全文获取类型
收费全文 | 1166篇 |
免费 | 179篇 |
国内免费 | 310篇 |
专业分类
测绘学 | 46篇 |
大气科学 | 128篇 |
地球物理 | 289篇 |
地质学 | 635篇 |
海洋学 | 168篇 |
天文学 | 248篇 |
综合类 | 67篇 |
自然地理 | 74篇 |
出版年
2024年 | 4篇 |
2023年 | 16篇 |
2022年 | 40篇 |
2021年 | 36篇 |
2020年 | 46篇 |
2019年 | 43篇 |
2018年 | 35篇 |
2017年 | 29篇 |
2016年 | 43篇 |
2015年 | 46篇 |
2014年 | 51篇 |
2013年 | 51篇 |
2012年 | 43篇 |
2011年 | 60篇 |
2010年 | 57篇 |
2009年 | 79篇 |
2008年 | 81篇 |
2007年 | 100篇 |
2006年 | 73篇 |
2005年 | 64篇 |
2004年 | 58篇 |
2003年 | 50篇 |
2002年 | 53篇 |
2001年 | 49篇 |
2000年 | 59篇 |
1999年 | 61篇 |
1998年 | 47篇 |
1997年 | 30篇 |
1996年 | 35篇 |
1995年 | 27篇 |
1994年 | 27篇 |
1993年 | 21篇 |
1992年 | 26篇 |
1991年 | 20篇 |
1990年 | 14篇 |
1989年 | 19篇 |
1988年 | 7篇 |
1987年 | 16篇 |
1986年 | 6篇 |
1985年 | 12篇 |
1984年 | 9篇 |
1983年 | 2篇 |
1982年 | 2篇 |
1981年 | 1篇 |
1978年 | 3篇 |
1977年 | 2篇 |
1954年 | 2篇 |
排序方式: 共有1655条查询结果,搜索用时 31 毫秒
101.
The concentrations of lipids were determined in atmospheric particle, gas and rain samples collected from the tropical North Pacific to assess lipid sources, transport mechanisms and fluxes to the ocean surface. Four lipid compound classes (aliphatic hydrocarbons, fatty alcohols, fatty acid esters, and salts) all unequivocally show a terrestrial vascular plant source. These aerosol lipids originate from wind erosion of Asian and American soils and direct emission from vegetation. The major fluxes result from rain rather than dry deposition. These fluxes are large enough to have a major potential impact on the inventory of terrestrially derived lipid material found in deep-sea sediments. This has been showm for n-alkanes, fatty alcohols, fatty acids, total lipids and for organic carbon. By comparing atmospheric and sediment trap fluxes with sediment accumulation rates, it is suggested that some biogenic terrestrial material is more protected from degradation than marine-derived material. 相似文献
102.
Results from long term microerosion meter (MEM) studies indicate that microenvironments are characterized by different erosion rates. A complementary study, carried out on the same sites, examines the microscale morphology in each environment. Optical and scanning electron microscopy were used to identify a range of features thought to reflect different processes. Under subaerial conditions micromorphology is dominated by biological weathering features owing to the presence of microflora. Sites permanently covered by soil or stream water have only dissolution etch features, though stream sites may exhibit microfractures. Sites with periodic exposure exhibit both dissolution and biological etch features. The natural environment is more accurately reflected by allowing some microflora colonization of MEM sites rather than preventing colonization by artificial means. 相似文献
103.
电子探针分析与系列矿物研究 总被引:2,自引:0,他引:2
本文论述以电子探针为主要测试手段对下述几种系列矿物的研究成果:(1)铂族元素系列矿物的研究(包括Ru-Os-Fe金属互化物矿物系列和RuAs_2-OsAs_2二元固溶体矿物系);(2)Au-Cu系列矿物的研究;(3)黝铜矿族矿物的研究。 相似文献
104.
尽管角闪石在地壳中大量存在 ,但是蓝色钾 钠透闪石却是角闪石中的稀有品种。本文采用X射粉晶线衍射和晶胞参数计算 ,确定该矿物为蓝色的钾 钠透闪石 ,用电子探针 (EMPA)精确分析了它的化学成分 ,确定为钾 钠透闪石 ,而不是蓝色软玉 (即不是透闪石或阳起石 )。 相似文献
105.
106.
