全文获取类型
收费全文 | 39083篇 |
免费 | 3208篇 |
国内免费 | 4770篇 |
专业分类
测绘学 | 2899篇 |
大气科学 | 5025篇 |
地球物理 | 8082篇 |
地质学 | 19104篇 |
海洋学 | 3307篇 |
天文学 | 2274篇 |
综合类 | 3499篇 |
自然地理 | 2871篇 |
出版年
2024年 | 89篇 |
2023年 | 268篇 |
2022年 | 761篇 |
2021年 | 915篇 |
2020年 | 738篇 |
2019年 | 843篇 |
2018年 | 5426篇 |
2017年 | 4639篇 |
2016年 | 3343篇 |
2015年 | 1003篇 |
2014年 | 1059篇 |
2013年 | 1029篇 |
2012年 | 1902篇 |
2011年 | 3603篇 |
2010年 | 2894篇 |
2009年 | 3151篇 |
2008年 | 2665篇 |
2007年 | 3062篇 |
2006年 | 722篇 |
2005年 | 832篇 |
2004年 | 866篇 |
2003年 | 851篇 |
2002年 | 783篇 |
2001年 | 560篇 |
2000年 | 505篇 |
1999年 | 629篇 |
1998年 | 523篇 |
1997年 | 502篇 |
1996年 | 475篇 |
1995年 | 369篇 |
1994年 | 362篇 |
1993年 | 314篇 |
1992年 | 281篇 |
1991年 | 225篇 |
1990年 | 172篇 |
1989年 | 154篇 |
1988年 | 126篇 |
1987年 | 61篇 |
1986年 | 72篇 |
1985年 | 46篇 |
1984年 | 37篇 |
1983年 | 22篇 |
1982年 | 40篇 |
1981年 | 39篇 |
1980年 | 37篇 |
1979年 | 14篇 |
1978年 | 8篇 |
1977年 | 6篇 |
1976年 | 10篇 |
1958年 | 14篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
991.
Fine-grained marine sediments containing large undissolved gas bubbles are widely distributed around the world. Presence of the bubbles could degrade the undrained shear strength (su ) of the soil, when the gas pressure ug is relatively high as compared with the effective stress in the saturated soil matrix. Meanwhile, the addition of bubbles may also increase su when the difference between ug and pore water pressure uw becomes smaller than the water entry value, causing partial water drainage from the saturated matrix into the bubbles (bubble flooding) during globally undrained shearing. A new constitutive model for describing the two competing effects on the stress-strain relationship of fine-grained gassy soil is proposed within the framework of critical state soil mechanics. The gassy soil is considered as a three-phase composite material with compressible cavities, which allows water entry from the saturated matrix. Bubble flooding is modelled by introducing an additional positive volumetric strain increment of the saturated clay matrix, which is dependent on the difference between pore gas and pore water pressure based on experimental observations. A modified hardening law based on that of the modified Cam clay model is employed, which in conjunction with the expression for bubble flooding, can describe both the detrimental and beneficial effects of gas bubbles on soil strength and plastic hardening in shear. Only two extra parameters in addition to those in the modified Cam clay model are used. It is shown that the key features of the stress-strain relationship of three fine-grained gassy soils can be reproduced satisfactorily. 相似文献
992.
Frequency analysis of nonstationary annual maximum flood series using the time‐varying two‐component mixture distributions
下载免费PDF全文
![点击此处可从《水文研究》网站下载免费的PDF全文](/ch/ext_images/free.gif)
The most popular practice for analysing nonstationarity of flood series is to use a fixed single‐type probability distribution incorporated with the time‐varying moments. However, the type of probability distribution could be both complex because of distinct flood populations and time‐varying under changing environments. To allow the investigation of this complex nature, the time‐varying two‐component mixture distributions (TTMD) method is proposed in this study by considering the time variations of not only the moments of its component distributions but also the weighting coefficients. Having identified the existence of mixed flood populations based on circular statistics, the proposed TTMD was applied to model the annual maximum flood series of two stations in the Weihe River basin, with the model parameters calibrated by the meta‐heuristic maximum likelihood method. The performance of TTMD was evaluated by different diagnostic plots and indexes and compared with stationary single‐type distributions, stationary mixture distributions and time‐varying single‐type distributions. The results highlighted the advantages of TTMD with physically‐based covariates for both stations. Besides, the optimal TTMD models were considered to be capable of settling the issue of nonstationarity and capturing the mixed flood populations satisfactorily. Copyright © 2016 John Wiley & Sons, Ltd. 相似文献
993.
