全文获取类型
收费全文 | 44765篇 |
免费 | 7958篇 |
国内免费 | 13196篇 |
专业分类
测绘学 | 3410篇 |
大气科学 | 4487篇 |
地球物理 | 6286篇 |
地质学 | 35724篇 |
海洋学 | 4070篇 |
天文学 | 3363篇 |
综合类 | 3101篇 |
自然地理 | 5478篇 |
出版年
2024年 | 373篇 |
2023年 | 975篇 |
2022年 | 1696篇 |
2021年 | 1930篇 |
2020年 | 1784篇 |
2019年 | 2160篇 |
2018年 | 1619篇 |
2017年 | 1981篇 |
2016年 | 2145篇 |
2015年 | 2277篇 |
2014年 | 2931篇 |
2013年 | 2654篇 |
2012年 | 3033篇 |
2011年 | 3050篇 |
2010年 | 2713篇 |
2009年 | 3055篇 |
2008年 | 2929篇 |
2007年 | 3046篇 |
2006年 | 2933篇 |
2005年 | 2748篇 |
2004年 | 2455篇 |
2003年 | 2308篇 |
2002年 | 2048篇 |
2001年 | 1888篇 |
2000年 | 1873篇 |
1999年 | 1673篇 |
1998年 | 1506篇 |
1997年 | 1152篇 |
1996年 | 992篇 |
1995年 | 834篇 |
1994年 | 751篇 |
1993年 | 602篇 |
1992年 | 450篇 |
1991年 | 394篇 |
1990年 | 256篇 |
1989年 | 212篇 |
1988年 | 168篇 |
1987年 | 99篇 |
1986年 | 56篇 |
1985年 | 41篇 |
1984年 | 19篇 |
1983年 | 16篇 |
1982年 | 12篇 |
1981年 | 9篇 |
1980年 | 8篇 |
1979年 | 7篇 |
1978年 | 17篇 |
1977年 | 5篇 |
1976年 | 4篇 |
1954年 | 7篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
71.
采用相对资源承载力的理论与研究方法,定量分析了盐池县8个乡镇相对资源承载力时空变化及差异特征,并对承载力匹配特征进行空间表达。结果表明:盐池县辖各乡镇相对于盐池县的综合人口承载力表现出严重超载、超载和富余3种不同的状态;相对自然资源经济承载力时空变化稳定,自然资源的经济潜力不断增大;相对草地资源牲畜承载力随时间推移不断递增且空间差异明显;依据3种承载力状态组合划分了不同的承载力匹配类型,并提出不同类型的可持续发展策略。本研究可为盐池县禁牧10年后各乡镇发展提供建议,并为“退牧还草”政策的继续实施提供理论支持。 相似文献
72.
本文简述了沙柳河南区有色金属矿床的基本特征。通过对成矿热液及成矿物质的来源与性质的研究,认为该矿床的成因类型为火山沉积—接触交代改造叠加型,进而探讨了该矿床的成矿模式。 相似文献
73.
Designing a spatial pattern to rebalance the orientation of development and protection in Wuhan 总被引:1,自引:5,他引:1
Patterns of spatial development and protection form a basic category of geoscience,and redesigning them is a popular subject of research in regional sustainable development that is important for ecological civilization construction.The authors here report a case study of Wuhan city using the circuit theory model and minimum cumulative resistance(MCR)model to rebalance its spatial protection and development.The results show the following:(1)Using the density of the gross domestic product(GDP),density of population,rate of urbanization,and access to transportation as evaluation indicators,seven core areas of development in Wuhan were identified,accounting for 59%of the total number of streets,that exhibited a “circular-satellite”spatial structure.(2)According to the importance of ecosystem services,ecological sensitivity,land use type,and slope of the terrain,the resistance surface of spatial development in Wuhan had a stereoscopic spatial form of an“inverted pyramid,”with high surroundings and a low center.The area of low resistance accounted for 6.64%of the total area of Wuhan.(3)Based on coupling analysis using the MCR and spatial morphological characteristics of current,nine axes of spatial development with a total area of 427.27 km2 and eight key strategic points with a total area of 40.02 km2 were identified.Streets that were prioritized for development accounted for 9.63%of Wuhan's total area.(4)By combining the characterization of the development axis with the structure of the three-level core area,we extracted the structure of spatial development of "one heart,two wings,and three belts" in Wuhan.The research framework and empirical results can provide scientific guidance for the urban spatial layout,the development of regional linkages,and ecological environmental protection in China. 相似文献
74.
