全文获取类型
收费全文 | 1367篇 |
免费 | 283篇 |
国内免费 | 512篇 |
专业分类
测绘学 | 32篇 |
大气科学 | 185篇 |
地球物理 | 367篇 |
地质学 | 870篇 |
海洋学 | 208篇 |
天文学 | 24篇 |
综合类 | 51篇 |
自然地理 | 425篇 |
出版年
2024年 | 9篇 |
2023年 | 28篇 |
2022年 | 101篇 |
2021年 | 83篇 |
2020年 | 109篇 |
2019年 | 113篇 |
2018年 | 83篇 |
2017年 | 74篇 |
2016年 | 85篇 |
2015年 | 100篇 |
2014年 | 106篇 |
2013年 | 97篇 |
2012年 | 81篇 |
2011年 | 99篇 |
2010年 | 75篇 |
2009年 | 109篇 |
2008年 | 88篇 |
2007年 | 83篇 |
2006年 | 88篇 |
2005年 | 71篇 |
2004年 | 73篇 |
2003年 | 76篇 |
2002年 | 47篇 |
2001年 | 40篇 |
2000年 | 36篇 |
1999年 | 31篇 |
1998年 | 25篇 |
1997年 | 28篇 |
1996年 | 28篇 |
1995年 | 16篇 |
1994年 | 21篇 |
1993年 | 17篇 |
1992年 | 10篇 |
1991年 | 6篇 |
1990年 | 6篇 |
1989年 | 13篇 |
1988年 | 3篇 |
1987年 | 3篇 |
1986年 | 1篇 |
排序方式: 共有2162条查询结果,搜索用时 734 毫秒
961.
The family Mesopsychidae Tillyard,1917 presently consists of ten described genera from the Early Permian to the Early Cretaceous of Australia,China,Kyrgyzstan,Russia,South Africa,Tajikistan,and Ukraine.Herein,a new genus and a new species of fossil mesopsychid,Epicharmesopsyche pentavenulosa gen.et sp.nov.,is described from the Middle Jurassic Jiulongshan Formation of Inner Mongolia,China,supplementing the family-level diversity.The well-preserved,new material reveals many characters,including antennae,wing venation,shape and genitalia that increase our morphological understanding of the family,although unfortunately the mouthparts and most of the legs are not preserved.Diagnosis of the Mesopsychidae is emended to include a four-or five-branched MP in both the fore-and hind wings.This is the first documentation of a wing coupling structure in the Mesopsychidae,consisting of three to four frenula bristles on the humeral lobe at the base of the costal margin of both hind wings.Asymmetrical shape and size of the left and right wings on a specimen of E.pentavenulosa gen.et sp.nov.,seems to be a common condition for mesopsychid taxa from northeastern China. 相似文献
962.
塔里木盆地台盆区寒武系已发现的碳酸盐岩潜山油气藏显示了巨大的勘探潜力,然而由于塔里木盆地经历了多期的构造运动,早期形成的寒武系碳酸盐岩油气藏经历了后期的调整、改造、破坏和再聚集,油气成藏过程复杂,已有的油气成藏理论难以指导该地区的油气勘探。目前寒武系地层钻探成功率偏低,仅为25%。从地质角度出发,采用“功能要素匹配-构造过程叠合-晚期相势源复合的新方法对塔里木盆地寒武系失利井原因进行分析,试图揭示该地区油气成藏机理,以便有效地指导油气勘探。通过对塔里木盆地寒武系的15口失利井的分析结果表明:80%的失利井由于功能要素不匹配,落在有利成藏领域之外而失利;7%的失利井由于构造过程叠合改造破坏了早期形成的油气藏,落在有利勘探区带之外而失利;13%的失利井由于晚期相-势-源不复合,落在有利勘探目标外而失利。该方法完美地解释了塔里木盆地寒武系钻探失利的原因,说明其在指导油气勘探中的可行性和有效性,对以后的油气勘探具有重大的现实意义。 相似文献
963.
