全文获取类型
收费全文 | 110270篇 |
免费 | 1902篇 |
国内免费 | 1530篇 |
专业分类
测绘学 | 2548篇 |
大气科学 | 7701篇 |
地球物理 | 21624篇 |
地质学 | 40038篇 |
海洋学 | 10081篇 |
天文学 | 24712篇 |
综合类 | 445篇 |
自然地理 | 6553篇 |
出版年
2022年 | 820篇 |
2021年 | 1343篇 |
2020年 | 1412篇 |
2019年 | 1546篇 |
2018年 | 3076篇 |
2017年 | 2854篇 |
2016年 | 3352篇 |
2015年 | 1851篇 |
2014年 | 3270篇 |
2013年 | 5756篇 |
2012年 | 3688篇 |
2011年 | 4864篇 |
2010年 | 4364篇 |
2009年 | 5501篇 |
2008年 | 4799篇 |
2007年 | 4905篇 |
2006年 | 4615篇 |
2005年 | 3332篇 |
2004年 | 3225篇 |
2003年 | 2990篇 |
2002年 | 2924篇 |
2001年 | 2513篇 |
2000年 | 2496篇 |
1999年 | 1978篇 |
1998年 | 2079篇 |
1997年 | 1915篇 |
1996年 | 1645篇 |
1995年 | 1657篇 |
1994年 | 1389篇 |
1993年 | 1305篇 |
1992年 | 1244篇 |
1991年 | 1246篇 |
1990年 | 1256篇 |
1989年 | 1099篇 |
1988年 | 1019篇 |
1987年 | 1230篇 |
1986年 | 1057篇 |
1985年 | 1276篇 |
1984年 | 1487篇 |
1983年 | 1375篇 |
1982年 | 1283篇 |
1981年 | 1207篇 |
1980年 | 1118篇 |
1979年 | 1027篇 |
1978年 | 995篇 |
1977年 | 896篇 |
1976年 | 874篇 |
1975年 | 833篇 |
1974年 | 824篇 |
1973年 | 925篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
992.
The separation of the influence of nutrients and climate on the varve time-series of baldeggersee, Switzerland 总被引:7,自引:2,他引:7
The varve data-set from a freeze-core taken in the deepest part of Baldeggersee was subjected to different multivariate statistical
analyses in order to estimate the amount of variance in the varve thickness measures explicable by past climate and by the
trophic state of the lake. A comparison of two different time-periods (1902 to1992 versus 1920 to 1980) revealed that the
lake restoration programme since 1982 has had a significant impact on the formation of the seasonal layers. Results of the
partitioning of the variance in the varve thickness measures showed that about two thirds of the variance are unexplained
by a climate and trophic state model and that trophic state explains 6%, whereas climate accounts for about 28% of the variance
before the effect of lake restoration had a strong impact on the varves. Among the climate parameters the amount of annual
precipitation is a strong predictor for explaining the thickness of both dark layer and total couplet thickness, whereas summer
precipitation is important for the thickness of the light layer. 相似文献
993.
—An algorithm has been developed to compute the dispersive and dissipative seismic response using FUTTERMAN’S (1962) third attenuation-dispersion relationship. In the computation, frequency-dependent velocity and quality factor Q have been used but in the case of the nondispersive synthetic seismogram, frequency-independent velocity has been used. The model’s parameters are density, phase velocity, quality factors and thicknesses of the layers. Dispersive and nondispersive synthetic seismograms have been computed with and without absorption for a layered earth geological model. Fast Fourier transform (FFT) technique has been adopted for converting the frequency domain response into the time domain. The frequency spacing, Δf = 0.976?Hz, has been considered to avoid the aliasing effect. The results have revealed changes in the reflected waveforms in the frequency domain as well as in the time domain for absorption and dispersion cases. It is also concluded that dispersion reduces the arrival time and this effect is increasing with the travel time. The effect of constant Q on the seismic response has also been studied. 相似文献
994.
—The clustering structure of the Vesuvian earthquakes occurring is investigated by means of statistical tools: the inter-event time distribution, the running mean and the multifractal analysis. The first cannot clearly distinguish between a Poissonian process and a clustered one due to the difficulties of clearly distinguishing between an exponential distribution and a power law one. The running mean test reveals the clustering of the earthquakes, but looses information about the structure of the distribution at global scales. The multifractal approach can enlighten the clustering at small scales, while the global behaviour remains Poissonian. Subsequently the clustering of the events is interpreted in terms of diffusive processes of the stress in the earth crust. 相似文献
995.
A. Grafe P. A. Bespalov V. Y. Trakhtengerts A. G. Demekhov 《Annales Geophysicae》1997,15(12):1537-1547
For four geomagnetic storms of middle intensity the relationship between the low-latitude magnetic field asymmetry using ASY indices and the intensity of the auroral eastward and westward electro-jet was considered. It was asked whether there exists a connection between ASY and the eastward electrojet. To answer this question equivalent current systems were estimated in mid-latitudes. It was found that the observations obviously show no correlative relationship between the low-latitude magnetic-field asymmetry and the eastward electrojet, whereas one exists between ASY and the westward electrojet. To explain the generally accepted common three-dimensional current system between the partial ring current and the eastward electrojet, a condensor model of the three-dimensional current system was developed. It could be shown that the short periodic variations of the partial ring current are shielded by the condensor and cannot influence the eastward-electrojet current. 相似文献
996.
The electron cyclotron waves that originate at low altitudes (<0.5 RE) and observed by ground facilities have been studied in the presence of a weak parallel electric field in auroral magnetoplasma consisting of trapped energetic auroral electrons and cold background electrons of ionospheric origin. The model distribution for auroral trapped electrons is taken as Maxwellian ring distribution. An expression for the growth rate has been obtained in the presence of parallel electric field assuming that the real frequency in the whistler mode is not affected by the presence of the electric field. The results show that waves grow (or damp) in amplitude for a parallel (or antiparallel) electric field. The influence of the electric field is more pronounced at a shorter wavelength spectrum. An increase in population of energetic electrons increases the growth rate and thus, plays a significant role in the wave excitation process in the auroral regions. 相似文献
997.
Study of sporadic-E clouds by backscatter radar 总被引:1,自引:0,他引:1
It is shown that swept-frequency backscatter ionograms covering a range of azimuths can be used to study the dynamics of sporadic-E clouds. A simple technique based on analytic ray tracing can be used to simulate the observed narrow traces associated with Es patches. This enables the location and extent of the sporadic-E clouds to be determined. The motion of clouds can then be determined from a time sequence of records. In order to demonstrate the method, results are presented from an initial study of 5 days of backscatter ionograms from the Jindalee Stage B data base obtained during March-April 1990. Usually 2–3 clouds were observed each day, mainly during the evening and up to midnight. The clouds lasted from 1–4 h and extended between 30°–80° in azimuth and 150–800 km in range. The clouds were mostly stationary or drifted generally westward with velocities of up to 80 ms–1. Only one cloud was observed moving eastward. 相似文献
998.
999.
1000.