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   638篇
  免费   21篇
  国内免费   19篇
测绘学   5篇
大气科学   37篇
地球物理   135篇
地质学   262篇
海洋学   45篇
天文学   145篇
综合类   18篇
自然地理   31篇
  2024年   1篇
  2023年   5篇
  2022年   4篇
  2021年   17篇
  2020年   6篇
  2019年   22篇
  2018年   34篇
  2017年   34篇
  2016年   34篇
  2015年   30篇
  2014年   33篇
  2013年   42篇
  2012年   21篇
  2011年   39篇
  2010年   40篇
  2009年   38篇
  2008年   40篇
  2007年   43篇
  2006年   25篇
  2005年   19篇
  2004年   20篇
  2003年   20篇
  2002年   17篇
  2001年   19篇
  2000年   14篇
  1999年   8篇
  1998年   13篇
  1997年   3篇
  1996年   2篇
  1995年   6篇
  1994年   5篇
  1993年   5篇
  1992年   2篇
  1991年   1篇
  1990年   2篇
  1989年   3篇
  1981年   2篇
  1972年   1篇
  1965年   1篇
  1964年   1篇
  1963年   1篇
  1962年   2篇
  1961年   1篇
  1960年   2篇
排序方式: 共有678条查询结果,搜索用时 312 毫秒
661.
Uranium minerals from the San Marcos District, Chihuahua, Mexico   总被引:1,自引:0,他引:1  
The mineralogy of the two uranium deposits (Victorino and San Marcos I) of Sierra San Marcos, located 30 km northwest of Chihuahua City, Mexico, was studied by optical microscopy, powder X-ray diffraction with Rietveld analysis, scanning electron microscopy with energy dispersive X-ray analysis, inductively coupled plasma spectrometry, and gamma spectrometry. At the San Marcos I deposit, uranophane Ca(UO2)2Si2O7·6(H2O) (the dominant mineral at both deposits) and metatyuyamunite Ca(UO2)(V2O8)·3(H2O) were observed. Uranophane, uraninite (UO2+x), masuyite Pb(UO2)3O3(OH)·3(H2O), and becquerelite Ca(UO2)6O4(OH)6 ·(8H2O) are present at the Victorino deposit. Field observations, coupled with analytical data, suggest the following sequence of mineralization: (1) deposition of uraninite, (2) alteration of uraninite to masuyite, (3) deposition of uranophane, (4) micro-fracturing, (5) calcite deposition in the micro-fractures, and (6) formation of becquerelite. The investigated deposits were formed by high-to low-temperature hydrothermal activity during post-orogenic evolution of Sierra San Marcos. The secondary mineralization occurred through a combination of hydrothermal and supergene alteration events. Becquerelite was formed in situ by reaction of uraninite with geothermal carbonated solutions, which led to almost complete dissolution of the precursor uraninite. The Victorino deposit represents the second known occurrence of becquerelite in Mexico, the other being the uranium deposits at Peña Blanca in Chihuahua State.  相似文献   
662.
The long term systematic investigations of ion composition components (bicarbonates, chlorides, sulphates, magnesium, calcium, sodium, potassium) and trophic status components (suspension, chlorophyll-a, mineral and organic forms phosphorus and nitrogen, carbon, silica) of water from the lake proper, its tributaries and atmospheric precipitation, which make up the main share of substance supply in to the lake, as well as the Angara river, being the source of substance discharge allowed the lake recent state to be evaluated in terms of both chemical pollution and possible eutrophication of its water. The similar (in terms of river runojf) periods of 50-ies and 80-ies have been compared. It was revealed that as a result of industrial activity 409 thousand tons of mineral substances (27.2 thousand tons of chlorides, 162 thousand tons of sulphates and over 200 thousand tons of organic substances) are supplied in the lake annually. The supply of substances of the anthropogenic origin is higher for the South Baikal than that for the North and Middle Baikal (the sum of mineral substances in 3.6 times; sulphates in 5 times, organic substances, including hydrocarbons in 7 times).The absence of abundant phytoplankton in the period studied when the ratio of silica to phosphorus is optimum (over 100), as well as a revers correlation between winter nitrogen content and spring of chlorophyll-a concentration in the Baikal water and revers dependence between the suspension and chlorophyll-a along the Selenga river valley lead to the conclusion that Baikal water contains toxicants. It is verified by the presence of polychlorbiphenyls (PCB), poly-chlordibenzo-p-dioxins (PCDD) and dibenzofurans (PCDF) for the whole food web with the maximum PCDD/PCDF (TEQ to 175 P g · g-1) concentration in the seal blubber. The levels are comparable with those reported for ringed seal (phoca hispida), living in the Baltic sea and Bar-row Strait Inlet in the Canadian Arctic.  相似文献   
663.
