全文获取类型
收费全文 | 1079篇 |
免费 | 55篇 |
国内免费 | 5篇 |
专业分类
测绘学 | 12篇 |
大气科学 | 66篇 |
地球物理 | 225篇 |
地质学 | 344篇 |
海洋学 | 68篇 |
天文学 | 296篇 |
自然地理 | 128篇 |
出版年
2021年 | 18篇 |
2020年 | 12篇 |
2019年 | 16篇 |
2018年 | 31篇 |
2017年 | 30篇 |
2016年 | 44篇 |
2015年 | 25篇 |
2014年 | 26篇 |
2013年 | 63篇 |
2012年 | 22篇 |
2011年 | 40篇 |
2010年 | 43篇 |
2009年 | 49篇 |
2008年 | 52篇 |
2007年 | 61篇 |
2006年 | 61篇 |
2005年 | 38篇 |
2004年 | 38篇 |
2003年 | 33篇 |
2002年 | 32篇 |
2001年 | 27篇 |
2000年 | 25篇 |
1999年 | 24篇 |
1998年 | 36篇 |
1997年 | 18篇 |
1996年 | 10篇 |
1995年 | 14篇 |
1994年 | 18篇 |
1993年 | 10篇 |
1992年 | 6篇 |
1991年 | 6篇 |
1990年 | 11篇 |
1989年 | 12篇 |
1988年 | 11篇 |
1987年 | 11篇 |
1986年 | 12篇 |
1985年 | 14篇 |
1984年 | 8篇 |
1983年 | 9篇 |
1982年 | 10篇 |
1981年 | 6篇 |
1980年 | 13篇 |
1979年 | 10篇 |
1978年 | 6篇 |
1977年 | 7篇 |
1976年 | 8篇 |
1975年 | 5篇 |
1973年 | 5篇 |
1972年 | 12篇 |
1971年 | 5篇 |
排序方式: 共有1139条查询结果,搜索用时 31 毫秒
71.
T.E.C. Keith J.M. Thompson R.A. Hutchinson L.D. White 《Journal of Volcanology and Geothermal Research》1992,49(3-4)
Meteoric waters from cold springs and streams outside of the 1912 eruptive deposits filling the Valley of Ten Thousand Smokes (VTTS) and in the upper parts of the two major rivers draining the 1912 deposits have similar chemical trends. Thermal springs issue in the mid-valley area along a 300-m lateral section of ash-flow tuff, and range in temperature from 21 to 29.8°C in early summer and from 15 to 17°C in mid-summer. Concentrations of major and minor chemical constituents in the thermal waters are nearly identical regardless of temperature. Waters in the downvalley parts of the rivers draining the 1912 deposits are mainly mixtures of cold meteoric waters and thermal waters of which the mid-valley thermal spring waters are representative. The weathering reactions of cold waters with the 1912 deposits appear to have stabilized and add only subordinate amounts of chemical constituents to the rivers relative to those contributed by the thermal waters. Isotopic data indicate that the mid-valley thermal spring waters are meteoric, but data is inconclusive regarding the heat source. The thermal waters could be either from a shallow part of a hydrothermal system beneath the 1912 vent region or from an incompletely cooled, welded tuff lens deep in the 1912 ash-flow sheet of the upper River Lethe area.Bicarbonate-sulfate waters resulting from interaction of near-surface waters and the cooling 1953–1968 southwest Trident plug issue from thermal springs south of Katmai Pass and near Mageik Creek, although the Mageik Creek spring waters are from a well-established, more deeply circulating hydrothermal system. Katmai caldera lake waters are a result of acid gases from vigorous drowned fumaroles dissolving in lake waters composed of snowmelt and precipitation. 相似文献
72.
Minor and trace elements in HF-soluble zircons 总被引:2,自引:0,他引:2
Electron probe analysis of a group of HF-soluble zircons from porphyroids of the Thuringian Forest, Germany, established yttrium, phosphorus, and iron to be the significant minor constituents. It is believed that these elements render the zircon structure HF-soluble. The Y content varies from about 6500–48000 ppm=0.83–6.10% Y2O3; the P concentrations range from a low of about 790 to a maximum of 4000 ppm=0.18–0.92% P2O5; the Fe content varies from about 400–15000 ppm=0.06–2.10% Fe2O3. Although both Y and P are distributed throughout each given grain, they are sometimes mutually enriched in growth zones. These zones are also resolved in reflected light micrographs and the electron images. 相似文献
73.
