首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   579篇
  免费   34篇
  国内免费   44篇
测绘学   5篇
大气科学   23篇
地球物理   176篇
地质学   232篇
海洋学   180篇
综合类   6篇
自然地理   35篇
  2024年   1篇
  2023年   4篇
  2022年   5篇
  2021年   4篇
  2020年   9篇
  2019年   15篇
  2018年   15篇
  2017年   25篇
  2016年   18篇
  2015年   18篇
  2014年   16篇
  2013年   59篇
  2012年   18篇
  2011年   22篇
  2010年   20篇
  2009年   26篇
  2008年   34篇
  2007年   28篇
  2006年   26篇
  2005年   23篇
  2004年   39篇
  2003年   21篇
  2002年   17篇
  2001年   23篇
  2000年   18篇
  1999年   16篇
  1998年   20篇
  1997年   16篇
  1996年   31篇
  1995年   11篇
  1994年   10篇
  1993年   9篇
  1992年   10篇
  1991年   5篇
  1990年   1篇
  1989年   2篇
  1988年   4篇
  1987年   2篇
  1986年   5篇
  1985年   2篇
  1984年   1篇
  1983年   3篇
  1982年   2篇
  1980年   1篇
  1978年   2篇
排序方式: 共有657条查询结果,搜索用时 15 毫秒
591.
Abstract. The breeding biology of the cave-dwelling Mediterranean triplefin blenny Tripterygion melanurus (Pisces, Blennioidei) was studied in the field. Two types of reproductive male were observed, territorial males and non-territorial males. Territorial males establish territories for mating, but not necessarily for feeding, in caves and crevices at depths between 0.5 and 33  m. Territories can be settled in clusters in a single cave. Non-territorial males occur in the vicinity of territories and attempt to parasitise matings of territorial males. Gravid females wait near territories where another female is spawning, rather than spawn with currently unmated territorial males. Waiting females intrude in matings and sometimes displace a spawning female. The frequency of mating of the territorial male, of sneaking and of female intrusions are higher in territories that border others than in isolated territories. Females may minimise predation risks both on themselves and on their eggs by spawning in clustered territories and by spawning with males that have more than one clutch of eggs. Territorial males must find a compromise between the benefits of attracting a female and the costs of parasitic spawning by sneaker males. The possibility that territorial males tolerate the proximity of other territorial males and accept a risk of cuckoldry is discussed. Parasitic spawning in T. melanurus is relatively rare and the gonadosomatic index in both non-territorial and territorial males is low, indicating that sperm competition is probably not intense.  相似文献   
592.
Submarine caves at the Cape of Otranto (SE, Italy) host pseudostalactites. Only recently have they been recognised as originating mainly from the marine tube worm Protula tubularia (Serpulidae, Polychaeta). Their formation depends on the adaptability, life history and behaviour of that species in the cave habitat. The dating of calcareous tubes in the pseudostalactite core represents the first attempt to describe the formation of such bio-construction, and the gregarious behaviour of the species. Calcareous tubes along the core axis of three pseudostalactites removed from the cave lu Lampiùne (Otranto, South East Italy) have been dated using the 14C method. Measures established ages from 2600 to 5000 years for the three structures. This extremely long period (when compared with the life span of each worm) suggests that each tube aggregation is the result of punctuated growth periods alternated with long standstill phases. At the end of a growth period, each tube aggregation was covered with a brownish concretion, completing the pseudostalactite.  相似文献   
593.
The knowledge of the transportation mode used by humans (e.g. bicycle, on foot, car and train) is critical for travel behaviour research, transport planning and traffic management. Nowadays, new technologies such as the Global Positioning System have replaced traditional survey methods (paper diaries, telephone) because they are more accurate and problems such as under reporting are avoided. However, although the movement data collected (timestamped positions in digital form) have generally high accuracy, they do not contain the transportation mode. We present in this article a new method for segmenting movement data into single-mode segments and for classifying them according to the transportation mode used. Our fully automatic method differs from previous attempts for five reasons: (1) it relies on fuzzy concepts found in expert systems, that is membership functions and certainty factors; (2) it uses OpenStreetMap data to help the segmentation and classification process; (3) we can distinguish between 10 transportation modes (including between tram, bus and car) and propose a hierarchy; (4) it handles data with signal shortages and noise, and other real-life situations; (5) in our implementation, there is a separation between the reasoning and the knowledge, so that users can easily modify the parameters used and add new transportation modes. We have implemented the method and tested it with a 17-million point data set collected in the Netherlands and elsewhere in Europe. The accuracy of the classification with the developed prototype, determined with the comparison of the classified results with the reference data derived from manual classification, is 91.6%.  相似文献   
594.
