全文获取类型
收费全文 | 27529篇 |
免费 | 5166篇 |
国内免费 | 7919篇 |
专业分类
测绘学 | 4355篇 |
大气科学 | 5916篇 |
地球物理 | 6389篇 |
地质学 | 12485篇 |
海洋学 | 4317篇 |
天文学 | 1314篇 |
综合类 | 2121篇 |
自然地理 | 3717篇 |
出版年
2024年 | 105篇 |
2023年 | 344篇 |
2022年 | 931篇 |
2021年 | 1085篇 |
2020年 | 1285篇 |
2019年 | 1510篇 |
2018年 | 1266篇 |
2017年 | 1392篇 |
2016年 | 1456篇 |
2015年 | 1640篇 |
2014年 | 1833篇 |
2013年 | 2005篇 |
2012年 | 1890篇 |
2011年 | 1981篇 |
2010年 | 1594篇 |
2009年 | 1863篇 |
2008年 | 1839篇 |
2007年 | 2051篇 |
2006年 | 1943篇 |
2005年 | 1666篇 |
2004年 | 1534篇 |
2003年 | 1313篇 |
2002年 | 1153篇 |
2001年 | 964篇 |
2000年 | 918篇 |
1999年 | 831篇 |
1998年 | 779篇 |
1997年 | 635篇 |
1996年 | 547篇 |
1995年 | 515篇 |
1994年 | 448篇 |
1993年 | 352篇 |
1992年 | 226篇 |
1991年 | 197篇 |
1990年 | 124篇 |
1989年 | 93篇 |
1988年 | 88篇 |
1987年 | 54篇 |
1986年 | 30篇 |
1985年 | 32篇 |
1984年 | 16篇 |
1982年 | 10篇 |
1981年 | 10篇 |
1980年 | 9篇 |
1979年 | 9篇 |
1978年 | 9篇 |
1977年 | 5篇 |
1976年 | 10篇 |
1973年 | 5篇 |
1954年 | 13篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
992.
993.
994.
The concept of the equivalent free surface has been extended to determine the seismic horizontal pullout capacity of shallow
vertical strip plate anchors buried in sand. The analysis has been done rigorously by using the method of stress characteristics.
The results have been expressed in the form of non-dimensional charts. The pullout resistance has been found to reduce quite
extensively with increase in the magnitude of horizontal earthquake acceleration. The results were compared with the previously
published data, and it was seen that the computed pullout resistance with the proposed method was found to be lowest.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
995.
Simulation of subsurface heterogeneity is important for modeling subsurface flow and transport processes. Previous studies have indicated that subsurface property variations can often be characterized by fractional Brownian motion (fBm) or (truncated) fractional Levy motion (fLm). Because Levy-stable distributions have many novel and often unfamiliar properties, studies on generating fLm distributions are rare in the literature. In this study, we generalize a relatively simple and computationally efficient successive random additions (SRA) algorithm, originally developed for generating Gaussian fractals, to simulate fLm distributions. We also propose an additional important step in response to continued observations that the traditional SRA algorithm often generates fractal distributions having poor scaling and correlation properties. Finally, the generalized and modified SRA algorithm is validated through numerical tests. 相似文献
996.
Numerical simulations for coupled rock deformation and gas leak flow in parallel coal seams 总被引:3,自引:0,他引:3
Peide Sun> 《Geotechnical and Geological Engineering》2004,22(1):1-17
Based on the new viewpoint of interaction mechanics for solid and gas, gas leakage in parallel deformable coal seams can be
understood. That is, under the action of varied geophysical fields, the methane gas flow in a double deformable coal seam
can be essentially considered to be compressible with time-dependent and mixed permeation and diffusion through a pore-cleat
deformable, heterogeneous and anisotropic medium. From this new viewpoint, coupled mathematical models for coal seam deformation
and gas leak flow in parallel coal seams were formulated and the numerical simulations for slow gas emission from the parallel
coal seams are presented. It is found that coupled models might be close to reality. Meanwhile, a coupled model for solid
deformation and gas leak flow can be applied to the problems of gas leak flow including mining engineering, gas drainage engineering
and mining safety engineering in particular the prediction of the safe range using protective layer mining where coal and
gas outbursts can efficiently be prevented.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
997.
