全文获取类型
收费全文 | 27910篇 |
免费 | 484篇 |
国内免费 | 295篇 |
专业分类
测绘学 | 686篇 |
大气科学 | 2141篇 |
地球物理 | 6060篇 |
地质学 | 9708篇 |
海洋学 | 2269篇 |
天文学 | 5779篇 |
综合类 | 51篇 |
自然地理 | 1995篇 |
出版年
2020年 | 176篇 |
2019年 | 179篇 |
2018年 | 347篇 |
2017年 | 327篇 |
2016年 | 493篇 |
2015年 | 378篇 |
2014年 | 505篇 |
2013年 | 1340篇 |
2012年 | 632篇 |
2011年 | 932篇 |
2010年 | 764篇 |
2009年 | 1060篇 |
2008年 | 976篇 |
2007年 | 941篇 |
2006年 | 948篇 |
2005年 | 823篇 |
2004年 | 860篇 |
2003年 | 809篇 |
2002年 | 806篇 |
2001年 | 651篇 |
2000年 | 660篇 |
1999年 | 628篇 |
1998年 | 597篇 |
1997年 | 596篇 |
1996年 | 498篇 |
1995年 | 484篇 |
1994年 | 471篇 |
1993年 | 439篇 |
1992年 | 404篇 |
1991年 | 350篇 |
1990年 | 399篇 |
1989年 | 318篇 |
1988年 | 360篇 |
1987年 | 399篇 |
1986年 | 347篇 |
1985年 | 510篇 |
1984年 | 548篇 |
1983年 | 548篇 |
1982年 | 444篇 |
1981年 | 441篇 |
1980年 | 453篇 |
1979年 | 394篇 |
1978年 | 402篇 |
1977年 | 352篇 |
1976年 | 379篇 |
1975年 | 344篇 |
1974年 | 383篇 |
1973年 | 367篇 |
1972年 | 236篇 |
1971年 | 188篇 |
排序方式: 共有10000条查询结果,搜索用时 9 毫秒
61.
The Culgoora radioheliograph has been modified for observing at 327.4 MHz, which is in addition to the three frequencies (43.25, 80, and 160 MHz) previously available. At the new frequency the array beamwidth is 56, which represents the highest resolution yet available for metre-wavelength solar mapping.At 327.4 MHz the sources of radio emission are mainly in the lowest layers of the corona. Some preliminary four-frequency observations have been made of type I storms. It is found that the source size generally decreases with increasing observing frequency. This result confirms earlier suggestions that the sources of both type I and type III emission are contained in structures whose boundaries diverge outwards in the corona. 相似文献
62.
Quick-look assessments to identify optimal CO2 EOR storage sites 总被引:1,自引:0,他引:1
Vanessa Núñez-López Mark H. Holtz Derek J. Wood William A. Ambrose Susan D. Hovorka 《Environmental Geology》2008,54(8):1695-1706
A newly developed, multistage quick-look methodology allows for the efficient screening of an unmanageably large number of
reservoirs to generate a workable set of sites that closely match the requirements for optimal CO2 enhanced oil recovery (EOR) storage. The objective of the study is to quickly identify miscible CO2 EOR candidates in areas that contain thousands of reservoirs and to estimate additional oil recovery and sequestration capacities
of selected top options through dimensionless modeling and reservoir characterization. Quick-look assessments indicate that
the CO2 EOR resource potential along the US Gulf Coast is 4.7 billion barrels, and CO2 sequestration capacity is 2.6 billion metric tons. In the first stage, oil reservoirs are screened and ranked in terms of
technical and practical feasibility for miscible CO2 EOR. The second stage provides quick estimates of CO2 EOR potential and sequestration capacities. In the third stage, a dimensionless group model is applied to a selected set
of sites to improve the estimates of oil recovery and storage potential using appropriate inputs for rock and fluid properties,
disregarding reservoir architecture and sweep design. The fourth stage validates and refines the results by simulating flow
in a model that describes the internal architecture and fluid distribution in the reservoir. The stated approach both saves
time and allows more resources to be applied to the best candidate sites. 相似文献
63.
G. Caro V.C. Bennett B. Bourdon T.M. Harrison A. von Quadt S.J. Mojzsis J.W. Harris 《Chemical Geology》2008,247(1-2):253-265
146Sm–142Nd and 147Sm–143Nd systematics were investigated in garnet inclusions in diamonds from Finsch (S. Africa) and Hadean zircons from Jack Hills (W. Australia) to assess the potential of these systems as recorders of early Earth evolution. The study of Finsch inclusions was conducted on a composite sample of 50 peridotitic pyropes with a Nd model age of 3.3 Ga. Analysis of the Jack Hills zircons was performed on 790 grains with ion microprobe 207Pb/206Pb spot ages from 3.95 to 4.19 Ga. Finsch pyropes yield 100 × ?142Nd = ? 6 ± 12 ppm, ?143Nd = ? 32.5, and 147Sm/144Nd = 0.1150. These results do not confirm previous claims for a 30 ppm 142Nd excess in South African cratonic mantle. The lack of a 142Nd anomaly in these inclusions suggests that isotopic heterogeneities created by early mantle differentiation were remixed at a very fine scale prior to isolation of the South African lithosphere. Alternatively, this result may indicate that only a fraction of the mantle experienced depletion during the first 400 Myr of its history. Analysis of the Jack Hills zircon composite yielded 100 × ?142Nd = 8 ± 10 ppm, ?143Nd = 45 ± 1, and 147Sm/144Nd = 0.5891. Back-calculation of this present-day ?143Nd yields an unrealistic estimate for the initial ?143Nd of ? 160 ?-units, clearly indicating post-crystallization disturbance of the 147Sm–143Nd system. Examination of 146,147Sm–142,143Nd data reveals that the Nd budget of the Jack Hills sample is dominated by non-radiogenic Nd, possibly contained in recrystallized zircon rims or secondary subsurface minerals. This secondary material is characterized by highly discordant U–Pb ages. Although the mass fraction of altered zircon is unlikely to exceed 5–10% of total sample, its high LREE content precludes a reliable evaluation of 146Sm–142Nd systematics in Jack Hills zircons. 相似文献
64.
65.
66.
J. W. Norman 《The Photogrammetric Record》1968,6(32):133-149
The geologist's use of air photographs and the characteristics studied by photogeologists are briefly described. The influence of photographic factors such as scale, season, time of day, film type, processing, overlap and dimensional accuracy are considered from the geologist's viewpoint. 相似文献
67.
D. W. Proctor 《The Photogrammetric Record》1968,6(31):49-58
An analytical relative orientation of a stereo-pair of photographs was computed in an unusual system of co-ordinates. The disparity in the widely mismatched photographs, their tilts and narrow angles subtended resulted in a rather weak solution from which sections of the lunar surface have been plotted. The results were not good but were better than expected. The main outcome of the work is a, probably unique, demonstration of the versatility of analytical photogrammetry. 相似文献
68.
69.
W. Löscher 《The Photogrammetric Record》1967,5(30):419-432
After a brief review of the history of the orthophoto mapping process and its methods, the paper describes the automatic production of orthophotographs in the BS-Stereomat. Technical details, especially of the electronics of the instrument, are given. The range of applications of an orthophoto is discussed, as well as the method's limitations. 相似文献
70.
Ohne Zusammenfassung 相似文献