全文获取类型
收费全文 | 4929篇 |
免费 | 709篇 |
国内免费 | 887篇 |
专业分类
测绘学 | 421篇 |
大气科学 | 608篇 |
地球物理 | 1181篇 |
地质学 | 2370篇 |
海洋学 | 817篇 |
天文学 | 6篇 |
综合类 | 271篇 |
自然地理 | 851篇 |
出版年
2024年 | 40篇 |
2023年 | 117篇 |
2022年 | 184篇 |
2021年 | 230篇 |
2020年 | 218篇 |
2019年 | 254篇 |
2018年 | 181篇 |
2017年 | 254篇 |
2016年 | 253篇 |
2015年 | 232篇 |
2014年 | 372篇 |
2013年 | 376篇 |
2012年 | 334篇 |
2011年 | 341篇 |
2010年 | 321篇 |
2009年 | 315篇 |
2008年 | 318篇 |
2007年 | 327篇 |
2006年 | 348篇 |
2005年 | 246篇 |
2004年 | 215篇 |
2003年 | 168篇 |
2002年 | 184篇 |
2001年 | 120篇 |
2000年 | 101篇 |
1999年 | 82篇 |
1998年 | 86篇 |
1997年 | 51篇 |
1996年 | 51篇 |
1995年 | 43篇 |
1994年 | 36篇 |
1993年 | 38篇 |
1992年 | 35篇 |
1991年 | 10篇 |
1990年 | 10篇 |
1989年 | 6篇 |
1988年 | 4篇 |
1987年 | 3篇 |
1986年 | 7篇 |
1985年 | 5篇 |
1984年 | 3篇 |
1983年 | 1篇 |
1981年 | 1篇 |
1978年 | 3篇 |
1977年 | 1篇 |
排序方式: 共有6525条查询结果,搜索用时 79 毫秒
1.
A coupled continuum‐discrete hydromechanical model was employed to analyse the liquefaction of a saturated loose deposit of cohesionless particles when subjected to a dynamic base excitation. The pore fluid flow was idealized using averaged Navier–Stokes equations and the discrete element method was employed to model the solid phase particles. A well established semi‐empirical relationship was utilized to quantify the fluid–particle interactions. The conducted simulations revealed a number of salient micro‐mechanical mechanisms and response patterns associated with the deposit liquefaction. Space and time variation of porosity was a major factor which affected the coupled response of the solid and fluid phases. Pore fluid flow was within Darcy's regime. The predicted response exhibited macroscopic patterns consistent with experimental results and case histories of the liquefaction of granular soil deposits. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
2.
Roberto Mínguez Fernando Delgado Ignacio Escuder Manuel G. de Membrillera 《国际地质力学数值与分析法杂志》2006,30(10):1019-1037
Empirical criteria have been used successfully to design filters of most embankment large dam projects throughout the world. However, these empirical rules are only applicable to a particular range of soils tested in laboratory and do not take into account the variability of the base material and filter particle sizes. In addition, it is widely accepted that the safety of fill dams is mainly dependent on the reliability of their filter performance. The work herein presented consists in a new general method for assessing the probability of fulfilling any empirical filter design criteria accounting for base and filter heterogeneity by means of first‐order reliability methods (FORM), so that reliability indexes and probabilities of fulfilling any particular criteria are obtained. This method will allow engineers to estimate the safety of existing filters in terms of probability of fulfilling their design criteria and might also be used as a decision tool on sampling needs and material size tolerances during construction. In addition, sensitivity analysis makes possible to analyse how reliabilities are influenced by different sources of input data. Finally, in case of a portfolio risk assessment, this method will allow engineers to compare the safety of several existing dams in order to prioritize safety investments and it is expected to be a very useful tool to evaluate probabilities of failure due to internal erosion. Copyright © 2006 John Wiley & Sons, Ltd. 相似文献
3.
随着太湖流域社会与经济的发展,多环芳烃(PAHs)在各种环境介质中逐渐累积,污染日益严重,可能对太湖生态环境及周边人体健康构成威胁。为探究太湖沉积物PAHs的来源及生态风险,于2021年12月在太湖采集30个表层沉积物样品,利用气相色谱-质谱联用仪(GC-MS)检测样品中16种PAHs含量;利用受体模型和苯并[a]芘(BaP)毒性当量法进行来源解析及生态风险评估,并将各来源贡献与毒性当量浓度相结合,量化源风险。结果表明,太湖表层沉积物中16种PAHs总含量介于124~592 ng/g之间,平均值为294 ng/g,中值为279 ng/g;高环多环芳烃(HMW PAHs)为主要组分,占∑PAHs的67%。高含量区域位于竺山湾、梅梁湾、贡湖湾和西太湖,与国内外其他湖泊沉积物相比,太湖沉积物PAHs含量处于较低水平。源解析的结果表明,太湖表层沉积物中PAHs交通排放源贡献率为29.1%、煤炭燃烧源贡献率为26.7%、生物质燃烧源贡献率为28.7%、石油源贡献率为15.6%。生态风险评价结果表明,交通排放源、生物质燃烧源、煤炭燃烧源和石油源的BaP毒性当量含量(TEQBaP)均值分别为19.34、17.81、16.33和9.1 ng/g,均小于70 ng/g,几乎处于无风险水平。西太湖、贡湖湾和梅梁湾的部分区域ΣTEQBaP大于70 ng/g属于潜在风险区,具有一定潜在毒性。在后续的污染治理中应重点关注太湖西北部地区污染物的排放。本研究可为沉积物中PAHs污染的研究提供数据支撑,为地方政府精准、高效地管控PAHs污染提供理论依据。 相似文献
4.
