全文获取类型
收费全文 | 40496篇 |
免费 | 3543篇 |
国内免费 | 5424篇 |
专业分类
测绘学 | 3080篇 |
大气科学 | 5415篇 |
地球物理 | 8592篇 |
地质学 | 19951篇 |
海洋学 | 3537篇 |
天文学 | 2273篇 |
综合类 | 3638篇 |
自然地理 | 2977篇 |
出版年
2024年 | 137篇 |
2023年 | 409篇 |
2022年 | 803篇 |
2021年 | 981篇 |
2020年 | 831篇 |
2019年 | 930篇 |
2018年 | 5500篇 |
2017年 | 4746篇 |
2016年 | 3453篇 |
2015年 | 1164篇 |
2014年 | 1179篇 |
2013年 | 1164篇 |
2012年 | 2022篇 |
2011年 | 3732篇 |
2010年 | 3026篇 |
2009年 | 3337篇 |
2008年 | 2814篇 |
2007年 | 3233篇 |
2006年 | 866篇 |
2005年 | 914篇 |
2004年 | 901篇 |
2003年 | 873篇 |
2002年 | 772篇 |
2001年 | 592篇 |
2000年 | 559篇 |
1999年 | 636篇 |
1998年 | 545篇 |
1997年 | 492篇 |
1996年 | 468篇 |
1995年 | 366篇 |
1994年 | 353篇 |
1993年 | 318篇 |
1992年 | 279篇 |
1991年 | 215篇 |
1990年 | 172篇 |
1989年 | 143篇 |
1988年 | 121篇 |
1987年 | 60篇 |
1986年 | 72篇 |
1985年 | 45篇 |
1984年 | 38篇 |
1983年 | 21篇 |
1982年 | 41篇 |
1981年 | 34篇 |
1980年 | 41篇 |
1979年 | 14篇 |
1978年 | 7篇 |
1977年 | 7篇 |
1976年 | 11篇 |
1958年 | 13篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
41.
洪泛平原农村居民地空间分布特征定量研究及应用探讨 总被引:23,自引:2,他引:23
通过对影响居民地空间分布因素的分析,探讨了洪泛平原农村居民地空间分布特征;利用地理信息系统技术对不同高程带内居民地分布规模、等级和空间分布密度作了定量研究,得到了居民地空间分布密度和人口空间分布密度参数;并结合洪灾淹没区居民地损失的估算,对研究结果的应用作了探讨。 相似文献
42.
The application of the saddlepoint approximation to reliability analysis of dynamic systems is investigated. The failure event in reliability problems is formulated as the exceedance of a single performance variable over a prescribed threshold level. The saddlepoint approximation technique provides a choice to estimate the cumulative distribution function (CDF) of the performance variable. The failure probability is obtained as the value of the complement CDF at a specif ied threshold. The method requires computing the saddlepoint from a simple algebraic equation that depends on the cumulant generating function (CGF) of the performance variable. A method for calculating the saddlepoint using random samples of the performance variable is presented. The applicable region of the saddlepoint approximation is discussed in detail. A 10-story shear building model with white noise excitation illustrates the accuracy and effi ciency of the proposed methodology. 相似文献
43.
The Ocean Drilling Program (ODP) initiated drilling at Site 1256D in the Guatemala Basin, about 1,000 km off the East Pacific
Rise to penetrate plutonic rocks, anticipated to be relatively shallow in this region, formed at an ultra-fast spreading rate.
IODP Expedition E312 successfully drilled into gabbros at ~1,150 m in basement. Multi-channel seismic traces show weak laterally
coherent sub-basement reflections at borehole depths. Synthetic reflectivity seismograms were computed using a Ricker wavelet
and impedance profiles from borehole sonic logs. These seismograms show significant sub-basement amplitude peaks. A zero-offset
vertical seismic profile, shot on E312, was processed to investigate the authenticity of these reflections and their relationship
to borehole geology. A dual scheme of the median filtering and F–K dip filtering was used. Tests with synthetic seismograms
indicate the approach is effective at reasonable SNR levels. Downgoing energy is clearly identified but negligible upgoing
energy is visible over random noise. These results indicate that lava flows and igneous contacts in upper ocean crust have
significant topography on lateral scales less than the Fresnel Zone (~300 m) due to igneous and tectonic processes. 相似文献
44.
