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   1784篇
  免费   261篇
  国内免费   300篇
测绘学   138篇
大气科学   224篇
地球物理   486篇
地质学   833篇
海洋学   238篇
天文学   184篇
综合类   88篇
自然地理   154篇
  2024年   9篇
  2023年   30篇
  2022年   103篇
  2021年   121篇
  2020年   99篇
  2019年   108篇
  2018年   125篇
  2017年   124篇
  2016年   127篇
  2015年   110篇
  2014年   121篇
  2013年   110篇
  2012年   126篇
  2011年   137篇
  2010年   82篇
  2009年   119篇
  2008年   80篇
  2007年   67篇
  2006年   62篇
  2005年   70篇
  2004年   52篇
  2003年   42篇
  2002年   44篇
  2001年   32篇
  2000年   33篇
  1999年   40篇
  1998年   16篇
  1997年   22篇
  1996年   15篇
  1995年   12篇
  1994年   13篇
  1993年   6篇
  1992年   11篇
  1991年   13篇
  1990年   7篇
  1989年   2篇
  1988年   3篇
  1987年   4篇
  1986年   3篇
  1985年   3篇
  1984年   5篇
  1983年   5篇
  1982年   4篇
  1981年   5篇
  1980年   3篇
  1979年   3篇
  1976年   3篇
  1973年   2篇
  1971年   2篇
  1970年   2篇
排序方式: 共有2345条查询结果,搜索用时 15 毫秒
991.
992.
The reconstruction of the main structural features of the Southern Apennines (Italy), in correspondence with the focal volume of some strong earthquakes that have affected this chain, can be attempted by analysing reflection seismic lines and deep well logs in comparison with surface geology.For instance, the Calore Valley and its surroundings have been the object of intense hydrocarbon exploration, and a wealth of subsurface data is available. Moreover, this area was affected by the 1688 Sannio earthquake (macroseismic magnitude 7.1), and a new location has recently been proposed for the related causative fault system. The present work defines the structural setting of the Southern Apennine chain in correspondence with this new location, and compares it with similar cases along the Italian peninsula.The analysis was focussed on the reconstruction of deep tectonic units (formed by the buried Apulia carbonate platform succession), which generally correspond to the hypocentral depths of strong earthquakes along the axis of the Southern Apennines. The results show that the Apulia platform succession is affected by three main thrusts, locally accompanied by backthrusts. The top of this succession is relatively shallow: the maximum depth does not exceed 1.8 s TWT (i.e. about 3500 m b.s.l.), while minimum depths occur in correspondence with the ramp anticlines culminations, at 0.5 s TWT (i.e. at about 500 m b.s.l.). Moreover, data suggest that the underlying Paleozoic basement is possibly involved in thrusting.In a regional perspective, extensional seismogenic structures along the axis of the Southern Apennines seem to share some common characteristics. Indeed, they develop (i) in correspondence with an uplifted Paleozoic basement; (ii) at the rear of a set of thrusts that account for the shallow Apulia units; (iii) at the surface, in proximity to the leading edge of a surficial tectonic unit formed by the Apennine carbonate platform succession. The 1688 seismogenic fault system fits in with these common traits. In the light of this, we finally speculate that these common characteristics in the architecture of the chain could provide a key to the location of the major seismicity along the axis of the Southern Apennines and an interpretative model for the identification of possible areas of seismic gap in this part of the Italian peninsula.  相似文献   
993.
利用1961-2016年夏季黄土高原地区64个气象监测站的逐日降水资料及同期NCEP/NCAR再分析数据,分析了近56年黄土高原夏季极端降水的时空变化特征,并对比了极端降水强弱年以及不同年代黄土高原地区夏季大气环流形势的异同.结果表明,黄土高原夏季极端降水占夏季总降水量的54%左右,总体上呈现出东多西少的空间分布特征....  相似文献   
994.
