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   42691篇
  免费   13184篇
  国内免费   6993篇
测绘学   4242篇
大气科学   5505篇
地球物理   7049篇
地质学   25622篇
海洋学   6459篇
天文学   2915篇
综合类   3117篇
自然地理   7959篇
  2025年   50篇
  2024年   1051篇
  2023年   1062篇
  2022年   1747篇
  2021年   1961篇
  2020年   1893篇
  2019年   2084篇
  2018年   1669篇
  2017年   1847篇
  2016年   1913篇
  2015年   2042篇
  2014年   2547篇
  2013年   2418篇
  2012年   2744篇
  2011年   2696篇
  2010年   2381篇
  2009年   2883篇
  2008年   2779篇
  2007年   3048篇
  2006年   2933篇
  2005年   2736篇
  2004年   2470篇
  2003年   2300篇
  2002年   2036篇
  2001年   1755篇
  2000年   1618篇
  1999年   1441篇
  1998年   1246篇
  1997年   965篇
  1996年   852篇
  1995年   736篇
  1994年   632篇
  1993年   557篇
  1992年   411篇
  1991年   379篇
  1990年   254篇
  1989年   231篇
  1988年   144篇
  1987年   97篇
  1986年   62篇
  1985年   46篇
  1984年   21篇
  1983年   22篇
  1982年   17篇
  1981年   10篇
  1980年   10篇
  1979年   10篇
  1978年   18篇
  1977年   7篇
  1954年   8篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
111.
Combined data of physical property, benthic foraminifera, and stable isotopes from ODP Sites 1148, 1146, and 1143 are used to discuss deep water evolution in the South China Sea (SCS) since the Early Miocene. The results indicate that 3 lithostratigraphic units, respectively corresponding to 21-17 Ma, 15-10 Ma, and 10-5 Ma with positive red parameter (a*) marking the red brown sediment color represent 3 periods of deep water ventilation. The first 2 periods show a closer link to contemporary production of the Antarctic Bottom Water (AABW) and Northern Component Water (NCW), indicating a free connection of deep waters between the SCS and the open ocean before 10 Ma. After 10 Ma, red parameter dropped but stayed higher than the modern value (a*=0), the CaCO3 percentage difference between Site 1148 from a lower deepwater setting and Site 1146 from an upper deepwater setting enlarged significantly, and benthic species which prefer oxygen-rich bottom conditions dramatically decreased. Coupled with a major negative excursion of benthic δ13C at ~10 Ma, these parameters may denote a weakening in the control of the SCS deep water by the open ocean. Probably they mark the birth of a local deep water due to shallow waterways or rise of sill depths during the course of sea basin closing from south to east by the west-moving Philippine Arc after the end of SCS seafloor spreading at 16-15 Ma. However, it took another 5 Ma before the dissolved oxygen approached close to the modern level. Although the oxygen level continued to stabilize, several Pacific Bottom Water (PBW) and Pacific Deep Water (PDW) marker species rapidly increased since ~6 Ma, followed by a dramatic escalation in planktonic fragmentation which indicates high dissolution especially after ~5 Ma. The period of 5-3 Ma saw the strongest stratified deepwater in the then SCS, as indicated by up to 40% CaCO3 difference between Sites 1148 and 1146. Apart from a strengthening PDW as a result of global cooling and ice cap buildup on northern high latitudes, a deepening sea basin due to stronger subduction eastward may also have triggered the influx of more corrosive waters from the deep western Pacific. Since 3 Ma, the evolution of the SCS deep water entered a modern phase, as characterized by relative stable 10% CaCO3 difference between the two sites and increase in infaunal benthic species which prefer a low oxygenated environment. Thesubsequent reduction of PBW and PDW marker species at about 1.2 Ma and 0.9 Ma and another significant negative excursion of benthic δ13C to a Neogene minimum at ~0.9 Ma together convey a clear message that the PBW largely disappeared and the PDW considerably weakened in the Mid-Pleistocene SCS. Therefore, the true modern mode SCS deep water started to form only during the "Mid-Pleistocene climatic transition" probably due to the rise of sill depths under the Bashi Strait.  相似文献   
112.
