全文获取类型
收费全文 | 70731篇 |
免费 | 918篇 |
国内免费 | 628篇 |
专业分类
测绘学 | 2143篇 |
大气科学 | 4936篇 |
地球物理 | 14073篇 |
地质学 | 24828篇 |
海洋学 | 5786篇 |
天文学 | 16872篇 |
综合类 | 233篇 |
自然地理 | 3406篇 |
出版年
2021年 | 640篇 |
2020年 | 676篇 |
2019年 | 728篇 |
2018年 | 1736篇 |
2017年 | 1576篇 |
2016年 | 2020篇 |
2015年 | 1144篇 |
2014年 | 1953篇 |
2013年 | 3599篇 |
2012年 | 2147篇 |
2011年 | 2775篇 |
2010年 | 2405篇 |
2009年 | 3264篇 |
2008年 | 2753篇 |
2007年 | 2758篇 |
2006年 | 2626篇 |
2005年 | 2038篇 |
2004年 | 1981篇 |
2003年 | 1897篇 |
2002年 | 1840篇 |
2001年 | 1647篇 |
2000年 | 1586篇 |
1999年 | 1383篇 |
1998年 | 1362篇 |
1997年 | 1340篇 |
1996年 | 1174篇 |
1995年 | 1146篇 |
1994年 | 1044篇 |
1993年 | 898篇 |
1992年 | 830篇 |
1991年 | 862篇 |
1990年 | 934篇 |
1989年 | 836篇 |
1988年 | 788篇 |
1987年 | 952篇 |
1986年 | 805篇 |
1985年 | 996篇 |
1984年 | 1110篇 |
1983年 | 1049篇 |
1982年 | 966篇 |
1981年 | 900篇 |
1980年 | 814篇 |
1979年 | 750篇 |
1978年 | 741篇 |
1977年 | 691篇 |
1976年 | 652篇 |
1975年 | 645篇 |
1974年 | 647篇 |
1973年 | 695篇 |
1972年 | 437篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
131.
The eleventh list of faint late M and carbon type stars detected on the plates of the First Byurakan Spectral Survey in zone -7° -3° and covering about 1000 square degrees is presented. From 126 detected stars, 88 are newly discovered objects: they are 6 carbon stars, 8 carbon star candidates, and 74 M-type stars; among the latter 38 (26 PSC + 12 FSC) are unclassified IRAS sources, and one object is an unclassified ROSAT source. Distances to the 6 newly discovered early-type carbon stars are estimated. Equatorial coordinates, red magnitudes, and spectral classes determined from the Palomar E-charts are provided. The lack of optical counterparts on Palomar O and E maps for two detected late M-type stars indicates a large variability in brightnesses of these objects (amplitude not smaller than 7.0 magnitude). 相似文献
132.
133.
Impacts of primary deforestation upon limestone slopes in northern Vancouver Island, British Columbia 总被引:1,自引:0,他引:1
Effects of deforestation upon slopes in limestones and in volcanic rocks in the Benson River valley, northern Vancouver Island, have been investigated quantitatively. Postlogging soil erosion and vegetal regeneration success were assessed by measuring soil depth, percent bare rock and moss cover, and the numbers and diversity of trees, shrubs, and plants on 25 sampling sites, each containing ten measuring quadrats selected at random. Sixteen sites were on the Quatsino Formation, a well-karstified limestone, and nine on the Karmutsen Formation of basaltic lavas. Eight sites were of virgin forest, 16 were logged between 1970 and 1983, and one (on limestone) was logged in 1911. Both bedrock types were significantly affected by the cutting. There was greater loss of soil and an increase in bare rock on the limestones. Erosion was increased significantly by burning on the limestones but not on the volcanics. Within-group comparisons on the limestones determined that steeper slopes and harder burned areas suffered the most and are slowest to regenerate. Volume of timber on the 1911 site was 19 percent of that in similar uncut forest sites. It appears that complete recovery on the barren limestone slopes will require at least some centuries. 相似文献
134.
135.
