全文获取类型
收费全文 | 353篇 |
免费 | 5篇 |
国内免费 | 6篇 |
专业分类
测绘学 | 25篇 |
大气科学 | 17篇 |
地球物理 | 63篇 |
地质学 | 191篇 |
海洋学 | 4篇 |
天文学 | 46篇 |
综合类 | 5篇 |
自然地理 | 13篇 |
出版年
2022年 | 7篇 |
2021年 | 8篇 |
2020年 | 9篇 |
2019年 | 5篇 |
2018年 | 27篇 |
2017年 | 30篇 |
2016年 | 27篇 |
2015年 | 22篇 |
2014年 | 28篇 |
2013年 | 29篇 |
2012年 | 17篇 |
2011年 | 22篇 |
2010年 | 18篇 |
2009年 | 17篇 |
2008年 | 13篇 |
2007年 | 6篇 |
2006年 | 15篇 |
2005年 | 4篇 |
2004年 | 8篇 |
2003年 | 4篇 |
2002年 | 1篇 |
2001年 | 2篇 |
2000年 | 2篇 |
1999年 | 2篇 |
1998年 | 2篇 |
1997年 | 2篇 |
1996年 | 3篇 |
1995年 | 1篇 |
1994年 | 1篇 |
1993年 | 3篇 |
1992年 | 3篇 |
1991年 | 4篇 |
1990年 | 3篇 |
1989年 | 1篇 |
1987年 | 1篇 |
1984年 | 5篇 |
1981年 | 2篇 |
1980年 | 1篇 |
1979年 | 1篇 |
1974年 | 1篇 |
1973年 | 4篇 |
1971年 | 3篇 |
排序方式: 共有364条查询结果,搜索用时 781 毫秒
1.
The kinetic Alfven waves in the presence of homogeneous magnetic field plasma with multi-ions effect are investigated. The dispersion relation and normalised damping rate are derived for low-\(\beta\) plasma using kinetic theory. The effect of density variation of \(\text{H}^{+}\), \(\text{He}^{+}\) and \(\text{O}^{+}\) ions is observed on frequency and damping rate of the wave. The variation of frequency (\(\omega\)) and normalised damping rate (\(\gamma / \varOmega_{H^{ +}} \)) of the wave are studied with respect to \(k_{ \bot} \rho_{j}\), where \(k_{ \bot} \) is the perpendicular wave number, \(\rho_{j}\) is the ion gyroradius and \(j \) denotes \(\text{H}^{+}\), \(\text{He}^{+}\) and \(\text{O}^{+}\) ions. The variation with \(k_{ \bot} \rho_{j}\) is considered over wide range. The parameters appropriate to cusp region are used for the explanation of results. It is found that with hydrogen and helium ions gyration, the frequency of wave is influenced by the density variation of \(\text{H}^{+}\) and \(\text{He}^{+}\) ions but remains insensitive to the change in density of \(\text{O}^{+}\) ions. For oxygen ion gyration, the frequency of wave varies over a short range only for \(\text{O}^{+}\) ion density variation. The wave shows damping at lower altitude due to variation in density of lighter \(\text{H}^{+}\) and \(\text{He}^{+}\) ions whereas at higher altitude only heavy \(\text{O}^{+}\) ions contribute in wave damping. The damping of wave may be due to landau damping or energy transfer from wave to particles. The present study signifies that the both lighter and heavier ions dominate differently to change the characteristics of kinetic Alfven wave and density variation is also an important parameter to understand wave phenomena in cusp region. 相似文献
2.
SHRIMP zircon dating and Sm/Nd isotopic investigations of Neoproterozoic granitoids, Eastern Desert, Egypt 总被引:2,自引:0,他引:2
Ewais M.M. Moussa Robert J. Stern William I. Manton Kamal A. Ali 《Precambrian Research》2008,160(3-4):341-356
There is an increasing evidence for the involvement of pre-Neoproterozoic zircons in the Arabian–Nubian Shield, a Neoproterozoic crustal tract that is generally regarded to be juvenile. The source and significance of these xenocrystic zircons are not clear. In an effort to better understand this problem, older and younger granitoids from the Egyptian basement complex were analyzed for chemical composition, SHRIMP U–Pb zircon ages, and Sm–Nd isotopic compositions. Geochemically, the older granitoids are metaluminous and exhibit characteristics of I-type granites and most likely formed in a convergent margin (arc) tectonic environment. On the other hand, the younger granites are peraluminous and exhibit the characteristics of A-type granites; these are post-collisional granites. The U–Pb SHRIMP dating of zircons revealed the ages of magmatic crystallization as well as the presence of slightly older, presumably inherited zircon grains. The age determined for the older granodiorite is 652.5 ± 2.6 Ma, whereas the younger granitoids are 595–605 Ma. Xenocrystic zircons are found in most of the younger granitoid samples; the xenocrystic grains are all Neoproterozoic, but fall into three age ranges that correspond to the ages of other Eastern Desert igneous rocks, viz. 710–690, 675–650 and 635–610 Ma. The analyzed granitoids have (+3.8 to +6.5) and crystallization ages, which confirm previous indications that the Arabian–Nubian Shield is juvenile Neoproterozoic crust. These results nevertheless indicate that older Neoproterozoic crust contributed to the formation of especially the younger granite magmas. 相似文献
3.
M. Tiwari R. Ramesh B. L. K. Somayajulu A. J. T. Jull G. S. Burr 《Geo-Marine Letters》2006,26(1):23-30
Rapid shifts in past climate recorded in polar ice sheets have elicited various explanations relating to either thermohaline
circulation changes by ice-rafting or natural greenhouse gas concentrations modulated by climatic conditions in the tropics.
