全文获取类型
收费全文 | 76719篇 |
免费 | 881篇 |
国内免费 | 1470篇 |
专业分类
测绘学 | 2743篇 |
大气科学 | 5677篇 |
地球物理 | 14335篇 |
地质学 | 29927篇 |
海洋学 | 5902篇 |
天文学 | 14416篇 |
综合类 | 2314篇 |
自然地理 | 3756篇 |
出版年
2022年 | 365篇 |
2021年 | 633篇 |
2020年 | 662篇 |
2019年 | 710篇 |
2018年 | 6075篇 |
2017年 | 5281篇 |
2016年 | 4328篇 |
2015年 | 1211篇 |
2014年 | 1820篇 |
2013年 | 2916篇 |
2012年 | 2773篇 |
2011年 | 4907篇 |
2010年 | 3963篇 |
2009年 | 4830篇 |
2008年 | 4081篇 |
2007年 | 4580篇 |
2006年 | 2257篇 |
2005年 | 1820篇 |
2004年 | 2028篇 |
2003年 | 1932篇 |
2002年 | 1711篇 |
2001年 | 1350篇 |
2000年 | 1272篇 |
1999年 | 1012篇 |
1998年 | 1070篇 |
1997年 | 956篇 |
1996年 | 820篇 |
1995年 | 787篇 |
1994年 | 690篇 |
1993年 | 607篇 |
1992年 | 595篇 |
1991年 | 598篇 |
1990年 | 625篇 |
1989年 | 495篇 |
1988年 | 508篇 |
1987年 | 538篇 |
1986年 | 490篇 |
1985年 | 613篇 |
1984年 | 674篇 |
1983年 | 593篇 |
1982年 | 569篇 |
1981年 | 520篇 |
1980年 | 490篇 |
1979年 | 484篇 |
1978年 | 459篇 |
1977年 | 373篇 |
1976年 | 354篇 |
1975年 | 361篇 |
1974年 | 317篇 |
1973年 | 350篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
461.
青藏铁路唐古拉山-拉萨段全新世控震断裂研究 总被引:8,自引:0,他引:8
地表调查表明,沿青藏铁路唐古拉山-拉萨段存在5条重要的全新世控震断裂带,从北到南分别是温泉盆地西缘断裂带、安多盆地北缘断裂带、崩错断裂带、谷露西缘断裂带和当雄-羊八井断裂带.构造-地貌和年代学分析结果表明,北部的温泉盆地西缘断裂和安多盆地北缘断裂带的活动强度相对比较小,平均垂直活动速率约为0.2~0.5mm/a.南侧的谷露西缘断裂带和当雄-羊八井断裂带的全新世垂直活动速率为约(15±0.5)mm/a.而中部的崩错走滑断裂带的活动强度最大,晚第四纪期间的走滑速率可达(11±4.5)mm/a.全新世断裂活动和古地震研究表明,其中温泉盆地西缘断裂带、安多盆地北缘断裂带、崩错断裂带的西北分支、当雄-羊八井断裂带的当雄段等区域未来发生强震的概率相对更大. 相似文献
462.
463.
S. Miyamoto 《Planetary and Space Science》1960,2(4):256-260
This interpretation of lunar surface features is based on the differentiation of siliceous and basaltic substances similar to those of the earth. Our hypothesis is that lunar maria are composed of basaltic matter similar to that of terrestrial ocean basins, and that the lunar continents (bright regions) are composed of siliceous blocks. 相似文献
464.
Abstract This paper experimentally investigates the convective planform near critical in a fluid layer whose temperature-dependent viscosity varies from top to bottom by up to a factor of 1500. Convection occurs in three different planforms: rolls, hexagons and squares. The square planform, which appears only for fluids with viscosity variation greater than about 50, replaces the hexagonal convection pattern as the Rayleigh number increases much above critical. The large amplitude of hexagonal convection with strong viscosity variation precludes studying the hexagon-square transition with perturbation methods of the type used to study the hexagon-roll transitions at smaller viscosity variations. 相似文献
465.
Volkov A. V. Galyamov A. L. Savchuk Yu. S. 《Izvestiya Atmospheric and Oceanic Physics》2020,56(12):1528-1536
Izvestiya, Atmospheric and Oceanic Physics - Gravimetric data obtained by the GOCE spacecraft contributes to the development of global models of the deep structure of the Earth’s crust and... 相似文献
466.
Natural environmental changes and human impact reflected in sediments of a high alpine lake in Switzerland 总被引:1,自引:0,他引:1
From the high alpine Sägistalsee (1935 m a.s.l.), 13.50 m of continuously laminated sediments comprising the last 9050 years, were analyzed. Even though Sägistalsee is a high elevation site, human-induced environmental changes start as early as 4300 cal. BP and leave a clearly detectable signal in the mineralogy of the sediments, which is much stronger than the signal from natural environmental changes that occurred before this time. Variations in the physical and mineralogical sediment properties of this clastic sequence reflect erosional changes in the catchment, where almost pure limestone contrasts with carbonaceous, quartz-bearing marl, and shist. The calcite/quartz (Cc/Qz) ratio was found to be most indicative of these changes, which occurred around AD 1850 and at 650, 2000, 3700, and 6400 cal. BP. The first four are interpreted as erosion events, which are related to human-induced changes in the vegetation cover and land use. We associate them to the recent development of tourism and grazing, the medieval intensification of pasturing, Roman forest clearance, and Bronze Age forest clearance, respectively. The Cc/Qz-ratio increases significantly within less than 100 years during these events, reflecting the erosion of unweathered or poorly weathered soils. The time intervals in between are characterized by a gradually decreasing Cc/Qz-ratio and reflect the stabilization or formation of new soils. Only the change at 6400 cal. BP, which represents the initial gradual stabilization of the catchment, is related to the immigration of Picea abies. 相似文献
467.
