全文获取类型
收费全文 | 2439篇 |
免费 | 410篇 |
国内免费 | 667篇 |
专业分类
测绘学 | 22篇 |
大气科学 | 110篇 |
地球物理 | 421篇 |
地质学 | 1753篇 |
海洋学 | 458篇 |
天文学 | 4篇 |
综合类 | 80篇 |
自然地理 | 668篇 |
出版年
2024年 | 12篇 |
2023年 | 35篇 |
2022年 | 98篇 |
2021年 | 92篇 |
2020年 | 143篇 |
2019年 | 139篇 |
2018年 | 128篇 |
2017年 | 114篇 |
2016年 | 136篇 |
2015年 | 136篇 |
2014年 | 141篇 |
2013年 | 187篇 |
2012年 | 161篇 |
2011年 | 175篇 |
2010年 | 145篇 |
2009年 | 148篇 |
2008年 | 140篇 |
2007年 | 169篇 |
2006年 | 160篇 |
2005年 | 127篇 |
2004年 | 138篇 |
2003年 | 125篇 |
2002年 | 97篇 |
2001年 | 90篇 |
2000年 | 73篇 |
1999年 | 73篇 |
1998年 | 61篇 |
1997年 | 62篇 |
1996年 | 44篇 |
1995年 | 39篇 |
1994年 | 28篇 |
1993年 | 22篇 |
1992年 | 15篇 |
1991年 | 9篇 |
1990年 | 11篇 |
1989年 | 10篇 |
1988年 | 9篇 |
1987年 | 3篇 |
1986年 | 4篇 |
1985年 | 3篇 |
1984年 | 6篇 |
1983年 | 2篇 |
1982年 | 4篇 |
1981年 | 2篇 |
排序方式: 共有3516条查询结果,搜索用时 15 毫秒
191.
西藏羌塘盆地侏罗系白云岩储层中油砂资源评价 总被引:2,自引:0,他引:2
西藏羌塘盆地是一个烃源岩发育厚度大、分布广的大型中生代海相沉积盆地,构造活动对早期形成的油气藏破坏和改造强烈,为油砂富集成矿提供了条件。通过对盆地中发现的隆鄂尼和昂达尔错两个矿点进行勘察。矿点油砂主要富集于中侏罗统布曲组,分析认为其形成和分布受燕山晚期形成的大型古隆起带的控制,早期形成的古油藏带在后期构造过程中遭受破坏和抬升,使顶部布曲组含油白云岩被抬升剥蚀风化形成现今的油砂矿。采用含油率法对油砂矿带进行了资源量的计算,结果表明两个矿带的油砂地质资源总量为93 099.76×104t。通过类比认为整个盆地油砂资源潜力巨大,可以成为油砂资源主要的勘探地区。 相似文献
192.
The strength of frozen soils has been one of the most extensively studied aspects in frozen soil mechanics. When carrying
out deep excavations using freezing methods, high stress states are often encountered. Therefore, the strength of frozen soils
under high confining pressures is of major concern. This paper first reviews the present failure criteria for frozen soils,
especially with regard to the effect of confining pressure on the strength. It is suggested that the strength consists of
two components, cohesion and friction, and can be expressed by the Mohr–Coulomb criterion and in the framework of the Drucker–Prager
criterion. However, these two components are both dependent on the stress state. Duncan’s equation for the friction angle
is extended to frozen soils. A frozen Lanzhou fine sand is taken as study subject. The frozen sand is compressed at a high
strain rate under three different temperatures and under a wide range of confining pressure. Experimental results have confirmed
the generally recognized principle that the strength increases with the confining pressure up to certain value. Thereafter,
it decreases with continued increase in confining pressure. Experimental data from our own tests and literature are fitted
to the new criterion, which shows its validity. 相似文献
193.
194.
195.
Qu JianJun Zhang KeCun Sun Bo Jiang ShengXiang Dong GuangRong Zu RuiPing Fang HaiYan 《中国科学D辑(英文版)》2007,50(9):1351-1358
The sound-producing mechanism of booming sand has long been a pending problem in the blown sand physics. Based on the earlier researches, the authors collected some silent sand samples from Teng- ger Desert, Australian Desert, Kuwait Desert, beaches of Hainan Island and Japanese coast as well as the soundless booming sand samples from the Mingsha Mountain in Dunhuang to make washing ex- periments. In the meantime the chemical corrosion experiment of glass micro-spheres, surface coating experiment and SEM examination were also conducted. The experimental results show that the sound production of booming sand seems to have nothing to do with the presence of SiO2 gel on the surface of sand grains and unrelated to the surface chemical composition of sand grains but is related to the resonance cavities formed by porous (pit-like) physical structure resulting from a number of factors such as wind erosion, water erosion, chemical corrosion and SiO2 gel deposition, etc. Its resonance mechanism is similar to that of Hemholz resonance cavity. Under the action of external forces, nu- merous spherical and sand grains with smooth surface and porous surface are set in motion and rub with each other to produce extremely weak vibration sound and then become audible sound by human ears through the magnification of surface cavity resonance. However the booming sands may lose their resonance mechanism and become silent sand due to the damping action caused by the invasion of finer particles such as dust and clay into surface holes of sand grains. Therefore, clearing away fine pollutants on the quartz grain surface is an effective way to make silent sand emit audible sound. 相似文献
196.
