全文获取类型
收费全文 | 3896篇 |
免费 | 123篇 |
国内免费 | 38篇 |
专业分类
测绘学 | 91篇 |
大气科学 | 323篇 |
地球物理 | 751篇 |
地质学 | 1230篇 |
海洋学 | 393篇 |
天文学 | 864篇 |
综合类 | 17篇 |
自然地理 | 388篇 |
出版年
2021年 | 30篇 |
2020年 | 41篇 |
2019年 | 51篇 |
2018年 | 74篇 |
2017年 | 48篇 |
2016年 | 75篇 |
2015年 | 82篇 |
2014年 | 90篇 |
2013年 | 219篇 |
2012年 | 111篇 |
2011年 | 188篇 |
2010年 | 125篇 |
2009年 | 197篇 |
2008年 | 151篇 |
2007年 | 177篇 |
2006年 | 146篇 |
2005年 | 124篇 |
2004年 | 111篇 |
2003年 | 104篇 |
2002年 | 114篇 |
2001年 | 84篇 |
2000年 | 106篇 |
1999年 | 83篇 |
1998年 | 85篇 |
1997年 | 61篇 |
1996年 | 62篇 |
1995年 | 54篇 |
1994年 | 65篇 |
1993年 | 48篇 |
1992年 | 44篇 |
1991年 | 52篇 |
1990年 | 41篇 |
1989年 | 55篇 |
1988年 | 40篇 |
1987年 | 61篇 |
1986年 | 45篇 |
1985年 | 65篇 |
1984年 | 79篇 |
1983年 | 67篇 |
1982年 | 62篇 |
1981年 | 62篇 |
1980年 | 64篇 |
1979年 | 46篇 |
1978年 | 39篇 |
1977年 | 49篇 |
1976年 | 38篇 |
1975年 | 47篇 |
1974年 | 20篇 |
1973年 | 31篇 |
1972年 | 22篇 |
排序方式: 共有4057条查询结果,搜索用时 15 毫秒
971.
972.
973.
Volcanic ash (tephra) erupted from the frequently active Campi Flegrei volcano forms layers in many palaeoenvironmental archives across Italy and the Mediterranean. Proximal deposits of 50 of the post-15 ka eruptions have been thoroughly sampled and analysed to produce a complete database of glass compositions (>1900 analyses) to aid identification of these units. The deposits of individual eruptions are compositionally diverse and this variability is often greater than that observed between different units. Many of the tephra units do not have a unique glass chemistry, with compositionally similar tephra often erupted over long periods of time (1000s years). Thus, glass chemistry alone is not enough to robustly correlate most of the tephra from Campi Flegrei, especially in the last 10 kyrs. In order to reliably correlate the eruption units it is important to take into account the stratigraphy, chronology, magnitude, and dispersal of the eruptions, which has been collated to aid identification. An updated chronology is also presented, which was constrained using Bayesian analysis (OxCal) of published radiocarbon dates and 40Ar/39Ar ages. All the data presented can be employed to help correlate post-15 ka tephra units preserved in archaeological and Holocene palaeoenvironmental archives. The new database of proximal glass compositions has been used to correlate proximal volcanic deposits through to distal tephra layers in the Lago di Monticchio record (Wulf et al., 2004, Wulf et al., 2008) and these correlations provide information on eruption stratigraphy and the tempo of volcanism at Campi Flegrei. 相似文献
974.
