全文获取类型
收费全文 | 3048篇 |
免费 | 92篇 |
国内免费 | 30篇 |
专业分类
测绘学 | 53篇 |
大气科学 | 250篇 |
地球物理 | 725篇 |
地质学 | 1231篇 |
海洋学 | 269篇 |
天文学 | 366篇 |
综合类 | 6篇 |
自然地理 | 270篇 |
出版年
2021年 | 44篇 |
2020年 | 45篇 |
2019年 | 60篇 |
2018年 | 90篇 |
2017年 | 46篇 |
2016年 | 83篇 |
2015年 | 64篇 |
2014年 | 80篇 |
2013年 | 153篇 |
2012年 | 106篇 |
2011年 | 117篇 |
2010年 | 105篇 |
2009年 | 139篇 |
2008年 | 132篇 |
2007年 | 125篇 |
2006年 | 132篇 |
2005年 | 106篇 |
2004年 | 110篇 |
2003年 | 107篇 |
2002年 | 104篇 |
2001年 | 63篇 |
2000年 | 64篇 |
1999年 | 50篇 |
1998年 | 50篇 |
1997年 | 39篇 |
1996年 | 37篇 |
1995年 | 36篇 |
1994年 | 38篇 |
1993年 | 39篇 |
1992年 | 41篇 |
1991年 | 28篇 |
1990年 | 31篇 |
1989年 | 37篇 |
1988年 | 31篇 |
1987年 | 41篇 |
1986年 | 33篇 |
1985年 | 34篇 |
1984年 | 43篇 |
1983年 | 43篇 |
1982年 | 38篇 |
1981年 | 34篇 |
1980年 | 43篇 |
1979年 | 23篇 |
1978年 | 20篇 |
1977年 | 33篇 |
1976年 | 18篇 |
1975年 | 20篇 |
1974年 | 20篇 |
1973年 | 19篇 |
1972年 | 14篇 |
排序方式: 共有3170条查询结果,搜索用时 15 毫秒
101.
Sets of 20 soda ash glasses, 16 soda lime glasses and 23 wood ash glasses mainly from excavations in Europe (additional soda ash glasses from Egypt) were analysed on 61 chemical elements. Average SiO2 is about 62% in soda glasses and 50% in wood ash glasses. The three groups of glasses contain on average 13% Na2O, 18% Na2O and 13% K2O as fluxes to lower the melting temperature of quartz at their production. The starting materials beside quartz were halophytic plant ash for soda ash glass, trona (Na3H(CO3)2·2H2O) and lime (clamshells) for soda lime glass and beech ash for wood ash glass. Each of the three major glass types contains specific Rare Earth Element (REE) concentrations mainly contained in quartz and its intergrown minerals. 50 Paleozoic and Mesozoic sandstones from Central Europe represent the quartz composition. The REE pattern of these glasses apparently indicates major compositional stages of the Continental Earth's Crust. The boron to lithium and sodium to potassium ratios as in seawater suggest reactions of materials for soda glass with seawater. Negative Ce anomalies in the three glasses are caused by reactions of quartz with seawater. 相似文献
102.
Stanley N. Davis Stephen Moysey DeWayne L. Cecil Marek Zreda 《Hydrogeology Journal》2003,11(2):217-227
Natural production of the radionuclide chlorine-36 (36Cl) has provided a valuable tracer for groundwater studies. The nuclear industry, especially the testing of thermonuclear
weapons, has also produced large amounts of 36Cl that can be detected in many samples of groundwater. In order to be most useful in hydrologic studies, the natural production
prior to 1952 should be distinguished from more recent artificial sources. The object of this study was to reconstruct the
probable preanthropogenic levels of 36Cl in groundwater in the United States. Although significant local variations exist, they are superimposed on a broad regional
pattern of 36Cl/Cl ratios in the United States. Owing to the influence of atmospherically transported ocean salt, natural ratios of 36Cl/total Cl are lowest near the coast and increase to a maximum in the central Rocky Mountains of the United States.
Electronic Publication 相似文献
103.
