全文获取类型
收费全文 | 30619篇 |
免费 | 526篇 |
国内免费 | 337篇 |
专业分类
测绘学 | 646篇 |
大气科学 | 1973篇 |
地球物理 | 5583篇 |
地质学 | 11691篇 |
海洋学 | 3092篇 |
天文学 | 7107篇 |
综合类 | 69篇 |
自然地理 | 1321篇 |
出版年
2022年 | 305篇 |
2021年 | 478篇 |
2020年 | 533篇 |
2019年 | 574篇 |
2018年 | 1128篇 |
2017年 | 1075篇 |
2016年 | 1192篇 |
2015年 | 574篇 |
2014年 | 1092篇 |
2013年 | 1754篇 |
2012年 | 1194篇 |
2011年 | 1510篇 |
2010年 | 1341篇 |
2009年 | 1641篇 |
2008年 | 1401篇 |
2007年 | 1461篇 |
2006年 | 1359篇 |
2005年 | 810篇 |
2004年 | 748篇 |
2003年 | 722篇 |
2002年 | 719篇 |
2001年 | 649篇 |
2000年 | 601篇 |
1999年 | 485篇 |
1998年 | 494篇 |
1997年 | 471篇 |
1996年 | 409篇 |
1995年 | 366篇 |
1994年 | 393篇 |
1993年 | 311篇 |
1992年 | 313篇 |
1991年 | 300篇 |
1990年 | 332篇 |
1989年 | 228篇 |
1988年 | 235篇 |
1987年 | 281篇 |
1986年 | 221篇 |
1985年 | 335篇 |
1984年 | 300篇 |
1983年 | 283篇 |
1982年 | 302篇 |
1981年 | 213篇 |
1980年 | 263篇 |
1979年 | 209篇 |
1978年 | 221篇 |
1977年 | 173篇 |
1976年 | 164篇 |
1975年 | 176篇 |
1974年 | 172篇 |
1973年 | 172篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
241.
242.
Influence of Structural Non-Stationarity of Surface Roughness on Morphological Characterization and Mechanical Deformation of Rock Joints 总被引:2,自引:2,他引:0
N. Fardin 《Rock Mechanics and Rock Engineering》2008,41(2):267-297
Summary Structural non-stationarity of surface roughness affects accurate morphological characterization as well as mechanical behaviour
of rock joints at the laboratory scale using samples with a size below the stationarity threshold. In this paper, the effect
of structural non-stationarity of surface roughness is investigated by studying the scale dependence of surface roughness
and mechanical behaviour of rock joints. The results show that the structural non-stationarity mainly affects the accurate
characterization of the surface roughness of the fracture samples. It also controls the amount and location of the contact
areas during shear tests, which in turn affects the mechanical properties and asperity degradation of the samples. It is concluded
that for accurate determination of the morphological and mechanical properties of rock joints at laboratory and field scales,
samples with size equal to or larger than the stationarity threshold are required.
Author’s address: Nader Fardin, Rock Mechanics Group, Department of Mining Engineering, Faculty of Engineering, University
of Tehran, P.O. Box: 11365/4563, Tehran, Iran 相似文献
243.
H. M. Hussein K. M. Abou Elenean I. A. Marzouk A. Peresan I. M. Korrat E. Abu El-Nader G. F. Panza M. N. El-Gabry 《Natural Hazards》2008,47(3):525-546
The aim of the present work is to compile and update a catalogue of the instrumentally recorded earthquakes in Egypt, with
uniform and homogeneous source parameters as required for the analysis of seismicity and seismic hazard assessment. This in
turn requires a detailed analysis and comparison of the properties of different available sources, including the distribution
of events with time, the magnitude completeness, and the scaling relations between different kinds of magnitude reported by
different agencies. The observational data cover the time interval 1900–2004 and an area between 22°–33.5° N and 25°–36° E.
