全文获取类型
收费全文 | 302篇 |
免费 | 29篇 |
国内免费 | 15篇 |
专业分类
安全科学 | 97篇 |
废物处理 | 1篇 |
环保管理 | 15篇 |
综合类 | 87篇 |
基础理论 | 109篇 |
环境理论 | 1篇 |
污染及防治 | 7篇 |
评价与监测 | 8篇 |
社会与环境 | 6篇 |
灾害及防治 | 15篇 |
出版年
2024年 | 2篇 |
2023年 | 4篇 |
2022年 | 8篇 |
2021年 | 14篇 |
2020年 | 12篇 |
2019年 | 9篇 |
2018年 | 3篇 |
2017年 | 12篇 |
2016年 | 8篇 |
2015年 | 12篇 |
2014年 | 6篇 |
2013年 | 7篇 |
2012年 | 15篇 |
2011年 | 20篇 |
2010年 | 14篇 |
2009年 | 30篇 |
2008年 | 23篇 |
2007年 | 22篇 |
2006年 | 19篇 |
2005年 | 15篇 |
2004年 | 21篇 |
2003年 | 5篇 |
2002年 | 4篇 |
2001年 | 7篇 |
2000年 | 9篇 |
1999年 | 8篇 |
1998年 | 8篇 |
1997年 | 3篇 |
1996年 | 7篇 |
1995年 | 6篇 |
1994年 | 7篇 |
1993年 | 3篇 |
1992年 | 1篇 |
1990年 | 1篇 |
1986年 | 1篇 |
排序方式: 共有346条查询结果,搜索用时 109 毫秒
211.
212.
针对火灾图像识别过程中颜色特征数量多、特征间相关性复杂、难以在多维特征融合过程中有效融合图像颜色特征等问题,提出1种考虑颜色特征最优组合的CART决策树火灾图像识别方法。首先,在Lab、RGB、HSV 3种色彩模式下基于图像颜色特征提取火灾图像特征序列;其次,分别在3种色彩模式下基于精细决策树与特征随机排列组合方法提取颜色特征中最优组合特征;最后,将提取的火灾图像最优组合特征序列作为CART决策树输入进行模型训练,并通过测试样本以及其他机器学习方法进行模型泛化能力的分析。研究结果表明:本文方法寻找出识别火灾图像的最优颜色特征组合为“Kb1+Var1+Kg+Kb2+Var2+Kh+Ks+Kv”;CART决策树方法对于火灾图像识别的测试准确度可达84.5%,其分类效果明显优于其他决策树类与集成树类方法;9折为最佳交叉验证折数,其测试准确度可达86.47%,与5折交叉验证相比明显提升14.77%。研究结果可为火灾图像识别提供方法基础。 相似文献
213.
基于高分影像的城市黑臭水体遥感识别:以南京为例 总被引:4,自引:0,他引:4
城市黑臭水体的遥感识别对于黑臭水体的监测、治理有着重要作用.2016年对南京城市河段进行了地面调查,并在沙洲西河、秃尾河、玄武湖、金川河设置了55个样点,采集水样分析水质参数并同步测量水面光谱.在此基础上,利用绿光波段遥感反射率的单波段阈值、蓝、绿波段差值、红、绿波段比值,以及色度值,分别构建了基于GF-2影像的城市黑臭水体遥感识别算法,并分析南京市主城区黑臭水体的空间分布和环境特点.结果表明:(1)和其他类型水体相比,城市黑臭水体遥感反射率最低,在整个可见光范围峰谷不突出,而且在400~550 nm范围光谱斜率最小;(2)采用地面同步调查结果检验,比值算法的识别精度最高,对城市河道黑臭水体的识别更准确;(3)利用比值法对2016年11月3日的GF-2影像进行计算,共提取研究区黑臭河段11条,总长度40.7 km,总面积0.749 km~2.黑臭河段分布具有范围广且不连续的特征,集中分布于各城区人口密集的区域;水体发生黑臭主要受到生活污水、工业废水、断头浜的影响. 相似文献
214.
Summary. In the past decades, several studies have focused on the identification of feeding stimulants for specialists that feed on
solanaceous plants, especially potato (Solanum tuberosum). In the 1950's, a phagostimulant was isolated from potato and tomato for Manduca sexta and characterised as a glycoside. It was suggested that the phagostimulant for M. sexta and Leptinotarsa decemlineata is identical. We tested whether these insects indeed share the same recognition factor. Previous bioassays for L. decemlineata larvae and adults were found to be unsatisfactory, so a new assay system was developed, using starch and wheat flour-based
wafers as a neutral substrate. An aqueous extract of potato foliage that was highly active as a stimulant was fractionated
by reverse phase medium pressure liquid chromatography (MPLC). Both insects were stimulated by a MPLC fraction that eluted
with 25% methanol in water. Further separation of this and the following fraction (35% methanol, stimulatory for the beetle
only) by semi-preparative high performance liquid chromatography (HPLC), using gradients of water and acetonitrile, resulted
in the isolation of two distinct fractions that stimulated feeding by L. decemlineata. None of the HPLC fractions was active for M. sexta. However, fractions of the flash chromatography with less polarity (45–75% methanol) had a stimulatory effect only on M. sexta. Thus, the two insects do not use a common feeding stimulant, and for both at least two compounds of different polarity are
active.