Sustainability of urban transportation and residents' daily mobility is increasingly important for the future of urban regions. In this paper, we present a novel methodology combining mapped survey responses (gathered using public participation GIS) and sophisticated multimodal routing analysis to understand patterns of suburban residents' daily mobility. Our particular focus is on residents' mode choices and their optimality in terms of travel time. We also examine the carbon-intensity of potential mode choice mismatches where a comparatively slower travel mode is chosen for a particular trip. This paper shows how the combination of the two data sources create new possibilities for analysing daily mobility by enabling the comparison of self-reported, geographically referenced mobility patterns and modelled, objectively “optimal” patterns. Our case study results from a rapidly growing metropolitan area, Greater Helsinki in Finland, indicate that people have rather sustainable daily mobility practices. Their daily trips are typically short (median 1.8 km), and non-motorised travel modes are the most popular. Also the share of car travel is significant. While public transport clearly has the lowest modal share, it is frequently used on longer trips to destinations where public transport connections are good (particularly the city centre). In 44% of trips, the respondents choose the fastest travel mode. When a comparatively slower travel mode is chosen, it most often produces less CO2 emissions than the fastest mode. However, particularly on shopping trips – the most commonly reported type of trip – car driving dominates as the fastest means of transportation. When a slower mode is chosen on a shopping trip, it is typically more carbon-intensive than the fastest mode would have been in that case. The methodology presented here can be used to support local decision making, particularly when daily services and transport infrastructure are planned or reorganised.  相似文献   
595.
Volcanic tuffs from Hesse (Germany) and their weathering behaviour   总被引:1,自引:0,他引:1  
In the Hessian region of Germany numerous deposits of tuffs are known from which natural building stones were extracted. Many buildings show examples of the use of this material in the past. In contrast to their attractive appearance and ease of extraction from quarries, tuffs are liable to destruction by weathering. Most common damages are disintegration, crumbling, cracking, scaling and flaking. In this paper, the causes of damage of selected varieties have been analysed using petrographical and petrophysical methods. The measured values are disadvantageous in comparison to other natural building stones. Large amounts of swellable clay minerals, mostly forming the cementing material, cause scaling due to repeated humidification. High porosity including negative pore structures and high adsorption effects results in structural damages while freezing. Consequently, most varieties of the described tuffs possess a limited usage as an outdoor building material.  相似文献   
596.
高层建筑结构的抗震可靠度分析与优化设计   总被引:12,自引:4,他引:12  
本文根据我们在文献2中给出的等效随机地震静力作用模型,紧密结合规范和利用我们在文献6中提出的结构体系可靠度分析的最弱失效模式法,提出了结构构件和体系“小震不坏”和“大震不倒”及结构体系在设计基准期内的抗震可靠度分析方法;重新校准了结构构件的目标可靠度指标;综合考虑结构造价和损失期望,提出了结构体系抗震目标可靠度的优化决策方法;分别给出了满足构件抗震目标可靠指标与同时满足构件和体系抗震目标可靠指标的  相似文献   
597.
A dynamic model for rainfall-induced landslides on natural slopes   总被引:18,自引:0,他引:18  
H. Chen  C. F. Lee   《Geomorphology》2003,51(4):269-288
  相似文献   
598.
本文用确定性和随机概率的方法研究了工桥墩台和小跨拱桥在恒载及活载(包括地震效应)作用下决定某截面上的偏心距,以便使其在设计规范规定的限度时所能通过的活载最大等级是什么,由于地震随机性强烈,故基于概率理论的结构可靠性方法优于确定性的方法,但在本文中的结果显示,这两种方法的最终结果十分接近。  相似文献   
599.
The presence of suitable gravel size alone can stimulate female rainbow trout (Salmo gairdneri) to excavate nests for spawning on the shores of Lake Rotoma. Emergent fry result from lake‐shore spawning, and could contribute significantly to the trout fishery in Lake Rotoma. Lake‐shore spawning could be enhanced by the use of stable artificial spawning beds that have adequate depth and appropriate sizes of gravel.  相似文献   
600.
季燕南 《地学前缘》2010,17(1):378-385
简要回顾了我国山东省诸城市吕标乡库沟村龙骨涧晚白垩世早期辛格庄组中产出的巨型山东龙的研究历史,并讨论了巨型山东龙与巨大诸城龙的系统分类关系。对比两者的产出状况、骨骼结构和个体发育特征,作者认为两者为同物异名。依据古生物命名法规的优先律,巨型山东龙命名在先,合法有效;巨大诸城龙命名较晚,是一无效名称,应予取消。还讨论了巨型山东龙的生活习性和生态环境,认为巨型山东龙属陆栖性、群居性、植食性恐龙,生活在气候温暖、雨水充沛、植被繁茂的低山或丘陵地区。此外研究了巨型山东龙赋存地层的沉积特征,认为是一套辫状河流相或冲积扇相沉积,而不是湖相沉积。恐龙化石的密集产出是非正常集群死亡于突发性灾害事件的结果,而与国际上的K/T事件无关。  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号