全文获取类型
收费全文 | 52448篇 |
免费 | 8730篇 |
国内免费 | 11190篇 |
专业分类
测绘学 | 5205篇 |
大气科学 | 6299篇 |
地球物理 | 10735篇 |
地质学 | 28672篇 |
海洋学 | 6955篇 |
天文学 | 3469篇 |
综合类 | 3382篇 |
自然地理 | 7651篇 |
出版年
2024年 | 373篇 |
2023年 | 934篇 |
2022年 | 1784篇 |
2021年 | 2212篇 |
2020年 | 2052篇 |
2019年 | 2468篇 |
2018年 | 1825篇 |
2017年 | 2152篇 |
2016年 | 2139篇 |
2015年 | 2390篇 |
2014年 | 3059篇 |
2013年 | 3231篇 |
2012年 | 3179篇 |
2011年 | 3327篇 |
2010年 | 2940篇 |
2009年 | 3420篇 |
2008年 | 3369篇 |
2007年 | 3597篇 |
2006年 | 3474篇 |
2005年 | 3175篇 |
2004年 | 2813篇 |
2003年 | 2655篇 |
2002年 | 2328篇 |
2001年 | 2043篇 |
2000年 | 2002篇 |
1999年 | 1727篇 |
1998年 | 1513篇 |
1997年 | 1131篇 |
1996年 | 949篇 |
1995年 | 806篇 |
1994年 | 771篇 |
1993年 | 614篇 |
1992年 | 453篇 |
1991年 | 382篇 |
1990年 | 256篇 |
1989年 | 230篇 |
1988年 | 176篇 |
1987年 | 113篇 |
1986年 | 62篇 |
1985年 | 56篇 |
1984年 | 39篇 |
1983年 | 27篇 |
1982年 | 15篇 |
1981年 | 18篇 |
1980年 | 15篇 |
1979年 | 6篇 |
1978年 | 23篇 |
1977年 | 5篇 |
1971年 | 5篇 |
1954年 | 7篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
171.
传统的算法复杂度分析方法重点对算法的内在流程进行分析,而且对于数据挖掘时具有很大不确定性,执行时间不能准确确定.用物理学原理从能量的角度对数据挖掘中的经典聚类算法K-means进行分析,并从算法的本质上推导出其复杂度下限,推导出该算法能优化到的极限值. 相似文献
172.
There continues to be reluctance in some jurisdictions to use chemical dispersants as a viable countermeasure for accidental oil spills. One argument used by some opponents to dispersant use is that “chemical dispersants do not work effectively in cold water”. To address this issue, the U.S. Minerals Management Service (MMS) funded and conducted two series of large-scale dispersant experiments in very cold water at Ohmsett - The National Oil Spill Response Test Facility, located in Leonardo, New Jersey in February-March 2006 and January-March 2007. Alaska North Slope, Endicott, Northstar and Pt. McIntyre crude oils and Corexit 9500 and Corexit 9527 dispersants were used in the two test series. The crude oils were tested both when fresh and after weathering. Results demonstrated that both Corexit 9500 and Corexit 9527 dispersants were 85-99% effective in dispersing the fresh and weathered crude oils tested at cold temperatures. The MMS expects that results from these test series will assist government regulators and responders in making science based decisions on the use of dispersants as a response tool for oil spills in the Arctic. 相似文献
173.
本文概述了新、老矿区的战略找矿方向和基础理论找矿模式,提出测试、钻探及最佳找矿技术方法组合,强化组织管理是地质勘查的正确思路 相似文献
174.
????CNES???????????????????λ???????д?????????????????о????????????????????????????????????????????????CNES??????????????????SuomiNet????????????????IGS??????????????????????????????????????????????????к??????? 相似文献
175.
176.