海底水合物丘与泥火山均属于不同相态流体向上运移排出至地表过程中的产物,与这2种特殊地质体相关的浅表层天然气水合物具有独特的成藏过程和赋存规律,同时,它们也都是富碳流体排放的重要途径.然而,由于对这2种地质体缺少系统的调查,加之对浅表层天然气水合物资源和碳泄漏过程的研究程度不高,当前在海底水合物丘与泥火山特征刻画及准确甄...  相似文献   
995.
In the Beni Issef Massif, nearly 30 km west of Chefchaouen (Morocco), the thickest post-nappe succession within the Rifian sector of the Maghrebian Chain seals the tectonic contact between the Intrarifian External Tanger and Loukkos Units, related to the Rifian External Domain. This succession is very important for the reconstruction of the deformation timing of the Rifian Maghrebids. The age of its base, in fact, is an important constraint for defining an upper boundary to the stacking of both the Intrarifian and Maghrebian Flysch Basin Units, because clasts fed by the Melloussa and Numidian Flysch Nappes are abundant in the conglomerate layers. Field and biostratigraphic analyses pointed out the presence of a Lower Beni Issef Fm, unconformable on the Intrarifian External Tanger and Loukkos Units, and an Upper Beni Issef Fm, unconformable on both the Intrarifian Units and the Lower Beni Issef Fm. The Lower Beni Issef Fm, 150 m thick, consists of lenticular conglomerates with huge blocks in a marly-clayey matrix, followed by marls and minor sandstones. It deposited in a siliciclastic platform, shows a fining upward trend and is affected by metre- to hectometre-sized, locally reversed, folds. Samples collected 45–50 m above the base of the formation resulted not older than Late Tortonian in age, but an older age for the base of the formation cannot be excluded. The Upper Beni Issef Fm, up to 550 m thick, starts with coarse conglomerates followed by medium- to coarse-grained well-bedded sandstones and by grey-blue marls and mudrocks. It indicates deposition in a channelized marine delta, with evolution towards pro-delta pelites, and shows sub-horizontal or gently dipping beds towards the east. Biostratigraphic data indicate a probable Messinian age for this formation. The composition of the arenites of both Lower Beni Issef and Upper Beni Issef Fms is quartzolithic and all samples show a notable content of monocrystalline well-rounded quartz and sedimentary lithic fragments. Detrital modes, all falling in the Quartzose Recycled and Transitional Recycled fields, suggest a provenance from recycling of sedimentary successions, easily recognizable in the Flysch Basin and External Units, mainly the Numidian Nappe sandstones. A Tortonian age of the Lower Beni Issef Fm would agree with the Late Serravallian age of the uppermost beds of the External Tanger Unit and indicate that the most probable age for the stacking of the Intrarifian Units falls in the Late Serravallian-Middle Tortonian time span. The Lower Beni Issef Fm was involved in a compressive tectonic phase testified by north-south striking folds. Later, probably during Messinian, the Upper Beni Issef Fm deposited in a younger intramontane basin, resting on both the Intrarifian Units and the Lower Beni Issef Fm. Successively, the Upper Beni Issef Fm was passively transported piggyback on top of the fold and thrust belt during later tectonic evolution of the Rifian Maghrebids. This tectonic evolution results quite similar to that recognized in the Tellian and Sicilian Maghrebids and also in the southern Apennines.  相似文献   
996.
地质资料开发利用被国家列为新时期地质工作的六大任务之一,足见其重要性。分析认为,地质资料具有科学性、经济性、社会性、原始记录性、历史再现性等多重属性,是地质资源信息的宝库,包含着巨大的经济社会价值。提出以下4点:①开展综合编研,以集成新的复合型地质资料;②开展有用信息的筛选提取,以放大和增强找矿信息;③开展多元信息叠加研究,以进行成矿预测;④再审视、再利用、回头看,以寻找新矿种、新资源等的地质资料编研开发途径,并列举了一些典型实例。还分析了河南省地质资料工作中存在的突出问题,提出了加强地质资料编研开发的若干措施和建议。  相似文献   
997.
