全文获取类型
收费全文 | 26708篇 |
免费 | 4708篇 |
国内免费 | 6166篇 |
专业分类
测绘学 | 1440篇 |
大气科学 | 5407篇 |
地球物理 | 7388篇 |
地质学 | 13014篇 |
海洋学 | 3401篇 |
天文学 | 1242篇 |
综合类 | 2551篇 |
自然地理 | 3139篇 |
出版年
2024年 | 140篇 |
2023年 | 478篇 |
2022年 | 1152篇 |
2021年 | 1334篇 |
2020年 | 1110篇 |
2019年 | 1243篇 |
2018年 | 1413篇 |
2017年 | 1292篇 |
2016年 | 1483篇 |
2015年 | 1221篇 |
2014年 | 1518篇 |
2013年 | 1545篇 |
2012年 | 1380篇 |
2011年 | 1493篇 |
2010年 | 1545篇 |
2009年 | 1428篇 |
2008年 | 1241篇 |
2007年 | 1304篇 |
2006年 | 984篇 |
2005年 | 892篇 |
2004年 | 718篇 |
2003年 | 785篇 |
2002年 | 776篇 |
2001年 | 724篇 |
2000年 | 885篇 |
1999年 | 1258篇 |
1998年 | 1053篇 |
1997年 | 1060篇 |
1996年 | 979篇 |
1995年 | 863篇 |
1994年 | 831篇 |
1993年 | 690篇 |
1992年 | 565篇 |
1991年 | 411篇 |
1990年 | 326篇 |
1989年 | 259篇 |
1988年 | 276篇 |
1987年 | 151篇 |
1986年 | 155篇 |
1985年 | 108篇 |
1984年 | 92篇 |
1983年 | 87篇 |
1982年 | 85篇 |
1981年 | 62篇 |
1980年 | 53篇 |
1979年 | 46篇 |
1978年 | 19篇 |
1977年 | 10篇 |
1975年 | 10篇 |
1958年 | 20篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
孔隙液体的可压缩性对非饱和孔隙介质弹塑性分析的影响 总被引:2,自引:0,他引:2
首先在多相孔隙介质的混合物理论框架内探讨了如何利用固-液两相孔隙介质模型描述介质的非饱和问题。其次,讨论了一个实用的描述孔隙液体可压缩性的方法,并建立了小变形条件下固-液两相孔隙介质的弹塑性本构方程。最后,采用俞茂宏统一强度理论,针对一个具体的结构例子,分析了孔隙液体的可压缩性对结构弹塑性分析的影响。 相似文献
992.
本研究通过对长江口现代潮滩6个柱状样沉积物的粒度精细分析,试图提取研究区潮滩沉积相识别的粒度敏感指标,并通过对长江三角洲南部平原一个全新世钻孔(SL67孔)潮滩沉积物的粒度研究,检验现代沉积粒度敏感指标在全新世钻孔潮滩沉积相识别中的应用。研究显示,长江口现代高潮滩和中潮滩的黏性颗粒(8μm)和粗粉砂(32~63μm)含量差异显著,可以成为区分高潮滩和中潮滩的敏感组分;在开敞型的中、低潮滩之间,砂(63μm)和细粉砂含量(8~32μm)也差异明显。长江口现代潮滩沉积物的粒度频率曲线及众数值也可以有效协助区分高潮滩、中潮滩和低潮滩沉积物。利用上述粒度敏感组分和众数指标检验SL67孔全新世早中期在海平面波动控制下的潮滩演替过程,并揭示了7.5 cal ka BP前后的一次海平面加速上升事件。 相似文献
993.
本文采用T-矩阵方法从理论计算的角度分析了在X、C和S波段下不同轴此,大小和相态的扁椭球降水粒子的差反射率因子ZDR,对为三种波长的双偏振雷达实际动用提出了更合理的解释,得到若干有用的结论,文中还对T-矩阵方法的准确性进行地评价。 相似文献
994.
选取阿尔山气象站1981—2015年冷季(10月—次年4月)气象资料,利用滑动平均、线性倾向估计和Mann-Kendall等方法,对年最大积雪深度、积雪日数、气温和降水量进行分析。结果表明,阿尔山地区年最大积雪深度主要发生在1月至3月,其中2月份概率最大,达50%;34 a内最大积雪深度呈上升趋势(2.77 cm/10a),年平均增加0.98%,且年最大积雪深度在1998年发生了突变,即在1998年之前增长缓慢,在2000年以后上升趋势显著。积雪日数的统计分析表明,初始积雪日数和有效积雪日数呈现略微减少趋势,而稳定积雪日数有微弱的增加趋势;通常初始积雪日数比有效积雪日数大30天左右。年最大积雪深度与稳定积雪时期的降水量、积雪日数、日照时数有显著的相关性,相关系数分别为0.647、0.515、0.584,但与稳定积雪时期的气温没有明显的相关性。在全球变暖的大环境下,积雪深度随着降水量和日照时数的增加而增加,且积雪深度受降水量的影响大于日照时数的影响。 相似文献
995.
