全文获取类型
收费全文 | 632篇 |
免费 | 129篇 |
国内免费 | 31篇 |
专业分类
测绘学 | 10篇 |
大气科学 | 10篇 |
地球物理 | 310篇 |
地质学 | 285篇 |
海洋学 | 54篇 |
天文学 | 1篇 |
综合类 | 19篇 |
自然地理 | 103篇 |
出版年
2024年 | 2篇 |
2023年 | 8篇 |
2022年 | 11篇 |
2021年 | 30篇 |
2020年 | 32篇 |
2019年 | 33篇 |
2018年 | 24篇 |
2017年 | 24篇 |
2016年 | 28篇 |
2015年 | 38篇 |
2014年 | 32篇 |
2013年 | 59篇 |
2012年 | 43篇 |
2011年 | 32篇 |
2010年 | 25篇 |
2009年 | 35篇 |
2008年 | 37篇 |
2007年 | 25篇 |
2006年 | 31篇 |
2005年 | 31篇 |
2004年 | 20篇 |
2003年 | 27篇 |
2002年 | 17篇 |
2001年 | 21篇 |
2000年 | 16篇 |
1999年 | 6篇 |
1998年 | 13篇 |
1997年 | 22篇 |
1996年 | 6篇 |
1995年 | 7篇 |
1994年 | 11篇 |
1993年 | 7篇 |
1992年 | 7篇 |
1991年 | 6篇 |
1990年 | 7篇 |
1989年 | 1篇 |
1988年 | 1篇 |
1987年 | 1篇 |
1986年 | 2篇 |
1985年 | 6篇 |
1984年 | 3篇 |
1983年 | 1篇 |
1982年 | 1篇 |
1981年 | 2篇 |
1976年 | 1篇 |
排序方式: 共有792条查询结果,搜索用时 250 毫秒
261.
三峡水库建成后,长江中下游防洪形势显著改善,但由于经济社会发展,防洪要求的提高和江湖关系的变化,长江防洪形势发生了一些新的变化。以1954年和1998年典型大洪水为例,分析了三峡水库建成后长江中下游防洪形势出现的新变化,讨论了长江中下游蓄滞洪空间格局调整及江湖关系变化对于防洪的影响。根据长江水沙变化、河道演变、水库群调控和分蓄洪区使用几率变化等出现的新问题,提出未来防洪战略及对策。结果表明:三峡建成后,百年一遇以下洪水防御形势明显好转,而百年一遇以上特大防洪的防洪形势仍然严峻,洪水风险主要转移到水库群上;今后需要在加强蓄滞洪区建设的基础上,重点推动防洪非工程措施建设,以减轻特大洪水带来的灾害损失。 相似文献
262.
263.
鄂尔多斯盆地苏里格气田是我国陆上产能最大的气田,其主力产气层---二叠纪石盒子群八段岩性致密,储层的分布主要受沉积微相控制。但S47区块作为苏里格气田西区一个重要的新区,有关沉积微相的认识仍存在分歧。在岩心观察、钻井资料及样品分析的基础上,重点研究了S47区块盒8段沉积微相,认为该区盒8段属辫状河三角洲前缘沉积,并识别出水下分流河道、河口坝、席状砂、水下天然堤、分流间湾等5种微相;最有利的储集砂体主要为水下分流河道叠置砂体,其次为河口坝砂体。沉积微相及其展布特征的新认识为该区储层预测、水平井整体开发提供了地质理论依据。 相似文献
264.
莱州湾河流表层沉积物中六溴环十二烷的含量水平、分布及组成特征 总被引:1,自引:0,他引:1
从莱州湾附近河流采集了36个表层沉积物样品,采用高效液相色谱耦合三重四极杆串联质谱的分析方法对沉积物中的六溴环十二烷(HBCDsl3种异构体进行了测定,目的是表征研究区域内HBCDs的污染水平和组成特征,并探讨其分布、来源及影响因素。结果表明,莱州湾河流沉积物中ZHBCDs的含量范围为0.03~20.17ng/gdw(均值是2.14ng/gdw):较国内外其他河流,该研究区域HBCDs含量水平较低。HBCDs异构体组成各不相同,除1个站位外,其他站位样品中7-HBCD占主导地位(52.3%~97.3%,均值72.4%),但是在农业区a-HBCD相对丰度较高。对HBCDs的分布特征及来源分析得出,含量较高的站点多集中在工业区,呈现出明显的点源特征:而远离工业区的采样点,HBCDs可能主要来自大气的传输和沉降,且HBCDs的含量与TOC含量呈现出较好的相关性,表明TOC是非工业区分布的一个控制因素。 相似文献
265.
