全文获取类型
收费全文 | 34918篇 |
免费 | 18200篇 |
国内免费 | 43062篇 |
专业分类
测绘学 | 2776篇 |
大气科学 | 21616篇 |
地球物理 | 12712篇 |
地质学 | 35008篇 |
海洋学 | 16082篇 |
天文学 | 994篇 |
综合类 | 4533篇 |
自然地理 | 2459篇 |
出版年
2024年 | 149篇 |
2023年 | 337篇 |
2022年 | 586篇 |
2021年 | 870篇 |
2020年 | 2023篇 |
2019年 | 5420篇 |
2018年 | 5710篇 |
2017年 | 5382篇 |
2016年 | 5487篇 |
2015年 | 4828篇 |
2014年 | 4331篇 |
2013年 | 4857篇 |
2012年 | 4557篇 |
2011年 | 4351篇 |
2010年 | 4303篇 |
2009年 | 3629篇 |
2008年 | 2924篇 |
2007年 | 2941篇 |
2006年 | 2521篇 |
2005年 | 2411篇 |
2004年 | 2778篇 |
2003年 | 2495篇 |
2002年 | 2235篇 |
2001年 | 2007篇 |
2000年 | 1656篇 |
1999年 | 1776篇 |
1998年 | 1880篇 |
1997年 | 1892篇 |
1996年 | 1527篇 |
1995年 | 1439篇 |
1994年 | 1300篇 |
1993年 | 1233篇 |
1992年 | 1047篇 |
1991年 | 796篇 |
1990年 | 760篇 |
1989年 | 629篇 |
1988年 | 529篇 |
1987年 | 418篇 |
1986年 | 342篇 |
1985年 | 263篇 |
1984年 | 284篇 |
1983年 | 167篇 |
1982年 | 209篇 |
1981年 | 149篇 |
1980年 | 115篇 |
1979年 | 111篇 |
1978年 | 47篇 |
1977年 | 43篇 |
1971年 | 53篇 |
1970年 | 44篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
71.
This paper considers the problem of estimatingm, the number of components in a finite mixture of distributions from a parametric family. A step-up procedure using the bootstrap method is proposed. Some properties of the procedure are illustrated with simulation studies. An example of the method, applied to orientation of beach clasts, is given. 相似文献
72.
1 INTRODUCTIONCompact Symmetric Objects (CSOs) are powerful and compact sources (overall size <1 bpc) with lobe emission on both sides of the central engine. The small size of these sourcesis almost certainly to be attributed to the youth of the sources themselves (ages < 104 yr) andnot due to a dense coallning medium (Readhead 1996). The unification scenario assumes thatCSOs evolve into compact steep spectrum (CSS) sotirces and then into Fanaroff-Riley type 11objects (Fanti 1995). … 相似文献
73.
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
74.
75.
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
76.
This paper presents a numerical model for predicting the dynamic response of rock mass subjected to large‐scale underground explosion. The model is calibrated against data obtained from large‐scale field tests. The Hugoniot equation of state for rock mass is adopted to calculate the pressure as a function of mass density. A piecewise linear Drucker–Prager strength criterion including the strain rate effect is employed to model the rock mass behaviour subjected to blast loading. A double scalar damage model accounting for both the compression and tension damage is introduced to simulate the damage zone around the charge chamber caused by blast loading. The model is incorporated into Autodyn3D through its user subroutines. The numerical model is then used to predict the dynamic response of rock mass, in terms of the peak particle velocity (PPV) and peak particle acceleration (PPA) attenuation laws, the damage zone, the particle velocity time histories and their frequency contents for large‐scale underground explosion tests. The computed results are found in good agreement with the field measured data; hence, the proposed model is proven to be adequate for simulating the dynamic response of rock mass subjected to large‐scale underground explosion. Extended numerical analyses indicate that, apart from the charge loading density, the stress wave intensity is also affected, but to a lesser extent, by the charge weight and the charge chamber geometry for large‐scale underground explosions. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
77.
78.
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
79.
科技进步是测绘事业发展的动力 ,讨论运用科技进步的评价指标 ,对测绘单位的科技进步速度和对经济增长的贡献等进行测算和分析 ,可以为测绘规划管理及决策提供参考。 相似文献
80.