国家森林乡村建设是实施乡村振兴战略和改善农村人居环境的重要措施,但森林乡村在不同尺度的分异特征及优化方向尚不明晰。本文综合运用探索性空间数据分析、耦合度模型和Voronoi图分析等研究方法,从全国、区域和行政尺度分析了国家森林乡村空间格局、数量规模和内部分异特征,并基于森林乡村与人口耦合关系及乡村可达类型识别,分析了全国森林乡村的分区格局及其优化方向。结果表明:全国森林乡村密度具有明显的东高西低分异特征,长江三角洲地区是森林乡村高密度集中分布最大的连片区。基于县域单元的省域森林乡村泰尔指数差异较大,东南区域森林乡村分布相对均衡,西南和东北区域内部差异显著。森林乡村空间可达性呈现自东向西的梯度递减,森林乡村与人口密度的耦合类型分布具有较强的内部分异性。文章基于森林乡村可达类型与耦合分区的组合分析,将全国划分为重点发展区、特色挖掘区、集聚提升区、适度引导区和功能转型区五类,明确了不同类型区森林乡村建设的重点方向,将为面向乡村振兴的全国森林乡村建设与分区优化提供决策依据。 相似文献
964.
Previous studies have highlighted reversals in the Beaufort Gyre on regional scales during summer months, and more recently, throughout the annual cycle. In this study we investigate coherent ice drift features associated with individual ice beacons during winter 2008 that may be a signature of ice–coast interactions, atmospheric and/or oceanic forcing. Examined in particular are three case studies associated with reversals in ice beacon trajectories in January and April of 2008; case I corresponds to a meander reversal event in January, case II to a loop reversal event in April, and case III to a meander reversal event located to the northeast of the Mackenzie Canyon in April. An assessment of atmospheric and oceanic conditions during these reversal events shows enhanced ocean–sea-ice–atmosphere dynamical coupling during the Case I meander reversal event in January and comparatively weak coupling during the Case II loop and Case III meander reversal event in April. Absolute (single-particle/beacon) and relative (two-particle/beacon) dispersion results demonstrate dominant meridional ice drift displacement and inter-beacon separation for Case I relative to Cases II and III indicative of ice–ice and ice–coast interactions in January. The results from this investigation provide an ice drift case study analysis relevant to, and template for, high-resolution sea ice dynamic modeling studies essential for safety and hazard assessments of transportation routes and shipping lanes, ice forecasting, and nutrient and contaminant transport by sea ice in the Arctic. 相似文献
965.
针对多元线性回归系统,利用耦合辨识概念和多新息辨识理论,讨论了多元随机梯度算法、多元多新息随机梯度算法,以及变递推间隔多元多新息梯度算法,进一步分解多元系统为一些子系统,给出了耦合子系统随机梯度算法、耦合随机梯度算法、耦合子系统多新息随机梯度算法、耦合多新息随机梯度算法,并将这些方法推广到多元伪线性滑动平均系统和多元伪线性自回归滑动平均系统.文中给出了几个典型耦合随机梯度算法、耦合多新息随机梯度算法的计算步骤和示意图. 相似文献
966.
实施人工岬角和海滩养护的软硬措施相结合的方法是探索解决海滩侵蚀问题的新举措之一。基于验证良好的水动力模型建立了泥沙输运和海床演变模型,对秦皇岛市新开河口至南山岸线海滩经整治修复工程实施后的泥沙输运和海床演变规律进行模拟分析。结果表明:大潮与常浪耦合作用下,研究区域周围海域流速整体上落急时刻大于涨急时刻,沙坝掩护水域的流速较弱,波浪在沙坝处增强并在沙坝后破碎并减弱;在沙坝处出现高悬沙浓度区域,悬沙浓度分布主要由底床切应力控制,部分受水动力影响。大潮与强浪耦合作用下,在沙坝处出现远大于常浪时的增强水流(波生流),方向与强浪向一致,波浪在沙坝处已破碎且沙坝对强浪的削减效率大于常浪;与常浪作用下不同,悬沙浓度场在强浪作用下沙坝处没有出现明显的高浓度区域。波流耦合作用下,人工岬角单独存在时护岸效果有限,工程实施后在人工岬角和人工沙坝的配合下,沙坝处侵蚀而沙坝后侧掩护区域淤积,沙坝起到了有效养护海滩的作用。 相似文献
967.
