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   8304篇
  免费   2239篇
  国内免费   1672篇
测绘学   203篇
大气科学   351篇
地球物理   3189篇
地质学   5964篇
海洋学   1137篇
天文学   24篇
综合类   502篇
自然地理   845篇
  2024年   7篇
  2023年   108篇
  2022年   232篇
  2021年   299篇
  2020年   352篇
  2019年   326篇
  2018年   343篇
  2017年   338篇
  2016年   375篇
  2015年   365篇
  2014年   492篇
  2013年   525篇
  2012年   498篇
  2011年   515篇
  2010年   455篇
  2009年   583篇
  2008年   586篇
  2007年   614篇
  2006年   547篇
  2005年   508篇
  2004年   492篇
  2003年   468篇
  2002年   400篇
  2001年   331篇
  2000年   328篇
  1999年   283篇
  1998年   270篇
  1997年   241篇
  1996年   234篇
  1995年   233篇
  1994年   185篇
  1993年   161篇
  1992年   123篇
  1991年   100篇
  1990年   78篇
  1989年   75篇
  1988年   46篇
  1987年   33篇
  1986年   20篇
  1985年   9篇
  1984年   13篇
  1983年   5篇
  1981年   2篇
  1980年   3篇
  1979年   5篇
  1978年   2篇
  1954年   7篇
排序方式: 共有10000条查询结果,搜索用时 18 毫秒
1.
唐爽  张晋瑞  姜灵  魏春景  初航 《地质学报》2023,97(3):705-724
位于内蒙古中西部地区的宝音图群,主要由云母片岩、斜长角闪岩、石英岩和大理岩等组成。其中云母片岩中发育典型的巴罗式递增变质带,目前对于该递增变质带的研究非常薄弱。本文对宝音图群的云母片岩开展了系统的岩相学、矿物化学、相平衡模拟以及锆石U-Pb年代学的研究。岩相学研究结果显示,三个云母片岩样品具有不同的峰期矿物组合,分别为石榴子石+白云母+黑云母+斜长石+石英,十字石+白云母+黑云母+斜长石+钛铁矿+石英,石榴子石+蓝晶石+白云母+黑云母+斜长石+钛铁矿+石英。此外,样品LS93和LS07存在晚期红柱石的叠加。相平衡模拟研究限定三个云母片岩的峰期温度压力分别为:577℃、0.73 GPa, 620℃、0.78 GPa和670℃、0.8 GPa,具有递增型特征,其峰期温度压力指示的地热梯度为18~20℃/km,为典型的中压相系;此外,三个样品的P-T轨迹均显示早期升温升压直至峰期阶段,随后降温降压的顺时针型,反映地壳加厚和后期抬升的演化过程。云母片岩中的锆石发育典型的变质边结构,变质边的Th/U=0.004~0.02,锆石U-Pb年代学结果显示该变质年龄为398±6 Ma,代表峰期或近峰期阶...  相似文献   
2.
浅海和俯冲海沟等海域,不仅是矿产和油气资源主潜力区,也是构造地震频发区,其浅表热流和深部温度信息对于了解板块俯冲和岩浆活动等过程至关重要.这些区域浅层地温场和热流场受到底水温度波动(BTV)强烈扰动,其背景热流需由长期观测来获取.在全面分析了国内外海底热流长期观测技术特点后,我们提出了系缆式海底热流长期观测方案,2013年起陆续开展了部分核心技术的预研究及一系列海底、湖底及浅孔试验.结果表明:(1)自主研制的长周期低功耗微型测温单元,在2~36℃的环境下可连续观测1年;系缆式投放与回收方案即使在地形陡峭、1.5 kn流速及无动力定位等条件下仍然可行.(2)南海北部BTV总体随水深变浅而增强,在浅水区对浅层地温场扰动不可忽略.例如,在水深2600~3200 m和850~1200 m海域分别为0.025~0.053℃(17天内)、0.182~0.417℃(2天内),而台西南盆地北坡(水深763 m)夏季的海底热流由浅表的0.69 W·m-2转变为0.83 m以深的-0.25~-0.05 W·m-2.(3)兴伊措和湖光岩玛珥湖BTV向深部传导过程中其幅度逐渐减弱、相位滞后,进而导致热流方向与强度随季节发生变化.而康定中谷浅层(7 m内)地温在不同深度处同步波动,且冬高(35~36℃)夏低(28~32℃).推测为夏季大量降雨所致;其热流浅部低(0.504 W·m-2)深部高(0.901 W·m-2),指示着鲜水河断裂带深部热流体上涌.这些预研究工作为后续系缆式海底热流长期观测系统的正式研制与应用奠定了扎实基础.  相似文献   
3.