“山水林田湖草沙生命共同体”理念为沙地生态环境治理指明了新方向,但是在沙区开展什么研究却是新课题。本文基于生态建设面临的挑战,从山水林田湖草沙一体化治理的角度,探析了科尔沁沙地生态治理的研究方向及应重点关注的问题。 相似文献
266.
天然河流水资源开发中的水环境保护问题 总被引:1,自引:0,他引:1
人类在发展社会经济的同时,出于商业目的和个人利益,争相控制水资源,随意向河流排污,导致天然河流、湖泊及附近地区水环境恶化.保护天然河流和水资源是人类共同的责任,不能以牺牲环境为代价,要有统一的认识、统一的措施来进行水资源的保护工作. 相似文献
267.
M. F. Araújo J. -M. Jouanneau P. Valrio T. Barbosa A. Gouveia O. Weber A. Oliveira A. Rodrigues J. M. A. Dias 《Progress in Oceanography》2002,52(2-4)
The northern Portuguese coastal zone is drained by large Iberian rivers, along which there is intensive industrial, agricultural and urban activity. Offshore of the two main river basins of the Douro and Minho, two fine-grained sedimentary formations can be characterised. A geochemical study of this sediment cover has been performed focussing in particular on the fine deposits and the adjacent estuaries. The chemical composition of superficial sediments collected from the northern Portuguese shelf and from the river basins of the Minho and Douro rivers was determined for major, minor and trace elements; the rare earth elements (REE) composition was also determined in selected samples. Statistical analyses of these data for the shelf sediments allow us to identify elements with similar behaviours and the different sediment types along the shelf, and to evaluate of their origins. Downcore profiles of the chemical composition of the fine sediments reveal no evidence for changes induced by recent human activities. To assess the influence of the rivers on the fine sedimentary formations, the elemental distribution patterns of selected sediments have been compared with the estuarine sediments. Lithogenic elements composition normalised to Al and the shale normalised REE distributions showed there are similarities between the Douro River sediments and the sediment composition of the Douro and Galicia mud patches. However, the Galicia mud field showed no relationship in its chemical composition to sediments derived from the Minho River. Nor was any evidence for heavy metal contamination originating from the Douro estuary detected in anthropogenic element/Al ratios in the shelf sediments. This may be because of processes associated with resuspension, complexation and bioturbation that occur during sediment transport and deposition. Nevertheless, the Douro River seems to be the main continental source of fine sediments being deposited on the northern Portuguese shelf. 相似文献
268.
Metal fluxes through the Strait of Gibraltar: the influence of the Tinto and Odiel rivers (SW Spain)
Franoise Elbaz-Poulichet Nicholas H. Morley Jean-Marie Beckers Philippe Nomerange 《Marine Chemistry》2001,73(3-4)
A large set of new data concerning dissolved metal concentrations has been acquired in the Gulf of Cadiz and in the Strait of Gibraltar from 1996 to 1999. These data, associated with models (hydrodynamic, tracer advection–dispersion and mixing), have been used to assess the influence of rivers draining the South Iberian Pyrite Belt on the Gulf of Cadiz and on the Atlantic inflow in the Strait of Gibraltar.Metal concentrations in surface waters from the Gulf of Cadiz are maximal near the mouth of the Tinto/Odiel rivers with values exceeding 50 nmol/kg (Mn), 5 nmol/kg (Ni), 30 nmol/kg (Cu), 100 nmol/kg (Zn), 0.9 nmol/kg (Cd) and 45 nmol/kg (As). From the Tinto/Odiel river, a plume of contamination follows the coast in the direction of the Strait of Gibraltar. The computation of a tracer advection–dispersion model confirms that the coastal currents carry the metals discharged from the Tinto and Odiel to the Strait of Gibraltar.From temperature–salinity and metal–salinity plots, four water masses can be recognised in the Gulf of Cadiz and in the Strait of Gibraltar: North Atlantic Surface Water (NASW), North Atlantic Central Water (NACW) and metal-enriched Spanish Shelf Waters from the Gulf of Cadiz (SSW). The Mediterranean Outflow Water (MOW) is also clearly seen at depths greater than 300 m.The chemical characteristics of these various water masses have been used in a mixing model to evaluate their relative contribution to the Atlantic inflow through the Strait of Gibraltar. These contributions are seasonally variable. In June 1997, the contribution was: 80±20%, 5±5% and 15±10% for NASW, NACW and SSW, respectively. In September, the SSW contribution was apparently negligible.Finally, these relative contributions allow the evaluation of the metal fluxes in the Strait of Gibraltar. The presence of SSW in the Strait increases the metal flux to the Mediterranean Sea by a factor of 2.3 (Cu), 2.4 (Cd), 3 (Zn) and 7 (Mn). It does not modify significantly As and Ni fluxes. 相似文献
269.
270.