全文获取类型
收费全文 | 4415篇 |
免费 | 506篇 |
国内免费 | 698篇 |
专业分类
测绘学 | 1394篇 |
大气科学 | 438篇 |
地球物理 | 476篇 |
地质学 | 1755篇 |
海洋学 | 391篇 |
天文学 | 67篇 |
综合类 | 376篇 |
自然地理 | 722篇 |
出版年
2024年 | 28篇 |
2023年 | 69篇 |
2022年 | 148篇 |
2021年 | 181篇 |
2020年 | 227篇 |
2019年 | 213篇 |
2018年 | 140篇 |
2017年 | 218篇 |
2016年 | 222篇 |
2015年 | 206篇 |
2014年 | 225篇 |
2013年 | 278篇 |
2012年 | 257篇 |
2011年 | 252篇 |
2010年 | 199篇 |
2009年 | 261篇 |
2008年 | 252篇 |
2007年 | 298篇 |
2006年 | 254篇 |
2005年 | 194篇 |
2004年 | 180篇 |
2003年 | 156篇 |
2002年 | 140篇 |
2001年 | 126篇 |
2000年 | 133篇 |
1999年 | 115篇 |
1998年 | 95篇 |
1997年 | 94篇 |
1996年 | 68篇 |
1995年 | 60篇 |
1994年 | 69篇 |
1993年 | 60篇 |
1992年 | 48篇 |
1991年 | 22篇 |
1990年 | 31篇 |
1989年 | 26篇 |
1988年 | 11篇 |
1987年 | 11篇 |
1986年 | 10篇 |
1985年 | 6篇 |
1984年 | 6篇 |
1982年 | 2篇 |
1980年 | 4篇 |
1978年 | 3篇 |
1977年 | 3篇 |
1976年 | 4篇 |
1973年 | 7篇 |
1972年 | 1篇 |
1971年 | 3篇 |
1928年 | 1篇 |
排序方式: 共有5619条查询结果,搜索用时 0 毫秒
71.
Statistical facies classification from multiple seismic attributes: comparison between Bayesian classification and expectation–maximization method and application in petrophysical inversion 下载免费PDF全文
We present here a comparison between two statistical methods for facies classifications: Bayesian classification and expectation–maximization method. The classification can be performed using multiple seismic attributes and can be extended from well logs to three‐dimensional volumes. In this work, we propose, for both methods, a sensitivity study to investigate the impact of the choice of seismic attributes used to condition the classification. In the second part, we integrate the facies classification in a Bayesian inversion setting for the estimation of continuous rock properties, such as porosity and lithological fractions, from the same set of seismic attributes. The advantage of the expectation–maximization method is that this algorithm does not require a training dataset, which is instead required in a traditional Bayesian classifier and still provides similar results. We show the application, comparison, and analysis of these methods in a real case study in the North Sea, where eight sedimentological facies have been defined. The facies classification is computed at the well location and compared with the sedimentological profile and then extended to the 3D reservoir model using up to 14 seismic attributes. 相似文献
72.
场地条件对地震动具有较大影响, 研究不同场地条件下的地震动特征对地震动的校正具有重要意义。 本文以九寨沟地震为例, 收集了66个台站的198条三分量强震观测记录和SRTM(Shuttle Radar Topography Mission)公里格网的DEM(Digital Elevation Model)数据, 从多个角度对场地特征进行分析。 首先根据坡度法使用DEM数据对九寨沟特征进行了场地分类, 然后讨论了不同场地类型下的加速度时程、 加速度反应谱、 地震动衰减等地震动特征。 研究表明, 缺少实测资料的情况下, 地形坡度可以作为Vs30(地表以下30 m范围的平均剪切波速度)的一种替代指标, 利用坡度法可以较快速地对场地进行分类; 利用强震观测记录能够从多角度对台站的场地特征进行分析, 不同场地类型对地震动影响不同, 其中, 土层对地震动具有明显的放大效应。 该结论可以为地震动结果的校正提供依据。 相似文献
73.
74.
75.
湘南地区风化壳型稀土资源丰富,对其开发的采、选工艺和流程简单,投入资金较少,产出颇丰、经济、社会效益明显,是湘南盆困山区脱贫致富的重要途径。本文就开发湘南稀土矿的成本、效益等进行了初步测算,可供参考。 相似文献
76.
传统的基于象元的分类方法在处理海岸带这类动态多变、混杂度大的地物时常出现“椒盐现象”,难以矢量化形成专题图。提出了一种新的遥感图像分类方法——基于分割图斑的分类法,即选择一个合适的尺度,利用光谱信息按照一定的策略将图像分割为一系列的图斑,并确保图斑内大多数象元的光谱特征相近,分别对图斑内诸象元进行统计,求出各个波段的均值,用该均值替换图斑内所有象元的原始亮度值,这样分类时可以确保同一图斑内的所有象元被分到同一个类别中,从而有效地避免了“椒盐现象”,使分类结果便于矢量化成图,也便于对特定地物的提取。实验结果表明,该方法还能在一定程度上提高分类精度。 相似文献
77.