“双循环”发展背景下,城市创新能力成为影响经济高质量发展的关键因素。选取2010年、2014年和2018年作为时间节点,运用改进TOPSIS和耦合协调模型对安徽省城市创新能力及其子系统间耦合协调的时空演化规律进行实证评价与分析。研究表明(:1)安徽省内城市综合创新能力差异逐渐扩大,呈现“东高西低”的空间分布特征。(2)安徽省城市创新投入能力整体逐渐上升,呈现“南高北低”的空间分布特征;创新产出能力差距逐渐缩小,空间分布差异不显著;城市环境支持能力差距逐渐拉大,呈现“北高南低”的空间分布特征。(3)安徽省城市“创新投入-创新产出-环境支持”能力的耦合协调度整体水平较低,在空间上表现出显著的“中心-外围”分布特征,以中度失调和轻度失调两种类型为主。(4)安徽省城市创新能力耦合协调性无法进一步提升的原因主要受2个或3个复合限制因素的影响,其中城市创新投入、创新产出和环境支持能力同时滞后的城市数量最多。 相似文献
968.
969.
Shiang‐Jung Wang Jenn‐Shin Hwang Kuo‐Chun Chang Meng‐Hui Lin Bo‐Han Lee 《地震工程与结构动力学》2013,42(2):201-219
In a midstory isolated building, the isolation system is incorporated into the midstory rather than the base of the building. Because of the flexibility of the substructure below the isolation system in a midstory isolated building, the contribution of higher modes to the seismic responses of the midstory isolated structure may not be negligible, especially when the coupling of higher modes exists. To investigate this modal coupling effect, a simplified three‐lumped‐mass structural model of the midstory isolated building is assumed in this study. Through the equivalent linear analysis and shaking table tests, it is found that the coupling of higher modes may lead to enlarged acceleration responses at the super‐floor and superstructure above the isolation layer. Accordingly, a simple method to prevent the midstory isolation design from the coupling of higher modes attributed to the improper design of the substructure and superstructure is proposed in this paper. Copyright © 2012 John Wiley & Sons, Ltd. 相似文献
970.
ABSTRACTIn this paper we develop a coupled analytical model for salinity and tidal propagation in estuaries where the cross-sectional area varies exponentially. A simple analytical model for tidal dynamics has been used to estimate the tidal excursion, which has an important influence on the salt intrusion process since it determines the extreme salinities (i.e. salinity distribution for high water slack and low water slack). The objective of the coupling is to reduce the number of calibration parameters, which subsequently strengthens the reliability of the salt intrusion model. Moreover, the coupling enables us to assess the potential impacts of external changes, both human-induced interventions (e.g. dredging) and natural changes (e.g. global sea level rise), on the salt intrusion process. In addition, the fully analytical solution for hydrodynamics allows immediate estimation of the tidally averaged depth and friction coefficient for given water level recordings and salinity measurements. This is particularly useful when a geometric survey is not available. The coupled model has been applied to six previously unsurveyed estuaries in Malaysia and the results show that the correspondence between analytical estimations and observations is very good. Thus, the coupled model proves to be a useful tool to obtain estimates of salt intrusion in estuaries based on a minimum amount of information required and for assessing the effect of human-induced or natural changes.
EDITOR D. Koutsoyiannis ASSOCIATE EDITOR B. Dewals 相似文献