration methods might produce different impacts on soil physical properties. In this study, two vegetation restoration methods (i.e., natural restoration and artificial restoration) were compared on abandoned farmland in the typically hilly and gullied areas of the Loess Plateau of Northwest China. In the natural restoration method, the farmland was abandoned to natural vegetation succession without irrigation, fertilization or other artificial disturbances. In the artificial restoration method, the farmland was planted with black locust (Robinia pseudoacacia L.) and watered and cultivated for the first two years. Three soil physical properties (i.e., soil moisture, bulk density and aggregation) were investigated under the two vegetation restoration methods. The results showed that the soil moisture and soil bulk density were higher under artificial restoration than under natural restoration within the first three years of vegetation restoration. By the fourth year, the soil moisture and soil bulk density were higher under natural restoration than under artificial restoration. For the stability of soil aggregates?>?0.25 mm, the soil aggregates in the 0-20 cm soil layer were more stable under artificial restoration than under natural restoration, while the results were the opposite for the 40-60 cm soil layer. Overall, the soil physical properties were continuously improved during the restoration of vegetation on abandoned farmland. In choosing between vegetation restoration methods, natural restoration is preferable to artificial restoration, but artificial intervention is needed during the first three years.  相似文献   
43.
松辽盆地是上叠于海西褶皱带基底之上的中新生代陆相沉积盆地。舍伯吐地区位于松辽盆地西南部,区内上第三系松散堆积层赋存有3层稳定的砂体。砂体中均发育有不同规模的区域性层间氧化带。经钻探揭露,榆树屯地段Ⅱ层砂体的层间氧化-还原过渡带比较典型,并有铀和钼的次生富集,这一发现为松辽盆地找矿工作的突破展现了良好的前景。  相似文献   
44.
The late Archean (~3.0–2.5 Ga) was a key period of continental growth globally, which is widely considered to reflect the onset of vigorous plate tectonic activity, although related continental growth modes remain contentious. Here we investigate a suite of late Neoarchean metavolcanic rocks from the southwest Qixia area of the Jiaobei terrane in the North China Craton. The rocks in this suite include amphibolites, clinopyroxene amphibolites, and hornblende plagioclase gneisses. We present zircon U-Pb isotopic data which indicate that the protoliths of these rocks formed during ~2549–2511 Ma.The (clinopyroxene) amphibolites correspond to meta-basaltic rocks, with some containing high modal content of titanite. These rocks show moderate to high FeOT (8.96–13.62 wt.%) and TiO2 (0.59–1.59 wt.%), flat to less fractionated REE patterns, and mildly negative Th, Nb, and Ta anomalies, resembling those of Fe-tholeiites. In addition, they display positive zircon ?Hf(t) values (+2.6 to +8.7), and are devoid of crustal contamination or fractional crystallization. Combined with the low Nb/Yb (mostly < 1.60) and (Hf/Sm)N (mostly < 0.95), low to moderate Th/Yb (0.08–0.54), and low V/Sc (5.53–9.19) ratios, these basaltic rocks are interpreted to have been derived from a relatively reduced and depleted mantle source that was mildly metasomatized by hydrous fluids. The hornblende plagioclase gneisses are meta-andesitic rocks, and occur interlayered with the basaltic rocks. They are transitional between tholeiitic and calc-alkaline rock series, and show fractionated REE patterns with evidently negative Th, Nb, and Ta anomalies. The depleted zircon ?Hf(t) values (+2.4 to +8.4) and quantitative chemical modeling suggest that the andesitic rocks were most likely generated by injection and mixing of juvenile felsic magmas with the tholeiitic basaltic magmas.In general, the chemical features and genesis of late Neoarchean meta-basaltic rocks in our study area resemble those of Mariana back-arc basin basalts. Combined with regional geological data, it is proposed that the Jiaobei terrane witnessed late Neoarchean crustal growth under a paired continental arc-back arc setting. On a regional context, we propose two distinct geodynamic mode of late Neoarchean continental growth across North China Craton (particularly the Eastern Block), i.e., (1) arc-continent accretion along northwestern part of the Eastern Block; and (2) paired continental arc-back arc system surrounding the ~3.8–2.7 Ga continental nuclei to the southeast.  相似文献   
45.
