全文获取类型
收费全文 | 231篇 |
免费 | 25篇 |
国内免费 | 28篇 |
专业分类
测绘学 | 52篇 |
大气科学 | 11篇 |
地球物理 | 74篇 |
地质学 | 72篇 |
海洋学 | 33篇 |
天文学 | 8篇 |
综合类 | 17篇 |
自然地理 | 17篇 |
出版年
2024年 | 3篇 |
2023年 | 4篇 |
2022年 | 8篇 |
2021年 | 5篇 |
2020年 | 7篇 |
2019年 | 6篇 |
2018年 | 4篇 |
2017年 | 3篇 |
2016年 | 5篇 |
2015年 | 7篇 |
2014年 | 8篇 |
2013年 | 7篇 |
2012年 | 14篇 |
2011年 | 19篇 |
2010年 | 9篇 |
2009年 | 8篇 |
2008年 | 15篇 |
2007年 | 24篇 |
2006年 | 17篇 |
2005年 | 7篇 |
2004年 | 8篇 |
2003年 | 7篇 |
2002年 | 9篇 |
2001年 | 7篇 |
2000年 | 7篇 |
1999年 | 3篇 |
1998年 | 5篇 |
1997年 | 7篇 |
1996年 | 7篇 |
1995年 | 11篇 |
1994年 | 6篇 |
1993年 | 6篇 |
1992年 | 6篇 |
1991年 | 6篇 |
1990年 | 6篇 |
1989年 | 2篇 |
1986年 | 1篇 |
排序方式: 共有284条查询结果,搜索用时 0 毫秒
91.
92.
93.
针对分形海面模型的功率谱在空间波数小于基波波数时不能满足正幂率的问题,提出一种统计模型和归一化带限Weierstrass分形模型相结合的1维海面模型,确定了表面功率谱的闭式解,表面功率谱和有关文献的数据进行了对比。在满足Kirchhoff近似的条件下推导该模型电磁散射系数的闭式解,计算了散射系数随时间变化的Hurst指数,进而求得其分形维数。从数值仿真结果可知,表面功率谱和PM谱、南海的实测数据都拟合得很好,验证了模型的有效性,海面所接收信号随时间变化是分形的,并且其分形维数与分形海表面的分形维数几乎相等。 相似文献
94.
95.
MODFLOW 2000 head uncertainty,a first-order second moment method 总被引:1,自引:0,他引:1
A computationally efficient method to estimate the variance and covariance in piezometric head results computed through MODFLOW 2000 using a first-order second moment (FOSM) approach is presented. This methodology employs a first-order Taylor series expansion to combine model sensitivity with uncertainty in geologic data. MODFLOW 2000 is used to calculate both the ground water head and the sensitivity of head to changes in input data. From a limited number of samples, geologic data are extrapolated and their associated uncertainties are computed through a conditional probability calculation. Combining the spatially related sensitivity and input uncertainty produces the variance-covariance matrix, the diagonal of which is used to yield the standard deviation in MODFLOW 2000 head. The variance in piezometric head can be used for calibrating the model, estimating confidence intervals, directing exploration, and evaluating the reliability of a design. A case study illustrates the approach, where aquifer transmissivity is the spatially related uncertain geologic input data. The FOSM methodology is shown to be applicable for calculating output uncertainty for (1) spatially related input and output data, and (2) multiple input parameters (transmissivity and recharge). 相似文献
96.
Quintaneiro C Monteiro M Pastorinho R Soares AM Nogueira AJ Morgado F Guilhermino L 《Marine pollution bulletin》2006,52(11):1406-1413
The degradation of estuaries is a result of human activities which overloads the environment with substances of both industrial and/or natural origins. Bioindicators have been consistently used to interpret effects of contaminants in the environment. In this study, the use of biomarkers (particular measurable characteristics of a bioindicator organism) was used to evaluate the contamination by xenobiotics of Crangon crangon natural populations. The central aim was to evaluate the capability of a battery of biomarkers to discriminate sites with different types of contamination. The activity of the enzymes cholinesterases (ChE), lactate dehydrogenase (LDH) and glutathione S-transferases (GST) were used as biomarkers. In addition, the ChE form(s) present in the cephalotorax of C. crangon were characterised. Organisms were seasonally sampled from winter 2001/2002 to autumn of 2002, at "reference" sites and at sites that receive agricultural, industrial and/or urban effluents. Results obtained in the characterisation of ChE with different substrates and selective inhibitors demonstrate that the form of ChE present in the cephalotorax of C. crangon shows proprieties of vertebrates' AChE and therefore it may be classified as true AChE-like ChE. The battery of biomarkers exhibited seasonal and local variations, apparently related to agricultural, industrial or urban effluent contamination. The tested biomarkers proved to be able to discriminate sources of environmental contamination, and confirms C. crangon as a sensitive species suitable to be used as a bioindicator. 相似文献
97.
98.
Distinguishing between natural and aquaculture-derived sediment concentrations of heavy metals in the Broughton Archipelago, British Columbia 总被引:5,自引:0,他引:5
Marine sediment samples were collected in the Broughton Archipelago, British Columbia, to assess the use of a geochemical normalization technique in the identification of a chemical tracer of aquaculture waste material. Zinc and copper were suggested as tracers of feed pellets, while copper was considered an indicator of anti-foulant agents used on netpen systems. The sediment samples were analyzed for carbon, nitrogen, organic matter, water, trace-element, and free sulfide concentrations, and sediment grain-size distribution. Sediment texture analysis revealed a wide range of substrate types from sand to silty loam categories. Strong relationships between sediment texture, sediment porosity, and organic content were observed across both near-field and far-field stations. Excess zinc and copper sediment concentrations, identified using a lithium-normalization technique, were restricted to near-field sampling stations (0 and 30m from netpen systems). The relationships between these metal tracers and organic content and sulfur concentrations were explored to account for variations in sediment concentrations of zinc and copper. 相似文献
99.
Smith AJ 《Ground water》2008,46(2):228-238
Published frequency-domain solutions of periodic flow in aquifers apply strictly to mathematically linear systems that arise when aquifer diffusivity is assumed constant in space and time. This assumption can be invalid in phreatic aquifers that experience spatiotemporal variation in the free surface position and consequent variation in saturated thickness. A weakly nonlinear approach to formulating and solving periodic flow problems in the frequency domain can be applied in situations where conventional linearized approximations break down. The weakly nonlinear equations provide robust approximations of the true nonlinear response and require much less computational effort and time to solve than the full nonlinear problem. Nondimensional rules of thumb are presented for choosing between linear, weakly nonlinear and nonlinear solution strategies. 相似文献
100.
UNIX操作系统与Internet网络 总被引:1,自引:0,他引:1
根据作者的工作实践,本文提出了UNIX操作系统在互联网络中的作用和对Internet网络发展的重要技术及其应用进行了介绍。 相似文献