Transport and deposition of suspended particles in saturated porous media: hydrodynamic effect 总被引:1,自引:1,他引:1
Nasre-Dine Ahfir Hua Qing Wang Ahmed Benamar Adbellah Alem Nicolas Massei Jean-Paul Dupont 《Hydrogeology Journal》2007,15(4):659-668
A laboratory study was undertaken to determine the transport and deposition rate of suspended particles in columns of saturated porous media (gravel and glass beads), where the porous media were subjected to steady-state flow. Silt particles with a mode of 14 μm diameter (used as the suspended particles) and fluorescein (as the conservative tracer) were injected into the columns in short pulses. The breakthrough curves were competently described with the analytical solution of a convection–dispersion equation with a first-order deposition rate. The experiments were performed using different flow rates. The suspended particle size distribution, the porous media, and the flow rates themselves were the main factors retained in this study to investigate the mechanisms governing the transport and deposition kinetics in detail. The results showed the existence of a flow rate, beyond which suspended particles travel faster than the conservative tracer. A decrease of the deposition rate of suspended particles beyond a critical flow velocity was also observed. Such behaviour led to consideration of the couple hydrodynamic-gravity forces at high flow rates. As the hydrodynamic force increases, particle deposition rates are reduced due to the effect of hydrodynamic forces inhibiting the deposition. 相似文献
107.
North End Lake is a polluted and eutrophic freshwater system located in Port Elizabeth, South Africa. Since the lake is expected
to be used for recreational/tourist purposes by 2010, a rehabilitation program will have to be designed. For this reason,
we retrieved a sediment core from the central region of the lake to decipher the effect of historical human impacts on the
water body. Pre-disturbance paleolimnological inferences indicate that the lake was likely mesotrophic. After ∼1831, when
sheep farming activities were undertaken in the catchment, increases in trophic state and changes in sediment composition
were observed. After ∼1937, increases in trace metal levels, organic matter, spheroidal carbonaceous particles (SCP) and changes
in sediment composition were recorded. The system became eutrophic as indicated by the dominance of the diatom Actinocyclus normanii, a cosmopolitan species often observed in systems where water quality has been dramatically degraded. The conditions worsened
after 1986 because of the construction of a storm-water retention system, which intentionally channeled storm-water runoff
into the lake. Because of this, extremely high values of fecal coliforms (i.e. 2 × 106 every 100 ml) have been measured in the water column. The paleolimnological information identified the sharp increase in
organic content in the uppermost section of the core, and this could be correlated to the operation of the storm-water retention
system. Therefore, as an immediate management measure, we suggest that the storm-water retention system should either no longer
be utilized, or the storm-water runoff should be treated before disposal into the lake. In addition, an effective sewage system
has to be constructed. 相似文献
108.
109.
110.
E. L. Afraimovich E. A. Kosogorov N. P. Perevalova A. V. Plotnikov 《Journal of Atmospheric and Solar》2001,63(18)
This paper is concerned with the parameters of shock-acoustic waves (SAW) generated during rocket launchings. We have developed the interferometric method for determining SAW parameters (including angular characteristics of the wave vector, and the SAW phase velocity, as well as the direction towards the source) using GPS-arrays. Contrary to the conventional radio-probing techniques, the proposed method provides an estimate of SAW parameters without a priori information about the site and time of a rocket launching. The application of the method is illustrated by a case study of ionospheric effects from launchings of rockets PROTON, SOYUZ and SPACE SHUTTLE from Baikonur and Kennedy Space Center cosmodromes in 1998–2000. In spite of a difference of rocket characteristics, the ionospheric response for all launchings had the character of an N-wave corresponding to the form of a shock wave. The SAW period T is 270–360 s, and the amplitude exceeds the standard deviation of total electron content background fluctuations in this range of periods under quiet and moderate geomagnetic conditions by factors of 2–5 as a minimum. The angle of elevation of the SAW wave vector varies from 30° to 60°, and the SAW phase velocity (900–1200 m/s) approaches the sound velocity at heights of the ionospheric F-region maximum. The position of the SAW source, inferred by neglecting refraction corrections, corresponds to the segment of the rockets path at a distance no less than 200–900 km from the launch pad, and to the rocket flying altitude no less than 100 km. Our data are consistent with the existing view that SAW are generated during a nearly horizontal flight of the rocket with its engine in operation in the acceleration segment of the path at 100–130 km altitudes in the lower atmosphere. 相似文献