全文获取类型
收费全文 | 791篇 |
免费 | 31篇 |
专业分类
测绘学 | 12篇 |
大气科学 | 52篇 |
地球物理 | 189篇 |
地质学 | 256篇 |
海洋学 | 78篇 |
天文学 | 165篇 |
综合类 | 2篇 |
自然地理 | 68篇 |
出版年
2020年 | 12篇 |
2019年 | 6篇 |
2018年 | 17篇 |
2017年 | 25篇 |
2016年 | 20篇 |
2015年 | 21篇 |
2014年 | 26篇 |
2013年 | 53篇 |
2012年 | 22篇 |
2011年 | 26篇 |
2010年 | 30篇 |
2009年 | 54篇 |
2008年 | 43篇 |
2007年 | 41篇 |
2006年 | 34篇 |
2005年 | 32篇 |
2004年 | 29篇 |
2003年 | 26篇 |
2002年 | 28篇 |
2001年 | 28篇 |
2000年 | 19篇 |
1999年 | 13篇 |
1998年 | 12篇 |
1997年 | 15篇 |
1996年 | 3篇 |
1995年 | 13篇 |
1994年 | 17篇 |
1993年 | 12篇 |
1992年 | 8篇 |
1991年 | 8篇 |
1990年 | 8篇 |
1989年 | 12篇 |
1988年 | 6篇 |
1987年 | 10篇 |
1986年 | 6篇 |
1985年 | 3篇 |
1984年 | 4篇 |
1983年 | 4篇 |
1981年 | 7篇 |
1980年 | 8篇 |
1979年 | 2篇 |
1978年 | 5篇 |
1977年 | 9篇 |
1976年 | 3篇 |
1975年 | 8篇 |
1974年 | 4篇 |
1973年 | 9篇 |
1972年 | 6篇 |
1971年 | 3篇 |
1970年 | 2篇 |
排序方式: 共有822条查询结果,搜索用时 546 毫秒
481.
The Atlantic Coastal Plain of the United States has long been considered to have negligible soil erosion owing to low relief and permeable soils. Several recent studies point to a need for reassessment. Post-settlement erosion on the North Carolina Coastal Plain was estimated at the regional scale based on soil-profile truncation of a representative upland soil (Norfolk series). Apparent post-settlement soil-profile truncation ranges from 2.5 to 30 cm (mean 14.5 cm), implying a minimum regional average rate of 0.55 mm/yr since the region was settled in the 1700s. This amounts to a conservative estimated average rate of 9.3 t/ha/yr, a rate of the same general magnitude as estimated erosion rates in the North Carolina Piedmont, long recognized as a problem erosion area. Results imply accelerated water erosion despite the low slopes and permeable soils, and suggest the possibility of significant eolian erosion in the region. This rapid erosion in an area noted for its geological stability is a testament to the importance of vegetation cover and human agency in landscape evolution in the Coastal Plain. [Key words: accelerated erosion, soil profile, geomorphology, coastal plain, North Carolina.] 相似文献
482.
Jane Xinxin Zhang Kang‐Tsung Chang Joan Qiong Wu 《International journal of geographical information science》2013,27(8):925-942
Digital elevation models (DEMs) vary in resolution and accuracy by the production method. DEMs with different resolutions and accuracies can generate varied topographic and hydrological features, which can in turn affect predictions by soil erosion models, such as the WEPP (Water Erosion Prediction Project) model. This study investigates the effects of DEMs on deriving topographic and hydrological attributes, and on predicting watershed erosion using WEPP v2006.5. Six DEMs at three resolutions from three sources were prepared for two small forested watersheds located in northern Idaho, USA. These DEMs were used to calculate topographic and hydrological parameters that served as inputs to WEPP. The model results of sediment yields and runoffs were compared with field observations. For both watersheds, DEMs with different resolutions and sources generated varied watershed shapes and structures, which in turn led to different extracted hill slope and channel lengths and gradients, and produced substantially different erosion predictions by WEPP. 相似文献
483.
Arch-shaped coronal loops that are isolated from the background are typically acquired manually from massive online image
databases to be used in solar coronal research. The manual search for special coronal loops is not only subject to human mistakes
but is also time consuming and tedious. In this study, we propose a completely automated image-retrieval system that identifies
coronal-loop regions located outside of the solar disk from 17.1 nm EIT images. To achieve this aim, we first apply image-preprocessing
techniques to bring out loop structures from their background and to reduce the effect of undesired patterns. Then we extract
principal contours from the solar image regions. The geometrical attributes of the extracted principal contours reveal the
existence of loops in a given region. Our completely automated decision-making procedure gives promising results in separating
the regions with loops from the regions without loops. Based on our loop-detection procedure, we have developed an automated
image-retrieval tool that is capable of retrieving images containing loops from a collection of solar images. 相似文献
484.