根据泰安市区域矿产开发特点,及时掌握矿山企业开采情况,通过加强采矿登记现场预审查、开展矿山开采督察和采矿权人矿产资源开发利用年查等措施,强化了矿产资源开发过程中的监督管理,维护了正常开采秩序及采矿权人的合法权益。  相似文献   
113.
王启宇 《地质与勘探》2013,49(5):990-998
英买力凝析气田群区层组划分与对比研究是该地区陆相地层油气藏地质和开发工程中最基本和最重要的精细地质研究内容之一,也是急需解决的关键技术难题之一。本次研究采取"取心标准井出发,岩心标定测井,寻找辅助标志层,进行旋回对比、分级控制,将隔夹层纳入到同一沉积旋回进行识别、划分对比,以隔层确定小层,并兼顾‘就近、闭合’的原则"展开层组划分与对比,重新确定出更适用于气田开发的气层组、砂层组、小层等划分单元。而对可以作为层组划分与对比辅助手段的白垩系与古近系不整合界面、库姆格列木群底砂岩段顶界以及层组中的隔夹层等的识别,为层组的精细划分对比提供了重要保证,很好地解决了巴什基奇克组、库姆格列木群的层组划分问题,对英买力地区凝析气藏的后期开发等问题具有积极的指导意义。  相似文献   
114.
从围护结构节能与防火一体化思路出发,提倡钢筋混凝土结构保温一体化建筑体系和砌体结构保温一体化建筑体系;尤其主张在我国建筑工业化全面升级的基础上大力开发与推广保温型混凝土预制装配化技术,统一建筑模数,实现标准化与多样化;针对预制装配化保温型混凝土在梁、构造柱等容易出现的冷、热桥问题的部位,创新式提出以相变储能材料与建筑结构相融合来提高建这些部位的热工性能,减少温度梯度;从系统上做到保温与建筑同寿命、免维护、工序少,工期短、耐火等,从而真正实现低碳,构建建筑围护结构节能防火一体化建筑体系。  相似文献   
115.
砂石土指第四纪堆积的泥、土、砂,为一种极为重要的资源。我国具有丰富的海洋砂石土资源。以广东海域为例,通过资料的综合分析,探讨了海岸砂石土资源的重要作用,并提出其开采应注意的问题。  相似文献   
116.
Mechanisms of fractional crystallization with simultaneous crustalassimilation (AFC) are examined for the Kutsugata and Tanetomilavas, an alkali basalt–dacite suite erupted sequentiallyfrom Rishiri Volcano, northern Japan. The major element variationswithin the suite can be explained by boundary layer fractionation;that is, mixing of a magma in the main part of the magma bodywith a fractionated interstitial melt transported from the mushyboundary layer at the floor. Systematic variations in SiO2 correlatewith variations in the Pb, Sr and Nd isotopic compositions ofthe lavas. The geochemical variations of the lavas are explainedby a constant and relatively low ratio of assimilated mass tocrystallized mass (‘r value’). In the magma chamberin which the Kutsugata and Tanetomi magmas evolved, a strongthermal gradient was present and it is suggested that the marginalpart of the reservoir was completely solidified. The assimilantwas transported by crack flow from the partially fused floorcrust to the partially crystallized floor mush zone throughfractures in the solidified margin, formed mainly by thermalstresses resulting from cooling of the solidified margin andheating of the crust. The crustal melt was then mixed with thefractionated interstitial melt in the mushy zone, and the mixedmelt was further transported by compositional convection tothe main magma, causing its geochemical evolution to be characteristicof AFC. The volume flux of the assimilant from the crust tothe magma chamber is suggested to have decreased progressivelywith time (proportional to t–1/2), and was about 3 x 10–2m/year at t = 10 years and 1 x 10–2 m/year at t = 100years. It has been commonly considered that the heat balancebetween magmas and the surrounding crust controls the couplingof assimilation and fractional crystallization processes (i.e.absolute value of r). However, it is inferred from this studythat the ratio of assimilated mass to crystallized mass canbe controlled by the transport process of the assimilant fromthe crust to magma chambers. KEY WORDS: assimilation and fractional crystallization; mass balance model; magma chamber; melt transport; Pb isotope  相似文献   
117.
