全文获取类型
收费全文 | 51947篇 |
免费 | 508篇 |
国内免费 | 291篇 |
专业分类
测绘学 | 1376篇 |
大气科学 | 3848篇 |
地球物理 | 9347篇 |
地质学 | 21723篇 |
海洋学 | 3948篇 |
天文学 | 9924篇 |
综合类 | 211篇 |
自然地理 | 2369篇 |
出版年
2021年 | 275篇 |
2020年 | 301篇 |
2019年 | 307篇 |
2018年 | 3665篇 |
2017年 | 3426篇 |
2016年 | 2374篇 |
2015年 | 598篇 |
2014年 | 835篇 |
2013年 | 1566篇 |
2012年 | 1974篇 |
2011年 | 3878篇 |
2010年 | 3517篇 |
2009年 | 3872篇 |
2008年 | 3120篇 |
2007年 | 3764篇 |
2006年 | 1223篇 |
2005年 | 1323篇 |
2004年 | 1204篇 |
2003年 | 1217篇 |
2002年 | 1026篇 |
2001年 | 752篇 |
2000年 | 741篇 |
1999年 | 556篇 |
1998年 | 524篇 |
1997年 | 603篇 |
1996年 | 472篇 |
1995年 | 476篇 |
1994年 | 480篇 |
1993年 | 388篇 |
1992年 | 388篇 |
1991年 | 363篇 |
1990年 | 380篇 |
1989年 | 345篇 |
1988年 | 348篇 |
1987年 | 375篇 |
1986年 | 330篇 |
1985年 | 432篇 |
1984年 | 421篇 |
1983年 | 449篇 |
1982年 | 418篇 |
1981年 | 381篇 |
1980年 | 424篇 |
1979年 | 328篇 |
1978年 | 303篇 |
1977年 | 297篇 |
1976年 | 271篇 |
1975年 | 266篇 |
1974年 | 274篇 |
1973年 | 257篇 |
1971年 | 176篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
101.
Himalayan magmatism and porphyry copper–molybdenum mineralization in the Yulong ore belt, East Tibet
Summary ?The NW–SE-trending Yulong porphyry Cu–Mo ore belt, situated in the Sanjiang0 area of eastern Tibet, is approximately 400 km
long and 35 to 70 km wide. Complex tectonic and magmatic processes during the Himalayan epoch have given rise to favorable
conditions for porphyry-type Cu–Mo mineralization.
Porphyry masses of the Himalayan epoch in the Yulong ore belt are distributed in groups along regional NW–SE striking tectonic
lineaments. They were emplaced mainly into Triassic and Lower Permian sedimentary-volcanic rocks. K–Ar und U–Pb isotopic datings
give an intrusion age range of 57–26 Ma. The porphyries are mainly of biotite monzogranitic and biotite syenogranitic compositions.
Geological and geochemical data indicate that the various porphyritic intrusions in the belt had a common or similar magma
source, are metaluminous to peraluminous, Nb–Y–Ba-depleted, I-type granitoids, and belong to the high-K calc-alkaline series.
Within the Yulong subvolcanic belt a number of porphyry stocks bear typical porphyry type Cu–Mo alteration and mineralization.
The most prominent porphyry Co–Mo deposits include Yulong, Malasongduo, Duoxiasongduo, Mangzong and Zhanaga, of which Yulong
is one of the largest porphyry Cu (Mo) deposits in China with approximately 8 × 106 tons of contained Cu metal. Hydrothermal alteration at Yulong developed around a biotite–monzogranitic porphyry stock that
was emplaced within Upper Triassic limestone, siltstone and mudstone. The earliest alteration was due to the effects of contact
metamorphism of the country rocks and alkali metasomatism (potassic alteration) within and around the porphyry body. The alteration
of this stage was accompanied by a small amount of disseminated and veinlet Cu–Mo sulfide mineralization. Later alteration–mineralization
zones form more or less concentric shells around the potassic zone, around which are distributed a phyllic or quartz–sericite–pyrite
zone, a silicification and argillic zone, and a propylitic zone.
Fluid inclusion data indicate that three types of fluids were involved in the alteration–mineralization processes: (1) early
high temperature (660–420 °C) and high salinity (30–51 wt% NaCl equiv) fluids responsible for the potassic alteration and
the earliest disseminated and/or veinlet Cu–Mo sulfide mineralization; (2) intermediate unmixed fluids corresponding to phyllic
alteration and most Cu–Mo sulfide mineralization, with salinities of 30–50 wt% NaCl equiv and homogenization temperatures
of 460–280 °C; and (3) late low to moderate temperature (300–160 °C) and low salinity (6–13 wt% NaCl equiv) fluids responsible
for argillic and propylitic alteration. Hydrogen and oxygen isotopic studies show that the early hydrothermal fluids are of
magmatic origin and were succeeded by increasing amounts of meteoric-derived convective waters. Sulfur isotopes also indicate
a magmatic source for the sulfur in the early sulfide mineralization, with the increasing addition of sedimentary sulfur outward
from the porphyry stock.
