全文获取类型
收费全文 | 63499篇 |
免费 | 969篇 |
国内免费 | 446篇 |
专业分类
测绘学 | 1415篇 |
大气科学 | 4482篇 |
地球物理 | 12557篇 |
地质学 | 23052篇 |
海洋学 | 5619篇 |
天文学 | 13789篇 |
综合类 | 147篇 |
自然地理 | 3853篇 |
出版年
2022年 | 378篇 |
2021年 | 652篇 |
2020年 | 745篇 |
2019年 | 803篇 |
2018年 | 1588篇 |
2017年 | 1579篇 |
2016年 | 1764篇 |
2015年 | 954篇 |
2014年 | 1686篇 |
2013年 | 3168篇 |
2012年 | 1948篇 |
2011年 | 2643篇 |
2010年 | 2274篇 |
2009年 | 2910篇 |
2008年 | 2570篇 |
2007年 | 2625篇 |
2006年 | 2474篇 |
2005年 | 1761篇 |
2004年 | 1764篇 |
2003年 | 1733篇 |
2002年 | 1622篇 |
2001年 | 1454篇 |
2000年 | 1378篇 |
1999年 | 1160篇 |
1998年 | 1146篇 |
1997年 | 1207篇 |
1996年 | 990篇 |
1995年 | 983篇 |
1994年 | 905篇 |
1993年 | 778篇 |
1992年 | 751篇 |
1991年 | 730篇 |
1990年 | 777篇 |
1989年 | 696篇 |
1988年 | 670篇 |
1987年 | 722篇 |
1986年 | 715篇 |
1985年 | 882篇 |
1984年 | 901篇 |
1983年 | 930篇 |
1982年 | 861篇 |
1981年 | 777篇 |
1980年 | 778篇 |
1979年 | 694篇 |
1978年 | 637篇 |
1977年 | 646篇 |
1976年 | 587篇 |
1975年 | 579篇 |
1974年 | 573篇 |
1973年 | 587篇 |
排序方式: 共有10000条查询结果,搜索用时 10 毫秒
111.
112.
In our study, 48 U.S. departments that grant geography PhDs are compared along four measures of academic productivity. Using 1980–1994 as the base period, research productivity of faculty was assessed by counting book titles, as well as authorships in 77 journals. Teaching productivity was determined by placement of PhDs in graduate programs of geography in North America, and by a weighted index of teaching outcomes. Data were summed for departments, but are reported primarily as productivity per person (per FTE count). The results point to a wide variety of departmental profiles, from those departments that appear to emphasize teaching outcomes to those that are more productive in book publishing and journal authorship, with several exhibiting a balance between teaching and research. 相似文献
113.
114.
The paper presents the first quantitative results of a laboratory study of the velocity field in a two-arm spiral-wave pattern generated in a steady-state fashion by a hydrodynamical instability in a differentially rotating, thin layer of liquid. The liquid layer has a free surface, and the rotational profile includes an interval where the velocity drops abruptly, as in the gaseous disks of spiral galaxies. The properties of anticyclonic vortices observed between the arms of this pattern at the corotation radius are considered. 相似文献
115.
116.
117.
118.
J. García-Abdeslem J.M. Romo E. Gómez-Treviño J. Ramírez-Hernández F.J. Esparza-Hernández C.F. Flores-Luna 《Geophysical Prospecting》2005,53(6):755-765
The subsurface geometry of the Sebastián Vizcaíno Basin is obtained from the 2D inversion of gravity data, constrained by a density-versus-depth relationship derived from an oil exploration deep hole. The basin accumulated a thick pile of marine sediments that evolved in the fore-arc region of the compressive margin prevalent along western North America during Mesozoic and Tertiary times. Our interpretation indicates that the sedimentary infill in the Sebastián Vizcaíno Basin reaches a maximum thickness of about 4 km at the centre of a relatively symmetric basin. At the location of the Suaro-1 hole, the depth to the basement derived from this work agrees with the drilled interface between calcareous and volcaniclastic members of the Alisitos Formation. A sensitivity analysis strongly suggests that the assumed density function leads to a nearly unique solution of the inverse problem. 相似文献
119.
120.
Contamination of groundwater has become a major concern in recent years. Since testing of water quality of all domestic and irrigation wells within large watersheds is not economically feasible, one frequently used monitoring strategy is to develop contamination potential maps of groundwater, and then prioritize those wells located in the potentially highly contaminated areas for testing of contaminants. However, generation of contamination potential maps based on groundwater sensitivity and vulnerability is not an easy task due inherent uncertainty. Therefore, the overall goal of this research is to improve the methodology for the generation of contamination potential maps by using detailed landuse/pesticide and soil structure information in conjunction with selected parameters from the DRASTIC model. The specific objectives of this study are (i) to incorporate GIS, GPS, remote sensing and the fuzzy rule-based model to generate groundwater sensitivity maps, and (ii) compare the results of our new methodologies with the modified DRASTIC Index (DI) and field water quality data. In this study, three different models were developed (viz. DIfuzz, VIfuzz and VIfuzz_ped) and were compared to the DI. Once the preliminary fuzzy logic-based (DIfuzz) was generated using selected parameters from DI, the methodology was further refined through VIfuzz and VIfuzz_ped models that incorporated landuse/pesticide application and soil structure information, respectively. This study was conducted in Woodruff County of the Mississippi Delta region of Arkansas. Water quality data for 55 wells were used to evaluate the contamination potential maps. The sensitivity map generated by VIfuzz_ped with soil structure showed significantly better coincidence results when compared with the field data. 相似文献