Received 3 July 2000; accepted 24 October 2000 相似文献
215.
An absence of aggression between non-nestmates in the bull ant <Emphasis Type="Italic">Myrmecia nigriceps</Emphasis> 总被引:2,自引:0,他引:2
van Wilgenburg E Dang S Forti AL Koumoundouros TJ Ly A Elgar MA 《Die Naturwissenschaften》2007,94(9):787-790
The ability of social insects to discriminate against non-nestmates is vital for maintaining colony integrity, and in most
social insect species, individuals act aggressively towards non-nestmates that intrude into their nest. Our experimental field
data revealed that intra-colony aggression in the primitive bulldog ant Myrmecia nigriceps is negligible; our series of bioassays revealed no significant difference in the occurrence of aggression in trials involving
workers from the same, a close (less than 300 m) or a far (more than 1.5 km) nest. Further, non-nestmate intruders were able
to enter the nest in 60% of our trials; a similar level was observed in trials involving nestmates. These results suggest
that workers of M. nigriceps are either unable to recognize alien conspecifics or that the costs of ignoring workers from foreign colonies are sufficiently
low to favor low levels of inter-colony aggression in this species. 相似文献
216.
阐释了长江经济带港口代际识别的意义和UNCTAD框架下港口代际识别的标准。运用模糊模式识别的方法对长江经济带主要港口进行代际识别测度,并对初步识别结果做进一步分析和修正。研究表明:①长江经济带港口代际等级总体上较低,大多数处于UNCTAD框架下的第一代港和第二代港水平;②不同代际港口在空间上呈现出明显的东西分异特征,港口代际等级和港口分布密度由东向西均表现出显著的下降趋势;③从局部空间看,不同等级的港口在地理分布上呈现出一定的"省域化"特征,即在省域范围内形成以一两个高等级港为中心、其他低等级港为支线港、喂给港的港口群形态。根据研究结论,提出长江经济带建设下的港口战略转向:充分认识港口在长江综合运输体系中的作用;有序推进长江经济带港口代际更替;以港口群的形式推进港口建设和适时推进长江沿岸自贸区建设。 相似文献
217.
运用未确知测度理论,建立煤矿奥灰岩岩溶水类底板突水评价模型。选取煤层底板的地质构造(断裂构造密度、褶皱、断层导水性、岩体的裂隙发育情况)、水文地质条件(水压、含水性、岩溶含水层发育情况、强水源补给情况)、底板隔水层条件(隔水层厚度、岩石力学强度以及隔水岩层的完整性)、开采条件(采厚和采深)4个一级指标、13项二级指标作为未确知测度模型的判别指标;根据指标值的类型采用定量或定性的方法对其进行分级和赋值,建立评判集,进而构建单指标测度函数。将13个突水评价指标值代入单指标测度函数计算得到单指标测度评价矩阵。利用信息熵理论获得各判别指标的权重,依照置信度识别准则进行等级判定,最后得出煤层底板突水危险性评价结果。利用该方法对肥城矿区14个地质块段的煤层底板突水危险性进行评价,结果表明,该评价结果与矿山实际情况相符。 相似文献
218.
Foundresses of the social wasp Polistes biglumis were tested to see whether they were able to recognize alien eggs experimentally introduced into their own nests. Foundresses
removed alien conspecific reproductive-destined eggs while they accepted worker-destined eggs. The results indicate that social
wasps discriminate among eggs and that they discriminate against alien eggs destined to produce unrelated reproductives. P. biglumis is a strictly solitary founding species, with no reproductive competition within colonies; thus, brood discrimination abilities
could have evolved as a counteradaptation against intra- and inter-specific brood parasitism.
Received: 12 May 2000 / Revised: 15 July 2000 / Accepted 20 July 2000 相似文献
219.
A conflict over male production arises in social insects where workers are able to lay unfertilized male eggs. This happens
because each female (queen or worker) is most closely related to her own sons and is thus predicted to reproduce. The conflict
is modulated by worker policing where workers prevent each other from reproducing by aggression or egg cannibalism. In this
study, we show that in the ant Formica fusca, worker policing occurs by egg cannibalism rather than by overt aggression among workers. Furthermore, we show that, contrary
to bees, wasps and other ant species, egg discrimination in F. fusca is not based only on a universal queen signature chemical and that nest mate recognition of eggs occurs. 相似文献
220.
We asked whether switching among song type variants functions as a graded signal of aggression in song sparrows (Melospiza melodia). Song type variants are different versions of a given song type and are defined by much smaller acoustic differences than
those defining separate song types. The principal evidence that switching between song types functions as an aggressive signal,
in song sparrows and other species, is that song type switching frequencies increase during aggressive contexts, for example
during simulated territorial intrusions. We tested whether variant switching frequencies change in the same way, using playback
of conspecific song to simulate intrusions. Male song sparrows showed a significant increase in variant switching frequencies
during intrusion relative to before intrusion. After intrusions, variant switching frequencies returned to pre-intrusion levels.
These results are consistent with the hypothesis that variant switching is a graded aggressive signal, but other interpretations
are possible, for example, that variant switching is a signal of submission or represents simple production error.
Received: 20 January 2000 / Revised: 15 July 2000 / Accepted: 30 July 2000 相似文献