Received August 29, 2001; revised version accepted May 1, 2002
Published online: November 29, 2002 相似文献
102.
The collisional instability of the drift wave in a multi-component plasma is investigated. It is shown that when the electron and ion density gradients are different, e.g., due to the presence of a static third component or due to neutral drag effects, the drift mode becomes unstable. The instability is caused by the simultaneous action of the electron collisions with all other plasma species and the spatial difference of the density of the plasma components. This instability may be expected as a natural consequence of the stratification of a multi-component plasma placed in an external gravity field where it can operate for any amount of charge on heavy particles. Therefore it could develop in weakly ionized cold interstellar regions for example, when the heavy particles, i.e. charged grains, are a few tens of Å in size, and carry typically ±1,±2 charge. In the solar atmosphere, it may appear in the weakly ionized photospheric layers due to the convective motion of the neutral component. 相似文献
103.
We have studied the variations of the height of polar crown prominences according to daily observations of the Sun at the Kodaikanal Observatory (India) during 1905–1975. Polar ring filaments at latitudes 60°–80° are related to the polar magnetic field reversal. A double decrease of the height of polar ring filaments was found in the course of their migration from 40°to the poles. We estimated the limiting height of the equilibrium of polar ring filaments from the stability condition of a strong electric current. We found that the transition from large-scale to small-scale ring filaments reduces the critical height of the stability for the prominences. A model of an inverse-polarity filament was used. 相似文献
104.
105.
106.
Globorotalia puncticulata and Globorotalia margaritae are critical species that define internationally recognized planktonic foraminiferal biozones in the Pliocene. These biozones are defined from stratotype sections on Sicily and their fauna are commonly considered to have been introduced to the Mediterranean after an influx of Atlantic water that terminated the late Miocene desiccation of the basin. Herein new discoveries of these species in rocks that predate the late Miocene desiccation are described. These data are supported by magneto- and lithostratigraphy that have been integrated at a single continuous section. Not only do these discoveries question the existing foraminiferal biozone stratigraphy, they also suggest new models for the dispersal of planktonic species. It is proposed that Globorotalia puncticulata and perhaps even Globorotalia margaritae evolved in the Mediterranean during earliest Messinian times (during or before chron C3Bn1n) and dispersed into the Atlantic. This suggests that a marine connection remained between the two sea areas until at least chron C3An.1n. Using the existing geomagnetic polarity time scale, these occurrences are some 2 Myr earlier than previously recorded in the Mediterranean. The distribution of G. margaritae and G. puncticulata in Mediterranean sections is likely to reflect palaeoenvironment or the preservation of deposits rather than the absolute age of the sediments. 相似文献
107.
108.
A new method of initial orbit determination 总被引:2,自引:0,他引:2
Up to now we have been dealing with the construction of entirely analytical planetary theories such as VSOP82 (Bretagnon, 1982) and TOP82 (Simon, 1983). These theories take into account the whole of the Newtonian perturbations of nine point masses: the Sun, the Earth-Moon barycentre, the planets Mercury, Venus, Mars, Jupiter, Saturn, Uranus and Neptune. They also take into account perturbations due to some minor planets, to the action of the Moon and the relativistic effects. The perturbations of these last three types are in a very simple way under analytical form but they considerably increase the computations when introduced in the numerical integration programs.In the present paper we thus study a solution in which the Newtonian perturbations for the ten point masses are treated through numerical integration, the other perturbations being analytically added. 相似文献
109.
Bjerrum-Niese C. Bjorno L. Pinto M.A. Quellec B. 《Oceanic Engineering, IEEE Journal of》1996,21(2):143-149
The paper discusses the development of a simulation tool to model high data-rate acoustic communication in shallow water. The simulation tool is able to generate synthetic time series of signals received at a transducer array after transmission across a shallow-water communication channel. The simulation tool is suitable for testing advanced signal processing techniques for message recovery. A channel model has been developed based on the physical aspects of the acoustic channel. Special emphasis has been given to fluctuations of the signal transmission caused by time-varying multipath effects. At shorter ranges, the temporal variations are dominated by acoustic scattering from the moving sea surface. Therefore, the channel model produces a coherence function which may be interpreted as a time-varying reflection coefficient for the surface scattered acoustical path. A static, range-independent ray model identifies the significant multipaths, and the surface path is modulated with the time-varying reflection coefficient. The advantages and limitations of the channel model are discussed and assumptions necessary to overcome the limitations are emphasised. Based on the assumptions, an algorithm has been developed and implemented to model how a binary message will be modulated when transmitted by a transducer, is distorted in the channel and finally is received by a transducer array 相似文献
110.