Wenzhi Zhao Shuichang Zhang Feiyu Wang Jianping Chen Zhongyao Xiao Fuqing Song 《Organic Geochemistry》2005,36(12):1602-1616
Oil and gas exploration in eastern Tarim Basin, NW China has been successful in recent years, with several commercial gas accumulations being discovered in a thermally mature to over-mature region. The Yingnan2 (YN2) gas field, situated in the Yingnan structure of the Yingjisu Depression, produces gases that are relatively enriched in nitrogen and C2+ alkanes. The δ13C1 (−38.6‰ to −36.2‰) and δ13C2 values (−30.9‰ to −34.7‰) of these gases are characteristic of marine sourced gases with relatively high maturity levels. The distributions of biomarkers in the associated condensates suggest close affinities with the Cambrian–Lower Ordovician source rocks which, in the Yingjisu Sag, are currently over-mature (with 3–4%Ro). Burial and thermal maturity modeling results indicate that paleo-temperatures of the Cambrian–Lower Ordovician source rocks had increased from 90 to 210 °C during the late Caledonian orogeny (458–438 Ma), due to rapid subsidence and sediment loading. By the end of Ordovician, hydrocarbon potential in these source rocks had been largely exhausted. The homogenization temperatures of hydrocarbon fluid inclusions identified from the Jurassic reservoirs of the YN2 gas field suggest a hydrocarbon emplacement time as recent as about 10 Ma, when the maturity levels of Middle–Lower Jurassic source rocks in the study area were too low (<0.7%Ro) to form a large quantity of oil and gas. The presence of abundant diamondoid hydrocarbons in the associated condensates and the relatively heavy isotopic values of the oils indicate that the gases were derived from thermal cracking of early-formed oils. Estimation from the stable carbon isotope ratios of gaseous alkanes suggests that the gases may have been formed at temperatures well above 190 °C. Thus, the oil and gas accumulation history in the study area can be reconstructed as follows: (1) during the late Caledonian orogeny, the Cambrian–Lower Ordovician marine source rocks had gone through the peak oil, wet gas and dry gas generation stages, with the generated oil and gas migrating upwards along faults and fractures to form early oil and gas accumulations in the Middle–Upper Ordovician and Silurian sandstone reservoirs; (2) since the late Yanshanian orogeny, the early oil accumulations have been buried deeper and oil has undergone thermal cracking to form gas; (3) during the late Himalayan orogeny, the seals for the deep reservoirs were breached; and the gas and condensates migrated upward and eventually accumulating in the relatively shallow Jurassic reservoirs. 相似文献
996.
利用2010年7月22日苏皖中尺度对流复合体(MCC)数值模拟输出结果,考察了模式对MCC的模拟能力,并对模拟结果做了动力和热力诊断分析,以揭示盛夏江淮下游MCC的特征。结果表明:1) 三重嵌套网格距为3.3 km的区域WRF模式的模拟效果较佳,结果与实况一致,并可利用模拟降水的范围及强度来确定MCC的位置及演变。2) 此MCC维持约10 h,其南北不对称,并随西太平洋副热带高压西伸北抬而随之北抬。MCC核心区对流层低层有水汽丰沛的入流,并有强辐合区,呈对流不稳定层结;中层有深厚的强上升运动,并因凝结潜热大量释放呈中性层结;高层则有出流;MCC核心区对流降水非常强。3) 在垂直剖面上,该核心区散度存在中低层辐合、高层辐散的柱状结构,此配置有利于强对流维持和加强,中低层以上有深厚的强上升气流柱,这些都是MCC核心区存在强对流的标志。该MCC的螺旋结构表明其中的强对流高度有组织。 相似文献
997.