In this paper, the particle acceleration processes around magnetotail dipolarization fronts(DFs) were reviewed. We summarize the spacecraft observations(including Cluster, THEMIS, MMS) and numerical simulations(including MHD, testparticle, hybrid, LSK, PIC) of these processes. Specifically, we(1) introduce the properties of DFs at MHD scale, ion scale, and electron scale,(2) review the properties of suprathermal electrons with particular focus on the pitch-angle distributions,(3)define the particle-acceleration process and distinguish it from the particle-heating process,(4) identify the particle-acceleration process from spacecraft measurements of energy fluxes, and(5) quantify the acceleration efficiency and compare it with other processes in the magnetosphere(e.g., magnetic reconnection and radiation-belt acceleration processes). We focus on both the acceleration of electrons and ions(including light ions and heavy ions). Regarding electron acceleration, we introduce Fermi,betatron, and non-adiabatic acceleration mechanisms;regarding ion acceleration, we present Fermi, betatron, reflection, resonance, and non-adiabatic acceleration mechanisms. We also discuss the unsolved problems and open questions relevant to this topic, and suggest directions for future studies.  相似文献   
664.
 A complex method for assessing the ecological state of groundwater by using indicators and indexes of underground hydrosphere sustainable development, and of characterizing its quality, is given for an oil field. The concept of forming indicators and indexes of sustainable development has been elaborated for this purpose. When forming a structure of environment indicators, the developments of the European Organization for Economic Co-operation and Development (OECD) in this sphere were considered. New indicators are given and a structure of indexes quantitatively characterizing them is worked out. Indexes can be simple, complex, muldidisciplinary and inter-multidisciplinary by their structure. By their essence indexes are subdivided into five classes: indexes of quality, damage, interaction, hydrogeochemical and dynamic ones. The ecological state of groundwater is assessed by the following categories of sustainability: sustainable, slightly unsustainable, medium unsustainable, unsustainable, highly unsustainable, catastrophically unsustainable. Received: 23 November 1998 · Accepted: 14 September 1999  相似文献   
665.
666.
 Analysis and hydrogeochemical modeling of hydrocarbonate groundwater, including its buffering geochemical properties, have demonstrated that this water has undergone a geochemical transformation almost throughout the whole of one industrial region. It is known that hydrocarbonate groundwater possesses a high protective natural geochemical potential, supporting neutralization of acid atmospheric precipitation. Natural hydrocarbonate water undergoes three stages of anthropogenic transformation caused by acid atmospheric precipitation over more than 50 years. The first stage is transformation of neutral hydrocarbonate water into alkaline water, accompanied by calcite precipitation and sodium carbonate dissolution from the unsaturated zone. The second stage is transformation of alkaline water into neutral hydrocarbonate water; the hydrocarbonate content, being less than the lower limit for background concentrations, showed reduced water buffering properties or protective potential relative to acid precipitation. The third stage is transformation of neutral hydrocarbonate water into acidic water, with a complete loss of protective geochemical potential. This unfavorable ecological situation with natural geochemically hydrocarbonate groundwater shows that natural groundwater less protected from the impact of acid atmospheric precipitation is in a worse ecological condition, which is confirmed by observations in northern and Western Europe and other regions of the world. Received: 30 October 1997 · Accepted: 23 March 1998  相似文献   
667.