To compare the tropical paleoclimate record with the polar record, one must choose sediment cores from highly productive ocean
regions. Necessarily, such regions reflect the wind records in the tropics, because high productivity is associated with upwelling
driven by winds. Comparing tropical precipitation records with high-latitude records is, however, a more difficult task because
sediments recording paleoprecipitation usually have low sedimentation rates, and offer coarser resolution relative to polar
ice cores. Here, we present δ
18O data of three planktonic species of Foraminifera (a proxy for precipitation) from such a sediment core, spanning the past
35 ka for the equatorial Indian Ocean, which falls under the southwest monsoon (SWM) realm. Results show that minimum SWM
precipitation occurred at the Last Glacial Maximum, with a subsequent increase at Termination IA. During the Holocene, SWM
precipitation intensified uniformly up to the core top (∼2.2 ka b.p.), as revealed by generally decreasing δ
18O values. Variations in precipitation are consistent with climate changes recorded in polar ice sheets. Although the different resolutions of the two records preclude
a rigorous comparison, abrupt cooling/warming events appear to be accompanied by sudden reduction/enhancement in (SWM) rainfall.
Thus, mechanisms with time scales much shorter than a millennium, such as natural greenhouse warming (e.g., CH4 concentration), controlled by emissions from the tropics, could have played a major role in high-latitude climate change. 相似文献
4.
Airborne LiDAR has found application in an increasing number of mapping and Geo-data acquisition tasks. Apart from terrain
information generation, applications such as automatic detection and modeling of objects like buildings or vegetation for
the generation of 3-D city models have been explored. Besides the height itself, height texture defined by local variations
of the height is a significant parameter for object recognition. The paper explores the potential of the analysis of height
texture as a cue for the automatic detection of objects in LiDAR datasets. A number of texture measures were computed. Based
on their definition and computation these measures were used as bands in a classification algorithm, and objects like buildings,
single trees, and roads could be recognized. 相似文献
5.
A great sunspot group of October 1991 (NOAA 6891 atL = 189,B = –10) was observed, in which an eruptive flare of importance (X2.5/31B) has been reported. A Caii-line 3933 filter was used, and during a time interval of 3 hours more than 90 frames were taken, using a CCD camera. The progress and the distribution in size of the two main flare points have been obtained; they covered an area of about 1800 millionths of the solar disk within a time interval of about 24 min. The average apparent horizontal velocity was 70 km s–1. 相似文献
6.
Periodicities in natural remanent magnetization and palaeoclimatic records detected by walsh spectra
The time series consisting of nonsinusoidal Natural Remanent Magnetization (NRM) and palaeoclimatic variations for the past two million years have been spectrally examined by using a new Walsh transform technique. The results show statistically significant periodicities (at 95% confidence level) of approximately 92,000 years; 43,000 years and 21,000 years in the ensemble spectra of the NRM intensity (mineralogic) variations. These NRM periodicities are remarkably close to the well-known Milankovitch cycles. The ensemble spectra of palaeoclimatic records reveal only 41,000 years statistically significant (95%) periodicity corresponding to the obliquity cycle. The study suggests that NRM variations in deep sea sediments probably are more sensitive recorders of palaeoclimatic memory than the oxygen isotope variations. 相似文献
7.
Subrata Kumar Majumder Kamal ChandnaDhiren Sankar De Gautam Kundu 《International Journal of Mineral Processing》2006
Experiments were carried out to investigate the rheological properties of coal–oil–water suspension containing solids of different sizes. Two different coal samples with mean particle sizes of 120 mesh, 175 mesh and 220 mesh were used. The coal concentration was varied from 5% to 25% by weight. Sodium silicate has been used as an additive to study the behavior of the variation of average viscosity of the suspension. A generalized correlation has been developed to predict the average viscosity of suspension in terms of particle diameter of the coal, concentration of coal, viscosity of the suspending medium and the concentration of water. Experimental investigations revealed that coal–oil–water suspensions show an increase in the viscosity with decrease in coal size but with the addition of an additive, the average viscosity tends to decrease initially up to a certain optimum dosages and thereafter it increases with further addition of additives. Two empirical correlations are proposed for average viscosity of the coal–oil–water suspension, μsL in terms of physical properties of the solid and viscosity of the suspending medium with and without additives. 相似文献
8.
9.
10.
Neotectonic Control on the Geomorphic Evolution of the Gangotri Glacier Valley, Garhwal Himalaya 总被引:1,自引:0,他引:1
The paper records evidences of neotectonic activities in the Gangotri glacier valley that are found to be responsible for the present-day geomorphic set-up of the area since the last phase of major glaciation. Geomorphological features indicate the presence of a large glacier in the valley in the geological past. Prominent planar structures present in the rocks were later on modified into sets of normal faults in the present-day Himalayan tectonic set-up giving rise to graben structures. The block nearest the snout is traversed by the NW-SE trending Gaumukh fault. A number of terraces mark the entrenchment of Bhagirathi River in this part. The contrasting drainage morphometric parameters of two sides of the valley and asymmetric recessional patterns of the tributary glaciers further document movement along the fault. The distribution and orientation of debris fans also seem to be controlled by neotectonic activity. The neotectonic activity that followed the process of deglaciation has brought the glacially carved, wide U- shaped valley in contact with the present-day fluvially incised narrow and relatively deep valley. The wider segments have become sites of active deposition of glacially eroded debris. The low gradient and excessive filling has resulted in the river attaining a braided nature in these segments. 相似文献