通过野外地质调查和系统的岩石主量元素、微量元素、稀土元素等的研究,探讨红山铁矿床硅质岩的地球化学特征及其沉积环境。红山铁矿床中广泛发育一套与铁矿体呈共生关系的硅质岩。研究表明,与铁矿密切共生的呈层状和透镜状产出的灰色、紫红色硅质岩总体属于低硅类型的硅质岩。在SiO2-Al2O3判别图解中,SiO2与Al2O3负相关,样品数据点均落入热水沉积区域;稀土元素配分模式经北美页岩标准化后呈平缓型,与东太平洋热泉水、红海热卤水具有相似的正Eu异常和配分模式,δEu值均大于1,显示热水沉积的特征;硅质岩中的V、Ni、Cu、Ba等微量元素的含量具有陆缘海盆热水沉积的特征。 相似文献
75.
Quantification of landscape-based vegetation structural variation and pattern is a significant goal for a variety of ecological, monitoring and biodiversity studies. Vegetation structural metrics, derived from airborne laser scanning (ALS or aerial light detection and ranging—LiDAR) and QuickBird satellite imagery, were used to establish the degree of plot-based vegetation variation at a hillslope scale. Topographic position is an indicator of energy and water availability, and was quantified using DEM-based insolation and topographic wetness, respectively, stratifying areas into hot-warm-cold and wet-moist-dry topographic classes. A range of vegetation metrics—maximum and modal canopy height, crown cover, foliage cover, NDVI and semivariance—were compared among randomly selected plots from each topographic class. NDVI increases with increasing landscape wetness, whereas ALS-derived foliage cover decreases with increasing insolation. Foliage cover is well correlated with crown cover (R 2 =0.65), and since foliage cover is readily calculable for whole-of-landscape application, it will provide valuable and complementary information to NDVI. Between-plot heterogeneity increases with increasing wetness and decreasing insolation, indicating that more sampling is required in these locations to capture the full range of landscape-based variability. Pattern analysis in landscape ecology is one of the fundamental requirements of landscape ecology, and the methods described here offer statistically significant, quantifiable and repeatable means to realise that goal at a fine spatial grain. 相似文献
76.
岩石变形破坏的熵突变过程与破坏判据 总被引:2,自引:0,他引:2
在岩石的变形破坏过程中,当进入不稳定的破裂发展阶段之后,系统不断调整结构抵抗外力的扰动,裂纹向局域集中的有序方向发展,应变能不断耗散,并以应变能的耗散为媒介,使系统与外扰动所追加的负熵流产生能量流通,也使系统熵值和系统的维数逐次降低,这一阶段岩石系统远离平衡态,岩石的破坏是系统熵的突变过程。基于这一认识,并在验证应变能分布与结构块度分布模式的一致性的基础上,推导出包含了结构因素和能量分布的熵表达式;对熵表达式进行平衡分析获得局部突变的分岔集,得到了岩石局部破坏的熵折迭突变破坏准则;同时,探讨了熵表达式所表征的结构有序度的尖点突变性,解出岩石系统的分岔集,这个分岔集就是岩石系统熵突变的整体破坏准则。 相似文献
77.
78.
79.
80.
Geospatial data and tools are key in locating lost or missing persons in as short a time as possible. In this study, we used a geographic information system (GIS) to analyze four years of search and rescue (SAR) mission data from Colorado to determine the appropriate use of GIS for volunteer-based SAR organizations with limited resources and GIS expertise. GIS can provide more sophisticated analyses of geospatial data than simple mapping technologies, but our findings indicated that complex spatial analysis might not be required on all missions, because the majority of missions were completed within six to ten hours. Instead, new technologies such as tablets with mapping software and online GIS systems that provide quick and easy access to up-to-date geospatial data such as imagery offer capabilities that could improve mission planning. Here we provide a framework in which SAR missions can apply geospatial technologies to aid with missions, identify critical “hotspots,” and enhance postanalysis and training. The work here is highly applicable for nonprofit SAR groups when deciding on what GIS technologies to consider for their areas. 相似文献