74.
75.
76.
An analytical study is carried out of rays from a source in a magneto-ionic medium in which the magnetic field is linear in the position coordinates. A general solution is given, and special cases, including that in which the magnetic field vector is confined to a plane, are examined.In the latter case, the specific sub-case of null current density, in which the field lines are rectangular hyperbolas, has been the subject of detailed numerical calculations. It is shown that focusing of rays can occur, though perhaps less readily than in the case of parabolic field lines considered by previous workers. We relate our parametrisation quantitatively to the terrestrial magnetic field. 相似文献
77.
Djuro Novakovic Christopher D. White Rucsandra M. Corbeanu William S. Hammon III Janok P. Bhattacharya George A. McMechan 《Mathematical Geology》2002,34(7):857-893
Ground-penetrating radar (GPR) surveys, outcrop measurements, and cores provide a high-resolution 3D geologic model to investigate the hydraulic effects of shales in marine-influenced lower delta-plain distributary channel deposits within the Cretaceous-age Ferron Sandstone at Corbula Gulch in central Utah, USA. Shale statistics are computed from outcrop observations. Although slight anisotropy was observed in mean length and variogram ranges parallel and perpendicular to pale of low
, the anisotropy is not statistically significant and the estimated mean length is 5.4 m. Truncated Gaussian simulation was used to create maps of shales that are placed on variably dipping stratigraphic surfaces interpreted from high-resolution 3D GPR surveys, outcrop interpretations, and boreholes. Sandstone permeability is estimated from radar responses calibrated to permeability measurements from core samples. Experimentally designed flow simulations examine the effects of variogram range, shale coverage fraction, and trends in shale coverage on predicted upscaled permeability, breakthrough time, and sweep efficiency. Approximately 1500 flow simulations examine three different geologic models, flow in the 3 coordinate directions, 16 geostatistical parameter combinations, and 10 realizations for each model. ANOVA and response models computed from the flow simulations demonstrate that shales decrease sweep, recovery, and permeability, especially in the vertical direction. The effect on horizontal flow is smaller. Flow predictions for ideal tracer displacements at Corbula Gulch are sensitive to shale-coverage fraction, but are relatively insensitive to twofold variations in variogram range or to vertical trends in shale coverage. Although the hydraulic effects of shale are statistically significant, the changes in flow responses rarely exceed 20%. As a result, it may be reasonable to use simple models when incorporating analogous shales into models of reservoirs or aquifers. 相似文献
78.
We report the direct observation of motion associated with a solar flare at a speed of 26,000 km s-1. The motion is seen from a radio source at 0.33 GHz, which suddenly starts moving during the flare. At its peak, the radio source covers a quiet region of dimension 500&arcsec;. Emission from any given location is sporadic. The disturbance itself does not seem to radiate, but it excites coronal features that continue to radiate after it passes. The inferred velocity is larger than any previously inferred velocity of a disturbance in the solar atmosphere apart from freely streaming beams of accelerated electrons. The observed motion of the source at a fixed frequency, low polarization, and moderate bandwidth are more consistent with the typical properties of moving type IV radio bursts than with classical coronal shock-associated type II bursts, but any disturbance at such a high velocity must be highly supersonic and should drive a shock. We speculate that the disturbance is associated with the realignment of magnetic fields connecting different portions of an active region. 相似文献
79.
MacQueen R.M. Hendrickson M.A. Woods J.C. Lecinski A.R. Elmore D.F. White O.R. 《Solar physics》2000,191(1):85-96
The intensity of a sample of large, high-contrast and isolated dark points has been observed with full-disk images in the light of Hei 1083 nm from the Chromospheric Helium line Intensity Photometer (CHIP) on Mauna Loa, Hawaii. Temporal variations in the intensity encompassing a broad range of time scales have been recorded. Long-term changes in the intensity, although highly variable, are characterized by e-folding times on the order of 5 h. Superposed on these variations are frequent intensity variations, which occur over time scales ranging from the typical observing cadence of 3 min, to tens of minutes. Microflares-involving intensity changes of at least 50% over periods of minutes are observed frequently. Rapid cadence (
min) observations reveal differences between rise and decay times and shorter-term variations in the intensity profiles of these microflares. 相似文献
80.