全文获取类型
收费全文 | 866篇 |
免费 | 67篇 |
国内免费 | 12篇 |
专业分类
测绘学 | 16篇 |
大气科学 | 58篇 |
地球物理 | 286篇 |
地质学 | 312篇 |
海洋学 | 106篇 |
天文学 | 93篇 |
综合类 | 5篇 |
自然地理 | 69篇 |
出版年
2024年 | 5篇 |
2023年 | 8篇 |
2022年 | 7篇 |
2021年 | 24篇 |
2020年 | 34篇 |
2019年 | 35篇 |
2018年 | 36篇 |
2017年 | 39篇 |
2016年 | 47篇 |
2015年 | 39篇 |
2014年 | 46篇 |
2013年 | 50篇 |
2012年 | 50篇 |
2011年 | 59篇 |
2010年 | 60篇 |
2009年 | 48篇 |
2008年 | 41篇 |
2007年 | 40篇 |
2006年 | 29篇 |
2005年 | 30篇 |
2004年 | 35篇 |
2003年 | 22篇 |
2002年 | 24篇 |
2001年 | 16篇 |
2000年 | 18篇 |
1999年 | 12篇 |
1998年 | 9篇 |
1997年 | 11篇 |
1996年 | 11篇 |
1995年 | 5篇 |
1994年 | 5篇 |
1993年 | 6篇 |
1992年 | 3篇 |
1990年 | 4篇 |
1989年 | 3篇 |
1988年 | 3篇 |
1987年 | 3篇 |
1986年 | 4篇 |
1985年 | 4篇 |
1984年 | 1篇 |
1983年 | 1篇 |
1982年 | 4篇 |
1981年 | 4篇 |
1980年 | 1篇 |
1979年 | 1篇 |
1977年 | 2篇 |
1975年 | 3篇 |
1973年 | 1篇 |
1971年 | 1篇 |
1970年 | 1篇 |
排序方式: 共有945条查询结果,搜索用时 640 毫秒
841.
Pietro Mantovan Laura Menegazzo Vitturi Bruno Pavoni Sandro Rabitti 《Continental Shelf Research》1985,4(3):321-340
Data concerning grain-size distribution, mineralogy, geochemistry, and chlorinated hydrocarbon concentrations, obtained from 246 surface sediment samples collected in the Northern Adriatic sea (Italian area), provide the basis for a multivariate statistical analysis which evaluates the influence of riverine inputs, the differences among various depositional environments and the presence of polluted zones.Cluster analysis applied to grain size has identified nine groups of lithotypes associated with the morphology and the hydrology of the basin.In the Gulf of Venice, principal component analysis on clay minerals and carbonates has permitted recognition of two main areas located north and south of the Brenta river. More subareas correspond to other lithological types.Principal component analysis of nine metal (Hg, Cd, Cu, Pb, Cr, Zn, Fe, Ni, and Co) concentrations reveals that the studied sea area is more complex from the geochemical point of view. Three geochemical zones related to riverine inputs, different lithotypes and specific forms of contamination are identified. High pollution of both halocarbons and heavy metals is associated with fine grain size. 相似文献
842.
Ohne Zusammenfassung 相似文献
843.
Paleomagnetic apparent polar wander paths provide a method for estimating minimum plate velocities which can be extended much further into the geologic past than can be estimated based on sea-floor magnetic anomalies. Minimum velocities can be determined from the rate of change of latitude as derived relative to a fixed pole. Such latitudinal velocities have been determined for the center of mass of the North American, Baltic, Siberian and African shields since the Archean. The results indicate that plate velocities in the past have at times easily exceeded those for present-day continents, and that they often were equivalent to present-day oceanic velocities, although there are peaks and troughs through time. North American velocities are significantly greater prior to one billion years ago than those of Siberia or Baltica. 相似文献
844.
Bruno Capaccioni Laura Valentini Marco B. L. Rocchi Giovanni Nappi Damiano Sarocchi 《Bulletin of Volcanology》1997,58(7):501-514
Computer-assisted image analysis can be successfully used to derive quantitative textural data on pyroclastic rock samples.