Xiaochun Liu Yi Chen Wei‐ Wang Mengmeng Xia Juan Hu Yibing Li Daogong Hu Biao Song 《Journal of Metamorphic Geology》2021,39(1):101-132
High‐P (HP) eclogite and associated garnet–omphacite granulite have recently been discovered in the Mulantou area, northeastern Hainan Island, South China. These rocks consist mainly of garnet, omphacite, hornblende, quartz and rutile/ilmenite, with or without zoisite and plagioclase. Textural relationships, mineral compositions and thermobarometric calculations demonstrate that the eclogite and garnet–omphacite granulite share the same three‐stage metamorphic evolution, with prograde, peak and retrograde P?T conditions of 620–680°C and 8.7–11.1 kbar, 820–860°C and 17.0–18.2 kbar, and 700–730°C and 7.1–8.5 kbar respectively. Sensitive high‐resolution ion microprobe U–Pb zircon dating, coupled with the identification of mineral inclusions in zircon, reveals the formation of mafic protoliths before 355 Ma, prograde metamorphism at c. 340–330 Ma, peak to retrograde metamorphism at c. 310–300 Ma, and subsequent pegmatite intrusion at 295 Ma. Trace element geochemistry shows that most of the rocks have a MORB affinity, with initial εNd values of +2.4 to +6.7. As with similar transitional eclogite–HP granulite facies rocks in the thickened root in the European Variscan orogen, the occurrence of relatively high P?T metamorphic rocks of oceanic origin in northeastern Hainan Island suggests Carboniferous oceanic subduction leading to collision of the Hainan continental block, or at least part of it, with the South China Block in the eastern Palaeo‐Tethyan tectonic domain. 相似文献
994.
995.
In‐situ analysis of trace elements and Sr–Pb isotopes of K‐feldspars from Tongshankou Cu–Mo deposit,SE Hubei Province,China: Insights into early potassic alteration of the porphyry mineralization system
下载免费PDF全文
![点击此处可从《地学学报》网站下载免费的PDF全文](/ch/ext_images/free.gif)
In the Tongshankou porphyry deposit (SE Hubei Province, South China), three types of K‐feldspars are recognized: (I) the phenocryst type in the porphyry that crystalized during the magmatic stage, (II) the megacryst type and (III) the vein type in the altered porphyry and orebody that was produced by hydrothermal fluids. A detailed in‐situ analysis of trace elements and Sr–Pb isotopes was carried out on K‐feldspars in an attempt to unravel their formation processes and to trace the element sources during potassic alteration. The Type III K‐feldspars show lower Sr contents and Sr‐isotope ratios but higher Pb contents and Pb‐isotope ratios than the Type I and II K‐feldspars, possibly reflecting a contribution from the country carbonate rocks with less radiogenic Sr but more radiogenic Pb sources, and indicate that the ore‐forming fluids and materials may have been partially derived from external sources such as the host sedimentary rocks during the early potassic alteration stage. 相似文献
996.
Ranganath Navalgund 《Journal of the Indian Society of Remote Sensing》2018,46(3):483-489
997.
998.
999.
Natural Hazards - Meteorological disasters (MDs), including drought, flood, hail, low temperature and frost (LTF), are causing severe damage to human life and economic development in China. Mapping... 相似文献
1000.
Recent perspectives on temporary river ecology 总被引:1,自引:0,他引:1