全文获取类型
收费全文 | 48273篇 |
免费 | 8110篇 |
国内免费 | 11066篇 |
专业分类
测绘学 | 4202篇 |
大气科学 | 5205篇 |
地球物理 | 10257篇 |
地质学 | 29382篇 |
海洋学 | 5244篇 |
天文学 | 2175篇 |
综合类 | 3321篇 |
自然地理 | 7663篇 |
出版年
2024年 | 255篇 |
2023年 | 692篇 |
2022年 | 1384篇 |
2021年 | 1569篇 |
2020年 | 1631篇 |
2019年 | 1862篇 |
2018年 | 1494篇 |
2017年 | 1735篇 |
2016年 | 1917篇 |
2015年 | 2061篇 |
2014年 | 2513篇 |
2013年 | 2482篇 |
2012年 | 2788篇 |
2011年 | 2974篇 |
2010年 | 2575篇 |
2009年 | 3046篇 |
2008年 | 3038篇 |
2007年 | 3381篇 |
2006年 | 3277篇 |
2005年 | 2883篇 |
2004年 | 2707篇 |
2003年 | 2612篇 |
2002年 | 2347篇 |
2001年 | 2044篇 |
2000年 | 1907篇 |
1999年 | 1758篇 |
1998年 | 1529篇 |
1997年 | 1401篇 |
1996年 | 1292篇 |
1995年 | 1078篇 |
1994年 | 1140篇 |
1993年 | 944篇 |
1992年 | 763篇 |
1991年 | 546篇 |
1990年 | 485篇 |
1989年 | 378篇 |
1988年 | 290篇 |
1987年 | 193篇 |
1986年 | 129篇 |
1985年 | 87篇 |
1984年 | 33篇 |
1983年 | 30篇 |
1982年 | 30篇 |
1981年 | 21篇 |
1980年 | 24篇 |
1979年 | 29篇 |
1978年 | 36篇 |
1977年 | 26篇 |
1975年 | 4篇 |
1954年 | 15篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
101.
李新生 《地球科学与环境学报》1993,(Z2)
本文首先简述了斜坡演化过程与构造运动的关系,然后阐明了滑坡的空间分布、活动周期及活动规模与构造因素的联系。在此基础上指出:构造因素是滑坡产生的基础,对滑坡分布具宏观控制作用,并与滑坡的活动周期和活动规模密切相关,是滑坡研究及其灾害预测、预防研究工作中值得重视的重要因素。 相似文献
102.
运输系统的区域效应研究 总被引:1,自引:0,他引:1
运输系统改进—运距缩短—产业布局调整这一空间过程对不同的区域可能具有完全不同的意义。本文通过对一个两区域—两部门经济模型的研究得出结论:当较落后的资源产区与经济中心之间的距离由于运输系统的改进而缩短时,资源产区的经济结构向单一化转变,初级产品的生产和输出在经济建设中逐渐占主导地位,加工工业逐渐萎缩,地方经济发展受到阻碍,而且这种变化是在距离的某一特定值处突然出现的,带有跃变的性质。这一研究结果表明,对区域运输系统的任何改进措施都应该慎而行之,以免造成与本来目标相悖的区域效应。 相似文献
103.
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
104.
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
105.
106.
G. A. Krasinsky 《Celestial Mechanics and Dynamical Astronomy》2002,84(1):27-55
Differential equations describing the tidal evolution of the earth's rotation and of the lunar orbital motion are presented in a simple close form. The equations differ in form for orbits fixed to the terrestrial equator and for orbits with the nodes precessing along the ecliptic due to solar perturbations. Analytical considerations show that if the contemporary lunar orbit were equatorial the evolution would develop from an unstable geosynchronous orbit of the period about 4.42 h (in the past) to a stable geosynchronous orbit of the period about 44.8 days (in the future). It is also demonstrated that at the contemporary epoch the orbital plane of the fictitious equatorial moon would be unstable in the Liapunov's sense, being asymptotically stable at early stages of the evolution. Evolution of the currently near-ecliptical lunar orbit and of the terrestrial rotation is traced backward in time by numerical integration of the evolutional equations. It is confirmed that about 1.8 billion years ago a critical phase of the evolution took place when the equatorial inclination of the moon reached small values and the moon was in a near vicinity of the earth. Before the critical epoch t
cr
two types of the evolution are possible, which at present cannot be unambiguously distinguished with the help of the purely dynamical considerations. In the scenario that seems to be the most realistic from the physical point of view, the evolution also has started from a geosynchronous equatorial lunar orbit of the period 4.19 h. At t < t
cr
the lunar orbit has been fixed to the precessing terrestrial equator by strong perturbations from the earth's flattening and by tidal effects; at the critical epoch the solar perturbations begin to dominate and transfer the moon to its contemporary near-ecliptical orbit which evolves now to the stable geosynchronous state. Probably this scenario is in favour of the Darwin's hypothesis about originating the moon by its separation from the earth. Too much short time scale of the evolution in this model might be enlarged if the dissipative Q factor had somewhat larger values in the past than in the present epoch. Values of the length of day and the length of month, estimated from paleontological data, are confronted with the results of the developed model. 相似文献
107.
张赤军 《大地测量与地球动力学》2002,22(4):31-34
介绍当今确定天文大地垂偏差的新仪器-CCD自动天体测量仪和确定两点间高程异常差的新方法,该方法用单极坐标代替繁琐的双极坐标进行计算,讨论了天文重力水准的误差及垂线偏差非线性影响等问题,估计在不久的将来,用这一手段施测山区似大地水准面的精度可望达到厘米级。 相似文献
108.
张晓晖 《成都信息工程学院学报》2002,17(3):166-169
通过对影响能见度的因素的分析,提出了一种能见度预报方法-综合分析法,极大地提高了能见度的预报准确率。 相似文献
109.
110.