全文获取类型
收费全文 | 25615篇 |
免费 | 3859篇 |
国内免费 | 4638篇 |
专业分类
测绘学 | 5606篇 |
大气科学 | 3801篇 |
地球物理 | 5435篇 |
地质学 | 9447篇 |
海洋学 | 2115篇 |
天文学 | 1802篇 |
综合类 | 2228篇 |
自然地理 | 3678篇 |
出版年
2024年 | 83篇 |
2023年 | 164篇 |
2022年 | 482篇 |
2021年 | 629篇 |
2020年 | 631篇 |
2019年 | 754篇 |
2018年 | 531篇 |
2017年 | 815篇 |
2016年 | 734篇 |
2015年 | 737篇 |
2014年 | 974篇 |
2013年 | 1342篇 |
2012年 | 1189篇 |
2011年 | 1010篇 |
2010年 | 879篇 |
2009年 | 1060篇 |
2008年 | 1275篇 |
2007年 | 1700篇 |
2006年 | 1734篇 |
2005年 | 1715篇 |
2004年 | 1709篇 |
2003年 | 1344篇 |
2002年 | 1253篇 |
2001年 | 1168篇 |
2000年 | 1059篇 |
1999年 | 1191篇 |
1998年 | 1319篇 |
1997年 | 882篇 |
1996年 | 867篇 |
1995年 | 805篇 |
1994年 | 737篇 |
1993年 | 832篇 |
1992年 | 639篇 |
1991年 | 547篇 |
1990年 | 359篇 |
1989年 | 231篇 |
1988年 | 204篇 |
1987年 | 105篇 |
1986年 | 39篇 |
1985年 | 54篇 |
1984年 | 43篇 |
1983年 | 49篇 |
1982年 | 36篇 |
1981年 | 27篇 |
1980年 | 28篇 |
1979年 | 20篇 |
1978年 | 18篇 |
1977年 | 22篇 |
1976年 | 16篇 |
1973年 | 10篇 |
排序方式: 共有10000条查询结果,搜索用时 140 毫秒
1.
The 33 086 ha mixed land use Fall Creek watershed in upstate New York is part of the Great Lakes drainage system. Results from more than 3500 water samples are available in a data set that compiles flow data and measurements of various water quality analytes collected between 1972 and 1995 in all seasons and under all flow regimes in Fall Creek and its tributaries. Data is freely accessible at https://ecommons.cornell.edu/handle/1813/8148 and includes measurements of suspended solids, pH, alkalinity, calcium, magnesium, potassium, sodium, chloride, nitrate nitrogen (NO3-N), sulphate sulphur (SO4-S), phosphorus (P) fractions molybdate reactive P (MRP) and total dissolved P (TDP), percent P in sediment, and ammonium nitrogen (NH4-N). Methods, sub-watershed areas, and coordinates for sampling sites are also included. The work represented in this data set has made important scientific contributions to understanding of hydrological and biogeochemical processes that influence loading in mixed use watersheds and that have an impact on algal productivity in receiving water bodies. In addition, the work has been foundational for important regulatory and management decisions in the region. 相似文献
2.
A constitutive model that captures the material behavior under a wide range of loading conditions is essential for simulating complex boundary value problems. In recent years, some attempts have been made to develop constitutive models for finite element analysis using self‐learning simulation (SelfSim). Self‐learning simulation is an inverse analysis technique that extracts material behavior from some boundary measurements (eg, load and displacement). In the heart of the self‐learning framework is a neural network which is used to train and develop a constitutive model that represents the material behavior. It is generally known that neural networks suffer from a number of drawbacks. This paper utilizes evolutionary polynomial regression (EPR) in the framework of SelfSim within an automation process which is coded in Matlab environment. EPR is a hybrid data mining technique that uses a combination of a genetic algorithm and the least square method to search for mathematical equations to represent the behavior of a system. Two strategies of material modeling have been considered in the SelfSim‐based finite element analysis. These include a total stress‐strain strategy applied to analysis of a truss structure using synthetic measurement data and an incremental stress‐strain strategy applied to simulation of triaxial tests using experimental data. The results show that effective and accurate constitutive models can be developed from the proposed EPR‐based self‐learning finite element method. The EPR‐based self‐learning FEM can provide accurate predictions to engineering problems. The main advantages of using EPR over neural network are highlighted. 相似文献
3.
利用内蒙古西部12个台站的地脉动噪声数据,采用噪声谱比法研究台站的场地响应情况。分析表明,台站场地响应按曲线形态可分为3类,且可能受地形地貌、局部构造和台基状况等条件影响。对比分析噪声谱比法与Moya方法的场地响应结果发现,场地响应曲线形态基本一致,只有极少数台站存在明显差异。结果表明,内蒙古西部区域大部分台站的台基状况总体较好,场地响应曲线较为平坦,无明显频率放大点。 相似文献
4.
利用“地震预报计算机专家系统”的思想对大同 -阳高 Ms6 .1地震前每一前兆异常事件进行综合评估 ,以每一异常的最可能发震时间来计算发震概率 ,利用地震前兆综合加权信息熵研究了系统熵值与地震的关系。对华北地区的地震前兆综合加权信息熵研究表明 ,在大同 -阳高 Ms6 .1地震前 ,信息熵出现了明显的减熵有序变化 相似文献
5.
CHANGE OF NUTRIENT IMPORT AND EXPORT IN PROCESS OF RAINFALL IN AILAO MOUNTAIN OF YUNNAN PROVINCEGanJianmin(甘健民);XueJingyi(薛敬意... 相似文献
6.
Mercury (Hg) was investigated in bone tissues of skua ( Catharacta maccormick) and penguin (Pygoscelis adeliae) collected in the maritime Antarctic using atomic fluorescence spectrometry (AFS) and synchrotron radiation X-ray fluorescence (SR-XRF) method. The total levels of mercury in bone tissues of penguin and skua are much lower than those in other organs (e. g. , kidney, liver). The toxic effects of mercury in bone tissues of seabirds in polar region are not known. We have used SR- XRF method to map the distribution of trace levels of mercury in bones. The levels of mercury are found to be enriched somewhere near the periosteal surface and/or endosteal surface. The distribution of mercury shows strongly correlation with that of some essential elements and probably poses negative effect on the bone metabolism inferring from the relationship of mercury with the other elements. These studies represent a first step toward understanding the toxic effects of mercury on bone of polar animals by suggesting the possible microscopic investigation. 相似文献
7.
8.
The stability and evolution of cold, shock-bounded slabs is studied using numerical hydrodynamic simulations. We confirm the analysis of Vishniac (1994) [ApJ, 428, 186], who showed that such slabs are unstable if they are perturbed by a displacement larger than their width. The growth rate of this nonlinear thin shell instability (NTSI) is found to increase with decreasing wavelength, in qualitative agreement with Vishniac's analysis. The NTSI saturates when the bending angle becomes large and the growth in the width of the slab pinches off the perturbation. After saturation, the slab remains greatly extended with an average density much less than the original slab density, supported primarily by supersonic turbulence within the slab. Linear perturbations are also found to be unstable in that they can lead to turbulent flow within the slab, although this response to linear perturbations is distinct from, and much less violent than the NTSI.Richard McCray 相似文献
9.
10.
A highly automated moving object detection package 总被引:1,自引:0,他引:1