贵州水城矿区煤层气地质条件与可采潜力分析   总被引:1,自引:0,他引:1  
水城矿区煤层气资源丰富,在分析矿区煤储层物性和含气性的基础上,估算了煤层气可采性参数,评价了煤层气的可采潜力。结果显示:水城矿区煤层顶、底板以及含煤地层上覆地层均为良好的区域性盖层,煤层气保存条件较好;矿区良好的煤岩、煤质特征和孔隙结构特征为煤层气的大量生成和富集奠定了基础;煤层气理论平均采收率为31.66%,可采资源量为1158×108m3,平均可采资源丰度0.98×108m3/km2,对煤层气开发较为有利;煤层的渗透性较低,可能对煤层气地面排采带来不利影响。建议矿区在煤层气地面开发工作中,注重煤储层渗透性的有效改善,并探讨矿井—地面开发结合的可行性。  相似文献   
998.
We present a thorough review of the knowledge on the climate and environment in Sicily over the last 20 000 years, taking into account results of several studies carried using terrestrial and marine records. We obtain a coherent framework of the most important changes succeeded in the island, even if some points need further investigation. All the reconstructions of surface temperatures of the seas and the air surrounding Sicily point out severe climatic conditions during the last glacial period. The steppe- and semisteppe-like vegetation pattern testifies, together with additional evidence from geochemical data of lacustrine evidence, markedly arid conditions. Fi-nally, significant episodes of sea level drop connected Sicily to the Italian Peninsula and favoured the dispersion of faunal elements from southern Italy. The transition between the last glacial and the Holocene was not characterized by a gradual warming but was punctuated by two abrupt suborbital climatic fluctuations: Bølling-Allerød (warm) and Younger Dryas (cold), as recognized in the sediments recovered close to the northern and southern coast of Sicily. A denser arboreal cover is possibly indicated by the occurrence of dormouse and Arvicola remains. Finally the sensitivity of Sicily to climate perturbations is demonstrated by the occurrence of repeated subtle climatic anomalies during the Holocene, including the Little Ice Age, also known from historical chronicles. Forests, woods and Mediterranean maquis developed in the early-middle Holocene. Thereafter was a general decline of arboreal vegetation, following a general aridification trend that seems to be a common feature in southern Europe and North Africa. Science Greek colonization (7th century before Christ), the landscape was intensively modelled for agriculture and breeding, leading to a significant loss of vegetation cover.  相似文献   
999.
利用生物信息学方法,从龙须菜(Gracilaria lemaneiformis)公共核苷酸数据库中筛查到8条含有微卫星DNA的序列,并根据筛查出的微卫星DNA序列设计合成了5对引物,此外还使用了细江蓠的4对已知微卫星DNA引物共9对引物在采集自青岛的26株野生龙须菜个体中进行扩增,结果筛选到2对引物可扩增出具有多态性的产物.然后利用5对龙须菜的微卫星引物对6个龙须菜品系和另外2个种外物种细基江蓠和菊花心江蓠进行系统学分析.根据计算得到的不同样品之间Nei氏遗传相似系数进行聚类分析,显示青岛野生龙须菜QD与栽培品系981、石岛的龙须菜样品SD与龙须岛的龙须菜样品LD之间的遗传相似系数最高,而来自委内瑞拉的龙须菜样品LV与青岛野生龙须菜之间的差异远远高于种内水平的差异.  相似文献   
1000.
自然资源管理背景下的时空大数据平台建设   总被引:2,自引:0,他引:2  
由于自然资源机构改革,测绘地理信息工作成为自然资源统一管理工作的组成部分。时空大数据平台如何在新的组织机构下发挥作用,是当前急需研究的问题。为此,本文在分析了自然资源管理的需求后,提出了时空大数据平台建设的技术路线和时空大数据服务自然资源管理的建设思路。  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