全文获取类型
收费全文 | 7426篇 |
免费 | 1432篇 |
国内免费 | 1207篇 |
专业分类
测绘学 | 1478篇 |
大气科学 | 625篇 |
地球物理 | 2141篇 |
地质学 | 3198篇 |
海洋学 | 1062篇 |
天文学 | 55篇 |
综合类 | 589篇 |
自然地理 | 917篇 |
出版年
2024年 | 21篇 |
2023年 | 101篇 |
2022年 | 268篇 |
2021年 | 313篇 |
2020年 | 333篇 |
2019年 | 387篇 |
2018年 | 313篇 |
2017年 | 405篇 |
2016年 | 393篇 |
2015年 | 407篇 |
2014年 | 471篇 |
2013年 | 532篇 |
2012年 | 482篇 |
2011年 | 504篇 |
2010年 | 391篇 |
2009年 | 466篇 |
2008年 | 463篇 |
2007年 | 508篇 |
2006年 | 464篇 |
2005年 | 359篇 |
2004年 | 372篇 |
2003年 | 300篇 |
2002年 | 252篇 |
2001年 | 194篇 |
2000年 | 212篇 |
1999年 | 153篇 |
1998年 | 162篇 |
1997年 | 140篇 |
1996年 | 132篇 |
1995年 | 119篇 |
1994年 | 100篇 |
1993年 | 74篇 |
1992年 | 62篇 |
1991年 | 37篇 |
1990年 | 25篇 |
1989年 | 40篇 |
1988年 | 32篇 |
1987年 | 16篇 |
1986年 | 12篇 |
1985年 | 13篇 |
1984年 | 7篇 |
1983年 | 5篇 |
1982年 | 3篇 |
1981年 | 2篇 |
1980年 | 2篇 |
1979年 | 4篇 |
1978年 | 2篇 |
1976年 | 2篇 |
1971年 | 2篇 |
1954年 | 7篇 |
排序方式: 共有10000条查询结果,搜索用时 640 毫秒
61.
在对遗传算法交叉、变异后结果的处理方法进行改进并在将进化过程分段的基础上,提出1种新的混沌遗传算法。该算法利用混沌运动的遍历性择优产生初始群体,对每一轮遗传操作所得到的部分当前最优个体进行变尺度混沌搜索。仿真结果表明,该算法具有较高的优化效率,并能求得全局最优解。 相似文献
62.
The microhabitat preferences and depth distribution of blenniid species (Blenniidae) in the Gulf of Trieste (North Adriatic Sea) were surveyed using the all‐occurrence sampling method, a non‐destructive visual census method, aided by SCUBA diving. Fourteen species were identified during the entire survey. Four species showed to be indiscriminate in their microhabitat choice while 10 species were classified as infrequent in the surveyed coastal area. A statistically significant correlation was found between the blenniid assemblage and nine microhabitat variables. The blenniid assemblage was divided in two main groups, by the use of canonical correspondence analysis, electivity index, and the depth distribution analysis. The first group comprises species that dwell in surface waters and show a high positive correlation with boulders, the presence of Mytilus galloprovincialis, cirripeds and empty holes bored by Lithophaga lithophaga. The second group includes species that mostly inhabit deeper waters and show a high positive correlation with rocks covered by precoralligenous bioformations. 相似文献
63.
A series of over 6 000 research-trawl samples collected along the west and south coasts of South Africa between 1987 and 2014 were analysed for the presence and biomass of two parapagurid hermit crabs, Sympagurus dimorphus and Parapagurus bouvieri. The percentage of trawls that landed S. dimorphus and P. bouvieri and the mean caught biomass were higher on the west than on the south coast for both the more-abundant S. dimorphus (30.59 vs 5.81% success and 287.88 vs 31.37 kg km–2, respectively) and for the less-abundant P. bouvieri (13.76 vs 3.58% success and 38.56 vs 16.32 kg km–2, respectively). Very few parapagurids were caught shallower than 150 m; thereafter, the proportion of trawls containing hermit crabs increased, peaking over the depth range 201–250 m for S. dimorphus (54%) and 401–450 m for P. bouvieri (51%), and declining steadily thereafter. On the west coast, the relative caught biomass of S. dimorphus increased significantly from north to south, but there was no apparent latitudinal trend in relative biomass for P. bouvieri. Similarly, there was a significant decline in caught biomass of S. dimorphus with increasing longitude along the south coast, but no apparent trend for P. bouvieri. Although this represents by far the most comprehensive global analyses of distribution and abundance patterns for parapagurids to date, extremely little remains known about the biology and ecological relationships of these species, or indeed of other members of the group. 相似文献
64.
Unlike in the open sea, the use of wind information for forecasting waves may encounter more ambiguous uncertainties in the coastal or harbor area due to the influence of complicated geometric configurations. Thus this paper attempts to forecast the waves based on learning the characteristics of observed waves, rather than the use of the wind information. This is reported in this paper by the application of the artificial neural network (ANN), in which the back-propagation algorithm is employed in the learning process for obtaining the desired results. This model evaluated the interconnection weights among multi-stations based on the previous short-term data, from which a time series of waves at a station can be generated for forecasting or data supplement based on using the neighbor stations data. Field data are used for testing the applicability of the ANN model. The results show that the ANN model performs well for both wave forecasting and data supplement when using a short-term observed wave data. 相似文献
65.
66.
67.
68.
69.
两种涡鞭毛藻的周日垂直迁移特性研究 总被引:3,自引:3,他引:3
Alexandrium lusitanicum于1987年采自葡萄牙沿海,Y-100于1989年采自德国湾。在暗室中用一根柱长150cm,内径3.4cm的玻璃柱对两种涡鞭毛藻的垂直行特性进行研究。结果表明,两种藻均进行有规律的周日垂直迁移,且均在始前2h开始向上迁移,光照结束前3h开始向移速度约为280μm/s,向下约为140μm/s向下约为850μm/s;Y-100向上迁移速度约为280μm/s 相似文献
70.
台湾海峡金色小沙丁鱼幼鱼索饵群体研究 总被引:2,自引:1,他引:2
本文研究了台湾海峡南部和北部的金色小沙丁鱼当年生幼鱼计数性状和量度性状特征差异,并结合其生物学特性、产卵场、洄游及海区环境特征进行综合分析。结果表明,台湾海峡南部和北部的索饵幼鱼仍属于同一地方种群:闽南-台湾浅滩语地方种群。根据其索饵洄游特点,可分为台湾海峡南部和北部两个索饵群体. 相似文献