The linear regressions between various magnitude types have been evaluated for different magnitude ranges. Using the best
linear relationship determined for each available pair of magnitudes, as well as those identified between the magnitudes and
the seismic moment, we convert the different magnitude types into moment magnitudes M
W, through a multi-step conversion process. Analysis of the catalogue completeness, based on the M
W thus estimated, allows us to identify two different time intervals with homogeneous properties. The first one (1900–1984)
appears to be complete for M
W ≥ 4.5, while the second one (1985–2004) can be considered complete for magnitudes M
W ≥ 3. 相似文献
244.
245.
P. N. S. O'BRIEN 《Geophysical Prospecting》1969,17(4):511-547
Recordings were made with three types of detector of the primary compressional (P) and shear (S) wave pulses generated by explosions in boreholes. Charge weights varied from 0.08 kg to 9.5 kg and detector distances varied from about 3 m to about 80 m. Scaling by the simple factor W1/3 where W is the charge weight, enabled observations from different sized charges to be fitted to a single expression. Experiments were carried out in the Bunter sandstone and the London clay and both fluid and solid tamping were used. This variation in tamping had no significant effect on the P-waves but it may have affected the generation of SV-waves. In both media the P-wave energy carried at 30 m from the shot by frequencies less than 100 Hz decreased rapidly with depth and was usually 1–2 % of the available chemical energy for a shot depth of 15 m. The S-wave energy was much less than this, but was highly directional. The P-wave pulse had the appearance of a damped sinusoid in very good agreement with the predictions of the ‘equivalent radiator’ hypothesis. However, the surface of this radiator should be identified not with the blown cavity but with the surface at which the tensile stresses associated with the stress wave become less than the tensile strength of the rock. The predominant frequency for a 1 kg charge at a depth of 15 m was 24 Hz in the clay and 52 Hz in the sandstone. In these and similar media, therefore, an effort should be made to keep individual charges less than 1 kg in reflection shooting and less than 10 kg in refraction shooting. The value of Q was about 50 in clay and about 25 in the sandstone. These estimates are rather uncertain because of the small distances over which the pulses were observed. The Z-transforms of the sampled pulses indicated that they were all of minimum phase, or very near to it. 相似文献
246.
G. R. Cooper G. Petrie A. D. N. Smith M. Sowton K. B. Atkinson 《The Photogrammetric Record》1969,6(33):239-258
The International Society for Photogrammetry held the Xlth International Congress at Lausanne during July 1968. The authors review the activities of five of the seven I.S.P. Commissions. 相似文献
247.
The historical development of analytical aerial triangulation block adjustment is mentioned by way of introduction. The advantages of separating the plan and height adjustments are given and the observation equations of a typical adjustment section are analysed to show that this convenient practice is justified in normal circumstances. The principles of a direct solution of the normal equations suitable for a moderate-sized computer, are explained and developed in detail for the height adjustment. The advantages of tie-strips in the height adjustment are demonstrated and the difficulties which then arise are discussed. An approximate method which overcomes these difficulties is developed. Details of the computer programmes used are given in Appendix B. 相似文献
248.
M. A. Anastassiades L. N. Carapiperis N. K. Kariambas P. G. Paraskevopoulos 《Pure and Applied Geophysics》1962,52(1):143-152
Summary In this study the different forms of the electromagnetic field strength recordings at the Thera-Crete microwave line of link are examined, in comparison with the corresponding weather situations prevailing over that area. According to the amplitude of fluctuations, the different forms of recordings have been classified in three basic typesA, B, C and the annual frequency of the appearance of these types has been observed. Finally, the influence of weather situations on radio wave propagation was examined and it has been found that in most cases a close relationship exists between weather situations and the different fading forms of the electromagnetic field so that a prediction of the fading form be possible by means of the weather forecast.This study was sponsored by a research grant from NATO's Scientific Committee. 相似文献
249.
Ohne Zusammenfassung 相似文献
250.