Miguel A. Ortega-Huerta Enrique Martinez-Meyer Stephen L. Egbert Kevin P. Price A. Townsend Peterson 《国际地球制图》2013,28(3):7-20
Abstract An important methodological and analytical requirement for analyzing spatial relationships between regional habitats and species distributions in Mexico is the development of standard methods for mapping the country's land cover/land use formations. This necessarily involves the use of global data such as that produced by the Advanced Very High Resolution Radiometer (AVHRR). We created a nine‐band time‐series composite image from AVHRR Normalized Difference Vegetation Index (NDVI) bi‐weekly data. Each band represented the maximum NDVI for a particular month of either 1992 or 1993. We carried out a supervised classification approach, using the latest comprehensive land cover/vegetation map created by the Mexican National Institute of Geography (INEGI) as reference data. Training areas for 26 land cover/vegetation types were selected and digitized on the computer's screen by overlaying the INEGI vector coverage on the NDVI image. To obtain specific spectral responses for each vegetation type, as determined by its characteristic phenology and geographic location, the statistics of the spectral signatures were subjected to a cluster analysis. A total of 104 classes distributed among the 26 land cover types were used to perform the classification. Elevation data were used to direct classification output for pine‐oak and coastal vegetation types. The overall correspondence value of the classification proposed in this paper was 54%; however, for main vegetation formations correspondence values were higher (60‐80%). In order to obtain refinements in the proposed classification we recommend further analysis of the signature statistics and adding topographic data into the classification algorithm. 相似文献
104.
Josef Vesely Heather Almquist-Jacobson Lisa M. Miller Stephen A. Norton Peter Appleby Aruna S. Dixit John P. Smol 《Journal of Paleolimnology》1993,8(3):211-231
This multi-disciplinary investigation documents the longterm effects of atmospheric pollution of metals and acids on a geologically sensitive catchment in the umava Mountains, southwestern Czech Republic, a region with a long history of human disturbance. A 30 cm long sediment core (I) from ertovo Lake was analyzed for natural and artifical radionuclides, metals, diatoms, chrysophytes, and pollen in sediments accumulated during the last 200 years. A second core (II), extending to 95 cm, included sediment judged to be free of atmospheric deposition of pollutants associated with the Industrial Revolution. Chronostratigraphic markers include several changes in the pollen assemblages corresponding to well-documented changes in land-use, and distinct distributions of 137Cs, 134Cs and 241Am from weapons testing and the 1986 nuclear accident at Chernobyl, Russia. These markers corroborate the 210Pb dating and, together, produce a reliable chronology extending back nearly to 1800 A.D.Stratigraphic profiles of Cu, Pb, and Zn in Core I are unlike any previously reported in the literature. Concentrations of Cu, Pb, and Zn remain generally above 100, 400, and 200 g g-1, respectively, for the 200 years represented by Core I. These values are unusually high for sediments from a watershed with no known heavy-metal ore bodies. Accumulation rates for Cu, Pb, and Zn, which include both atmospheric and watershed contributions, are also high (ca 1, > 1 and > 1 g cm-2 yr-1, respectively) for the same period, although the anthropogenic contribution of Zn rose from nearly zero at 1800 A.D. The Cu and Pb accumulation rates rose dramatically about 1640 A.D.Accumulation rates of anthropogenically-derived Be, a relatively abundant element in the soft coals of the region, are also elevated by about 0.01 g cm-2 yr-1 in sediments of this period. Vanadium accumulation rates increased only since 1980 A.D., presumably along with increased consumption of oil.Diatom assemblages illustrate that the lake was acidic (pH between 4.5 and 5) through at least the past 200 years. The pH declined significantly (from ca 5 to 4) between 1960 and 1985 with a slight increase to 4.5 in the last few years. Recent diatom and chrysophyte assemblages suggest high trace metal concentrations, consistent with the present lake-water chemistry. 相似文献
105.
Ben Buse John C. Schumacher R. Stephen J. Sparks Matthew Field 《Contributions to Mineralogy and Petrology》2010,160(4):533-550
Metamorphic assemblages within Karoo basalt xenoliths, found within volcaniclastic kimberlite of the B/K9 pipe, Damtshaa, Botswana, constrain conditions of kimberlite alteration. Bultfonteinite and chlorite partially replace the original augite-plagioclase assemblage, driven by the serpentinisation of the kimberlite creating strong chemical potential gradients for Si and Mg. Hydrogarnet and serpentine replace these earlier metamorphic assemblages as the deposits cool. The bultfonteinite (ideally Ca2SiO2[OH,F]4) and hydrogarnet assemblages require a water-rich fluid containing F−, and imply hydrothermal alteration dominated by external fluids rather than autometamorphism from deuteric fluids. Bultfonteinite and hydrogarnet are estimated to form at temperatures of ca. 350–250°C, which are similar to those for serpentinisation. Alteration within the B/K9 kimberlite predominantly occurs between 250 and 400°C. We attribute these conditions to increased efficiency of mass transfer and chemical reactions below the critical point of water and a consequence of volume-increasing serpentinisation and metasomatic reactions that take place over this temperature range. A comparison of the B/K9 kimberlite with kimberlites from Venetia, South Africa suggests that the composition and mineralogy of included xenoliths affects the alteration assemblages within kimberlite deposits. 相似文献
106.