Shengrui Zhang Qinghai Xu Anne Birgitte Nielsen Hui Chen Yuecong Li Manyue Li Lingyun Hun Jianyong Li 《Boreas: An International Journal of Quaternary Research》2012,41(4):602-613
The Qaidam Basin is one of the most sensitive areas to climate change in China, owing to its unique geographical position and ecological condition. In this study, 32 surface‐soil pollen samples were collected to reveal the relationship between modern pollen assemblages, vegetation and precipitation in the eastern region of the Qaidam Basin. The results show that Chenopodiaceae (3.8–87%, average 48%), Artemisia (1.7–64.2%, average 17.5%) and Ephedra (0–90%, average 16.3%) are the dominant pollen types in all samples, and that different pollen assemblages correspond to different vegetation types. DCA and CCA of major pollen types demonstrate that precipitation is an important factor in the control of the distribution of vegetation in the study area. The content and concentration of the three major pollen types (Artemisia, Chenopodiaceae and Ephedra) change with the mean annual precipitation, and the optimum mean annual precipitation for Ephedra, Chenopodiaceae and Artemisia is <80, 80–200 and >160 mm, respectively. Correlation analysis between the variation in grain size of the three major pollen types and the main environmental variables shows that the grain size of the three pollen types is positively correlated with precipitation in the Qaidam Basin. The results confirm that precipitation is the most important environmental factor in the Qaidam Basin, and that it has an important effect on pollen grain size in the study area. 相似文献
998.
The increasing natural disasters, especially floods during the last quarter century, are raising the economic losses in Taiwan.
The most severe hazard in Taiwan is flooding induced by typhoons and storms in summer and autumn. By comparing the rivers
around the world, the ones in Taiwan have the steepest slopes, the largest discharge per unit drainage area, and the shortest
time of concentrations. Rapid urbanization without proper land uses managements usually worsen the flood problems. Consequently,
flood hazards mitigation has become the most essential task for Taiwan to deal with. Although the government keeps improving
flood defense structures, the flood damage grows continuously. In this article, possible flood mitigation strategies are identified
for coping with complex environmental and social decisions with flood risk involved. 相似文献
999.
30年来东南红壤丘陵区土壤侵蚀度时空演变研究——以长汀县为例 总被引:7,自引:0,他引:7
土壤侵蚀动态监测是侵蚀研究体系中的重要内容,当前国内大部分研究直接引用国外监测模型,不仅在因子选取以及模型通用性上存在不足,而且对区域土壤的潜在性侵蚀危险考虑不够。因此,研究提出土壤侵蚀度的概念,作为区域土壤实际侵蚀强度与潜在侵蚀危险的综合表征,并基于人地关系系统,通过多指标的量化与建模进行土壤侵蚀度测算,从而分析福建省长汀县土壤侵蚀时空演变规律。结果表明,长汀县土壤侵蚀分布存在集中连片的特点,汀中地区的侵蚀度最高,并向周边逐渐衰减。同时,侵蚀度在变化方向上虽然存在不确定性,但总体侵蚀状况近30a来得到了明显的改善,安全区的比重从1975年的3.87%增长至2006年的56.38%,相比而言,极度侵蚀危险区的比重仅从1975年的12.3%,减少至2006年的6.66%,改善幅度较其他等级土壤有所差距,这与区域在立地条件、经济发展水平以及政策导向上的差异密切相关。研究结果说明选取侵蚀影响因子具有一定的适宜性,也证明土壤侵蚀度可以有效表征土壤侵蚀时空演变趋势。 相似文献
1000.
A surface network based method for studying urban hierarchies by night time light remote sensing data 总被引:1,自引:0,他引:1
Bin Wu Qiusheng Wu Zuoqi Chen Jianping Wu 《International journal of geographical information science》2019,33(7):1377-1398
Urban hierarchies are closely related to economic growth, urban planning and sustainable urban development. Due to the limited availability of reliable statistical data at fine scales, most existing studies on urban hierarchy characterization failed to capture the detailed urban spatial structure information. Previous studies have demonstrated that night time light data are correlated with many urban socio-economic indicators and hence can be used to characterize urban hierarchies. This paper presents a novel method for studying urban hierarchies from night time light data. Night time light data were first conceptualized as continuous mathematical surfaces, termed night time light surfaces. From the morphology of these surfaces the corresponding surface networks were derived. Hereafter, a night time light intensity (NTLI) graph was defined to describe the morphology of the surface network. Then, structural similarity between the night time light surfaces of any two different cities was calculated via a threshold-based maximum common induced graph searching algorithm. Finally, urban hierarchies were defined on the basis of the structural similarities between different cities. Using the 2015 annual NPP-VIIRS night time light data, the urban hierarchies of 32 major cities in China were successfully examined. The results are highly consistent with the reference urban hierarchies. 相似文献