全文获取类型
收费全文 | 688篇 |
免费 | 12篇 |
国内免费 | 22篇 |
专业分类
测绘学 | 10篇 |
大气科学 | 43篇 |
地球物理 | 140篇 |
地质学 | 261篇 |
海洋学 | 53篇 |
天文学 | 139篇 |
自然地理 | 76篇 |
出版年
2021年 | 3篇 |
2020年 | 6篇 |
2019年 | 4篇 |
2018年 | 13篇 |
2017年 | 12篇 |
2016年 | 24篇 |
2015年 | 17篇 |
2014年 | 12篇 |
2013年 | 28篇 |
2012年 | 18篇 |
2011年 | 29篇 |
2010年 | 27篇 |
2009年 | 34篇 |
2008年 | 26篇 |
2007年 | 32篇 |
2006年 | 30篇 |
2005年 | 24篇 |
2004年 | 37篇 |
2003年 | 26篇 |
2002年 | 20篇 |
2001年 | 21篇 |
2000年 | 19篇 |
1999年 | 15篇 |
1998年 | 19篇 |
1997年 | 17篇 |
1996年 | 13篇 |
1995年 | 12篇 |
1994年 | 4篇 |
1993年 | 7篇 |
1992年 | 16篇 |
1991年 | 12篇 |
1990年 | 6篇 |
1989年 | 4篇 |
1988年 | 4篇 |
1987年 | 8篇 |
1986年 | 3篇 |
1985年 | 8篇 |
1984年 | 13篇 |
1983年 | 12篇 |
1982年 | 8篇 |
1981年 | 8篇 |
1980年 | 11篇 |
1979年 | 5篇 |
1978年 | 6篇 |
1977年 | 8篇 |
1976年 | 8篇 |
1975年 | 9篇 |
1974年 | 7篇 |
1972年 | 3篇 |
1964年 | 2篇 |
排序方式: 共有722条查询结果,搜索用时 265 毫秒
221.
Vertical profiles of the critical erosion threshold (τcrit) in sediment have been measured at 11 stations along the axis of the Tamar Estuary and at a single station in a tributary of the Tamar at St. John's Ford. The τcrit of surface sediment increased from 0.04 Pa in the upper, brackish estuary to 0.09 Pa in the lower estuary. In the upper estuary τcrit only increased slightly with depth whereas in the marine estuary τcrit increased rapidly from 0.09 Pa at the surface to 0.25 Pa at 15 cm below the sediment surface. The results showed that the relationship between τcrit and bulk density (ρb) obtained previously for surface sediment was also applicable to sediments from depths of 10–15 cm and probably deeper. Profiles of ρb were measured to depths of 70 cm using a corer. In the lower (marine) estuary ρb increased with depth in the sediment from 1580 kg m−3 at the surface to 1720 kg m−3 at 70 cm. In the upper estuary ρb values were lower at 1170–1200 kg m−3 and profiles were almost homogeneous indicating that consolidation was not occurring. The mid-estuary was transitional between these two situations. These results are consistent with the seasonal accumulation and loss of ‘mobile’ sediment observed previously in the upper estuary with changes in river flow, and with the apparent stability of intertidal mud in the lower marine estuary deduced from historical bathymetric survey records. The slopes of the intertidal mud banks ranged from 1–2% in the lower estuary to 20–25% in mid-estuary but, instead of continuing to increase in steepness towards the head as the estuary became narrower, the measured slopes actually decreased. It is speculated that the lack of consolidation through continual mobilisation and settlement cycles combined with an increase in silt content in the upper estuary resulted in sediment that lacked the mechanical strength to maintain steep slopes. 相似文献
222.
Matthew R. Patrick John L. Smellie Andrew J. L. Harris Robert Wright Ken Dean Pavel Izbekov Harold Garbeil Eric Pilger 《Bulletin of Volcanology》2005,67(5):415-422
The MODVOLC satellite monitoring system has revealed the first recorded eruption of Mount Belinda volcano, on Montagu Island in the remote South Sandwich Islands. Here we present some initial qualitative observations gleaned from a collection of satellite imagery covering the eruption, including MODIS, Landsat 7 ETM+, ASTER, and RADARSAT-1 data. MODVOLC thermal alerts indicate that the eruption started sometime between 12 September and 20 October 2001, with low-intensity subaerial explosive activity from the islands summit peak, Mount Belinda. By January 2002 a small lava flow had been emplaced near the summit, and activity subsequently increased to some of the highest observed levels in August 2002. Observations from passing ships in February and March 2003 provided the first visual confirmation of the eruption. ASTER images obtained in August 2003 show that the eruption at Mount Belinda entered a new phase around this time, with fresh lava effusion into the surrounding icefield. MODIS radiance trends also suggest that the overall activity level increased significantly after July 2003. Thermal anomalies continued to be observed in MODIS imagery in early 2004, indicating a prolonged low-intensity eruption and the likely establishment of a persistent summit lava lake, similar to that observed on neighboring Saunders Island in 2001. Our new observations also indicate that lava lake activity continues on Saunders Island.Editorial responsibility: J. Gilbert 相似文献
223.
