全文获取类型
收费全文 | 43975篇 |
免费 | 7549篇 |
国内免费 | 10649篇 |
专业分类
测绘学 | 5540篇 |
大气科学 | 7085篇 |
地球物理 | 11917篇 |
地质学 | 21691篇 |
海洋学 | 5968篇 |
天文学 | 1550篇 |
综合类 | 3112篇 |
自然地理 | 5310篇 |
出版年
2024年 | 230篇 |
2023年 | 772篇 |
2022年 | 1356篇 |
2021年 | 1829篇 |
2020年 | 1949篇 |
2019年 | 2346篇 |
2018年 | 1756篇 |
2017年 | 2113篇 |
2016年 | 2225篇 |
2015年 | 2334篇 |
2014年 | 2915篇 |
2013年 | 3069篇 |
2012年 | 2882篇 |
2011年 | 3004篇 |
2010年 | 2534篇 |
2009年 | 2950篇 |
2008年 | 2852篇 |
2007年 | 3075篇 |
2006年 | 2950篇 |
2005年 | 2437篇 |
2004年 | 2260篇 |
2003年 | 1917篇 |
2002年 | 1519篇 |
2001年 | 1367篇 |
2000年 | 1275篇 |
1999年 | 1182篇 |
1998年 | 1167篇 |
1997年 | 1019篇 |
1996年 | 852篇 |
1995年 | 723篇 |
1994年 | 660篇 |
1993年 | 570篇 |
1992年 | 462篇 |
1991年 | 327篇 |
1990年 | 257篇 |
1989年 | 280篇 |
1988年 | 201篇 |
1987年 | 122篇 |
1986年 | 84篇 |
1985年 | 72篇 |
1984年 | 36篇 |
1983年 | 18篇 |
1982年 | 27篇 |
1981年 | 23篇 |
1980年 | 29篇 |
1977年 | 19篇 |
1976年 | 25篇 |
1973年 | 17篇 |
1971年 | 15篇 |
1954年 | 23篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
11.
12.
Dan Gill 《Mathematical Geology》1993,25(4):471-482
The ability of association analysis to discriminate sedimentary facies was tested on Purdy's modal analyses of modern sediments of the Great Bahama Bank. Purdy's data set has served in the past as a standard reference for evaluating various multivariate classification algorithms. In order to adapt Purdy's data to association analysis, the percent abundance of the 12 constituents was converted to binary form by dichotomizing each variable on its mean value. The results obtained by association analysis are virtually identical to those obtained by Purdy and other authors. The same four main sedimentary facies were discriminated; 86% of the samples were identically classified (97% when misclassified borderline cases are counted as matches); the total partition variance of the classification is only negligibly greater (4%); and the grouping of the variables yielded the same four groups. The rank order of the three division-attributes responsible for the sample classification is fines, oolites, and corals. Association analysis has been employed by other authors to differentiate meaningful facies groups in studies of ancient reef carbonates, modern reef sediments, and heavy minerals in stream sediments. In all these studies, the results were found to be compatible with those obtained by using the continuous quantitative measurements, indicating that qualitative binary data may often be sufficient for the purpose of facies discrimination in many branches of geology and that association analysis is an effective method for this purpose. 相似文献
13.
Qin Chengzhi 《中国地震研究》2004,18(2):178-187
The migration of strong earthquakes is an important research topic because the migration phenomena reflect partly the seismic mechanism and involve the prediction of tendency of seismic activity. Research on migration of strong earthquakes has mostly focused on finding the phenomena. Some attempts on getting regularity were comparatively subjective. This paper suggests that there should be indices of migration in earthquake dataset and the indexes should have statistical meaning if there is regularity in the migration of strong earthquakes. In this study, three derivative attributes of migration, i.e., migration orientation, migration distance and migration time interval, were statistically analyzed. Results in the North China region show that the migration of strong earthquakes has statistical meaning. There is a dominant migration orientation (W by S to E by N), a dominant distance (≤100km and on the confines of 300~700km), and a dominant time interval (≤1a and on the confines of 3~4a). The results also show that the migration will differ slightly with different magnitude range or earthquake activity phase. 相似文献
14.
