全文获取类型
收费全文 | 47265篇 |
免费 | 19556篇 |
国内免费 | 43982篇 |
专业分类
测绘学 | 3505篇 |
大气科学 | 22717篇 |
地球物理 | 14988篇 |
地质学 | 40716篇 |
海洋学 | 17531篇 |
天文学 | 2601篇 |
综合类 | 5129篇 |
自然地理 | 3616篇 |
出版年
2024年 | 210篇 |
2023年 | 469篇 |
2022年 | 1147篇 |
2021年 | 1603篇 |
2020年 | 2667篇 |
2019年 | 6023篇 |
2018年 | 6482篇 |
2017年 | 6139篇 |
2016年 | 6194篇 |
2015年 | 5275篇 |
2014年 | 4967篇 |
2013年 | 5632篇 |
2012年 | 5356篇 |
2011年 | 5096篇 |
2010年 | 5014篇 |
2009年 | 4299篇 |
2008年 | 3575篇 |
2007年 | 3586篇 |
2006年 | 3116篇 |
2005年 | 2723篇 |
2004年 | 3012篇 |
2003年 | 2798篇 |
2002年 | 2607篇 |
2001年 | 2350篇 |
2000年 | 1901篇 |
1999年 | 1972篇 |
1998年 | 1919篇 |
1997年 | 2017篇 |
1996年 | 1591篇 |
1995年 | 1533篇 |
1994年 | 1365篇 |
1993年 | 1300篇 |
1992年 | 1103篇 |
1991年 | 825篇 |
1990年 | 772篇 |
1989年 | 665篇 |
1988年 | 554篇 |
1987年 | 428篇 |
1986年 | 362篇 |
1985年 | 280篇 |
1984年 | 319篇 |
1983年 | 199篇 |
1982年 | 234篇 |
1981年 | 166篇 |
1980年 | 132篇 |
1979年 | 137篇 |
1978年 | 64篇 |
1977年 | 59篇 |
1972年 | 53篇 |
1971年 | 63篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
91.
Yanjun ZHANG Fenglong ZHANG Full QI Fuquan NI 《东北亚地学研究》2006,9(2):199-203
In view of the situation of excavation of open coal mine for the underground water disaster,we should carry out simulation studies for the numerical value of the water lowering project and improve the accuracy and the level of the water lowering project.On the basis of the hydrological geological conditions of certain open mine digging,a more reasonable seepage numerical model was built according to MODFLOW.It was simulated in advance that the process of the confined water level descending with the time,and combining with the actual observations to test the correctness of the model.The calculation showed that the results coincided well with the results of actual measurement.Based on this,different water lowering numerical simulations were built for the open coal mine digging.It could be simulated and forecast that the changes of the groundwater level in drainage process within and outside the mine pit,and it was quantitatively assessed that the possible water lowering result of the opencast water drainage process,which provide an important basis for the actual water lowering project and the possible project disposal. 相似文献
92.
V. Yu. Terebizh 《Astrophysics and Space Science》1992,193(2):269-288
Image restoration, computerized tomography, and other similar problems are considered as a unified class of stochastic inverse problems. The conventional approach to these problems that proceeds from some integral or functional equations suffers from three main shortcomings: (i) subjectivity, (ii) inability to account for the inner (radiational) noise, and (iii) inability to include the fundamental concept of the natural limit of solution accuracy. A general approach is developed, the Statistical Parameterization of Inverse Problems (SPIPR), that takes into account both the inner and external random noise and gives an explicit form of the above-mentioned natural limit. Applications of the SPIPR to various problems show that the maximum likelihood method as the concrete way to obtain an object estimate has practically limiting efficiency.Two new fields of applications of the SPIPR are outlined along with the image restoration problem: the elimination of blurring due to atmosphere turbulence and reconstruction of an object structure in the computerized tomography. The expressions for the main distribution function in all these problems are found. The corresponding real examples and model cases are considered as well. 相似文献
93.