Precipitation is often the sole source of water replenishment in arid and semi‐arid areas and, thus, plays a pertinent role in sustaining desert ecosystems. Revegetation over 40 years using mainly Artemisia ordosica and Caragana korshinskii at Shapotou Desert Experimental Research Station near Lanzhou, China, has established a dwarf‐shrub and microbiotic soil crust cover on the stabilized sand dunes. The redistribution of infiltrated moisture through percolation, root extraction, and evapotranspiration pathways was investigated. Three sets of time‐domain reflectometry (TDR) probes were inserted horizontally at 5, 10, 15, 20, 30 and 40 cm depths below the ground surface in a soil pit. The three sets of TDR probes were installed in dwarf‐shrub sites of A. ordosica and C. korshinskii community with and without a microbiotic soil crust cover, and an additional set was placed in a bare sand dune area that had neither vegetation nor a microbiotic soil crust present. Volumetric soil moisture content was recorded at hourly intervals and used in the assessment of infiltration for the different surface covers. Infiltration varied greatly, from 7·5 cm to more than 45 cm, depending upon rainfall quantity and soil surface conditions. In the shrub community area without microbiotic soil crust cover, infiltration increased due to preferential flow associated with root tunnels. The microbiotic soil crust cover had a significant negative influence on the infiltration for small rainfall events (~10 mm), restricting the infiltration depth to less than 20 cm and increasing soil moisture content just beneath the soil profile of 10 cm, whereas it was not as strong or clear for larger rainfall events (~60 mm). For small rainfall events, the wetting front depth for the three kinds of surface cover was as follows: shrub community without microbiotic soil crust > bare area > shrub community with microbiotic soil crust. In contrast, for large rainfall events, infiltration was similar in shrub communities with and without microbiotic soil crust cover, but significantly higher than measured in the bare area. Soil water extraction by roots associated with evapotranspiration restricted the wetting front penetration after 1 to 3 h of rainfall. Copyright © 2006 John Wiley & Sons, Ltd. 相似文献
197.
我们研究中国西部某盆地一些气田时,发现地震子波旁瓣在很大程度上影响着薄砂体的识别和解释。该区目的层段H段地层中,辫状河流相薄砂岩储层与湖相泥岩成互层。在地震资料上,来自H段地层顶面的强波谷总跟随一个弱波峰(源于子波旁瓣),此弱波峰与薄砂体顶面反射波峰相互叠合,掩盖了砂体反射,造成了薄砂体识别和解释的困难。为此,我们设计了一套正演模型,利用不同频率的零相位雷克子波合成人工地震剖面,来研究砂体位置、频率与地震子波旁瓣效应的关系,以及振幅类属性与薄砂体关系。提出处于高位置的砂体反射形态清晰,易于识别解释;认为通过对比不同带通滤波处理后的地震资料,可很好辨别旁瓣效应和砂体反射;同时发现振幅类属性可作为薄砂体识别表征参数,可有效合理地描述砂体分布范围和特征。 相似文献
198.
通过对广东西部沿海高速公路路面基层现场压实度检测过程的详细观察,并在不同的条件下反复进行试验、分析比较,找出了影响压实度现场检测精度的主要非理想因素,即标准砂密度、试样含水量和路面基层平整度,并提出了解决方法。 相似文献
199.
软基固结度时空变化规律的研究对解决高速公路软基沉降预测问题具有十分重要的意义。本文在讨论固结度时空变化的实质与计算方法的基础上,结合京珠高速公路广珠段某软基试验资料,初步探讨了高速公路砂井地基固结度的时空变化规律,特别提出固结的滞后效应及其对固结沉降曲线形态的影响,并简要讨论了砂井直径和间距对固结度的影响,认为软基沉降中地下水微观动力学过程的研究将有助于揭示固结度时空变化规律。 相似文献
200.