用电测深研究地电断面、寻找隐伏多金属矿体时,以往采用ρ_s(或η_s)等值线断面图进行地质推断解释,因受浅源信号的干扰,效果差。本文提出了采用ρ_s(或η_s)差值法,能排除浅源干扰信号,反映出地电断面的相对变化,突出了深部微弱信息,取得了较好的地质解释效果。 相似文献
15.
Geoff Kite 《水文研究》1993,7(2):227-233
Using current meters to estimate flows in fast mountain streams is difficult and inaccurate. The salt dilution method offers an easier and more accurate alternative. The method has been used for many years but is time consuming and messy because of the need to mix chemical solutions and measure chemical concentrations in the field. A computer program has been developed which uses a laptop computer to calculate the mixing length and the mass of salt needed. The program controls the conductivity measurements and calculates the streamflow directly in the field. The development of this program is described and examples are presented of the application of the method in the Rocky Mountains of Alberta and British Columbia. 相似文献
16.
17.
The paper provides a new stereo‐analytical method, which is a combination of the stereographic method and analytical methods, to separate finite removable blocks from the infinite and tapered blocks in discontinuous rock masses. The methodology has applicability to both convex and concave blocks. Application of the methodology is illustrated through examples. Addition of this method to the existing block theory procedures available in the literature improves the capability of block theory in solving practical problems in rock engineering. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
18.
19.
《Mathematical Geology》1997,29(5):653-668
Filtering either through the electronics of an instrument or through digital procedure is performed routinely on geophysical
data. When velocity fluctuations are measured in turbulent flows using electromagnetic current meters (ECMs), a builtin lowpass
Butterworth filter of order n usually attenuates fluctuations at high frequencies. However, the effects of this filter may
not be acknowledged in turbulence studies, thus impeding comparisons between data collected with different ECMs. This paper
explores the implications of the filters on the characteristics of velocity signals, mainly on variance, power spectra, and
correlation analyses. Variance losses resulting from filtering can be important but will vary with the order n of the Butterworth
filter, decreasing as n increases. Knowing the filter response, it is possible to reconstruct the original signal spectrum
to evaluate the effect of filtering on variance and to allow comparisons between data collected with different instruments.
The autocorrelation function also is affected by filtering which increases the value of the coefficients in the first lags,
resulting in an overestimation of the integral length scale of coherent structures. These important effects add to those related
to size and shape differences in ECM sensors and must be taken into account in comparative studies. 相似文献
20.
Effect of Inter-Column Spacing on Soil Stresses due to Vibro-Installed Stone Columns: Interesting Findings 总被引:1,自引:0,他引:1
Hesham Elshazly Mohamed Elkasabgy Azza Elleboudy 《Geotechnical and Geological Engineering》2008,26(2):225-236
This paper reveals the interesting relation between the inter-column spacing and the corresponding alteration of soil state
of stresses due to the vibro-installation technique. This relation is inferred from analyses for load settlement records of
various field load tests, performed for stone columns arrangements with different inter-column spacing values. In order to
have adequate confidence in the findings, a well-documented case history, involving three columns patterns along with their
relevant field and laboratory test results, is utilized for this study. Moreover, a well-tested finite element model, capable
of simulating both elasto-plastic and time dependent soil deformations as well as pore water pressure building and dissipation,
is employed in the analysis. Instead of determining the soil response to the test load, based on known initial soil stresses
and material properties, the analysis is inversely posed to determine the soil initial stresses, based on the recorded settlements
and the post-installation material properties. The alteration in the soil state of stress is represented by the increase in
the post-installation horizontal to vertical stress ratio, K*, as a function of the inter-column spacing. It is found that this alteration experiences a systematic decrease in its magnitude
as the inter-column spacing increases. 相似文献