94.
Natural and agricultural wetlands are considered to be the major sources of global atmospheric methane (CH4). A one‐dimensional model was developed to simulate methane emission and used to examine the influence of various physical processes on the rate of methane emission. Three processes involved in the methane emission are implemented in the model: production, reoxidation and transport. Three transport pathways were considered: diffusion across water–air or soil–air interfaces, ebullition and diffusion through plants. These pathways are influenced by soil properties, plant growth, water‐table conditions, temperature and external inputs (e.g. fertilizer). The model was used to examine the seasonal variation of the methane emission at a rice field in Hunan, China, which was observed during a field experiment for consecutive (early and late) rice seasons in 1992. The observed seasonal variations of methane emission, and role of plants in transporting methane to the atmosphere, are captured by the model simulation. Further model applications were conducted to simulate effects of fertilizer and water‐level condition on the methane emission. The results indicate that unfermented organic fertilizer produces a higher methane emission rate than mineral fertilizer. The simulations with treatments of a deep‐water covering and constant moisture reduced the methane emission. The rice field study provides a framework for further development of the model towards simulations based on spatially distributed variables (e.g. water table, soil temperature and vegetation) at a regional scale. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
95.
Axial surveys were performed in the two river tributaries of the Cochin estuary, SW India during November 1988. Surficial sediments were subjected to sequential chemical extractions to delineate five metal fractions, namely, exchangeable, carbonate bound, easily reducible, organic/sulfide bound, and residual. The results indicated selective accumulation of Mn and Ni in carbonate bound and organic/sulfide forms, along with marginal amounts of Co in the exchangeable fraction. Large portions of Fe and Cr occurred in the residual fraction, whereas composite fractionation of Zn species was noticed. The exchangeable fractions of Fe and Cr as well as of easily reducible cobalt were below detection limits. The levels of Cr and Zn indicate anthropogenic inputs in this estuary, whereas Co and Ni show regional contamination exceeding natural levels. The analytical speciation procedure helps to deduce the sedimental diagenetic processes in the estuarine environment. 相似文献
96.
(姚建衢)(杨焕宗)COORDINATINGDEVELOPMENTOFAGRICULTURALRESOURCESANDENVIRONMENTOFTHETROPICREGIONINYUNNANPROVINCE¥YaoJianqu(Instituteof... 相似文献
97.
98.
Universal cokriging is used to obtain predictions when dealing with multivariate random functions. An important type of nonstationarity is defined in terms of multivariate random functions with increments which are stationary of orderk. The covariance between increments of different variables is modeled by means of the pseudo-cross-covariance function. Criteria are formulated to which the parameters of pseudo-cross-covariance functions must comply so as to ensure positive-definiteness. Cokriging equations and the induced cokriging equations are given. The study is illustrated by an example from soil science. 相似文献
99.
An analysis of our observations of the Geminga object with the GT-48 ground-based gamma-ray telescope has shown that its very-high-energy gamma-ray flux is modulated with a 59-s period. The 59-s period and its time derivative previously inferred from satellite data have been confirmed. According to our data, the period was 61.94 s in 1997 at MSD=50573. The statistical significance of this result is (1?4.5)×10?4. 相似文献
100.
介绍了近年来河外H2O超脉泽的主要观测结果。H2O超脉泽通常起源于活动星系核中央的拱核盘。它们主要寄生在 Serfert 2星系或低电离核区。至今为止,已有20个星系探测到H2O超脉泽。脉泽辐射的各向同性光度为10~6000L⊙。所有超脉泽星系显示出核的活动,显然,脉泽是由核活动所产生的射电和X射线光子或激波来抽运的。H2O超脉泽倾向存在于高倾斜度的星系,这使得沿视线上的分子柱密度增高,产生足够大的放大光深。最有可能产生H2O超脉泽辐射的星系应有一个包含着射电源的侧向的分子盘以及一个适当的抽运机制。 相似文献