全文获取类型
收费全文 | 1049篇 |
免费 | 139篇 |
国内免费 | 153篇 |
专业分类
测绘学 | 130篇 |
大气科学 | 76篇 |
地球物理 | 306篇 |
地质学 | 422篇 |
海洋学 | 188篇 |
天文学 | 21篇 |
综合类 | 49篇 |
自然地理 | 149篇 |
出版年
2024年 | 2篇 |
2023年 | 10篇 |
2022年 | 23篇 |
2021年 | 14篇 |
2020年 | 34篇 |
2019年 | 41篇 |
2018年 | 40篇 |
2017年 | 56篇 |
2016年 | 50篇 |
2015年 | 40篇 |
2014年 | 79篇 |
2013年 | 81篇 |
2012年 | 58篇 |
2011年 | 66篇 |
2010年 | 57篇 |
2009年 | 73篇 |
2008年 | 72篇 |
2007年 | 94篇 |
2006年 | 79篇 |
2005年 | 50篇 |
2004年 | 40篇 |
2003年 | 46篇 |
2002年 | 37篇 |
2001年 | 39篇 |
2000年 | 23篇 |
1999年 | 31篇 |
1998年 | 14篇 |
1997年 | 17篇 |
1996年 | 12篇 |
1995年 | 13篇 |
1994年 | 9篇 |
1993年 | 11篇 |
1992年 | 8篇 |
1991年 | 6篇 |
1990年 | 4篇 |
1989年 | 3篇 |
1988年 | 2篇 |
1987年 | 2篇 |
1986年 | 2篇 |
1982年 | 2篇 |
1978年 | 1篇 |
排序方式: 共有1341条查询结果,搜索用时 281 毫秒
941.
942.
943.
944.
945.
The present study considers the prediction of extreme values of the second-order hydrodynamic parameters related to offshore structures in waves, where the application of Gaussian distribution is not valid. Particularly, this study focuses on a characteristic function approach in the frequency domain to estimate the probability distribution of the second-order quantities, and the results are compared with direct simulations in the time domain. The stochastic behaviors of the second-order hydrodynamic quantities are investigated with the characteristic function approach, which involves eigenvalue analyses of Hermitian kernels constructed with quadratic transfer functions. Three different second-order responses are considered: the springing responses of TLP tendons representative of the sum-frequency problem, the slow-drift motions of a semi-submersible platform moored in waves as a representative of the difference-frequency problem, and the wave run-up around a vertical column for regular and irregular waves. The applicability of the present approach in predicting extreme values is assessed by comparing the results with the values obtained from time-domain signals. 相似文献
946.
《Marine Policy》2017
Offshore wind energy development can have major economic implications because of the potential impact on coastal recreation demand – particularly for countries that are characterised by high offshore wind power potential while also being popular tourist destinations. In this context, the impact of offshore wind farm projects on beach recreation demand in Catalonia (Spain) during the 2012 summer season was examined. A combined revealed and stated preference approach, which allows joint modelling of actual and hypothetical demands, was applied. The results demonstrate a significant welfare loss up to €203 million per season. The results further show that the installation of a wind farm mainly will cause a shift in trips to Catalan beaches without wind farms, which implies that the estimated negative economic impacts will occur mostly in areas where wind farms are located. From a political economy point of view, this may call for the design and implementation of re-distributive instruments to offset the negative impacts caused by wind farms. 相似文献
947.
