全文获取类型
收费全文 | 1881篇 |
免费 | 63篇 |
国内免费 | 15篇 |
专业分类
测绘学 | 29篇 |
大气科学 | 159篇 |
地球物理 | 401篇 |
地质学 | 683篇 |
海洋学 | 145篇 |
天文学 | 355篇 |
综合类 | 5篇 |
自然地理 | 182篇 |
出版年
2021年 | 27篇 |
2020年 | 18篇 |
2019年 | 25篇 |
2018年 | 33篇 |
2017年 | 43篇 |
2016年 | 38篇 |
2015年 | 34篇 |
2014年 | 48篇 |
2013年 | 93篇 |
2012年 | 40篇 |
2011年 | 86篇 |
2010年 | 58篇 |
2009年 | 91篇 |
2008年 | 63篇 |
2007年 | 50篇 |
2006年 | 72篇 |
2005年 | 62篇 |
2004年 | 67篇 |
2003年 | 74篇 |
2002年 | 45篇 |
2001年 | 47篇 |
2000年 | 37篇 |
1999年 | 46篇 |
1998年 | 52篇 |
1997年 | 23篇 |
1996年 | 28篇 |
1995年 | 27篇 |
1994年 | 25篇 |
1993年 | 26篇 |
1992年 | 31篇 |
1991年 | 23篇 |
1990年 | 24篇 |
1989年 | 17篇 |
1988年 | 18篇 |
1987年 | 22篇 |
1986年 | 11篇 |
1985年 | 22篇 |
1984年 | 38篇 |
1983年 | 34篇 |
1982年 | 23篇 |
1981年 | 36篇 |
1980年 | 26篇 |
1979年 | 29篇 |
1978年 | 38篇 |
1977年 | 35篇 |
1976年 | 24篇 |
1975年 | 26篇 |
1974年 | 19篇 |
1973年 | 21篇 |
1972年 | 13篇 |
排序方式: 共有1959条查询结果,搜索用时 9 毫秒
1.
The elm decline of 5000 14C yr ago has been the most widely discussed phenomenon in post‐glacial vegetation history. This pan‐European reduction of elm populations, echoed in the decimation of elmwoods in Europe during the twentieth century, has attracted a series of interrelated hypotheses involving climate change, human activity, disease and soil deterioration. The elm bark beetle (Scolytus scolytus L.) is an essential component of disease explanations. We present evidence for the presence of the beetle over a prolonged period (ca. 7950–4910 yr BP [8800–5660 cal. yr BP]) from a lowland raised mire deposit in northeast Scotland, with its final appearance at this site, and the first and only appearance in another mire of a single scolytid find, around the time of the elm decline. The subfossil S. scolytus finds are not only the first from Scotland, but they also represent the most comprehensive sequence of finds anywhere. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
2.
3.
Clark R. CHAPMAN 《Meteoritics & planetary science》1996,31(6):699-725
Abstract— New observations of the S-type asteroids Gaspra and Ida, especially, by the Galileo spacecraft demonstrate that a “space weathering” process operates, which modifies the reflectance spectra of fresh material to be redder, straighter, and have shallower absorption bands. The weathering process appears similar to but less potent than that observed on the Moon. It operates in the sense that it would tend to convert spectra of ordinary chondrites (OC) to having the spectral traits of S-type asteroids. These results appear to resolve the major obstacles of the long standing “S-type conundrum” about the provenance of ordinary chondrite meteorites. A wide body of recent research, which is reviewed here, builds on previous meteoritical evidence to support a new, developing consensus that the larger S-type asteroids are a diverse assortment of silicaceous assemblages, which includes the ordinary chrondite parent bodies. Recent fairly realistic laboratory simulations of space weathering processes have changed OC spectra to resemble S-type spectra. J. F. Bell's highly regarded earlier paradigm that OC parent bodies would be found only among sub-10 km main-belt asteroids has been tested by an extensive observational survey by R. Binzel and his proteges; the result is that no small, main-belt analog for the OCs has been found, not even the somewhat OC-like object Boznemcová. This article reviews the history of the S-type conundrum, which set the stage for Galileo's historic flybys. I review the findings about Gaspra and Ida, including results about their geology, their geophysical structure and probable origins, and about Ida's small moonlet, Dactyl. Density constraints on Ida set by Dactyl's orbit rule out (for Ida, at least) the classic view of S-types as metallic, stripped cores of differentiated precursor asteroids. New analysis of the Galileo spectral images of Ida is presented that provides strong evidence that space weathering occurs on Ida, and that Ida, and the Koronis family asteroids in general, are plausibly OC-like in composition. After reviewing 1990s developments on the S-type conundrum, I advocate a new perspective that the ordinary chondrite parent bodies are among the S-type asteroids, a diverse grouping that also contains other silicaceous and silicate/metal assemblages, presumably including various stony-irons and primitive achondrites represented in meteorite collections; Gaspra may be such a metal-rich assemblage. 相似文献
4.
Long-range sidescan sonar can be used to map sediment distributions over wide expanses of deep ocean floor. Seven acoustic facies that arise from differing sediment or rock types have been mapped over the low-relief Saharan continental rise and Madeira abyssal plain. These have been calibrated with sampling, profiling and camera studies and the facies can be traced confidently on a regional scale using the sidescan data. The mapping of the sediment distribution shows that a complex interplay of turbidity current and debris flow processes can occur at a continental rise/abysaal plain transition over 1000 km from the nearest continental slope. 相似文献
5.
In this paper the second order characteristic (discontinuous bifurcation) condition is derived for the granular flow (fully plastic) equations. This second order bifurcation equation is shown to be formally identical to the first order localization requirement during steady elastoplastic deformation provided the elastic compliance tensor is substituted for the product of the plastic multiplier with the flow Hessian. For isotropic yield and flow functions the invariant form of the characteristic condition is given in detail, as well as an alternative expression in adapted co‐ordinates. The characteristic condition can be regarded as defining a hardening function which is maximized to identify the critical angles. When the method is applied to 3D Coulomb flow, Mohr's 3D fracture plane conditions are obtained uniquely. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
6.
7.
Polarimetry in the far-infrared and submillimeter has been valuable in tracing magnetic fields in bright Galactic clouds. We discuss the techniques we have developed and trends we have found in polarization vs column density and wavelength. The polarization spectrum has proven to be more interesting than had been anticipated. It is potentially valuable in explaining grain alignment and in characterizing dust species. One can expect the infrared cirrus to be a much simpler environment than the molecular clouds that have heretofore been explored at multiple wavelengths. Although cirrus observations must deal with low signals and wide extent, it is becoming possible to measure accurately the spectral energy distribution from mid-infrared to microwave frequencies and it should soon become feasible to determine the polarization spectrum over the same range. 相似文献
8.
Roger T. Trindell 《The Professional geographer》1969,21(5):328-332
9.
10.