全文获取类型
收费全文 | 2753篇 |
免费 | 491篇 |
国内免费 | 688篇 |
专业分类
测绘学 | 169篇 |
大气科学 | 679篇 |
地球物理 | 526篇 |
地质学 | 1784篇 |
海洋学 | 232篇 |
天文学 | 21篇 |
综合类 | 189篇 |
自然地理 | 332篇 |
出版年
2024年 | 6篇 |
2023年 | 34篇 |
2022年 | 87篇 |
2021年 | 114篇 |
2020年 | 101篇 |
2019年 | 122篇 |
2018年 | 108篇 |
2017年 | 94篇 |
2016年 | 159篇 |
2015年 | 156篇 |
2014年 | 203篇 |
2013年 | 163篇 |
2012年 | 200篇 |
2011年 | 222篇 |
2010年 | 180篇 |
2009年 | 216篇 |
2008年 | 180篇 |
2007年 | 256篇 |
2006年 | 210篇 |
2005年 | 142篇 |
2004年 | 146篇 |
2003年 | 118篇 |
2002年 | 110篇 |
2001年 | 81篇 |
2000年 | 78篇 |
1999年 | 66篇 |
1998年 | 45篇 |
1997年 | 63篇 |
1996年 | 49篇 |
1995年 | 55篇 |
1994年 | 41篇 |
1993年 | 34篇 |
1992年 | 38篇 |
1991年 | 20篇 |
1990年 | 13篇 |
1989年 | 7篇 |
1988年 | 7篇 |
1987年 | 2篇 |
1986年 | 1篇 |
1985年 | 4篇 |
1978年 | 1篇 |
排序方式: 共有3932条查询结果,搜索用时 62 毫秒
71.
72.
73.
河南省洛宁县龙门店银铅矿地质特征及找矿前景 总被引:1,自引:0,他引:1
文章总结了龙门店矿区地质找矿、生产勘探和综合研究等地质成果,探讨了控矿因素和矿床成因;根据瞬变电磁法物探成果及成矿条件综合分析,推断K4、K6、K9、K11、及K1的200m标高左右存在成矿有利地段,预测本区远景银金属量近千吨,指出了本区具有中大型银铅多金属矿床找矿前景,深部找矿潜力巨大。 相似文献
74.
文章在1∶5万土壤地球化学测量成果基础上,对内蒙古苏尼特左旗巴彦洪格尔地区的地球化学参数进行计算和分析,对区内元素分布特征、元素相关性及组合特点、主要综合异常特征进行了研究,结果表明:区内Ag、Pb、Sn、W含量较高,分异性又强,所形成异常规模较大,强度较高,具有较大找矿价值。结合对主要综合异常的分析,划分出古尔班呼都格金、银找矿远景区,苏吉敖包钨、铅、锌找矿远景区,哈尔德勒铅、锌找矿远景区,查干诺尔钼找矿远景区。 相似文献
75.
76.
77.
78.
Carissa A. Raymond Luke A. McGuire Ann M. Youberg Dennis M. Staley Jason W. Kean 《地球表面变化过程与地形》2020,45(6):1349-1360
Wildfire significantly alters the hydrologic properties of a burned area, leading to increases in overland flow, erosion, and the potential for runoff-generated debris flows. The initiation of debris flows in recently burned areas is well characterized by rainfall intensity-duration (ID) thresholds. However, there is currently a paucity of data quantifying the rainfall intensities required to trigger post-wildfire debris flows, which limits our understanding of how and why rainfall ID thresholds vary in different climatic and geologic settings. In this study, we monitored debris-flow activity following the Pinal Fire in central Arizona, which differs from both a climatic and hydrogeomorphic perspective from other regions in the western United States where ID thresholds for post-wildfire debris flows are well established, namely the Transverse Ranges of southern California. Since the peak rainfall intensity within a rainstorm may exceed the rainfall intensity required to trigger a debris flow, the development of robust rainfall ID thresholds requires knowledge of the timing of debris flows within rainstorms. Existing post-wildfire debris-flow studies in Arizona only constrain the peak rainfall intensity within debris-flow-producing storms, which may far exceed the intensity that actually triggered the observed debris flow. In this study, we used pressure transducers within five burned drainage basins to constrain the timing of debris flows within rainstorms. Rainfall ID thresholds derived here from triggering rainfall intensities are, on average, 22 mm h−1 lower than ID thresholds derived under the assumption that the triggering intensity is equal to the maximum rainfall intensity recorded during a rainstorm. We then use a hydrologic model to demonstrate that the magnitude of the 15-min rainfall ID threshold at the Pinal Fire site is associated with the rainfall intensity required to exceed a recently proposed dimensionless discharge threshold for debris-flow initiation. Model results further suggest that previously observed differences in regional ID thresholds between Arizona and the San Gabriel Mountains of southern California may be attributed, in large part, to differences in the hydraulic properties of burned soils. © 2019 John Wiley & Sons, Ltd. 相似文献
79.
