全文获取类型
收费全文 | 998篇 |
免费 | 38篇 |
国内免费 | 5篇 |
专业分类
测绘学 | 16篇 |
大气科学 | 74篇 |
地球物理 | 262篇 |
地质学 | 270篇 |
海洋学 | 86篇 |
天文学 | 198篇 |
综合类 | 13篇 |
自然地理 | 122篇 |
出版年
2021年 | 10篇 |
2020年 | 10篇 |
2019年 | 8篇 |
2018年 | 15篇 |
2017年 | 19篇 |
2016年 | 13篇 |
2015年 | 14篇 |
2014年 | 25篇 |
2013年 | 66篇 |
2012年 | 31篇 |
2011年 | 35篇 |
2010年 | 42篇 |
2009年 | 43篇 |
2008年 | 43篇 |
2007年 | 39篇 |
2006年 | 43篇 |
2005年 | 50篇 |
2004年 | 40篇 |
2003年 | 27篇 |
2002年 | 26篇 |
2001年 | 22篇 |
2000年 | 30篇 |
1999年 | 27篇 |
1998年 | 25篇 |
1997年 | 23篇 |
1996年 | 13篇 |
1995年 | 20篇 |
1994年 | 20篇 |
1993年 | 15篇 |
1992年 | 14篇 |
1991年 | 13篇 |
1990年 | 15篇 |
1989年 | 11篇 |
1988年 | 9篇 |
1987年 | 14篇 |
1986年 | 9篇 |
1985年 | 18篇 |
1984年 | 15篇 |
1983年 | 13篇 |
1982年 | 15篇 |
1981年 | 16篇 |
1980年 | 9篇 |
1979年 | 9篇 |
1978年 | 9篇 |
1977年 | 8篇 |
1976年 | 6篇 |
1975年 | 10篇 |
1974年 | 7篇 |
1973年 | 8篇 |
1972年 | 9篇 |
排序方式: 共有1041条查询结果,搜索用时 500 毫秒
51.
52.
Spatial distribution patterns ofScirpus validus were studied in tidal marshes of the lower Savannah River. The hypothesis that changes in spatial pattern forS. validus would accompany differences in environmental parameters was tested by sampling densities and biomass along environmental gradients of salinity and elevation. Coefficients of dispersion were calculated forS. validus and used to compare spatial patterns among freshwater, midly oligohaline, strongly oligohaline, and mesohaline tidal marshes. Results indicated significantly greater clumping ofS. validus in mesohaline marsh than in freshwater marsh. Only the mildly oligohaline site supported a random population ofS. validus, while the strongly oligohaline marsh supported a uniform spatial distribution. Spatial pattern and relative importance ofS. validus, as well as composition of co-occurring species, changed significantly with changing salinity. The relations between changes in relative importance ofS. validus and differences in soil organic matter and elevation were also significant. 相似文献
53.
54.
Activities of 238U, 228Ra, 226Ra and 210Pb were determined in submarine hydrothermal massive sulfides by nondestructive, gamma-ray spectrometry. The samples were collected by the manned submersible DSRV “Alvin” from active hydrothermal fields on the Endeavour Segment and Axial Seamount of the Juan de Fuca Ridge. 210Pb activities are mostly below the equilibrium level with 226Ra, which is linearly correlated with the concentration of Ba, Sr and Ca in the deposits. The ratios of226Ra to the alkaline earth elements indicate that hydrothermal alteration of the underlying oceanic crust is the dominant source of Ra in the sulfide deposits.The 210Pb/Pb ratios measured in many of the sulfides are higher than the ratio obtained for a basalt source of 210Pb, probably because of 210Pb ingrowth from 226Ra within the sulfide deposits. An isochron approach employing ratios of 210Pb/Pb and 226Ra/Pb yielded an initial 210Pb/Pb ratio in the range of 0–0.57 dpm/μg Pb, implying that the crustal residence time of the hydrothermal fluid in the Endeavour Segment is very short ( < 10 years) and that basalt alteration is the only source of the 210Pb and Pb. For the hydrothermal fluid in Axial Seamount, a residence time of 10–20 years is estimated on the basis of the initial 210Pb/Pb and 228Ra/226Ra ratios of a sulfide chimney. Both residence time estimates are consistent with the results of previous studies. We have estimated the upper limits of sample ages ranging from tens to a hundred years for 12 samples, with the remaining seven samples indicating ages close to or older than 150 years, the practical limit of the 210Pb/Pb dating technique.A concentrically sectioned sulfide chimney from Axial Seamount yielded mean radial growth rates ranging from a few tenths to a few mm/year based on gradients of 210Pb/Pb and 228Ra/226Ra activity ratios, whereas a similarly sectioned chimney from the Endeavour Segment, although collected from the top of an active “black smoker” vent, displayed no 210Pb or 228Ra activity. Our results have implications for the duration and periodicity of hydrothermal circulation and associated mineral deposition at mid-ocean ridges. 相似文献
55.