G. Ajeagah Aghaindum S. Foto Menbohan 《International Journal of Environmental Science and Technology》2012,9(4):655-660
The optimum period for ciliated protozoa colonizing of an artificial substrate, the polyurethane foams have been assessed in a tropical aquatic ecosystem, the Ekozoa stream of the Mfoundi River Basin in Yaounde (Cameroon). 5?days were calculated as the highest period for the biological indicators of pollution to optimally colonize the artificial substrate. This time interval is the same for all the sampling stations assessed from upstream to downstream and the various microhabitats along the water course. The statistical method applied is that of the completely randomized blocks. The colonization of the substrate increases from the first day to the fifth day, before decreasing to the tenth day. The statistical analysis of variance between the maximum day and the other sampling period was significant at 5?% while the calculation of the value between different points of the same station was not significant. The average number of ciliated protozoan ranges from 20 to 23, from upstream to downstream. 相似文献
136.
It is suggested that the pattern of three-dimensional substorm current circuit varies significantly even during the lifetime of a single substorm. This gives rise to quite complex time variations of the magnetic field at low latitude stations even for relatively isolated substorms. To verify this, three-dimensional current models with time dependent spatial variations are used to simulate one type of complex low-latitude “substorm signature”. It is shown that the utmost care should be exercised in determining different substorm phases on the basis of such a signature. The results indicate also that, in certain longitudes in the evening sector, one should expect distinct differences in characteristics between positive bays observed on the ground and at the synchronous distance. 相似文献
137.
A model is proposed that shows the relation of the block structure of the crust and earthquake sources (Sadovskii, 1979; Rodionov, 1979, 1984, 1994; Bugaev, 1999, 2011, 2014, 2015). The model can formalize how to assess the prediction of seismic regime parameters depending on the elastic limit and conditions and rate of deformation of the Earth’s crust. The spent nuclear fuel repository site in Olkiluoto (Finland) and a site in the area of the Krasnoyarsk Mining and Chemical Combine are considered as examples. It is demonstrated that the parameters of the prediction graphs limit the location of the points of magnitude repeatability graphs calculated for a site based on samples of earthquakes in the area according to different authors. This makes it possible to recommend predictive assessment of seismic regime parameters for stability monitoring of the seismic regime and safety analysis of a geological environment’s insulation properties for waste sites from the results of seismological monitoring and high-precision observations of modern movements of the Earth’s crust. 相似文献
138.
Meysam Fadaee I. Anastasopoulos G. Gazetas M.K. Jafari M. Kamalian 《地震工程与工程振动(英文版)》2013,12(3):473-486
When seismic thrust faults emerge on the ground surface,they are particularly damaging to buildings,bridges and lifelines that lie on the rupture path.To protect a structure founded on a rigid raft,a thick diaphragm-type soil bentonite wall(SBW) is installed in front of and near the foundation,at sufficient depth to intercept the propagating fault rupture.Extensive numerical analyses,verified against reduced–scale(1 g) split box physical model tests,reveal that such a wall,thanks to its high deformability and low shear resistance,"absorbs" the compressive thrust of the fault and forces the rupture to deviate upwards along its length.As a consequence,the foundation is left essentially intact.The effectiveness of SBW is demonstrated to depend on the exact location of the emerging fault and the magnitude of the fault offset.When the latter is large,the unprotected foundation experiences intolerable rigid-body rotation even if the foundation structural distress is not substantial. 相似文献
139.
K. Schneider-Carius 《Pure and Applied Geophysics》1949,15(3-4):236-241
Ohne Zusammenfassung 相似文献
140.
Wiener optimal filtering of GRACE data 总被引:4,自引:0,他引:4
We present a spatial averaging method for Gravity Recovery and Climate Experiment (GRACE) gravity-field solutions based on
the Wiener optimal filtering.
The optimal filter is designed from the least-square minimization of the difference between the desired and filtered signals.
It requires information about the power spectra of the desired gravitational signal and the contaminating noise, which is
inferred from the average GRACE degree-power spectrum. We show that the signal decreases with increasing spherical harmonic
degree j with approximately j−b, where b = 1.5 for GRACE data investigations. This is termed the Second Kaula rule of thumb for temporal variations of the
Earth’s gravity field. The degree power of the noise increases, in the logarithmic scale, linearly with increasing j.
The Wiener optimal filter obtained for the signal model with b = 1.5 closely corresponds to a Gaussian filter with a spatial
half width of 4° (∼440 km). We find that the filtered GRACE gravity signal is relatively insensitive to the exponent b of
the signal model, which indicates the robustness of Wiener optimal filtering. This is demonstrated using the GFZ-GRACE gravity-field
solution for April 2004. 相似文献