Chris McGonigle Jonathan H. Grabowski Craig J. Brown Thomas C. Weber Rory Quinn 《Estuarine, Coastal and Shelf Science》2011
Benthic macroalgae form an important part of temperate marine ecosystems, exhibiting a complex three-dimensional character which represents a vital foraging and spawning ground for many juvenile fish species. In this research, image-based techniques for classification of multibeam backscatter are explored for the detection of benthic macroalgae at Cashes Ledge in the Gulf of Maine, USA. Two classifications were performed using QTC-Multiview, differentiated by application of a threshold filter, and macroalgal signatures were independently extracted from the raw sonar datagrams in Matlab. All classifications were validated by comparison with video ground-truth data. The unfiltered classification shows a high degree of complexity in the shallowest areas within the study site; the filtered demonstrates markedly less variation by depth. The unfiltered classification shows a positive agreement with the video ground-truth data; 82.6% of observations recording Laminaria sp., 39.1% of Agarum cribrosum and 100.0% (n = 3) of mixed macroalgae occur within the same acoustically distinct group of classes. These are discrete from the 8.1% recorded agreement with absences and nulls (>40 m) of macrophytes (n = 32) from a total of 86 ground-truth locations. The results of the water column data extraction (WCDE) show similar success, accurately predicting 78.3% of Laminaria sp. and 30.4% of A. cribrosum observations. 相似文献
78.
应用模式识别定量划分潜在震源区 总被引:2,自引:0,他引:2
本文以陕西关中及部分邻区为例,考虑地质构造、地震活动及地球物地场等因素,采用不同的模式识别方法和多种计算方案,以定量判定潜在震源区。其结果表明,模式识别方法有利于地质、地震活动及地球物理场等多种因素综合应用,并能选择和显示判定潜有震源区的主要特征。不同模式识别方法的比较和多种试验方案的综合,则可提高潜在震源区判定的可靠性和稳定性。 相似文献
79.
Descriptive Analysis and Classification of Benthic Communities in Some Mediterranean Coastal Lagoons (Central Italy) 总被引:3,自引:0,他引:3
Abstract. Benthic samples were taken seasonally at 32 stations in five coastal lagoons of Central Italy (Fogliano, Monaci, Caprolace, Fondi, and Lungo) during a two-year period (1982–1984). Composition and distribution of benthic populations in each lagoon as well as seasonal trends of species richness, density, diversity, and redundancy are presented and discussed.
The faunal differentiation appeared to be mainly due to the different degree of water exchange, both with marine and continental systems. 相似文献
The faunal differentiation appeared to be mainly due to the different degree of water exchange, both with marine and continental systems. 相似文献
80.
Anabranching rivers consist of multiple channels separated by vegetated semi-permanent alluvial islands excised from existing floodplain or formed by within-channel or deltaic accretion. These rivers occupy a wide range of environments from low to high energy, however, their existence has never been adequately explained. They occur concurrently with other types of channel pattern, although specific requirements include a flood-dominated flow regime and banks that are resistant to erosion, with some systems characterized by mechanisms to block or constrict channels, thereby triggering avulsion. The fundamental advantage of an anabranching river is that, by constructing a semi-permanent system of multiple channels, it can concentrate stream flow and maximize bed-sediment transport (work per unit area of the bed) under conditions where there is little or no opportunity to increase gradient. On the basis of stream energy, sediment size and morphological characteristics, six types of anabranching river are recognized; types 1–3 are lower energy and types 4–6 are higher energy systems. Type 1 are cohesive sediment rivers (commonly termed anastomosing) with low w/d ratio channels that exhibit little or no lateral migration. They are divisible into three subtypes based on vegetative and sedimentary environment. Type 2 are sand-dominated, island-forming rivers, and type 3 are mixed-load laterally active meandering rivers. Type 4 are sand-dominated, ridge-forming rivers characterized by long, parallel, channel-dividing ridges. Type 5 are gravel-dominated, laterally active systems that interface between meandering and braiding in mountainous regions. Type 6 are gravel-dominated, stable systems that occur as non-migrating channels in small, relatively steep basins. Anabranching rivers represent a relatively uncommon but widespread and distinctive group that, because of particular sedimentary, energy-gradient and other hydraulic conditions, operate most effectively as a system of multiple channels separated by vegetated floodplain islands or alluvial ridges. 相似文献