全文获取类型
收费全文 | 58034篇 |
免费 | 703篇 |
国内免费 | 583篇 |
专业分类
测绘学 | 1556篇 |
大气科学 | 4354篇 |
地球物理 | 10728篇 |
地质学 | 23880篇 |
海洋学 | 4563篇 |
天文学 | 11263篇 |
综合类 | 234篇 |
自然地理 | 2742篇 |
出版年
2022年 | 203篇 |
2021年 | 324篇 |
2020年 | 362篇 |
2019年 | 387篇 |
2018年 | 3761篇 |
2017年 | 3519篇 |
2016年 | 2557篇 |
2015年 | 736篇 |
2014年 | 1071篇 |
2013年 | 1942篇 |
2012年 | 2162篇 |
2011年 | 4170篇 |
2010年 | 3744篇 |
2009年 | 4181篇 |
2008年 | 3410篇 |
2007年 | 4002篇 |
2006年 | 1488篇 |
2005年 | 1599篇 |
2004年 | 1435篇 |
2003年 | 1450篇 |
2002年 | 1244篇 |
2001年 | 938篇 |
2000年 | 922篇 |
1999年 | 710篇 |
1998年 | 671篇 |
1997年 | 740篇 |
1996年 | 599篇 |
1995年 | 596篇 |
1994年 | 569篇 |
1993年 | 462篇 |
1992年 | 462篇 |
1991年 | 434篇 |
1990年 | 443篇 |
1989年 | 422篇 |
1988年 | 401篇 |
1987年 | 460篇 |
1986年 | 387篇 |
1985年 | 486篇 |
1984年 | 477篇 |
1983年 | 506篇 |
1982年 | 475篇 |
1981年 | 439篇 |
1980年 | 470篇 |
1979年 | 370篇 |
1978年 | 327篇 |
1977年 | 335篇 |
1976年 | 313篇 |
1975年 | 294篇 |
1974年 | 293篇 |
1973年 | 288篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
31.
A formula for the thickness of a shear band formed in saturated soils under a simple shear or a combined stress state has been proposed. It is shown that the shear band thickness is dependent on the pore pressure properties of the material and the dilatancy rate, but is independent of the details of the combined stress state. This is in accordance with some separate experimental observations. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
32.
Y.-B. WU S. GAO H.-J. GONG H. XIANG W.-F. JIAO S.-H. YANG Y.-S. LIU H.-L. YUAN 《Journal of Metamorphic Geology》2009,27(6):461-477
U–Pb age, trace element and Hf isotope compositions of zircon were analysed for a metasedimentary rock and two amphibolites from the Kongling terrane in the northern part of the Yangtze Craton. The zircon shows distinct morphological and chemical characteristics. Most zircon in an amphibolite shows oscillatory zoning, high Th/U and 176 Lu/177 Hf ratios, high formation temperature, high trace element contents, clear negative Eu anomaly, as well as HREE-enriched patterns, suggesting that it is igneous. The zircon yields a weighted mean 207 Pb/206 Pb age of 2857 ± 8 Ma, representing the age of the magmatic protolith. The zircon in the other two samples is metamorphic. It has low Th/U ratios, low trace element concentrations, variable HREE contents (33.8 ≥ LuN ≥2213; 14.7 ≤ LuN /SmN ≤ 354) and 176 Lu/177 Hf ratios (0.000030–0.001168). The data indicate that the zircon formed in the presence of garnet and under upper amphibolite facies conditions. The metamorphic zircon yields a weighted mean 207 Pb/206 Pb age of 2010 ± 13 Ma. These results combined with previously obtained Palaeoproterozoic metamorphic ages suggest a c. 2.0 Ga Palaeoproterozoic collisional event in the Yangtze Craton, which may result from the assembly of the supercontinent Columbia. The zircon in two samples yields weighted mean two-stage Hf model ( T DM2 ) ages of 3217 ± 110 and 2943 ± 50 Ma, respectively, indicating that their protoliths were mainly derived from Archean crust. 相似文献
33.
S. ENDO S.WALLIS T. HIRATA R. ANCZKIEWICZ J. PLATT M. THIRLWALL Y. ASAHARA 《Journal of Metamorphic Geology》2009,27(5):371-384
Two distinct age estimates for eclogite-facies metamorphism in the Sanbagawa belt have been proposed: (i) c. 120–110 Ma based on a zircon SHRIMP age for the Western Iratsu unit and (ii) c. 88–89 Ma based on a garnet–omphacite Lu–Hf isochron age from the Seba and Kotsu eclogite units. Despite the contrasting estimates of formation ages, petrological studies suggest the formation conditions of the Western Iratsu unit are indistinguishable from those of the other two units—all ∼20 kbar and 600–650 °C. Studies of the associated geological structures suggest the Seba and Western Iratsu units are parts of a larger semi-continuous eclogite unit. A combination of geochronological and petrological studies for the Western Iratsu eclogite offers a resolution to this discrepancy in age estimates. New Lu–Hf dating for the Western Iratsu eclogite yields an age of 115.9 ± 0.5 Ma that is compatible with the zircon SHRIMP age. However, petrological studies show that there was significant garnet growth in the Western Iratsu eclogite before eclogite facies metamorphism, and the early core growth is associated with a strong concentration of Lu. Pre-eclogite facies garnet (Grt1) includes epidote–amphibolite facies parageneses equilibrated at 550–650 °C and ∼10 kbar, and this is overgrown by prograde eclogite facies garnet (Grt2). The Lu–Hf age of c. 116 Ma is strongly skewed to the isotopic composition of Grt1 and is interpreted to reflect the age of the pre-eclogite phase. The considerable time gap ( c. 27 Myr) between the two Lu–Hf ages suggests they may be related to separate tectonic events or distinct phases in the evolution of the Sanbagawa subduction zone. 相似文献
34.
35.
Stress wave attenuation across fractured rock masses is a great concern of underground structure safety. When the wave amplitude is large, fractures experience nonlinear deformation during the wave propagation. This paper presents a study on normal transmission of P‐wave across parallel fractures with nonlinear deformational behaviour (static Barton–Bandis model). The results show that the magnitude of transmission coefficient is a function of incident wave amplitude, nondimensional fracture spacing and number of fractures. Two important indices of nondimensional fracture spacing are identified, and they divide the area of nondimensional fracture spacing into three parts (individual fracture area, transition area and small spacing area). In the different areas, the magnitude of transmission coefficient has different trends with nondimensional fracture spacing and number of fractures. In addition, the study reveals that under some circumstances, the magnitude of transmission coefficient increases with increasing number of fractures, and is larger than 1. Copyright © 2006 John Wiley & Sons, Ltd. 相似文献
36.
37.
38.
Measurements of 18O concentrations in precipitation, soil solution, spring and runoff are used to determine water transit time in the small granitic Strengbach catchment (0·8 km2; 883–1146 m above sea level) located in the Vosges Mountains of northeastern France. Water transit times were calculated by applying the exponential, exponential piston and dispersion models of the FlowPC program to isotopic input (rainfall) and output (spring and stream water) data sets during the period 1989–95. The input function of the model was modified compared with the former version of the model and estimated by a deterministic approach based on a simplified hydrological balance. The fit between observed and calculated output data showed marked improvements compared with results obtained using the initial version of the model. An exponential piston version of the model applied to spring water indicates a 38·5 month mean transit time, which suggests that the volume in the aquifer, expressed in water depth, is 2·4 m. A considerable thickness (>45 m) of fractured bedrock may be involved for such a volume of water to be stored in the aquifer. Copyright © 2005 John Wiley & Sons, Ltd. 相似文献
39.
40.