Marine ferromanganese nodules and crusts containing Mn, Cu, Ni and Co in the most promising resource-grade concentrations and quantities, together with Fe and Zn (all elements of biogeochemical importance) are found far from land on the deep seafloor of the Pacific Ocean. The biogeochemical, chemical and physical mechanisms contributing to their formation, distribution, abundance and – for these six elements – variability in their concentrations in these deposits, are the main focus of the present review. The mechanisms addressed include biological productivity, sedimentation types and rates, bottom water characteristics, the Calcium Carbonate Compensation Depth, the depth and intensity of the oxygen minimum zone, and biogeochemical characteristics of the six focal elements. Particular attention is given to comparisons between the deposits found in the north and the south Pacific, in order to present an overarching view of our current understanding of the mechanisms that apply to both nodules and crusts in both oceanic hemispheres, including examination of the possible existence of a marine ferromanganese oxide continuum. The renewed interest in the commercial exploitation of these deposits has stimulated a welcome increase in scientific research that is essential to informing the public discourse on seabed mining. We briefly reflect on the work addressed in this review in that context.  相似文献   
4.
大地震在哪里发生是地震预报首先要解决的问题.利用反演GNSS观测数据得到的2011年日本东北9级大地震前7年(2004—2010年)断层上的应力变化,我们发现了这次地震断层的孕震区.为了进一步研究该孕震区的演化过程,本文继续反演这次大地震在1997—2003年间的断层应力变化过程.通过这两期的反演工作,我们看到,在这14年中,断层应力的年变化图案的主要特征基本是稳定的,并存在明显的应力增加区和降低区.前者与地震的破裂区吻合,后者与前震、重复小地震和无震滑动的区域一致.显著的剪切应力增加区不但与主震,而且还与大余震的破裂区相符合.我们发现断层面上高应力积累区的零剪应力和零正应力变化的等值线不重合,前者在断层面上的深度大于后者,这意味着在剪应力增加区存在着正应力降低区或剪切强度降低区(由于剪切强度与正应力成正比).断层初始破裂点似乎更偏好零正应力等值线附近的位置,这是因为该处不但靠近剪切强度降低区,而且位于剪应力积累最显著的地方.研究结果表明,正应力变化对大地震的初始破裂有影响;本文所使用的断层应力变化反演方法,可以用来作为预测大地震发生位置的一种手段.  相似文献   
5.
利用国家测震台网固定台站和“中国地震科学台阵探测”项目在南北地震带北段布设的宽频带流动台阵记录到的极远震事件,通过SS前驱波震相研究,获得了阿留申—阿拉斯加俯冲带东段及邻区下方410 km和660 km间断面的埋深和起伏形态特征.为增强对SS前驱波震相的识别,我们采用了时差校正和共反射点叠加分析.叠加结果显示,毗邻阿留申俯冲带的白令海、阿拉斯加半岛、以及阿拉斯加中南部和东部地区下方,410 km和660 km间断面的埋深基本呈正相关关系,因而具有正常的过渡带厚度.这表明在阿留申—阿拉斯加俯冲带东段,北太平洋板块还没有俯冲到地幔过渡带深度范围内.其次,在阿拉斯加西部地区下方,660 km间断面出现明显下沉,而上覆的410 km间断面埋深接近于全球平均值,从而导致过渡带明显加厚.据此,我们推测在阿拉斯加西部地区下方地幔过渡带底部可能存在库拉残留板块.  相似文献   
6.
Abstract

The ground vibrations during pile driving operation have a drastic potential to undermine the surrounding structures both in land and reclaimed land. Particularly, reclaimed land necessitates ample application of pile driving due to the weak land condition. To prevent the structural damage, attenuation of the ground vibrations to an allowable level through active isolation of circular open trench is the scope of this study. In this research, finite element simulations of continuous impact pile driving process from the ground surface was executed with particular attention to the pile-soil interaction, and thereby, the efficiency of open trench application in attenuation of the unsafe distance of different structures was surveyed using the vibration sensitivity degree. Regarding the crucial parameters of an open trench (depth, width, and location), it was concluded that a sufficient high depth can attenuate the unsafe distance up to 68%, the trench width variations are less effective, and an average pile-trench distance is the most efficient option. The excavation volume was also concluded as another crucial parameter in open trench design which takes all three parameters into account. The trench depth equal to the pile’s maximum critical depth of vibration was inferred for an optimum design.  相似文献   
7.
