全文获取类型
收费全文 | 2365篇 |
免费 | 379篇 |
国内免费 | 269篇 |
专业分类
测绘学 | 107篇 |
大气科学 | 248篇 |
地球物理 | 792篇 |
地质学 | 1030篇 |
海洋学 | 138篇 |
天文学 | 9篇 |
综合类 | 177篇 |
自然地理 | 512篇 |
出版年
2025年 | 2篇 |
2024年 | 40篇 |
2023年 | 48篇 |
2022年 | 54篇 |
2021年 | 49篇 |
2020年 | 86篇 |
2019年 | 84篇 |
2018年 | 49篇 |
2017年 | 77篇 |
2016年 | 125篇 |
2015年 | 152篇 |
2014年 | 154篇 |
2013年 | 118篇 |
2012年 | 121篇 |
2011年 | 153篇 |
2010年 | 106篇 |
2009年 | 182篇 |
2008年 | 165篇 |
2007年 | 141篇 |
2006年 | 144篇 |
2005年 | 125篇 |
2004年 | 100篇 |
2003年 | 89篇 |
2002年 | 75篇 |
2001年 | 89篇 |
2000年 | 65篇 |
1999年 | 57篇 |
1998年 | 67篇 |
1997年 | 43篇 |
1996年 | 38篇 |
1995年 | 28篇 |
1994年 | 33篇 |
1993年 | 21篇 |
1992年 | 20篇 |
1991年 | 16篇 |
1990年 | 14篇 |
1989年 | 9篇 |
1988年 | 9篇 |
1987年 | 10篇 |
1986年 | 13篇 |
1985年 | 8篇 |
1984年 | 11篇 |
1983年 | 6篇 |
1981年 | 3篇 |
1980年 | 4篇 |
1979年 | 4篇 |
1978年 | 3篇 |
1977年 | 2篇 |
1976年 | 1篇 |
排序方式: 共有3013条查询结果,搜索用时 265 毫秒
51.
Spectroscopic techniques have become attractive to assess soil properties because they are fast, require little labor and may reduce the amount of laboratory waste produced when compared to conventional methods. Imaging spectroscopy (IS) can have further advantages compared to laboratory or field proximal spectroscopic approaches such as providing spatially continuous information with a high density. However, the accuracy of IS derived predictions decreases when the spectral mixture of soil with other targets occurs. This paper evaluates the use of spectral data obtained by an airborne hyperspectral sensor (ProSpecTIR-VS – Aisa dual sensor) for prediction of physical and chemical properties of Brazilian highly weathered soils (i.e., Oxisols). A methodology to assess the soil spectral mixture is adapted and a progressive spectral dataset selection procedure, based on bare soil fractional cover, is proposed and tested. Satisfactory performances are obtained specially for the quantification of clay, sand and CEC using airborne sensor data (R2 of 0.77, 0.79 and 0.54; RPD of 2.14, 2.22 and 1.50, respectively), after spectral data selection is performed; although results obtained for laboratory data are more accurate (R2 of 0.92, 0.85 and 0.75; RPD of 3.52, 2.62 and 2.04, for clay, sand and CEC, respectively). Most importantly, predictions based on airborne-derived spectra for which the bare soil fractional cover is not taken into account show considerable lower accuracy, for example for clay, sand and CEC (RPD of 1.52, 1.64 and 1.16, respectively). Therefore, hyperspectral remotely sensed data can be used to predict topsoil properties of highly weathered soils, although spectral mixture of bare soil with vegetation must be considered in order to achieve an improved prediction accuracy. 相似文献
52.
影响文山三七品质的土壤地质背景因素 总被引:6,自引:0,他引:6
应用农业地质学的理论和方法,初步探讨了影响云南文山地区的三七品质的土壤地质背景因素。结果表明:不同类型的土壤地质背景上种植出的三七,具有不同的品质特征。火山岩形成的土壤,种植出的三七,其总皂甙的得率高,碳酸盐形成的红壤,种植出的三七,则总皂甙得率最低,碎屑岩和碳酸岩形成的混合型土壤,种植出的三七,其总皂甙得率则居于二者之间。高钾的火山岩类土壤地质背景和高稀土的碳酸盐类土壤地质背景,是形成不同品质特征三七的根本原因,钾和稀土是形成三七不同品质特征的重要元素。 相似文献
53.