Using the FLOWGO thermo-rheological model we have determined cooling-limited lengths of channel-fed (i.e. a) lava flows from Mauna Loa. We set up the program to run autonomously, starting lava flows from every 4th line and sample in a 30-m spatial-resolution SRTM DEM within regions corresponding to the NE and SW rift zones and the N flank of the volcano. We consider that each model run represents an effective effusion rate, which for an actual flow coincides with it reaching 90% of its total length. We ran the model at effective effusion rates ranging from 1 to 1,000 m3 s–1, and determined the cooling-limited channel length for each. Keeping in mind that most flows extend 1–2 km beyond the end of their well-developed channels and that our results are non-probabilistic in that they give all potential vent sites an equal likelihood to erupt, lava coverage results include the following: SW rift zone flows threaten almost all of Mauna Loas SW flanks, even at effective effusion rates as low as 50 m3 s–1 (the average effective effusion rate for SW rift zone eruptions since 1843 is close to 400 m3 s–1). N flank eruptions, although rare in the recent geologic record, have the potential to threaten much of the coastline S of Keauhou with effective effusion rates of 50–100 m3 s–1, and the coast near Anaehoomalu if effective effusion rates are 400–500 m3 s–1 (the 1859 a flow reached this coast with an effective effusion rate of 400 m3 s–1). If the NE rift zone continues to be active only at elevations >2,500 m, in order for a channel-fed flow to reach Hilo the effective effusion rate needs to be 400 m3 s–1 (the 1984 flow by comparison, had an effective effusion rate of 200 m3 s–1). Hilo could be threatened by NE rift zone channel-fed flows with lower effective effusion rates but only if they issue from vents at 2,000 m or lower. Populated areas on Mauna Loas SE flanks (e.g. Phala), could be threatened by SW rift zone eruptions with effective effusion rates of 100 m3 s–1.Editorial responsibility: J Donnelly-Nolan 相似文献
224.
D. Baines R. D. Oudmaijer A. Mora C. Eiroa John M. Porter B. Merín B. Montesinos D. de Winter A. Cameron J. K. Davies H. J. Deeg R. Ferlet C. A. Grady A. W. Harris M. G. Hoare K. Horne S. L. Lumsden L. F. Miranda A. Penny A. Quirrenbach 《Monthly notices of the Royal Astronomical Society》2004,353(2):697-704
225.
226.
R. Srama A. Srowig M. Rachev E. Grün S. Auer T. Conlon A. Glasmachers D. Harris S. Helfert S. Kempf H. Linnemann G. Moragas-Klostermeyer V. Tschernjawski 《Earth, Moon, and Planets》2004,95(1-4):211-220
There are different types of dust particles in interplanetary space, such as dust from comets and asteroids, and interstellar
grains traversing the solar system. Based on experience with current space dust instruments, a novel dust telescope is being
developed. A dust telescope is a combination of a dust trajectory sensor for the identification and an analyzer for the elemental
composition of the dust. Dust particles’ trajectories are determined by the measurement of the electric signals that are induced
when a charged grain flies through a position-sensitive electrode system. The objective of the trajectory sensor is to measure
dust charges in the range 10−16–10−13 C and dust speeds in the range 6–100 km/s. First tests with a laboratory setup have been performed. The chemical analyzer
will have an impact area of 0.1 m2. It consists of a target with an acceleration grid and a single-stage reflectron for energy focusing, and a central ion detector.
Results from SIMION simulations show that a mass resolution of M/ΔM>150 can be obtained. 相似文献
227.
J. Allington-Smith P. Bettess E. Chadwick R. Content R. Davies G. Dodsworth R. Haynes D. Lee I. Lewis J. Webster E. Atad S. Beard R. Bennett J. Harris P. Hastings D. Kelly T. Paul M. Strachan P. Williams T. Bond D. Crampton T. Davidge B. Leckie C. Morbey R. Murowinski S. Roberts L. Saddlemyer J. Sebesta J. Stilburn K. Szeto 《Experimental Astronomy》1997,7(4):293-299
228.
229.
The Jacchia models are represented in terms of spherical harmonic functions. This representation has the advantage of ease of comparison with other global theoretical and empiric models that use this mathematical form. Furthermore it is analytic, continuous and has continuous derivatives all over the globe. The representation of the exospheric temperatures shows clearly the amplitudes of the various periodic terms and uses relatively few constants. An example of a similar representation for the total mass density at a particular height and level of solar activity is given as well. 相似文献
230.
I. Adler J. I. Trombka P. Lowman R. Schmadebeck H. Blodget E. Eller L. Yin R. Lamothe G. Osswald J. Gerard P. Gorenstein P. Bjorkholm H. Gursky B. Harris J. Arnold A. Metzger R. Reedy 《Earth, Moon, and Planets》1973,7(3-4):487-504
A number of experiments carried in orbit on the Apollo 15 and 16 spacecraft were used in the compositional mapping of the lunar surface. The observations involved measurements of secondary (fluorescent) X-rays, gamma rays and alpha particle emissions. A large scale compositional map of over 20% of the lunar surface was obtained for the first time. It was possible to demonstrate significant chemical differences between the mare and the highlands, to find specific areas of high radioactivity and to learn something about the composition of the Moon's hidden side.Paper dedicated to Professor Harold C. Urey on the occasion of his 80th birthday on 29 April 1973. 相似文献