全文获取类型
收费全文 | 281120篇 |
免费 | 11268篇 |
国内免费 | 14646篇 |
专业分类
测绘学 | 11784篇 |
大气科学 | 22976篇 |
地球物理 | 58370篇 |
地质学 | 106095篇 |
海洋学 | 26917篇 |
天文学 | 55814篇 |
综合类 | 3203篇 |
自然地理 | 21875篇 |
出版年
2022年 | 3020篇 |
2021年 | 4478篇 |
2020年 | 4368篇 |
2019年 | 4906篇 |
2018年 | 4743篇 |
2017年 | 4288篇 |
2016年 | 7220篇 |
2015年 | 6136篇 |
2014年 | 8797篇 |
2013年 | 16536篇 |
2012年 | 8762篇 |
2011年 | 9917篇 |
2010年 | 9059篇 |
2009年 | 11559篇 |
2008年 | 10438篇 |
2007年 | 9582篇 |
2006年 | 11920篇 |
2005年 | 9523篇 |
2004年 | 8783篇 |
2003年 | 7985篇 |
2002年 | 7555篇 |
2001年 | 6794篇 |
2000年 | 6634篇 |
1999年 | 5555篇 |
1998年 | 5321篇 |
1997年 | 5093篇 |
1996年 | 4712篇 |
1995年 | 4423篇 |
1994年 | 4112篇 |
1993年 | 3852篇 |
1992年 | 3658篇 |
1991年 | 3599篇 |
1990年 | 3784篇 |
1989年 | 3512篇 |
1988年 | 3297篇 |
1987年 | 3867篇 |
1986年 | 3424篇 |
1985年 | 4235篇 |
1984年 | 4739篇 |
1983年 | 4412篇 |
1982年 | 4323篇 |
1981年 | 3919篇 |
1980年 | 3637篇 |
1979年 | 3523篇 |
1978年 | 3477篇 |
1977年 | 3277篇 |
1976年 | 3049篇 |
1975年 | 2964篇 |
1974年 | 2926篇 |
1973年 | 3075篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
941.
This paper examines historical and spatial trends in hazardous materials transportation spills from 1971 to 1991. While the number of spills Inc.reased steadily during the 1970s, peaking in 1978–1979, there has been a decline in frequency sInc.e then largely due to modifications in reporting. Monetary damages have the opposite temporal pattern, with major Inc.reases recorded from 1982 onward. Death and injury statistics are more variable. Spatially, accidents are more prevalent in the Rust Belt extending from the Northeast corridor westward to the Great Lakes states, as well as in the Southeast. The greatest potential risk to the public is found in smaller, more densely populated industrial states such as New Jersey, Delaware, and Maryland. The spatial distribution, however, has not changed. Potential risk sources (e.g., chemical industry, number of hazardous waste facilities, number of railroad miles) are the best predictors of hazmat Inc.ident frequency. Mitigation efforts (statewide regulatory and/or management policies) also help explain the variability in hazmat Inc.idents. 相似文献
942.
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
943.
DAVID R. STODDART 《Geographical review》1997,87(4):537-541
944.
945.
946.
A generalized database of global palaeomagnetic data from 3719 lava flows and thin dykes of age 0–5 Ma has been constructed for use with a relational database management system. The database includes all data whose virtual geomagnetic poles (VGP) lie within 45 of the spin axis and can be used for studies of palaeosecular variation and for geomagnetic field modelling. Because many of these data were collected and processed more than 15–20 years ago, each result has been characterized according to the demagnetization procedures carried out. Analysis of these data in terms of the latitude variation of the angular dispersion of VGPs (palaeosecular variation from lavas) strongly suggests that careful data selection is required and that many of the older studies may need to be redone using more modern methods. Differences between the angular dispersions for separate normal- and reverse-polarity data sets confirm that many older studies have not been adequately cleaned magnetically. Therefore, the use of the database for geomagnetic field modelling should be carried out with some caution. Using a VGP cut-off angle that varies with latitude, the best data set consists of 2636 results that show a smooth increase of VGP angular dispersion with latitude. Model G for palaeosecular variation, which is based on modelling of the antisymmetric (dipole) and symmetric (quadrupole) dynamo families, provides a good fit to these results. 相似文献
947.
948.
949.
This paper presents an updated interpretation of seismic anisotropy within the uppermost mantle of southern Germany. The dense network of reversed and crossing refraction profiles in this area made it possible to observe almost 900 traveltimes of the Pn phase that could be effectively used in a time-term analysis to determine horizontal velocity distribution immediately below the Moho. For 12 crossing profiles, amplitude ratios of the Pn phase compared to the dominant crustal phase were utilized to resolve azimuthally dependent velocity gradients with depth. A P -wave anisotropy of 3–4 per cent in a horizontal plane immediately below the Moho at a depth of 30 km, increasing to 11 per cent at a depth of 40 km, was determined. For the axis of the highest velocity of about 8.03 km s−1 at a depth of 30 km a direction of N31°F was obtained. The azimuthal dependence of the observed Pn amplitude is explained by an azimuth-dependent sub-Moho velocity gradient decreasing from 0.06 s−1 in the fast direction to 0 s−1 in the slow direction of horizontal P -wave velocity. From the seismic results in this study a petrological model suggesting a change of modal composition and percentage of oriented olivine with depth was derived. 相似文献
950.
We evaluate the complete spectrum of the generalized fractal dimension of the spatial pattern of microearthquakes in Southern Italy, revealing a multifractal distribution structure. Our analysis is focused on the dependence of the multifractal distribution on the size of the selected area and the kind of seismicity in the area. As the size of the window varies, we observe that the capacity, information and correlation dimensions vary significantly, while both d ∞ and d −infin; remain unchanged within their errors limits. We interpret this result in terms of the observation that our data are mainly clustered around a linear fault (the Sisifo fault). When we restrict the selected windows around the fault, clustering around a line (the fault) is highlighted. The capacity dimension changes from about 1.8 to about 1.4 and the correlation dimension decreases because we observe in detail the clustering of the seismicity along the fault, which approximates the maximum intense clustering of the whole data set. Although our results are strongly influenced by the fact that the data are dominated by the epicentres located on the fault, we can conclude that multifractal analysis can be a very useful tool to discriminate the seismicity linked to a particular fault in a given area. 相似文献