A three-dimensional numerical model is described to study theresponse of a coastal ocean excited by a tropical cyclone in the Bay of Bengal. The numericalexperiments have been carried out using the model to understand the dynamics and thermodynamics ofthe ocean due to different cyclonic systems approaching in different directions. In the firstexperiment, the model is used to simulate the vertical thermal structure of the ocean as a response ofpassage of the less intensified 1997 cyclone, which was skirting the east coast of India before crossingthe Bangladesh coast. The simulations are compared with the buoy data available during the storm period.In the next experiment, it is considered an idealized cyclone with hurricane winds movingnormal to the east coast of India crossing between Visakhapatnam and Kakinada to evolve thermalstructure and currents of the ocean. A net decrease of the SST of 6–7 °C is simulated whenthe severe cyclonic storm moved over the coastal ocean. 相似文献
998.
In this study, a capillary barrier system was designed and tested for an arid land environment. To simulate arid land conditions
of high temperature and sub-irrigation systems, the barrier was subjected to thermal and hydraulic gradients in opposite directions;
to test the barrier system under these severe conditions, an experimental apparatus was designed and fabricated. The multilayer
capillary barrier consisted of three layers made of silica sand, a mixture of sand and bentonite in equal portions, and a
mixture of clay (25%) and aggregate (75%). Several one dimensional coupled heat and moisture tests were performed. Temperature
variations along the thickness of the barrier were recorded as a function of time, and at the end of each test, the barrier
was sliced into small sections, for the determination of volumetric water content as a function of distance from the heat
source. The experimental results were discussed in view of the barrier's intended purpose of its ability to store moisture
for long time durations.
Coupled heat and moisture flow equations were developed and solved numerically via a finite difference method. Diffusivity
parameters were calculated by using experimental results, a numerical model, and Powell's conjugate directions method of nonlinear
optimization. The model was calibrated and the results were discussed. Good agreement between calculated and experimental
results was obtained.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
999.
There are numerous hot springs with temperatures ranging from 30 to 100 °C in Biga peninsula and they occur throughout the peninsula. The result of this study shows that the region is under a tectonic compressional regime. The investigation of the faults and fractures in the region indicates that the region has been affected first by N–S and then E–W compression since the Middle Miocene. Opening fractures and antithetic and synthetic faults due to the compressional movements provide paths for the deep circulation of water. In addition, the tectonic movements, granitic intrusion and volcanic activity have also played important roles as heat sources for the geothermal systems. 相似文献
1000.
Charnockitic magmatism in southern India 总被引:2,自引:0,他引:2
Large charnockite massifs cover a substantial portion of the southern Indian granulite terrain. The older (late Archaean to
early Proterozoic) charnockites occur in the northern part and the younger (late Proterozoic) charnockites occur in the southern
part of this high-grade terrain. Among these, the older Biligirirangan hill, Shevroy hill and Nilgiri hill massifs are intermediate
charnockites, with Pallavaram massif consisting dominantly of felsic charnockites. The charnockite massifs from northern Kerala
and Cardamom hill show spatial association of intermediate and felsic charnockites, with the youngest Nagercoil massif consisting
of felsic charnockites. Their igneous parentage is evident from a combination of features including field relations, mineralogy,
petrography, thermobarometry, as well as distinct chemical features. The southern Indian charnockite massifs show similarity
with high-Ba-Sr granitoids, with the tonalitic intermediate charnockites showing similarity with high-Ba-Sr granitoids with
low K2O/Na2O ratios, and the felsic charnockites showing similarity with high-Ba-Sr granitoids with high K2O/Na2O ratios. A two-stage model is suggested for the formation of these charnockites. During the first stage there was a period
of basalt underplating, with the ponding of alkaline mafic magmas. Partial melting of this mafic lower crust formed the charnockitic
magmas. Here emplacement of basalt with low water content would lead to dehydration melting of the lower crust forming intermediate
charnockites. Conversely, emplacement of hydrous basalt would result in melting at higher {ie565-01} favoring production of
more siliceous felsic charnockites. This model is correlated with two crustal thickening phases in southern India, one related
to the accretion of the older crustal blocks on to the Archaean craton to the north and the other probably related to the
collision between crustal fragments of East and West Gondwana in a supercontinent framework. 相似文献