饱和砂土地基在爆炸荷载作用下会发生液化,地基上的结构物将受到爆炸荷载及地基液化的双重作用,从而产生不均匀沉降和破坏性变形。基于大型现场爆炸液化试验,对场地上钢筋混凝土(RC)结构的动力响应和地基液化后RC结构的变形进行了分析研究。结果表明:液化场地中浅埋RC结构产生了明显的不均匀沉降,且最大沉降量达到结构高度的10%,结构差异沉降达到最大沉降量的1/5,结构沉降变形在液化后15 h时基本稳定;RC结构表面未产生明显的裂缝,动态拉、压应变均在400??以内,不会对结构造成显著破坏;结构动力响应表现为柱侧加速度峰值明显大于梁侧,但柱侧动力稳定所需时间较梁侧短,即柱承受了更大的瞬时冲击力且其抵抗瞬时冲击力的能力更强。研究结果可以为在可液化地基中的浅埋RC结构稳定设计等工程情况提供参考。 相似文献
5.
Synthetic aperture radar (SAR) is an important alternative to optical remote sensing due to its ability to acquire data regardless of weather conditions and day/night cycle. The Phased Array type L-band SAR (PALSAR) onboard the Advanced Land Observing Satellite (ALOS) provided new opportunities for vegetation and land cover mapping. Most previous studies employing PALSAR investigated the use of one or two feature types (e.g. intensity, coherence); however, little effort has been devoted to assessing the simultaneous integration of multiple types of features. In this study, we bridged this gap by evaluating the potential of using numerous metrics expressing four feature types: intensity, polarimetric scattering, interferometric coherence and spatial texture. Our case study was conducted in Central New York State, USA using multitemporal PALSAR imagery from 2010. The land cover classification implemented an ensemble learning algorithm, namely random forest. Accuracies of each classified map produced from different combinations of features were assessed on a pixel-by-pixel basis using validation data obtained from a stratified random sample. Among the different combinations of feature types evaluated, intensity was the most indispensable because intensity was included in all of the highest accuracy scenarios. However, relative to using only intensity metrics, combining all four feature types increased overall accuracy by 7%. Producer’s and user’s accuracies of the four vegetation classes improved considerably for the best performing combination of features when compared to classifications using only a single feature type. 相似文献
6.
7.
QuickBird satellite imagery acquired in June 2003 and September 2004 was evaluated for detecting the noxious weed spiny aster [Leucosyris spinosa (Benth.) Greene] on a south Texas, USA rangeland area. A subset of each of the satellite images representing a diversity of cover types was extracted and used as a study site. The satellite imagery had a spatial resolution of 2.8 m and contained 11-bit data. Unsupervised and supervised classification techniques were used to classify false colour composite (green, red, and near-infrared bands) images of the study site. Imagery acquired in June was superior to that obtained in September for distinguishing spiny aster infestations. This was attributed to differences in spiny aster phenology between the two dates. An unsupervised classification of the June image showed that spiny aster had producer's and user's accuracies of 90% and 93.1%, respectively, whereas a supervised classification of the June image had producer's and user's accuracies of 90% and 81.8%, respectively. These results indicate that high resolution satellite imagery coupled with image analysis techniques can be used successfully for detecting spiny aster infestations on rangelands. 相似文献
8.
主要阐述了GPS基线解算的基本理论,分析了基线解算中的质量控制。并利用TBC软件以某市新城区控制网计算为例,在计算过程中对基线质量进行控制,最后对控制点成果进行评价。 相似文献
9.
利用小型低空航摄平台和非量测小相幅相机,搭载动态GNSS/INS组合系统进行空三加密,研究无人机搭载GNSS/INS辅助在不同分辨率、不同基线数量条件下的像控点布设以及空三加密,并对其平面精度、高程精度进行分析,以实现利用少量地面控制点进行大比例尺地形图测图。 相似文献
10.
运用高分一号卫星进行江西樟树市地质灾害评价对于科学防治地质灾害,提高国产卫星应用水平具有重要意义。本文通过正射校正、影像融合、几何校正等步骤对高分一号影像进行预处理,然后对处理后的影像进行地质灾害解译,获取樟树市地质灾害分布情况。在进行地质灾害评价过程中采用小流域评价单元,选取灾害点密度、地表水文、构造断裂带、工程活动、岩土类型、地形坡度等6个评价因子,并运用层次分析法确定因子权重,通过加权平均法最终获得樟树市地质灾害易发性综合评价图,其精度为78%。结果表明樟树市易发生地质灾害的地区占全市总面积的34.68%,易发程度最高的地区主要分布在东南部,占全市总面积的7.82%。 相似文献