David E. Smith Roger J. Phillips Gregory A. Neumann Stanton J. Peale Mark H. Torrence James W. Head III Catherine L. Johnson Olivier S. Barnouin Jürgen Oberst 《Icarus》2010,209(1):88-100
On 14 January and 6 October 2008 the MESSENGER spacecraft passed within 200 km of the surface of Mercury. These flybys by MESSENGER provided the first observations of Mercury from a spacecraft since the Mariner 10 flybys in 1974 and 1975. Data from the Mercury Laser Altimeter (MLA) provided new information on the equatorial shape of Mercury, and Doppler tracking of the spacecraft through the flybys provided new data on the planet’s gravity field. The MLA passes were on opposite hemispheres of the planet and span collectively ∼40% of the equatorial circumference. The mean elevation of topography observed during flyby 1, in the longitude range 0-90°E, is greater than that seen during flyby 2 in the longitude range 180-270°E, indicating an offset between centers of mass and figure having a magnitude and phase in general agreement with topography determined by Earth-based radar. Both MLA profiles are characterized by slopes of ∼0.015° downward to the east, which is consistent with a long-wavelength equatorial shape defined by a best-fitting ellipse. The Doppler tracking data show sensitivity to the gravitational structure of Mercury. The equatorial ellipticity of the gravitational field, C2,2, is well determined and correlates with the equatorial shape. The S2,2 coefficient is ∼0, as would be expected if Mercury’s coordinate system, defined by its rotational state, is aligned along its principal axes of inertia. The recovered value of the polar flattening of the gravitational potential, J2, is considerably lower in magnitude than the value obtained from Mariner 10 tracking, a result that is problematic for internal structure models. This parameter is not as well constrained as the equatorial ellipticity because the flyby trajectories were nearly in the planet’s equatorial plane. The residuals from the Doppler tracking data suggest the possibility of mascons on Mercury, but flyby observations are of insufficient resolution for confident recovery. For a range of assumptions on degree of compensation and crustal and mantle densities, the allowable crustal thickness is consistent with the upper limit of about 100 km estimated from the inferred depth of faulting beneath a prominent lobate scarp, an assumed ductile flow law for crustal material, and the condition that temperature at the base of the crust does not exceed the solidus temperature. The MESSENGER value of C2,2 has allowed an improved estimate of the ratio of the polar moment of inertia of the mantle and crust to the full polar moment (Cm/C), a refinement that strengthens the conclusion that Mercury has at present a fluid outer core. 相似文献
485.
486.
487.
488.
Luciano Telesca Michele Lovallo Joan Martì Molist Carmen López Moreno Rafael Abella Meléndez 《地学学报》2014,26(6):425-429
We investigated the dynamics of the continuous seismic signal recorded before and during the 2011–2012 El Hierro eruption (Canary Islands) using the innovative approach of the Fisher–Shannon method, a suitable statistical tool for detecting dynamic changes in complex systems. Our findings identify dynamic changes in the seismic signal that can be correlated with different stress states of the magmatic setting and the plumbing system in the volcano at El Hierro. The results contribute to the understanding of the fracturing pattern in the crust during a new intrusion and eruption of an overpressurized batch of magma. 相似文献
489.
490.
The Pliocene-Holocene Newer Volcanic Province (NVP) of southeastern Australia is an extensive, relatively well-preserved, intra-plate basaltic lava field containing more than 400 eruptive centres. This study reports new, high-precision 40Ar/39Ar ages for six young (300–600 ka) basalt flows from the NVP and is part of a broader initiative to constrain the extent, duration, episodicity and causation of NVP volcanism. Six fresh, holocrystalline alkali basalt flows were selected from the Warrnambool-Port Fairy area in the Western Plains sub-province for 40Ar/39Ar dating. These flows were chosen on the basis of pre-existing K-Ar age constraints, which, although variable, indicated eruption during a period of apparent relative volcanic quiescence (0.8–0.06 Ma).40Ar/39Ar ages were measured on multiple aliquots of whole rock basalt samples. Three separate flows from the Mount Rouse volcanic field yielded concordant 40Ar/39Ar age results, with a mean eruption age of 303 ± 13 ka (95% CI). An older weighted mean age of 382 ± 24 ka (2σ) was obtained for one sample from the central Rouse-Port Fairy Flow, suggesting extraneous argon contamination. Two basalt flows from the Mount Warrnambool volcano also yielded analogous results, with an average 40Ar/39Ar age of 542 ± 17 ka (95% CI). The results confirm volcanic activity during the interval of relative quiescence. Most previous K-Ar ages for these flows are generally older than the weighted mean 40Ar/39Ar ages, suggesting the presence of extraneous 40Ar. This study demonstrates the suitability of the 40Ar/39Ar incremental-heating method to obtain precise eruption ages for young, holocrystalline alkali basalt samples in the NVP. 相似文献