The growing attention to the development of coastal zone resources is notable not only for moderate and southern latitudes but also in the Arctic regions. In Russia this process has a history related to exploration of non-renewable resources and the long-term policy of moving labour forces to the Far North and East. The role of the Russian Arctic coastal zone in the national economy had been stimulated by active usage of the Northern Sea Route for 60 years as a mainstream Arctic transport line. Diversified human usage of all kinds of natural resources is getting more controversial. A deep revaluation of existing methods and management approaches is needed in accordance with the complexities of Russian social/economic and ecological factors in the Arctic. The modern challenge of transition towards sustainable development also needs to be reevaluated. The last decade has shown a real effort of all Arctic countries to expand cooperation in joint scientific multidisciplinary studies. The studies are aimed at elaboration of common views on Arctic development and protection of environment. Moreover, the Arctic shelf and coastal zone are getting a new arena of international investments for industrial projects and implementation due to recovery of huge deposits of oil and gas resources of sophisticated technologies. All this makes international scientific cooperation more of a reality.  相似文献   
668.
The Adige River drains 12 200 km2 of the Eastern Alps and flows for 213 km within this mountain range. Similar to other large rivers in Central Europe, the Adige River was subject to massive channelization works during the 19th century. Thanks to the availability of several historical maps, this river represents a very valuable case study to document the extent to which the morphology of the river changed due to channelization and to understand how much is left of its original morphodynamics. The study was based on the analysis of seven sets of historical maps dating from 1803–1805 to 1915–1927, on geomorphological analysis, on the application of mathematical morphodynamic theories and on the application of bar and channel pattern prediction models. The study concerns 115 km of the main stem and 29 km of its tributaries. In the pre‐channelization conditions, the Adige River presented a prevalence of single‐thread channel planforms. Multi‐thread patterns developed only immediately downstream of the main confluences. During the 19th century, the Adige underwent considerable channel adjustment, consisting of channel narrowing, straightening, and reduction of bars and islands. Multi‐thread and single‐thread reaches evolved through different evolutionary trajectories, considering both the channel width and the bar/vegetation interaction. Bar and channel pattern predictors showed good correspondence with the observed patterns, including the development of multi‐thread morphologies downstream of the confluences. Application of the free‐bar predictor helped to interpret the strong reduction – almost complete loss – of exposed sediment bars after the channelization works, quantifying the riverbed inclination to form alternate bars. This morphological evolution can be observed in other Alpine rivers of similar size and similar massive channelization, therefore, a simplified conceptual model for large rivers subjected to channelization is proposed, showing that a relatively small difference in the engineered channel width may have a strong impact on the river dynamics, specifically on bar formation. Copyright © 2017 John Wiley & Sons, Ltd.  相似文献   
669.
During the last glacial cycle an intriguing feature of the British-Irish Ice Sheet was the North Sea Lobe (NSL); fed from the Firth of Forth and which flowed south and parallel to the English east coast. The controls on the formation and behaviour of the NSL have long been debated, but in the southern North Sea recent work suggests the NSL formed a dynamic, oscillating terrestrial margin operating over a deforming bed. Further north, however, little is known of the behaviour of the NSL or under what conditions it operated. This paper analyses new acoustic, sedimentary and geomorphic data in order to evaluate the glacial landsystem imprint and deglacial history of the NSL offshore from NE England. Subglacial tills (AF2/3) form a discontinuous mosaic interspersed with bedrock outcrops across the seafloor, with the partial excavation and advection of subglacial sediment during both advance and retreat producing mega-scale glacial lineations and grounding zone wedges. The resultant ‘mixed-bed’ glacial landsystem is the product of a dynamic switch from a terrestrial piedmont-lobe margin with a net surplus of sediment to a partially erosive, quasi-stable, marine-terminating, ice stream lobe as the NSL withdrew northwards. Glaciomarine sediments (AF4) drape the underlying subglacial mixed-bed imprint and point to a switch to tidewater conditions between 19.9 and 16.5 ka cal BP as the North Sea became inundated. The dominant controls on NSL recession during this period were changing ice flux through the Firth of Forth ice stream onset zone and water depths at the grounding line; the development of the mixed-bed landsystem being a response to grounding line instability. © 2018 John Wiley & Sons, Ltd.  相似文献   
670.
Natural Hazards - This paper analyses data regarding the seismicity and ground deformations of the island of Ischia. The goal is to describe these phenomena as a space–time process, exploring...  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