《Marine Policy》2017
The present paper presents the practical implementation of the Ecosystem Approach to Fisheries Management (EAFM) in Norway. This involves defining management objectives and developing simple and efficient tools to achieve an overview of management needs and prioritise among these, while integrating broader conservation issues and ensuring stakeholder involvement. A new Marine Resources Act entered into force in Norway in 2009. By integrating conservation and sustainable use as basic principles, the law represents a paradigm shift in the management of Norwegian fisheries. The law indicates which concerns should be addressed, but neither how nor how often evaluations should take place. That is for management to decide. A management principle in the Marine Resources Act confers on the Ministry an obligation to evaluate whether continued fishing at the present scale is justifiable, or whether improved management is required to ensure sustainability. A Stock table, and a table of "Catches of data-poor species" constitute a comprehensive system for monitoring the management principle. Along with a Fisheries table, these tables establish a framework for developing an ecosystem-based fisheries management by providing a basis and tools for prioritising the needs of new and/or revised management measures. 相似文献
948.
Numerical approach to predict ground displacement caused by installing a horizontal jet grout column
When horizontal jet grouting is conducted in fine-grained soils, the injection of large volumes of fluid with high injection pressure into the ground may cause ground surface upheaval and the lateral movement of subsoil. This paper provides a numerical approach to calculate the ground displacement caused by installing a horizontal column based on determination of the influencing radius of injection pressure and the volumetric expansion strain caused by fluid injection. During the course of this study, the relationship between the influencing radius, column radius, volumetric expansion strain, and jetting parameters is established. The proposed numerical approach is applied to analyze a case history conducted in Shanghai soft soil deposits, which include the monitoring of vertical displacement of the ground surface and lateral displacement of the subsoil during construction. A comparison of the results between the simulated values with the Mohr Coulomb model and field data demonstrates that the numerical approach can yield a reasonable prediction of these field measurements when the influencing radius is six times the radius of the column. The influence range of the horizontal jet grouting activity is also discussed by numerical analysis with respect to two soil models (the Mohr Coulomb model and the hardening soil model). 相似文献
949.
通过对辽北平岗盆地中德仁组火山岩的岩石学、地球化学和年代学的研究,探讨了其岩石成因、形成时代、构造环境,以及华北克拉通东部岩石圈减薄时间和减薄机制。用LA-ICP-MS技术测定了平岗盆地中德仁组中部英安质角砾晶屑岩屑凝灰岩和上部流纹岩中的锆石U-Th-Pb同位素组成。获得前者的锆石~(206)Pb/~(238)U年龄为246.8±3.5Ma和169.6±4.6Ma,代表了2期岩浆活动的年龄,是平岗盆地基底早三叠世和中侏罗世岩浆活动的表现;后者的锆石~(206)Pb/~(238)U年龄为117.0±0.9Ma,表明流纹岩形成于早白垩世。德仁组火山岩富集大离子亲石元素K、Rb、Ba,亏损高场强元素Nb、Ta、Zr、Hf、Y、Yb;富集轻稀土元素,亏损重稀土元素,稀土元素配分曲线呈右倾型,表现出壳源特征。德仁组火山岩形成于活动大陆边缘火山弧环境,原始岩浆来源于盆地基底岩浆岩重熔。研究区在117~135Ma期间,发生了由正地形向负地形的重大转变,区域上发育伸展盆地和变质核杂岩,并在基底岩石重熔过程中受到地幔混染作用,表明研究区在早白垩世发生了岩石圈减薄,且减薄的起始时间早于117.0±0.9Ma,减薄机制为拆沉作用。 相似文献
950.
It is shown that the property of the scale invariance of the eigenvalues and eigenmodes of a finite element can be used as a basis to calculate good approximations to the analytical magnitudes of eigenvalues. This requires the subdivision of the element into a mesh of small elements with the same shape as the large element, the enforcement of the modal boundary displacements of the large element to the mesh of small elements and finally the application of the conditions of both the nodal equilibrium and the equality of the nodal work at both scales. Due to the self‐similarity of the elements at all scales the authors propose to call this method the fractal approach. The method is applied to calculate the hour‐glass eigenvalue of a plane square 4‐node quad for isotropic linear elastic material. The resulting hour‐glass eigenvalue is shown to be a good approximation of the analytical magnitude as derived in a companion paper. Copyright © 2000 John Wiley & Sons, Ltd. 相似文献