Behshad Jodeiri Shokri Faramarz Doulati Ardejani Hamidreza Ramazi 《Natural Resources Research》2016,25(3):347-363
In this paper, an abandoned waste coal pile, which is resulted from Alborz-Sharghi coal washing plant, NE of Iran was mineralogically and geochemically characterized to evaluate pyrite oxidation, acid mine drainage (AMD) generation, and trace element mobility. After digging ten trenches and vertical sampling, a quantitative method including the atomic absorption test, and the quality-based methods including optical study were carried out for determination of pyrite fractions in the waste pile. The geochemical results revealed that the fraction of remaining pyrite increased with depth, indicating that pyrite oxidation is limited to the shallower depths of the pile which were confirmed by variations of sulfate, pH, EC, and carbonate with depth of the pile. To evaluate the trend of trace elements and mineralogical constituents of the waste particles, the samples were analyzed by using XRD, ICP-MS, and ICP-OES methods. The results showed the secondary and neutralizing minerals comprising gypsum have been formed below the oxidation zone. Besides, positive values of net neutralization potential indicated that AMD generation has not taken in the waste pile. In addition, variations of trace elements with depth reveal that Pb and Zn exhibited increasing trends from pile surface toward the bottom sampling trenches while another of them such as Cu and Ni had decreasing trends with increasing depth of the waste pile. 相似文献
45.
46.
Mass-front velocity of dry granular flows influenced by the angle of the slope to the runout plane and particle size gradation 总被引:2,自引:0,他引:2
The mass-front velocities of granular flows results from the joint action of particle size gradations and the underlying surfaces.However,because of the complexity of friction during flow movement,details such as the slope-toe impedance effects and momentum-transfer mechanisms have not been completely explained by theoretical analyses,numerical simulations,or field investigations.To study the mass-front velocity of dry granular flows influenced by the angle of the slope to the runout plane and particle size gradations we conducted model experiments that recorded the motion of rapid and long-runout rockslides or avalanches.Flume tests were conducted using slope angles of 25°,35°,45°,and 55° and three particle size gradations.The resulting mass-front motions consisted of three stages:acceleration,velocity maintenance,and deceleration.The existing methods of velocity prediction could not explain the slowing effect of the slope toe or the momentum-transfer steady velocity stage.When the slope angle increased from 25° to 55°,the mass-front velocities dropped significantly to between 44.4% and59.6% of the peak velocities and energy lossesincreased from 69.1% to 83.7% of the initial,respectively.The velocity maintenance stages occurred after the slope-toe and mass-front velocity fluctuations.During this stage,travel distances increased as the angles increased,but the average velocity was greatest at 45°.At a slope angle of 45°,as the median particle size increased,energy loss around the slope toe decreased,the efficiency of momentum transfer increased,and the distance of the velocity maintenance stage increased.We presented an improved average velocity formula for granular flow and a geometrical model of the energy along the flow line. 相似文献
47.
There is a need to bridge theory and practice for incorporating parameter uncertainty in geostatistical simulation modeling workflows. Simulation workflows are a standard practice in natural resource and recovery modeling, but the incorporation of multivariate parameter uncertainty into those workflows is challenging. However, the objectives can be met without considerable extra effort and programming. The sampling distributions of statistics comprise the core theoretical notion with the addition of the spatial degrees of freedom to account for the redundancy in the spatially correlated data. Prior parameter uncertainty is estimated from multivariate spatial resampling. Simulation-based transfer of prior parameter uncertainty results in posterior distributions which are updated by data conditioning and the model domain extents and configuration. The results are theoretically tractable and practical to achieve, providing realistic assessments of uncertainty by accounting for large-scale parameter uncertainty, which is often the most important component impacting a project. A simulation-based multivariate workflow demonstrates joint modeling of intrinsic shale properties and uncertainty in estimated ultimate recovery in a shale gas project. The multivariate workflow accounts for joint prior parameter uncertainty given the current well locations and results in posterior estimates on global distributions of all modeled properties. This is achieved by transferring the joint prior parameter uncertainty through conditional simulations. 相似文献
48.
Eugene Yee 《Boundary-Layer Meteorology》2001,98(3):517-527
A simple analytical model is developed for the meanupcrossing rate of plume concentration fluctuations assuming that thisprocess can be well approximated by a lognormal process. The resultingexpression requires only the specification of the in-plume fluctuationintensity and in-plume Taylor micro-time scale and, hence, does notexplicitly involve the joint probability density function of theconcentration and its derivative. The analytical model provides agood fit to some field measurements of the mean upcrossing rate ina dispersing plume. 相似文献
49.
50.
This study analyzed the uncertainty of inversion and the resolution limit in the presence of noise by means of statistical experiments. The exhaustive method is adopted to obtain the global optimal solution in each experiment. We found that even with small level of noise, solutions fluctuate in a large range for the thin bed. The distribution of solutions in the presence of noise is closely related to the spread of the cost function in the absence of noise. As a result, the area of a certain neighborhood around the true solution on the spread of the cost function in the absence of noise is used to evaluate the uncertainty of inversion and the resolution limit in the presence of noise. In the case that the SNR (signal-to-noise ratio) is 5 in this study, solutions focus around the true solution with a very small uncertainty only when the bed thickness is greater than the reciprocal of the double predominant frequency of the convoluting wavelet. 相似文献