56.
J-P. Latham 《Journal of Structural Geology》1985,7(2):225-236
A reexamination of Biot's internal instability analysis, including the effects of bending resistance and nonlinear material properties, shows that internal buckling and oblique localized shearing into bands are the two end-member processes associated with internal instability. The first is a consequence of a high anisotropy which is an intrinsic property of the original unstressed layering or foliation. The second is a consequence of a highly anisotropic material response to perturbations which is induced during the initial uniform compression if material properties are nonlinear. The incremental anisotropy generating the instability is associated with planes of least resistance to shearing in directions parallel to the initial compression in the first case and at 45° to the initial compression in the second case. 相似文献
57.
Walter W. Loo Klaus Franz Gary R. Holzhausen 《Ground Water Monitoring & Remediation》1984,4(4):124-130
An advanced two-way radio link (transceiver units) and data logging system have provided a powerful means of real-time series data collection and analysis. The application of this telemetry system to surface tiltmeter surveys provides significant cost savings for testing and analysis of deep, large-scale horizontal anisotropic permeability and heterogeneity determinations.
The determination of horizontal anisotropic properties requires one pumping well and eight to 10 tiltmeter monitoring stations. The validity of the depth of the test may reach 4,000 feet below land surface. With the addition of one partially penetrating observation well, the entire three-dimensional anisotropic permeability can be determined. With the addition of a fully penetrating well, the storage coefficient can be determined as well as leakage property (including vertical permeability of the confining layers). An example test case was analyzed for horizontal anisotropic permeability with remarkable success despite relatively unfavorable detection conditions.
A thorough understanding of the geology overlying the aquifer system must be made in order to assess the applicability of the surface tiltmeter survey to large-scale horizontal anisotropic permeability determinations. 相似文献
The determination of horizontal anisotropic properties requires one pumping well and eight to 10 tiltmeter monitoring stations. The validity of the depth of the test may reach 4,000 feet below land surface. With the addition of one partially penetrating observation well, the entire three-dimensional anisotropic permeability can be determined. With the addition of a fully penetrating well, the storage coefficient can be determined as well as leakage property (including vertical permeability of the confining layers). An example test case was analyzed for horizontal anisotropic permeability with remarkable success despite relatively unfavorable detection conditions.
A thorough understanding of the geology overlying the aquifer system must be made in order to assess the applicability of the surface tiltmeter survey to large-scale horizontal anisotropic permeability determinations. 相似文献
58.
59.
A global atmospheric model is used to calculate the monthly river flow for nine of the world's major high latitude rivers for the present climate and for a doubled CO2 climate. The model has a horizontal resolution of 4° × 5°, but the model's runoff from each grid box is quartered and added to the appropriate river drainage basin on a 2° × 2.5° resolution. A routing scheme is used to move runoff from a grid box to its neighboring downstream grid box and ultimately to the mouth of the river. In a model simulation in which atmospheric carbon dioxide is doubled, mean annual precipitation and river flow increase for all of these rivers, increased outflow at the river mouths begins earlier in the spring, and the maximum outflow occurs approximately one month sooner due to an earlier snow melt season. In the doubled CO2 climate, snow mass decreases for the Yukon and Mackenzie rivers in North America and for rivers in northwestern Asia, but snow mass increases for rivers in northeastern Asia. 相似文献
60.
Characteristics of hydrothermal plumes from two vent fields on the Juan de Fuca Ridge, northeast Pacific Ocean 总被引:1,自引:0,他引:1
Deep CTD/transmissometer tows and water bottle sampling were used during 1985 to map the regional distribution of the neutrally-buoyant plumes emanating from each of two major vent fields on the Southern Symmetrical Segment (SSS) and Endeavour Segment (ES) of the Juan de Fuca Ridge. At both vent fields, emissions from point and diffuse hydrothermal sources coalesced into a single 200-m-thick plume elongated in the direction of current flow and with characteristic temperature anomalies of 0.02–0.05°C and light-attenuation anomalies of 0.01–0.08 m−1 (10–80 μg/l above background). Temperature anomalies in the core of each plume were uniform as far downcurrent as the plumes were mapped (10–15 km). Downcurrent light-attenuation trends were non-uniform and differed between plumes, apparently because different vent fluid chemistries at each field cause significant differences in the settling characteristics of the hydrothermal precipitates. Vent fluids from the SSS are metal-dominated and mostly precipitate very fine-grained hydrous Fe-oxides that remain suspended in the plume. Vent fluids from the ES are sulfur-dominated and precipitate a high proportion of coarser-grained Fe-sulfides that rapidly settle from the plume. The integrated flux of each vent field was estimated from measurements of the advective transport of each plume. Heat flux was 1700 ± 1100 MW from the ES and 580 ± 351 MW from the SSS. Particle flux varied from 546 ± 312 g/s to 204 ± 116 g/s at the ES depending on distance from the vent field, and was 92 ± 48 g/s from the SSS. 相似文献