全文获取类型
收费全文 | 1169篇 |
免费 | 197篇 |
国内免费 | 286篇 |
专业分类
测绘学 | 102篇 |
大气科学 | 241篇 |
地球物理 | 262篇 |
地质学 | 569篇 |
海洋学 | 161篇 |
天文学 | 77篇 |
综合类 | 127篇 |
自然地理 | 113篇 |
出版年
2024年 | 11篇 |
2023年 | 22篇 |
2022年 | 38篇 |
2021年 | 55篇 |
2020年 | 34篇 |
2019年 | 41篇 |
2018年 | 55篇 |
2017年 | 54篇 |
2016年 | 53篇 |
2015年 | 63篇 |
2014年 | 68篇 |
2013年 | 58篇 |
2012年 | 50篇 |
2011年 | 65篇 |
2010年 | 64篇 |
2009年 | 61篇 |
2008年 | 53篇 |
2007年 | 55篇 |
2006年 | 47篇 |
2005年 | 39篇 |
2004年 | 46篇 |
2003年 | 47篇 |
2002年 | 69篇 |
2001年 | 90篇 |
2000年 | 50篇 |
1999年 | 60篇 |
1998年 | 39篇 |
1997年 | 40篇 |
1996年 | 36篇 |
1995年 | 35篇 |
1994年 | 39篇 |
1993年 | 26篇 |
1992年 | 19篇 |
1991年 | 15篇 |
1990年 | 19篇 |
1989年 | 7篇 |
1988年 | 8篇 |
1987年 | 10篇 |
1986年 | 1篇 |
1985年 | 5篇 |
1984年 | 1篇 |
1983年 | 2篇 |
1982年 | 1篇 |
1964年 | 1篇 |
排序方式: 共有1652条查询结果,搜索用时 765 毫秒
981.
Water temperature is an important habitat factor in river ecosystems that exhibits the characteristics of continuous change. Dam construction disrupts the continuity of river water temperature and reset it, thus exerting sharp rise/decrease on the characteristics of water temperature change. The effect of a dam on river continuity is directly related to the dam size. To explain this relationship, two rivers in China were selected: one river without reservoirs and one river with cascade reservoirs. Through the analysis of the longitudinal change of water temperature in free-flowing rivers, we found that water temperature changes continuously and steadily in the longitudinal direction. Based on this, a temperature trend hypothesis in river was proposed, and the discontinuity of the water temperature in the reservoir section was evaluated. The results are as follows: (1) In mixed reservoirs, river water temperature remained as continuous as free-flowing rivers. However, the river water temperature had a large discontinuity in the stratified reservoir. (2) Water residence time was used as an indicator of the continuity of reservoir water temperature. (3) Selective withdrawal of stratified reservoirs in January could not remove the discontinuity caused by itself, but it worked in June. 相似文献
982.
983.
提出一种基于极大验后估计(MAP)超分辨率重建算法的新的改进算法.该算法将MAP算法或其他超分辨率重建算法所获得的高分辨率影像引入到MAP算法中,主要思想是基于所求取的分辨率应该与理想的高分辨率影像最大相关的原理.最后采取基于灰度的四参数仿射变换模型的配准算法,采用共轭梯度算法来迭代求解超分辨率重建方程.仿真试验证明,... 相似文献
984.
985.
高大建筑物和工业设施等的变形监测数据一般都是含噪的信号,需要对其进行降噪处理,获取其准确的变形信息,以便掌握真实的变形情况。由于监测数据都是非稳定的信号,通常采用小波阈值降噪法对其处理,但传统的软硬阈值进行降噪的效果不佳,需对其进行改进。文中采用3种改进的小波阈值降噪法与传统的方法进行比较,通过对信噪比和均方差的分析,获得适合于变形监测数据的非稳定信号的降噪方法。 相似文献
986.