Karl Lumme 《Icarus》1975,24(4):483-491
The true shape of the Cassini division of Saturn's rings in red, yellow, green, and blue light has been derived from photographs of high quality, taking into account the correction due to the spreading of light. It is evident that the division is not completely dark. Its minimum brightness is about 20% of that of ring B and the width of the division 0″.44±0″.05. 相似文献
107.
Guenther Eichhorn Alberto Accomazzi Carolyn S. Grant Michael J. Kurtz Stephen S. Murray 《Astrophysics and Space Science》2002,282(1):299-340
The Astrophysics Data System (ADS) provides access to the astronomical literature through the World Wide Web. It is a NASA
funded project and access to all the ADS services is free to everybody world-wide. The ADS Abstract Service allows the searching
of four databases with abstracts in Astronomy, Instrumentation, Physics/Geophysics, and the LANL Preprints with a total of
over 2.2 million references. The system also provides access to reference and citation information, links to on-line data,
electronic journal articles, and other on-line information. The ADS Article Service contains the full articles for most of
the astronomical literature back to volume 1. It contains the scanned pages of all the major journals (Astrophysical Journal,
Astronomical Journal, Astronomy & Astrophysics, Monthly Notices of the Royal Astronomical Society, and Solar Physics), as
well as most smaller journals back to volume 1.The ADS can be accessed through any web browser without signup or login. Alternatively
an email interface is available that allows our users to execute queries via email and to retrieve scanned articles via email.
This might be interesting for users on slow or unreliable links, since the email system will retry sending information automatically
until the transfer is complete. There are now 9 mirror sites of the ADS available in different parts of the world to improve
access. The ADS is available at: http://ads.harvard.edu
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
108.
Stephen R. Lewis Peter L. Read Barney J. Conrath John C. Pearl Michael D. Smith 《Icarus》2007,192(2):327-347
The Thermal Emission Spectrometer aboard the Mars Global Surveyor spacecraft has produced an extensive atmospheric data set, beginning during aerobraking and continuing throughout the extended scientific mapping phase. Temperature profiles for the atmosphere below about 40 km, surface temperatures and total dust and water ice opacities, can be retrieved from infrared spectra in nadir viewing mode. This paper describes assimilation of nadir retrievals from the spacecraft aerobraking period, LS=190°–260°, northern hemisphere autumn to winter, into a Mars general circulation model. The assimilation scheme is able to combine information from temperature and dust optical depth retrievals, making use of a model forecast containing information from the assimilation of earlier observations, to obtain a global, time-dependent analysis. Given sufficient temperature retrievals, the assimilation procedure indicates errors in the a priori dust distribution assumptions even when lacking dust observations; in this case there are relatively cold regions above the poles compared to a model which assumes a horizontally-uniform dust distribution. One major reason for using assimilation techniques is in order to investigate the transient wave behavior on Mars. Whilst the data from the 2-h spacecraft mapping orbit phase is much more suitable for assimilation, even the longer (45–24 h) period aerobraking orbit data contain useful information about the three-dimensional synoptic-scale martian circulation which the assimilation procedure can reconstruct in a consistent way. Assimilations from the period of the Noachis regional dust storm demonstrate that the combined assimilation of temperature and dust retrievals has a beneficial impact on the atmospheric analysis. 相似文献
109.
Raymond F. Smith K. Thomas Lorenz Darwin Ho Bruce A. Remington Alex Hamza John Rogers Stephen Pollaine Seokwoo Jeon Yun-Suk Nam J. Kilkenny 《Astrophysics and Space Science》2007,307(1-3):269-272
In recently developed laser-driven shockless compression experiments an ablatively driven shock in a primary target is transformed
into a ramp compression wave in a secondary target via unloading followed by stagnation across an intermediate vacuum gap.
Current limitations on the achievable peak longitudinal stresses are limited by the ability of shaping the temporal profile
of the ramp compression pulse. We report on new techniques using graded density reservoirs for shaping the loading profile
and extending these techniques to high peak pressures. 相似文献
110.