The threshold of motion of non-fragmented mollusc shells was studied for the first time under oscillatory flow. In this regard, flume experiments were used to investigate the threshold of motion of three bivalve and three gastropod species, two typical mollusc classes of coastal coquina deposits. The sieve diameters ranged from 2·0 to 15·9 mm. These experiments were performed on a flat-bottom setup under regular non-breaking waves (swell) produced by a flap-type wave generator. The critical Shields values for each species of mollusc were plotted against the sieve and nominal diameter. Moreover, the dimensionless Corey shape factor of the shells was evaluated in order to investigate the effect of mollusc shell shapes on the threshold of motion. According to their critical Shields parameter, the mollusc threshold data under oscillatory flow present smaller values than the siliciclastic sediments when considering their sieve diameter. In addition, the mollusc datasets are below the empirical curves built from siliciclastic grain data under current and waves. When considering the nominal diameter, the critical Shields parameter increases and the mollusc data are closer to siliciclastic sediments. Bivalves, which have a flat-concave shape (form factor: 0·27 to 0·37), have a higher critical Shields parameter for smaller particles and more uniform datasets than the gastropod scattered data, which have a rounded shape (form factor: 0·58 to 0·62) and have varied morphologies (ellipsoidal, conical and cubic). The comparison between previous current-driven threshold data of bioclastic sediment motion and the data of mollusc whole shells under oscillatory flow shows a fair correlation on the Shields diagram, in which all datasets are below the mean empirical curves for siliciclastic sediments. These findings indicate that the shape effect on the transport initiation is predominant for smaller shells. The use of the nominal diameter is satisfactory to improve the bioclastic and siliciclastic data correlation. 相似文献
80.
实验室和研究人员所使用的碳、氮同位素标准物质一般由国际原子能机构(IAEA)获得,然而近年来,随着碳氮同位素在实验室质量监控、方法评价、仪器校准等方面的广泛应用,市场需求量不断增加, IAEA研制的碳、氮同位素标准物质的种类与数量逐渐不能满足科学研究快速发展的需求。我国急需研制适应当今分析技术水平的有机质碳氮同位素国家标准物质用以进行质量监控、方法评价、仪器校准。为保证量值传递精度,本文研制了4个有机化学物质的碳氮稳定同位素标准物质,其中3个为尿素样品,1个为L-谷氨酸。经检验4种标准物质的均匀性通过F值检验,标准物质的δ~(13)C和δ~(15)N值经过一年的稳定性检验,特征量值变化在测量方法允许的不确定度范围内,由此判定δ~(13)C和δ~(15)N值稳定性良好。由包括研制单位实验室在内的12家实验室协同定值,采用高温燃烧-气体同位素质谱法测定了δ~(13)C和δ~(15)N值,系列标准物质δ~(13)C和δ~(15)N认定值区间呈梯度分布,δ~(13)C值为-40‰~0‰,δ~(15)N值为-10‰~30‰,涵盖了我国天然样品中有机质碳氮稳定同位素组成范围;研制的系列标准物质δ~(13)C的定值扩展不确定度不大于0.08‰,δ~(15)N的定值扩展不确定度不大于0.09‰,定值水平与国际标准物质相当。该系列标准物质已被国家质检总局批准为国家一级标准物质,批准号为GBW04494~GBW04497。可被用于地质、生态、环境等多种样品δ~(13)C和δ~(15)N比值测定时的分析监控、仪器校准、方法评价、质量保证和质量监控。 相似文献