Development of the ideas about the equilibrium between freshwater and saline water has received considerable attention in the literature, but little has been written so far about the earliest scientific works about well salinization. Based on a review of the literature from the second half of the 19th century and the early 20th century, this historical note explores how insights into groundwater abstraction and saltwater intrusion developed, and examples of the earliest field studies are provided. Fundamental research was driven by the need for increasing water supply, but the progress of science did not lead to sustainable management practices everywhere. Research outcomes were shared between scientists of different countries, marking the beginning of coastal hydrogeology as a scientific specialization in the first decade of the 20th century. 相似文献
468.
S. Venkatramanan S. Y. Chung S. Selvam J. H. Son Y. J. Kim 《Environmental Earth Sciences》2017,76(13):456
This research was conducted at Samrak Park Delta of Nakdong River Basin in Busan Metropolitan City, Korea. The main objective of this study was to evaluate the interrelationship of geochemical elements in sediments and groundwater through multivariate statistical analyses and a multilayer perceptron artificial neural network model. The mean concentrations of chemical elements were Si (46%), Fe (16.9%), Al (15.7%), K (7.5%) and Ca (4.5%) in sediments, and Na (8650 mg/L), Mg (999 mg/L), Ca (432 mg/L), K (293 mg/L) and Cl (17,640 mg/L) in groundwater, respectively. The principal component analysis produced 3 kinds of factors with the variances of 63.37, 27.02 and 9.62%, respectively. It is suggested that the chemical components of sediments and groundwater were mainly originated from source rocks and seawater intrusion, with the minor impacts of irrigation and industry. Cluster analysis also showed that chemical elements were mainly controlled by the natural geogenic sources and seawater intrusion. Multilayer perceptron of artificial neural network (ANN) presented the good interrelationship between sediment and groundwater. The determination coefficients (R 2) between ANN predicted values and observed values in groundwater showed the high values of 0.61–0.97 except Mg, Mn and Sr. It is revealed that the chemical components of sediment and groundwater were derived from local geological origin and from the minor impact of anthropogenic sources. Multivariate analyses and ANN contributed to the identification of the mutual relationship between the geochemical elements of sediment and those of groundwater. 相似文献
469.
Karst groundwater protection in the Kupa River catchment area and sustainable development 总被引:1,自引:0,他引:1
One of the most significant water resources in the Republic of Croatia is the catchment area of the Kupa River, located in
the region bordering the Republic of Slovenia. About 88% of the total amount of water in this catchment originates in Croatia
and just 12% from Slovenia; therefore, the largest part of the catchment area (about 1000 km2) is on the Croatian side of the border. It is a typical karst area of the Dinarides with aquifers characterized by a relatively
rapid water exchange, high groundwater flow velocities and aquifers open to human impact from the surface. Consequently, the
aquifers are highly vulnerable and at risk. Due to the availability of large quantities of high-quality spring water (about
6 m3/s), the entire area has a strategic importance within the context of any future development strategy pertaining to the western
part of Croatia. The catchment area on the Croatian side was investigated using a wide range of research methods that included
a classical hydrogeological approach, the detailed hydrologic calculation of water balance to the hydrogeochemical analyses
and modelling. The objective was to determine protection zones and protection measures for the whole area. The difficulties
are increased due to the fact that the karst catchment area is crossed by major traffic corridors, oil pipelines and a railway
and that many settlements and a highly developed wood industry are present. The combination of protecting water resources
with adequate prevention measures and necessary remedial activities that should satisfy the very strict requirements necessary
for the protection of the karst aquifers while still allowing for present and future human activities is difficult – but not
impossible – to achieve. One good example is the present highway with a closed dewatering system and waste water treatment
before the water passes into the karst underground system. 相似文献
470.
Hydraulic and flood-loss modeling of levee,floodplain, and river management strategies,Middle Mississippi River,USA 总被引:2,自引:2,他引:2
In this investigation, four scenarios were used to quantify the balance between the benefits of levees for flood protection
and their potential to increase flood risk using Hazards U.S. Multi-Hazard flood-loss software and hydraulic modeling of the
Middle Mississippi River (MMR). The goals of this study were (1) to quantify the flood exposure under different flood-control
configurations and (2) to assess the relative contributions of various engineered structures and flood-loss strategies to
potential flood losses. Removing all the flood-control structures along the MMR, without buyouts or other mitigation, reduced
the average flood stages between 2.3 m (100-year flood) and 2.5 m (500-year), but increased the potential flood losses by
$4.3–6.7 billion. Removing the agricultural levees downstream of St. Louis decreased the flood stages through the metro region
by ~1.0 m for the 100- and 500-year events; flood losses, without buyouts or other mitigation, were increased by $4.3–6.7 billion. Removing the agricultural levees downstream of St. Louis decreased the flood stages through the metro region
by ~1.0 m for the 100- and 500-year events; flood losses, without buyouts or other mitigation, were increased by 155 million
for the 100-year flood, but were decreased by $109 million for the 500-year flood. Thus, agricultural levees along the MMR
protect against small- to medium-size floods (up to the ~100-year flood level) but cause more damage than they prevent during
large floods such as the 500-year flood. Buyout costs for the all the buildings within the 500-year floodplain downstream
of urban flood-control structures near St. Louis are ~40% less than the cost of repairing the buildings damaged by the 500-year
flood. This suggests large-scale buyouts could be the most cost-effective option for flood loss mitigation for properties
currently protected by agricultural levees. 相似文献