全文获取类型
收费全文 | 2641篇 |
免费 | 101篇 |
国内免费 | 27篇 |
专业分类
测绘学 | 85篇 |
大气科学 | 398篇 |
地球物理 | 613篇 |
地质学 | 1014篇 |
海洋学 | 86篇 |
天文学 | 460篇 |
综合类 | 6篇 |
自然地理 | 107篇 |
出版年
2021年 | 39篇 |
2020年 | 34篇 |
2019年 | 28篇 |
2018年 | 67篇 |
2017年 | 65篇 |
2016年 | 107篇 |
2015年 | 91篇 |
2014年 | 104篇 |
2013年 | 159篇 |
2012年 | 101篇 |
2011年 | 97篇 |
2010年 | 103篇 |
2009年 | 120篇 |
2008年 | 100篇 |
2007年 | 91篇 |
2006年 | 94篇 |
2005年 | 66篇 |
2004年 | 50篇 |
2003年 | 61篇 |
2002年 | 63篇 |
2001年 | 54篇 |
2000年 | 57篇 |
1999年 | 43篇 |
1998年 | 36篇 |
1997年 | 47篇 |
1996年 | 37篇 |
1995年 | 42篇 |
1994年 | 43篇 |
1993年 | 28篇 |
1992年 | 18篇 |
1991年 | 40篇 |
1990年 | 28篇 |
1988年 | 24篇 |
1987年 | 29篇 |
1986年 | 20篇 |
1985年 | 19篇 |
1984年 | 30篇 |
1983年 | 40篇 |
1982年 | 19篇 |
1981年 | 30篇 |
1980年 | 28篇 |
1979年 | 22篇 |
1978年 | 33篇 |
1977年 | 19篇 |
1976年 | 28篇 |
1975年 | 23篇 |
1974年 | 25篇 |
1973年 | 30篇 |
1972年 | 20篇 |
1970年 | 16篇 |
排序方式: 共有2769条查询结果,搜索用时 15 毫秒
91.
92.
Understanding the degree of sea/saltwater intrusion in coastal regions is of great significance to treating the intrusion and improving the environment. Based on the character analysis of the sea/saltwater intrusion, five factors were selected in the fuzzy-synthetical evaluation approach to form the index system, so as to evaluate the degree of sea/saltwater intrusion in southern Laizhou Bay. The results show that the sea/saltwater intrusion is stronger in the middle and northern areas and weaker on the sides and in southern area; currently, the intrusion is relatively serious, and the intrusion area has covered about 68.2% of the areas under study, among which the heavily intruded area is over 50%. Based on the factors analysis of the occurrence and development of sea/saltwater intrusion, the thesis proposes treatment measures. 相似文献
93.
Falk Händel Gaisheng Liu Johann Fank Franz Friedl Rudolf Liedl Peter Dietrich 《Hydrogeology Journal》2016,24(8):2079-2091
An approach to establish the recharge component of managed aquifer recharge (MAR) has recently been proposed that uses small-diameter shallow wells installed using relatively inexpensive drilling methods such as direct push. As part of further development of that approach, a generalized procedure is presented for a technical and economic assessment of the approach’s potential in comparison to other systems. Following this procedure, the use of small-diameter wells was evaluated both experimentally and numerically for a site located in southern Styria, Austria. MAR is currently done at the site using a horizontal pipe infiltration system, and system expansion has been proposed with a target rate of 12 l/s using small-diameter wells as one possible option. A short-duration single-well field recharge experiment (recharge rate 1.3–3.5 l/s) was performed (recharge by gravity only). Numerical modeling of the injection test was used to estimate hydraulic conductivity (K). Quasi-steady-state, single-well recharge simulations for different locations, as well as a long-term transient simulation, were performed using the K value calibrated from the field injection test. Results indicate that a recharge capacity of 4.1 l/s was achievable with a maximum head rise of 0.2 m at the injection well. Finally, simulations were performed for three different well fields (4, 6 and 8 wells, respectively) designed to infiltrate a target rate of 12 l/s. The experimental and numerical assessments, supported by a cost analysis of the small-diameter wells, indicate that the small-diameter wells are a viable, cost-effective recharge approach at this and other similar sites. 相似文献
94.
In (semi-)arid regions, available water resources are scarce and groundwater resources are often overused. Therefore, the option to increase available water resources by managed aquifer recharge (MAR) via infiltration of captured surface runoff was investigated for two basins in northern Jordan. This study evaluated the general suitability of catchments to generate sufficient runoff and tried to identify promising sites to harvest and infiltrate the runoff into the aquifer for later recovery. Large sets of available data were used to create regional thematic maps, which were then combined to constraint maps using Boolean logic and to create suitability maps using weighted linear combination. This approach might serve as a blueprint which could be adapted and applied to similar regions. The evaluation showed that non-committed source water availability is the most restricting factor for successful water harvesting in regions with <200 mm/a rainfall. Experiences with existing structures showed that sediment loads of runoff are high. Therefore, the effectiveness of any existing MAR scheme will decrease rapidly to the point where it results in an overall negative impact due to increased evaporation if maintenance is not undertaken. It is recommended to improve system operation and maintenance, as well as monitoring, in order to achieve a better and constant effectiveness of the infiltration activities. 相似文献
95.
96.
97.
98.
Zircon M127 – A Homogeneous Reference Material for SIMS U–Pb Geochronology Combined with Hafnium,Oxygen and,Potentially, Lithium Isotope Analysis
下载免费PDF全文
![点击此处可从《Geostandards and Geoanalytical Research》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Lutz Nasdala Fernando Corfu John W. Valley Michael J. Spicuzza Fu‐Yuan Wu Qiu‐Li Li Yue‐Heng Yang Chris Fisher Carsten Münker Allen K. Kennedy Peter W. Reiners Andreas Kronz Michael Wiedenbeck Richard Wirth Chutimun Chanmuang Manuela Zeug Tamás Váczi Nicholas Norberg Tobias Häger Alfred Kröner Wolfgang Hofmeister 《Geostandards and Geoanalytical Research》2016,40(4):457-475
In this article, we document a detailed analytical characterisation of zircon M127, a homogeneous 12.7 carat gemstone from Ratnapura, Sri Lanka. Zircon M127 has TIMS‐determined mean U–Pb radiogenic isotopic ratios of 0.084743 ± 0.000027 for 206Pb/238U and 0.67676 ± 0.00023 for 207Pb/235U (weighted means, 2s uncertainties). Its 206Pb/238U age of 524.36 ± 0.16 Ma (95% confidence uncertainty) is concordant within the uncertainties of decay constants. The δ18O value (determined by laser fluorination) is 8.26 ± 0.06‰ VSMOW (2s), and the mean 176Hf/177Hf ratio (determined by solution ICP‐MS) is 0.282396 ± 0.000004 (2s). The SIMS‐determined δ7Li value is ?0.6 ± 0.9‰ (2s), with a mean mass fraction of 1.0 ± 0.1 μg g?1 Li (2s). Zircon M127 contains ~ 923 μg g?1 U. The moderate degree of radiation damage corresponds well with the time‐integrated self‐irradiation dose of 1.82 × 1018 alpha events per gram. This observation, and the (U–Th)/He age of 426 ± 7 Ma (2s), which is typical of unheated Sri Lankan zircon, enable us to exclude any thermal treatment. Zircon M127 is proposed as a reference material for the determination of zircon U–Pb ages by means of SIMS in combination with hafnium and stable isotope (oxygen and potentially also lithium) determination. 相似文献
99.
100.