古尔班通古特沙漠腹地土壤水分动态 总被引:3,自引:2,他引:3
采用土钻法对古尔班通古特沙漠腹地典型自然沙垄剖面土壤含水量进行长期监测.采取多重比较(LSD)、典型相关分析(MANOVA)、并在国内第一次采用季节交乘趋势等统计方法,对土壤水分与环境因子的关系进行分析,研究结果表明:土壤含水量时间变化规律分为补给期、失水期和冻结滞水期.土壤含水量均值为补给期>冻结滞水期>失水期,变异系数为补给期>冻结滞水期>失水期;土壤含水量空间变化规律表现为:阴阳坡间具有极显著差异(P<0.01,F=0.002 9).垄间分别与坡中、垄顶具有显著性差异(P<0.05).垂直变化分为活跃层、过渡层和稳定层,活跃层与其他土层间均有显著性差异(P<0.05),土壤含水量均值为阴坡>阳坡,垄顶>垄间>坡中.活跃层>过渡层和稳定层;变异系数为阴坡>阳坡,垄间>坡中>垄顶,活跃层>过渡层>稳定层;降雨量和温度是影响研究区土壤水分时间分布变化的重要影响因子,同时也是土壤含水量变化的决定性影响因子,与土壤含水量具有显著相关性(P<0.05,F=0.02). 相似文献
177.
This paper aims to determine the optimal fines content of coarse-grained soil required to simultaneously achieve weaker frost susceptibility and better bearing capacity. We studied the frost susceptibility and strength properties of coarse-grained soil by means of frost heaving tests and static triaxial tests, and the results are as follows:(1) the freezing temperature of coarse-grained soil decreased gradually and then leveled off with incremental increases in the percent content of fines; (2) the fines content proved to be an important factor influencing the frost heave susceptibility and strength properties of coarse-grained soil. With incremental increases in the percent content of fines, the frost heave ratio increased gradually and the cohesion function of fines effectively enhanced the shear strength of coarse-grained soil before freeze-thaw, but the frost susceptibility of fines weakened the shear strength of coarse-grained soil after freeze-thaw; (3) with increasing numbers of freeze-thaw cycles,the shear strength of coarse-grained soil decreased and then stabilized after the ninth freeze-thaw cycle, and therefore the mechanical indexes of the ninth freeze-thaw cycle are recommended for the engineering design values; and (4) considering frost susceptibility and strength properties as a whole, the optimal fines content of 5% is recommended for railway subgrade coarse-grained soil fillings in frozen regions. 相似文献
178.
中尺度流域土地利用/土地覆盖变化评估--以伊洛河中部地区为例 总被引:18,自引:7,他引:18
将规范分析与实证分析相结合,提出了一个评估区域各点位土地利用/土地覆盖变化的新方法:(1)采用相对原则观察一时段内的变化方向;(2)物理模型与统计模型相结合,基于区域实际情况建立生境适宜性参照基准;(3)用多变量分析方法量度各点位与参照基准的差异。结合GIS和遥感技术,这种方法被应用于伊洛河中部地区的研究中。 相似文献
179.
Through the study of mutual process between groundwater systems and eco-environmental water demand, the eco-environmental
water demand is brought into groundwater systems model as the important water consumption item and unification of groundwater’s
economic, environmental and ecological functions were taken into account. Based on eco-environmental water demand at Da’an
in Jilin province, a three-dimensional simulation and optimized management model of groundwater systems was established. All
water balance components of groundwater systems in 1998 and 1999 were simulated with this model and the best optimal exploitation
scheme of groundwater systems in 2000 was determined, so that groundwater resource was efficiently utilized and good economic,
ecologic and social benefits were obtained. 相似文献
180.
Water is a necessary element during gas hydrate formations. Therefore, by analyzing water depletion changes in media, the reaction characteristics of methane hydrate in media can be studied. In this study, two water sources supplying some liquid water which may be consumed by the methane hydrate formation reactions were designed and assembled. Using them, the full formation processes of methane hydrate was studied. Experimental results show the following: If heat released from nucleation reaction of methane hydrate is diffused rapidly, the nucleation ratios will be enhanced discernibly. While the hydrate is formed, a force is generated that sucks fresh water from the source into the vicinity of the hydrate, slowing down the cementation process and causing some hydrate grain dissociation. As a result of cementation differences, the hydrate reaction processes with different water sources present linear or quadratic equation characteristics. After a few repeated dissociation and formation processes of some hydrate grains caused by the fresh water, the gas amounts contained in hydrate will be significantly enhanced. 相似文献