沈世伟  吴飞  甘霖  姜满 《探矿工程》2021,48(1):120-128
对不同初始饱和度红砂岩冻融循环前后进行物理及力学试验研究,探讨初始饱和度对红砂岩冻融损伤的影响。本文设定红砂岩试样初始饱和度为20%、40%、60%、80%、100%,冻融次数设定为20次,对冻融前后试样分别测定质量、纵波波速以及进行单轴压缩试验。试验结果表明:1)冻融后不同初始饱和度红砂岩的物理性质发生变化,纵波波速降低、质量减小,但变化程度不同;2)随饱和度的增大,试样经冻融后峰值强度和弹性模量均呈降低趋势,但只有饱和度大于60%时,降低趋势较明显。本文研究为寒区隧道及地下工程建设以及岩土地质灾害监测与治理提供理论依据和试验基础。  相似文献   
8.
借助GPS数据作为边界条件,运用有限元对陇县-宝鸡断裂带及其邻区进行位移场与应力场数值模拟,分析该地区现今地壳活动与构造应力场的分布特征。结果表明:1)陇县-宝鸡断裂带与岐山-马昭断裂南端渭河盆地的张应力较弱;2)鄂尔多斯地块西南缘与陇县-宝鸡断裂带接壤的SE向条带的节点应力性质、方向与陇县-宝鸡断裂带一致,显示出平行于断裂带SE走向的张性活动特征;3)最大主应力表明,陇西地块及秦岭构造区具有较高强度的张性应力;4)第二主应力表明,陇县-宝鸡断裂带南端与秦岭构造区接壤地区南侧的SEE向压性应力显著。  相似文献   
9.
In the Veria-Naousa ophiolitic complex (north Greece), rodingite appears mainly in the form of cross cutting dykes within serpentinised peridotites. It is distinguished into three types, based upon the provenance of its protoliths, textural characteristics, mineralogical assemblages and geochemical affinities. Type I rodigites were derived from boninitic diabasic protoliths and their mineralogical assemblage include garnet + clinopyroxene + chlorite. Type II rodingites were formed at the expense of gabbroic precursors, comprising clinopyroxene + garnet + vesuvianite ± quartz, whereas Type III rodingites replaced diabasic tholeiitic protoliths comprising of garnets + vesuvianite + clinopyroxene + chlorite. Rodingitisation resulted in desilification, decrease of alkalies, Al, Fe, Mg and increase in Ca contents. In Type I rodingites the MREE (middle rare earth elements) and HREE (heavy rare earth elements) were slightly reduced. Type II rodingites experienced LREE (light rare earth elements) depletions, whereas MREE and HREE remained fairly stable. Restricted mobility of REE in Type III rodingites is assigned to shallow-level rodingitisation under decreasing pH.Rodingitisation occured in two distinct stages at fore-arc settings. The first stage occured under mildly oxidising conditions and enhanced CO2/H2O ratios. This stage affected the protoliths of all rodingite types. The second rodingitisation stage occured under more oxidising conditions and lower CO2/H2O ratios, which corresponds to the exhumation stage of the serpentinite-rodingite formations. Types II and III rodingites were subjected to further rodingitisation under the increasing influence of slab-derived hydrous phases at shallower depths, leading to the formation of late-stage andradite and vesuvianite. All stages of rodingitisation are estimated to have occurred under relatively moderate temperatures and pressure (~300 to 450 °C; ~2–6 kbar respectively).  相似文献   
10.
The Zedong ophiolites in the eastern Yarlung–Zangbo suture zone of Tibet represent a mantle slice of more than 45 km~2. This massif consists mainly of mantle peridotites, with lesser gabbros, diabases and volcanic rocks. The mantle peridotites are mostly harzburgite, lherzolite; a few dike-like bodies of dunite are also present. Mineral structures show that the peridotites experienced plastic deformation and partial melting. Olivine(Fo89.7–91.2), orthopyroxene(En_(88–92)), clinopyroxene(En_(45–49) Wo_(47–51) Fs_(2–4)) and spinel [Mg~#=100×Mg/(Mg+Fe)]=49.1–70.7; Cr~#=(100×Cr/(Cr+Al)=18.8–76.5] are the major minerals. The degree of partial melting of mantle peridotites is 10%–40%, indicating that the Zedong mantle peridotites may experience a multi–stage process. The peridotites are characterized by depleted major element compositions and low REE content(0.08–0.62 ppm). Their "spoon–shaped" primitive–mantle normalized REE patterns with(La/Sm)_N being 0.50–6.00 indicate that the Zedong ultramafic rocks belong to depleted residual mantle rocks. The PGE content of Zedong peridotites(18.19–50.74 ppb) is similar with primary mantle with Pd/Ir being 0.54–0.60 and Pt/Pd being 1.09–1.66. The Zedong peridotites have variable, unradiogenic Os isotopic compositions with ~(187)Os/~(188)Os=0.1228 to 0.1282. A corollary to this interpretation is that the convecting upper mantle is heterogeneous in Os isotopes. All data of the Zedong peridotites suggest that they formed originally at a mid-ocean ridge(MOR) and were later modified in supra–subduction zone(SSZ) environment.  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