全文获取类型
收费全文 | 73745篇 |
免费 | 1024篇 |
国内免费 | 659篇 |
专业分类
测绘学 | 1980篇 |
大气科学 | 6016篇 |
地球物理 | 14447篇 |
地质学 | 24915篇 |
海洋学 | 6053篇 |
天文学 | 17077篇 |
综合类 | 173篇 |
自然地理 | 4767篇 |
出版年
2020年 | 487篇 |
2019年 | 502篇 |
2018年 | 1214篇 |
2017年 | 1183篇 |
2016年 | 1612篇 |
2015年 | 1059篇 |
2014年 | 1688篇 |
2013年 | 3554篇 |
2012年 | 1704篇 |
2011年 | 2356篇 |
2010年 | 2115篇 |
2009年 | 2880篇 |
2008年 | 2586篇 |
2007年 | 2498篇 |
2006年 | 2450篇 |
2005年 | 2209篇 |
2004年 | 2165篇 |
2003年 | 2050篇 |
2002年 | 2009篇 |
2001年 | 1785篇 |
2000年 | 1757篇 |
1999年 | 1612篇 |
1998年 | 1536篇 |
1997年 | 1561篇 |
1996年 | 1301篇 |
1995年 | 1241篇 |
1994年 | 1177篇 |
1993年 | 1075篇 |
1992年 | 1046篇 |
1991年 | 1003篇 |
1990年 | 1092篇 |
1989年 | 992篇 |
1988年 | 953篇 |
1987年 | 1094篇 |
1986年 | 958篇 |
1985年 | 1198篇 |
1984年 | 1395篇 |
1983年 | 1353篇 |
1982年 | 1269篇 |
1981年 | 1216篇 |
1980年 | 1085篇 |
1979年 | 1051篇 |
1978年 | 1064篇 |
1977年 | 960篇 |
1976年 | 870篇 |
1975年 | 812篇 |
1974年 | 913篇 |
1973年 | 901篇 |
1972年 | 571篇 |
1971年 | 525篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
101.
In urban environments, one major concern with deep excavations in soft clay is the potentially large ground deformations in and around the excavation. Excessive movements can damage adjacent buildings and utilities. There are many uncertainties associated with the calculation of the ultimate or serviceability performance of a braced excavation system. These include the variabilities of the loadings, geotechnical soil properties, and engineering and geometrical properties of the wall. A risk‐based approach to serviceability performance failure is necessary to incorporate systematically the uncertainties associated with the various design parameters. This paper demonstrates the use of an integrated neural network–reliability method to assess the risk of serviceability failure through the calculation of the reliability index. By first performing a series of parametric studies using the finite element method and then approximating the non‐linear limit state surface (the boundary separating the safe and ‘failure’ domains) through a neural network model, the reliability index can be determined with the aid of a spreadsheet. Two illustrative examples are presented to show how the serviceability performance for braced excavation problems can be assessed using the reliability index. Copyright © 2005 John Wiley & Sons, Ltd. 相似文献
102.
R. K. Pant N. Basavaiah N. Juyal N. K. Saini M. G. Yadava E. Appel A. K. Singhvi 《第四纪科学杂志》2005,20(5):485-492
The southwest monsoon that dominated Central Himalaya has preserved loessic silt deposits preserved in patches that are proximal to periglacial areas. The occurrence of such silts suggests contemporary prevalence of cold and dry northwesterly winds. Field stratigraphy, geochemistry, mineral magnetism, infrared stimulated luminescence (IRSL) and radiocarbon dating has enabled reconstruction of an event chronology during the past 20 ka. Three events of loess accretion could be identified. The first two events of loess deposition occurred betweem 20 and 9 ka and were separated by a phase of moderate weathering. Pedogenesis at the end of this event gave rise to a well‐developed soil that was bracketed around 9 to > 4 ka. This was followed by the third phase of loess accretion that occurred around 4 to > 1 ka. Episodes of loess deposition and soil formation are interpreted in terms of changes in the strength of the Indian southwest monsoon. Copyright © 2005 John Wiley & Sons, Ltd. 相似文献
104.
105.
106.
J. van der Plicht B. van Geel S. J. P. Bohncke J. A. A. Bos M. Blaauw A. O. M. Speranza R. Muscheler S. Bjrck 《第四纪科学杂志》2004,19(3):263-269
Accurate chronologies are essential for linking palaeoclimate archives. Carbon‐14 wiggle‐match dating was used to produce an accurate chronology for part of an early Holocene peat sequence from the Borchert (The Netherlands). Following the Younger Dryas–Preboreal transition, two climatic shifts could be inferred. Around 11 400 cal. yr BP the expansion of birch (Betula) forest was interrupted by a dry continental phase with dominantly open grassland vegetation, coeval with the PBO (Preboreal Oscillation), as observed in the GRIP ice core. At 11 250 cal. yr BP a sudden shift to a humid climate occurred. This second change appears to be contemporaneous with: (i) a sharp increase of atmospheric 14C; (ii) a temporary decline of atmospheric CO2; and (iii) an increase in the GRIP 10Be flux. The close correspondence with excursions of cosmogenic nuclides points to a decline in solar activity, which may have forced the changes in climate and vegetation at around 11 250 cal. yr BP. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
107.
An analytical model for the constant-head pumping test is developed for a partially penetrating well that has a finite thickness skin, and intersects a single vertical fracture. In the model, the fracture is fully confined and flow occurs only in the fracture. The model is developed using Laplace transform and finite Fourier transform methods. The model is to be used for analysing well test data from vertically fractured media and for verifying numerical models. Dimensionless curves are used to study the effects of a finite thickness skin and a partially penetrating wellbore. In the presence of a finite thickness skin, or a partially penetrating wellbore, a typical flow response for the constant-head pumping test has three distinct periods of flow corresponding to small-, intermediate- and large-time. Small- and large-time approximations are presented for the model. For tests where the wellbore is fully penetrating, or the partial penetration ratio is known, these approximations can be used to analyse field data. 相似文献
108.
Using plane wave theory and assuming a given input wavelet the shape of the reflected (or transmitted) wavelet from a layered boundary is derived. Several types of boundaries are considered, among them the weathered layer and a wedge shaped intermediate layer. Different angles of incidence and all internal multiples are taken into account. The examples shown in the figures can be used for a direct comparison between theoretical and observed shapes of reflected (or transmitted) wavelets from special boundaries. 相似文献
109.
110.
The wadic project: A comprehensive field evaluation of directional wave instrumentation 总被引:1,自引:0,他引:1
J. Allender T. Audunson S. F. Barstow S. Bjerken H. E. Krogstad P. Steinbakke L. Vartdal L. E. Borgman C. Graham 《Ocean Engineering》1989,16(5-6)
The results of a comprehensive field trial of nearly all commercially available directional wave measurement systems at the Edda field in the North Sea during winter 1985-86 are presented. The results summarize the accuracy of the principal engineering wave parameters from each system and the dependence on sea state. Limiting factors on system performance and operational problems are also included in the assessment. Overall experience has been good with systems utilizing widely different measurement principles returning consistent results. 相似文献