测绘监理与测绘行业质量监督不同,测绘监理注重生产的全过程,控制测绘生产过程中各个工序的质量,从而保障最终测绘成果的质量;测绘行业质量监督则是代表政府行使对测绘项目进行监督的行为。因此,两者在工作范围、工作性质以及工作深度等方面存在差别。但是两者同时又有统一的关系,也就是都为保障测绘成果质量,更好地服务保障经济社会发展采取的保障措施。现今在国内已有很多项目引入了监理机制,但尚没有形成一套完善有效的测绘监理行业规范。笔者根据多年在广东、海南、新疆等地区从事监理工作的实际经验,结合工程监理中的部分内容和方法,总结出了一套较为完善有效的测绘工程监理方案。  相似文献   
118.
The anisotropy of magnetic susceptibility (AMS) of single crystals of biotite, muscovite and chlorite has been measured in order to provide accurate values of the magnetic anisotropy properties for these common rock-forming minerals. The low-field AMS and the high-field paramagnetic susceptibility are defined. For the high-field values, it is necessary to combine the paramagnetic deviatoric tensor obtained from the high-field torque magnetometer with the paramagnetic bulk susceptibility measured from magnetization curves of the crystals. This leads to the full paramagnetic susceptibility ellipsoid due to the anisotropic distribution of iron cations in the silicate lattice. The ellipsoid of paramagnetic susceptibility, which was obtained for the three phyllosilicates, is highly oblate in shape and the minimum susceptibility direction is subparallel to the crystallographic c-axes. The anisotropy of the susceptibility within the basal plane of the biotite has been evaluated and found to be isotropic within the accuracy of the instrumental measurements. The degree of anisotropy of biotite and chlorite is compatible with previously reported values while for muscovite the smaller than previously published values. The shape of the chlorite AMS ellipsoid for all the samples is near-perfect oblate in contrast with a wide distribution of oblate and prolate values reported in earlier studies. Reliable values are important for deriving models of the magnetic anisotropy where it reflects mineral fabrics and deformation of rocks.  相似文献   
119.
120.
LA-ICP-MS U-Pb dating, whole -rock geochemistry and Hf isotopic analyses for the Jianshanzi volcanic rocks in the Kaiyuan area are presented to explore the Late Paleozoic -Early Mesozoic tectonic evolution history of the eastern segment of the northern margin of the North China block. The detailed research indicates that the protolith of Jianshanzi metamorphic volcanic rocks includes rhyolite, dacite and andesite, they are calc-alkaline series and metaluminous-weak peraluminous rocks. According to the zircon U-Pb dating, the metamorphic rhyolite and metamorphic andesite formed in the Late Permian (256. 1 +/- 1. 5Ma, 252.4 +/- 1. 7Ma) and the Late Triassic (222. 4 +/- 1. 8Ma, 227. 9 +/- 1. 2Ma), respectively. They are characteristized by enrichment in light rare earth elements and large ion lithophile elements, and depletion of heavy rare earth elements and high field strength elements. Metamorphic rhyolite has an obvious negative Eu anomaly with epsilon(Hf) ( t) values of 10. 83 + 8. 10 and the second -stage model are 698 similar to 1967Ma. They are mainly due to the partial melting of the juvenile basaltic lower crust and the contamination of a small amount of ancient crustal materials. Metamorphic andesite has no obvious Eu anomaly, and the epsilon(Hf)( t) value and the second -stage model are +4.54 similar to + 6. 79 and 825 similar to 968 Ma, which may be the product of partial melting of basic lower crust. There are two periods of magmatism in Jianshanzi volcanic rocks: Late Permian -Early Triassic (247 similar to 256Ma) and Late Triassic (similar to 225Ma). The material composition of the Jianshanzi volcanic rocks is characterized by different times and different tectonic backgrounds. It is comprehensively believed that the eastern segment of the northern margin of the North China block was in an active continental margin tectonic environment during the Permian, and the Palaeo-Asian Ocean subducted in both directions; Mantle -derived magma ascends and emplaces, forming a nascent crust and causing crustal thickening. The Paleo-Asian Ocean finally closed in the early Middle Triassic, and the impact of collisional orogeny may have continued until the early Late Triassic.  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