全文获取类型
收费全文 | 701篇 |
免费 | 72篇 |
国内免费 | 113篇 |
专业分类
测绘学 | 213篇 |
大气科学 | 30篇 |
地球物理 | 119篇 |
地质学 | 165篇 |
海洋学 | 271篇 |
天文学 | 8篇 |
综合类 | 33篇 |
自然地理 | 47篇 |
出版年
2024年 | 4篇 |
2023年 | 11篇 |
2022年 | 24篇 |
2021年 | 39篇 |
2020年 | 45篇 |
2019年 | 64篇 |
2018年 | 43篇 |
2017年 | 44篇 |
2016年 | 24篇 |
2015年 | 43篇 |
2014年 | 46篇 |
2013年 | 38篇 |
2012年 | 45篇 |
2011年 | 47篇 |
2010年 | 38篇 |
2009年 | 44篇 |
2008年 | 37篇 |
2007年 | 44篇 |
2006年 | 42篇 |
2005年 | 43篇 |
2004年 | 28篇 |
2003年 | 19篇 |
2002年 | 14篇 |
2001年 | 14篇 |
2000年 | 6篇 |
1999年 | 7篇 |
1998年 | 6篇 |
1997年 | 7篇 |
1996年 | 4篇 |
1995年 | 4篇 |
1994年 | 1篇 |
1993年 | 3篇 |
1991年 | 3篇 |
1989年 | 1篇 |
1988年 | 1篇 |
1987年 | 1篇 |
1985年 | 1篇 |
1978年 | 1篇 |
排序方式: 共有886条查询结果,搜索用时 46 毫秒
141.
142.
公路装卸区是油库油品通过公路运输的一个重要区域,也是油库对外的一个重要窗口,作者从油库公路装卸区布置方式和要求,交通组织形式,车辆的种类及交通组织流线方面提出一些设计探讨,使业主在公路装卸区的使用管理上更方便,交通组织更顺畅。 相似文献
143.
144.
Giorgos Mountrakis 《International journal of geographical information science》2013,27(11):1389-1412
Moose–vehicle collisions (MVCs) pose a serious safety and environmental concern in many regions of Europe and North America. For example, in the state of Vermont, one‐third of all reported MVCs resulted in motorist injury or fatality while collisions have increased from two in 1982 to 164 in 2002. Our work used a MVC dataset from 1983 to 1999 in the Northeastern Highlands of Vermont (four major roads) to perform space, time and spatiotemporal analyses and guide future mitigation strategies. An adapted kernel density estimator was implemented for exploratory analyses to detect high density collision hotspots on roads. The kernel in space showed seven major density peaks which varied in magnitude and spread between roads. The kernel estimator in time for all roads showed an exponentially increasing trend with annual periodicity and a seasonal cyclic component, where the majority of collisions occurred from May to October. Spatiotemporal kernel estimation exhibited discontinuous density hotspots in time and space suggesting changing animal movement patterns across roads. We used an adapted Ripley's K‐function to test the hypothesis that MVCs clustering occurred at multiple scales in space, in time and in space–time combined. Statistically significant spatial clustering was evident on all roads at spatial scales from 2 to 10 km. A more consistent clustering in time occurred on all roads at a scale distance of 5 years. Similar to the kernel estimation, annual periodicity was also evident. Positive space–time clustering was present at small spatial (5 km) and temporal scales (2 years) indicating that where MVCs occur is also influenced by when they occur. In retrospect, using multiple road lengths, and the combined kernel estimation and Ripley's K‐function in time and space, provided a powerful methodology to study varying spatiotemporal patterns of wildlife collisions along roads. This can greatly assist transportation planners in identifying optimal mitigation strategies along specific roads, such as deciding on location and spatial length for permanent and expensive measures (e.g. crossing structures and associated fencing) versus less permanent and inexpensive structures (e.g. wildlife signage and reduced speed limits). 相似文献
145.
Zhixiang Fang Shih-Lung Shaw Shunqing Chen Bi Yu Chen 《International journal of geographical information science》2013,27(4):741-764
Local search heuristics for very large-scale vehicle routing problems (VRPs) have made remarkable advances in recent years. However, few local search heuristics have focused on the use of the spatial neighborhood in Voronoi diagrams to improve local searches. Based on the concept of a k-ring shaped Voronoi neighbor, we propose a Voronoi spatial neighborhood-based search heuristic and algorithm to solve very large-scale VRPs. In this algorithm, k-ring Voronoi neighbors of a customer are limited to building and updating local routings, and rearranging local routings with improper links. This algorithm was evaluated using four sets of benchmark tests for 200–8683 customers. Solutions were compared with specific examples in the literature, such as the one-depot VRP. This algorithm produced better solutions than some of the best-known benchmark VRP solutions and requires less computational time. The algorithm outperformed previous methods used to solve very large-scale, real-world distance constrained capacitated VRP. 相似文献
146.