This method provides a large number of different measurements such as grain size, particle shape and 2D orientation of particle
main axes (directional- or shape-fabric) automatically and in a relatively short time. Orientation data reduction requires
specific statistical tests, mainly devoted to defining the kind of particle distribution pattern, the possible occurrence
of preferred particle orientation, the confidence interval of the mean direction and the degree of randomness with respect
to pre-assigned theoretical frequency distributions. Data obtained from image analysis of seven lithified ignimbrite samples
from the Vulsini Volcanic District (Central Italy) are used to test different statistics and to provide insight about directional
fabrics. First, the possible occurrence of a significant deviation from a theoretical circular uniform distribution was evaluated
by using the Rayleigh and Tukey χ
2 tests. Then, the Kuiper test was performed to evaluate whether or not the observation fits with a unimodal, Von Mises-like
theoretical frequency distribution. Finally, the confidence interval of mean direction was calculated. With the exception
of one sample (FPD10), which showed a well-developed bimodality, all the analysed samples display significant anisotropic
and unimodal distributions. The minimum number of measurements necessary to obtain reasonable variabilities of the calculated
statistics and mean directions was evaluated by repeating random collections of the measured particles at increments of 100
particles for each sample. Although the observed variabilities depend largely on the pattern of distribution and an absolute
minimum number cannot be stated, approximately 1500–2000 measurements are required in order to get meaningful mean directions
for the analysed samples.
Received: 9 April 1996 / Accepted: 26 December 1996 相似文献
845.
Gerrit de Leeuw Leo Cohen Lise Marie Frohn Gary Geernaert Ole Hertel Bjarne Jensen Tim Jickells Laura Klein Gerard J. Kunz Soren Lund Marcel Moerman Frank Müller Britta Pedersen Knut von Salzen K. Heinke Schlünzen Michael Schulz Carsten A. Skjth Lise-Lotte Sorensen Lucinda Spokes Susanne Tamm Elisabetta Vignati 《Continental Shelf Research》2001,21(18-19)
The aim of the atmospheric nitrogen inputs into the coastal ecosystem (ANICE) project is to improve transport–chemistry models that estimate nitrogen deposition to the sea. To achieve this, experimental and modelling work is being conducted which aims to improve understanding of the processes involved in the chemical transformation, transport and deposition of atmospheric nitrogen compounds. Of particular emphasis within ANICE is the influence of coastal zone processes. Both short episodes with high deposition and chronic nitrogen inputs are considered in the project. The improved transport–chemistry models will be used to assess the atmospheric inputs of nitrogen compounds into the European regional seas (the North Sea is studied as a prototype) and evaluate the impact of various emission reduction strategies on the atmospheric nitrogen loads. Assessment of the impact of atmospheric nitrogen on coastal ecosystems will be based on comparisons of phytoplankton nitrogen requirements, other external nitrogen inputs to the ANICE area of interest and the direct nitrogen fluxes provided by ANICE. Selected results from both the experimental and modelling components are presented here. The experimental results show the large spatial and temporal variability in the concentrations of gaseous nitrogen compounds, and their influences on fluxes. Model calculations show the strong variation of both concentrations and gradients of nitric acid at fetches of up to 25 km. Aerosol concentrations also show high temporal variability and experimental evidence for the reaction between nitric acid and sea salt aerosol is provided by size-segregated aerosol composition measured at both sides of the North Sea. In several occasions throughout the experimental period, air mass back trajectory analysis showed connected flow between the two sampling sites (the Weybourne Atmospheric Observatory on the North Norfolk coast of the UK and Meetpost Noordwijk, a research tower at 9 km off the Dutch coast). Results from the METRAS/SEMA mesoscale chemistry transport model system for one of these cases are presented. Measurements of aerosol and rain chemical composition, using equipment mounted on a commercial ferry, show variations in composition across the North Sea. These measurements have been compared to results obtained with the transport–chemistry model ACDEP which calculates the atmospheric inputs into the whole North Sea area. Finally, the results will be made available for the assessment of the impact of atmospheric nitrogen on coastal ecosystems. 相似文献
846.
847.
Elio Antonello Massimo Fracassini Laura Enrica Pasinetti Livio Pastori 《Astrophysics and Space Science》1982,83(1-2):381-390
All the radial velocities (RV) available in the literature since the beginning of the century for the Be stars EW Lac, 28 Tau, Tau, KX And, KY And, CX Dra and 88 Her, are assembled. When possible, a periodogram analysis has been performed to search for eventual periodic phenomena.The following conclusions have been drawn: EW Lac could be a spectroscopic binary with a period of about 40 years; 28 Tau shows some indications of regular long-term RV variations on the time scale of decades; a probable recurrent shell activity of Tau could be hypothesized. Some useful indications for KX and KY And are given, whereas it is not possible to draw any conclusion for CX Dra and 88 Her owing to the lack of data. 相似文献
848.
849.
Quite frequently naval and ocean structural designers confront the problem of a plate or slab which supports a motor or engine which excites, dynamically, the structural element. Very often the excitation is treated as a concentrated force but, obviously, it acts over a finite area. The present study deals with the title problem for several combinations of boundary conditions by means of a variational approach and also using the finite element method. Dynamic displacement and bending moment amplitudes are determined. 相似文献
850.