根据全国739个气象台站1961年1月至2005年12月的逐日气象数据记录建立时空序列数据集,提取极端高温事件和极端低温事件。结合传统关联规则挖掘技术和地理空间数据分析方法,对极端气温事件数据集进行了空间关联模式的分析。实验结果显示,所得空间关联模式中涉及的区域在空间上具有明显的聚集性;在东北、华中两个局部地区的台站中,极端气温事件的发生存在较强的关联规则(支持度阈值6%,置信度阈值95%),而在其他区域的台站中,极端气温事件不存在类似的关联规则,且极端高温事件的关联规则数量要明显高于极端低温事件。对存在关联规则的台站进行空间分析发现,同一关联规则内的各台站具有空间邻近性,其邻近范围约为200 km。以上空间关联模式的挖掘分析,可以为我国极端气温事件的预警和防控提供有价值的参考。 相似文献
987.
A wave power device includes an energy harvesting system and a power take-off system. The power take-off system of a floating wave energy device is the key that converts wave energy into other forms. A set of hydraulic power take-off system, which suits for the floating wave energy devices, includes hydraulic system and power generation system. The hydraulic control system uses a special“self-hydraulic control system”to control hydraulic system to release or save energy under the maximum and the minimum pressures. The maximum pressure is enhanced to 23 MPa, the minimum to 9 MPa. Quite a few experiments show that the recent hydraulic system is evidently improved in efficiency and reliability than our previous one, that is expected to be great significant in the research and development of our prototype about wave energy conversion. 相似文献
988.
Characteristics of a drainage channel with staggered indented sills for controlling debris flows 总被引:2,自引:1,他引:1
The characteristics of a new type of drainage channel with staggered indented sills for controlling debris flows were studied. The intermediate fluid in the non-viscous debris flow exhibited a helical movement, whereas the fluid near the sidewall had a stop-start movement pattern; the viscous debris flow exhibited a stable structure between the indented sills. The experimental results indicate that the mean velocity of the debris flow increased with increasing channel gradients, and the debris flow velocity was slightly affected by the angle of the sills. The average velocity of the non-viscous debris flow increased in the range of(0.5–1.5) interval between the indented sills, whereas the average velocity of the viscous debris flow increased initially and then decreased in the range of(0.75–1.25) interval between the indented sills. The depth of the non-viscous debris flow tended to gradually increase as the channel gradients increased, whereas the depth of the viscous debris flow gradually decreased as the channel gradients increased. When the discharge of the debris flow was constant, the angle and the interval between the indented sills had a slight effect on the depth of the viscous debris flow, whereas the depth of the non-viscous debris flow exhibited a different trend, as the sill angles and intervals were varied. 相似文献
989.
This paper is concerned with the robust control synthesis of autonomous underwater vehicle(AUV) for general path following maneuvers.First,we present maneuvering kinematics and vehicle dynamics in a unified framework.Based on H∞ loop-shaping procedure,the 2-DOF autopilot controller has been presented to enhance stability and path tracking.By use of model reduction,the high-order control system is reduced to one with reasonable order,and further the scaled low-order controller has been analyzed in both the frequency and the time domains.Finally,it is shown that the autopilot control system provides robust performance and stability against prescribed levels of uncertainty. 相似文献
990.
Shi Yan Wu Qi You Zhou Gang Wang Lei Yang Cheng Peng Ling 《Environmental Earth Sciences》2011,62(5):999-1011
Based on batch experiments conducted on soil samples in the laboratory, the relationship between dielectric constant and water
content based on electrical capacitance measurement was investigated. Factors that may affect the relationship, such as measurement
frequency, electrode array methods, and soil particle sizes, were analyzed. A model fitting the relationship between dielectric
constant and water content was proposed. The results clearly indicate that even with the use of rod-like sensors, the measured
electrical capacitance with 1 kHz frequency can be used to construct a good relationship between dielectric constant and water
content. Although the electrode array methods (parallel or non-parallel electrode pair) and soil particle sizes do affect
the obtained absolute values of dielectric constant to some extent, the relationship between dielectric constant and water
content remains unchanged. A clear characteristic is the increase reduction in the dielectric constant at near saturation
condition, and the low increase speed when water content is small. The proposed modified VG model incorporated this characteristic
into it and fitted the data better than the reported models. These results suggest that the electrical capacitance-based dielectric
constant measurement can be extensively used and would be useful, especially in the situations where parallel sensor requirement
is difficult to satisfy. 相似文献