As the product of a variety of sediment sources and sedimentation (and re‐sedimentation) and erosion processes, the geomorphology and sedimentology of carbonate slopes are highly variable. The purpose of this study is to describe sub‐bottom profiles and side‐scan sonar, multibeam and optical data acquired by an autonomous underwater vehicle to explore variability in geomorphological and sedimentological character of the present‐day platform‐marginal, uppermost slope environments (< 240 m water depth) on the north, open‐ocean facing flank of Little Bahama Bank, Bahamas. Although at time scales of greater than 100 ka this margin is progradational, the data illustrate a complex juxtaposition of erosional and depositional processes and features. Erosion is evidenced by two prominent escarpments (70 m and 120 m) that expose eroded, bedded rocky outcrops. These escarpments are interpreted to represent relict features, related to past sea‐level positions, although they still may be shedding debris. Aside from erosional remnants, sedimentation and active transport is indicated by several features, including active bedforms (especially above the 70 m escarpment, but ripples occur to depths of ca 200 m), several mass transport complexes that overlie and cover the lower escarpment, gravity flow deposits and rare slump features. Similarly, a thick (up to 20 m) onlapping sediment wedge, interpreted to be Holocene in age, suggests lateral accretion of the slope by more than 75 m in this period. Data illustrate that this open‐ocean margin is distinct from windward margins in the Bahamas, which typically include near‐vertical walls of erosion or bypass, flanked downdip by rubble and talus, and leeward margins, which have onlapping muddy wedges, but that lack marked terraces or escarpments. Collectively, the results provide perspectives into the nature and controls on complex geomorphological patterns of erosion and deposition in Holocene uppermost slope systems, concepts potentially applicable to ancient analogues. 相似文献
147.
A Winkler model approach for vertically and laterally loaded piles in nonhomogeneous soil 总被引:1,自引:0,他引:1
Hiroyoshi Hirai 《国际地质力学数值与分析法杂志》2012,36(17):1869-1897
An investigation is made to present analytical solutions provided by a Winkler model approach for the analysis of single piles and pile groups subjected to vertical and lateral loads in nonhomogeneous soils. The load transfer parameter of a single pile in nonhomogeneous soils is derived from the displacement influence factor obtained from Mindlin's solution for an elastic continuum analysis, without using the conventional form of the load transfer parameter adopting the maximum radius of the influence of the pile proposed by Randolph and Wroth. The modulus of the subgrade reaction along the pile in nonhomogeneous soils is expressed by using the displacement influence factor related to Mindlin's equation for an elastic continuum analysis to combine the elastic continuum approach with the subgrade reaction approach. The relationship between settlement and vertical load for a single pile in nonhomogeneous soils is obtained by using the recurrence equation for each layer. Using the modulus of the subgrade reaction represented by the displacement influence factor related to Mindlin's solution for the lateral load, the relationship between horizontal displacement, rotation, moment, and shear force for a single pile subjected to lateral loads in nonhomogeneous soils is available in the form of the recurrence equation. The comparison of the results calculated by the present method for single piles and pile groups in nonhomogeneous soils has shown good agreement with those obtained from the more rigorous finite element and boundary element methods. It is found that the present procedure gives a good prediction on the behavior of piles in nonhomogeneous soils. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
148.
饱和土体中空沟对移动荷载被动隔振的 2.5D边界元法分析 总被引:1,自引:1,他引:0
根据饱和土Biot理论及移动荷载特性,采用积分变换方法,推导了半无限饱和土体频域-波数内的边界积分方程,讨论了2.5D边界积分方程中Cauchy的奇异性,并分析了空沟对移动荷载引起饱和土体振动的被动隔振效果。通过与已知文献结果比较,验证了算法的正确性。数值结果表明,采用2.5D BEM不仅可以解决具有复杂表面的移动荷载作用下动力响应问题,而且可极大减小计算工作量。对于饱和土体受高速移动荷载作用,空沟更能有效减小土体孔压、竖向位移的振幅;当荷载速度较小时,空沟对土体表面的水平向位移几乎不起衰减作用,甚至有放大效应。另外,随空沟距振源的距离增大,空沟能够更好阻隔土体表面波传播。 相似文献
149.
150.