Chemical degradations of coal resins, coal asphaltenes and coal residues have been performed with selective, non-oxidative reagents. The coal comprise samples a rank interval 0.30–1.21 %Rm. Degraded low-molecular-weight compounds were analyzed by GC-MS. The distributions of pentacyclic terpanes, isoprenoids, n-alkanes, acids and alcohols obtained as degradation products illustrate a considerable variability between the macromolecular coal fractions. A structural study on the molecular level of the degraded compounds from coal resins and coal asphaltenes show their potential to generate hydrocarbons during coalification process. 相似文献
54.
Seyyed Mohammad Mousavi Amir Hossein Alavi Ali Mollahasani Amir Hossein Gandomi 《Engineering Geology》2011,123(4):324
In this study, new empirical equations were developed to predict the soil deformation moduli utilizing a hybrid method coupling genetic programming and simulated annealing, called GP/SA. The proposed models relate secant (Es), unloading (Eu) and reloading (Er) moduli obtained from plate load–settlement curves to the basic soil physical properties. Several models with different combinations of the influencing parameters were developed and checked to select the best GP/SA models. The database used for developing the models was established upon a series of plate load tests (PLT) conducted on different soil types at various depths. The validity of the models was tested using parts of the test results that were not included in the analysis. The validation of the models was further verified using several statistical criteria. A traditional GP analysis was performed to benchmark the GP/SA models. The contributions of the parameters affecting Es, Eu and Er were analyzed through a sensitivity analysis. The proposed models are able to estimate the soil deformation moduli with an acceptable degree of accuracy. The Es prediction model has a remarkably better performance than the models developed for predicting Eu and Er. The simplified formulations for Es, Eu and Er provide significantly better results than the GP-based models and empirical models found in the literature. 相似文献
55.
56.
57.
全球冻融地区土壤是重要的N20释放源的综合分析 总被引:3,自引:0,他引:3
N2O是重要的温室气体之一,它是微生物硝化作用和脱氮化作用过程的产物。有多种释放源,其中土壤圈是重要的释放源之一。在影响N2O释放通量的诸多因素中温度是关键因素之一。本文根据土壤冻融加强有机质矿化作用,以及对微生物群体产生非生物应力的性质,结合冻融地区土壤和冻土带湿地所具有的特征,进行综合分析,论述冻融地区土壤是重要的N2O释放源。 相似文献
58.
设计并开展一系列土-结构群相互作用体系振动台试验,考虑结构数量、地震动类型与幅值等参数,研究土-结构群相互作用对结构及场地土响应的影响,并对模型土参数确定方法进行分析。研究结果表明,地表建筑物的存在并不总是减小自由场地面运动,但地面运动随着地表结构数量的增加而降低;土-结构群相互作用对位于结构群中心的结构响应影响最大,且会放大土体卓越频率附近的响应成分;不同评价指标之间具有不同的侧重点,但均可较好地评价结构群之间的相互作用;输入地震动的总能量越高,土-结构群相互作用越明显。 相似文献
59.
60.
Coral reefs in Jakarta Bay have been subjected to scientific studies since the 1920s. Also from that time on biological collections were made. The reefs in the Jakarta Bay have been under long-term natural and anthropogenic stress. With the biological collections and historical documents the coral species richness in Jakarta Bay around 1920 was reconstructed. New data from this bay and the adjacent offshore Thousand Islands archipelago were obtained during a 2005 research expedition. A comparison of the coral assemblages between 1920 and 2005 reveals a clear decline in species numbers. The most prominent results include the near-shore disappearance of species belonging to the families Acroporidae, Milleporidae, and to a lesser extent Poritidae. The overall coral species composition of the reefs has changed considerably, which is partly reflected in a strong decline in coral species richness. About half the number of species recorded in 1920 was found again in 2005. 相似文献