Clemens Reimann David B. Smith Laurel G. Woodruff Belinda Flem 《Applied Geochemistry》2011,26(9-10):1623-1631
Analytical results for Pb-concentrations and isotopic ratios from ca. 150 samples of soil A horizon and ca. 145 samples of soil C horizon collected along a 4000-km east–west transect across the USA are presented. Lead concentrations along the transect show: (1) generally higher values in the soil A-horizon than the C-horizon (median 21 vs. 16.5 mg/kg), (2) an increase in the median value of the soil A-horizon for central to eastern USA (Missouri to Maryland) when compared to the western USA (California to Kansas) (median 26 vs. 20 mg/kg) and (3) a higher A/C ratio for the central to eastern USA (1.35 vs. 1.14). Lead isotopes show a distinct trend across the USA, with the highest 206Pb/207Pb ratios occurring in the centre (Missouri, median A-horizon: 1.245; C-horizon: 1.251) and the lowest at both coasts (e.g., California, median A-horizon: 1.195; C-horizon: 1.216). The soil C-horizon samples show generally higher 206Pb/207Pb ratios than the A-horizon (median C-horizon: 1.224; A-horizon: 1.219). The 206Pb/207Pb-isotope ratios in the soil A horizon show a correlation with the total feldspar content for the same 2500-km portion of the transect from east-central Colorado to the Atlantic coast that shows steadily increasing precipitation. No such correlation exists in the soil C horizon. The data demonstrate the importance of climate and weathering on both Pb-concentration and 206Pb/207Pb-isotope ratios in soil samples and natural shifts thereof in the soil profile during soil-forming processes. 相似文献
975.
Martin C. Smith Gregory R. Ruchti Amina Helmi Rosemary F. G. Wyse J. P. Fulbright K. C. Freeman J. F. Navarro G. M. Seabroke M. Steinmetz M. Williams O. Bienaymé J. Binney J. Bland-Hawthorn W. Dehnen B. K. Gibson G. Gilmore E. K. Grebel U. Munari Q. A. Parker R.-D. Scholz A. Siebert F. G. Watson T. Zwitter 《Monthly notices of the Royal Astronomical Society》2007,379(2):755-772
976.
Array processing-a new method to detect and correct errors on array resistivity logging tool measurements 总被引:2,自引:0,他引:2
References: 《中国海洋湖沼学报》2007,25(1):51-58
In recent years more and more multi-array logging tools, such as the array induction and the array lateralog, are applied in place of conventional logging tools resulting in increased resolution, better radial and vertical sounding capability and other features. Multi-array logging tools acquire several times more individual measurements than conventional logging tools. In addition to new information contained in these data, there is a certain redundancy among the measurements. The sum of the measurements actu- ally composes a large matrix. Providing the measurements are error-free, the elements of this matrix show certain consistencies. Taking advantage of these consistencies, an innovative method is developed to de- tect and correct errors in the array resistivity logging tool raw measurements, and evaluate the quality of the data. The method can be described in several steps. First, data consistency patterns are identified based on the physics of the measurements. Second, the measurements are compared against the consis- tency patterns for error and bad data detection. Third, the erroneous data are eliminated and the measure- ments are re-constructed according to the consistency patterns. Finally, the data quality is evaluated by comparing the raw measurements with the re-constructed measurements. The method can be applied to all array type logging tools, such as array induction tool and array resistivity tool. This paper describes the method and illustrates its application with the High Definition Lateral Log (HDLL, Baker Atlas) instru- ment. To demonstrate the efficiency of the method, several field examples are shown and discussed. 相似文献
977.
978.
979.
980.
Comparisons of palaeo‐equilibrium line altitudes between the Western and Eastern Cordilleras in the Central Andes are commonly based on the assumption that the tall outermost moraines visible in remotely sensed images of the Western Cordillera date to the Last Glacial Maximum (LGM). However, field investigation and geomorphic mapping at Nevado Sajama, Bolivia, indicates the tall moraines are relic features with shorter moraines overlying and in some cases extending beyond them. 36Cl exposure ages from the shorter moraines suggest that they date to Lateglacial times ca. 16.9–10.2 ka. Although Lateglacial deposits have been found throughout the Central Andes, the extent of these deposits relative to LGM deposits varies both between the Western and Eastern Cordilleras and north‐to‐south along the Western Cordillera. In the Western Cordillera in the zone of easterly winds, the Lateglacial appears to be the most extensive glacial advance of the last glacial cycle. Geomorphic evidence also suggests that some Lateglacial moraines were deposited by cold‐based ice, a previously unreported finding in the tropical Andes. Retreat from other glacial features occurred at about 7.0–4.4 ka and 4.7–3.3 ka. These are the first directly dated Holocene glacial deposits in the Western Cordillera of Bolivia, and their presence suggests that the mid Holocene may not have been as warm and dry as previously thought. Copyright © 2008 John Wiley & Sons, Ltd. 相似文献