抚顺矿区城市下压煤面积6km^2,煤炭储量23901.5万t。通过对矿区地质特征、煤储层特征分析.认为区内煤层气含量高(9.81~24.53m^3/t),煤层中大空隙多,透气性好,渗透率较高(0.24~3.60mD)。煤层直接顶板为厚100m左右的致密油页岩,底板为泥岩、凝灰岩,具有良好的封闭条件。根据矿区多年测试资料,采用容积法得出区内煤层气资源量为30.53亿m^3。综合开发煤层气资源符合东北老矿区的实际,其将为复苏东北重工业基地提供有效的洁净能源。  相似文献   
46.
近2600年来黄河下游沉积量和上中游产沙量变化过程   总被引:2,自引:0,他引:2       下载免费PDF全文
主要基于华北平原上93个钻孔中沉积物详细观测和分析数据,结合182组14C测年和埋深数据、参考前人黄河下游河道历史变迁及其他相关研究成果,估算出2600年以来黄河下游在602BC~11A.D.,11~1034A.D.,1034~1128A.D.,1128~1855A.D.和1855~1997A.D.等5个历史时期的年平均沉积量分别是3.89×108t/a,2.24×108t/a,6.63×108t/a,6.78×108t/a和8.47×108t/a。通过建立黄河下游有无堤防和决溢频率与泥沙输移比的关系,计算出5个时期黄河上中游的平均年输沙量分别是6.2×108t/a,6.8×108t/a,8.3×108t/a,11.5×108t/a和15.3×108t/a。进而探讨了黄河输沙量变化的主要原因,以及历史上王景治河后出现的600年安流时期的原因。  相似文献   
47.
介绍该县堆积面滑坡和层面滑坡两种典型滑坡的发育特征,分析其形成条件和机理.在此基础上,利用分形分维理论,分析研究区水系特征对滑坡分布和发育特征的影响.  相似文献   
48.
东天山觉罗塔格造山带是塔里木板块和古亚洲洋碰撞对接的关键部位,为中亚造山带的重要组成部分,而其大地构造属性一直存在着争议。康古尔塔格断裂带作为觉罗塔格造山带的边界断裂,其构造特征的研究对区内大地构造演化有重要的启示作用。通过对康古尔塔格断裂带的地质特征、火山沉积建造及演化过程等方面的研究,发现康古尔塔格断裂带为韧性兼走滑特征深层次的剪切带;区内石炭系火山岩具有典型的沟-弧-盆体系火山-沉积建造特征;康古尔塔格断裂带到阿其克库都克断裂带之间的区域为准噶尔板块与塔里木板块之间的碰撞缝合带;早石炭-晚二叠世,康古尔大洋发生了颇具规模的南北双向俯冲,形成了康-阿缝合带,而康古尔塔格断裂带为康-阿缝合带的北部边界。  相似文献   
49.
柴北缘绿梁山地区辉长岩的锆石U-Pb年龄及意义   总被引:11,自引:0,他引:11  
柴达木盆地北缘绿梁山地区的辉长岩侵入到古生代滩间山群及超基性岩中,地球化学、微量元素、稀土元素显示出源幔特征.选自其中的锆石明显具岩浆型锆石特点,4个单颗粒锆石U-Pb同位素年龄均为谐和年龄,206Pb/238U表面年龄统计权重平均值为(496.3±6.2)Ma,代表了该岩体的结晶年龄,从而间接地否定了本区滩间山群的时代为晚奥陶世-志留纪的认识.辉长岩与滩间山群是柴北缘活动大陆边缘火山岛弧的组成部分.此类岩体同位素地质年龄的确定对柴北缘榴辉岩、滩间山群时代及大地构造性质的重新认识具有重要意义.  相似文献   
50.
汶川地震断裂带科学钻探项目一号孔(WFSD-1)钻遇了30多米的断层泥。针对断层泥钻探的主要问题,对断层泥钻探泥浆体系进行了研究与应用,形成了具有高密度、低失水、低渗透、润滑减阻特点的泥浆体系,再加上重泥浆体系对地应力的平衡作用,成功地解决了断层泥的钻进取心问题,为顺利完成钻孔施工任务奠定了坚实的基础。  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