全文获取类型
收费全文 | 1579篇 |
免费 | 69篇 |
国内免费 | 14篇 |
专业分类
测绘学 | 22篇 |
大气科学 | 95篇 |
地球物理 | 332篇 |
地质学 | 602篇 |
海洋学 | 118篇 |
天文学 | 347篇 |
综合类 | 5篇 |
自然地理 | 141篇 |
出版年
2021年 | 26篇 |
2020年 | 10篇 |
2019年 | 21篇 |
2018年 | 33篇 |
2017年 | 30篇 |
2016年 | 35篇 |
2015年 | 35篇 |
2014年 | 46篇 |
2013年 | 74篇 |
2012年 | 44篇 |
2011年 | 69篇 |
2010年 | 58篇 |
2009年 | 58篇 |
2008年 | 69篇 |
2007年 | 52篇 |
2006年 | 65篇 |
2005年 | 59篇 |
2004年 | 50篇 |
2003年 | 51篇 |
2002年 | 44篇 |
2001年 | 43篇 |
2000年 | 35篇 |
1999年 | 30篇 |
1998年 | 47篇 |
1997年 | 16篇 |
1996年 | 26篇 |
1995年 | 15篇 |
1994年 | 22篇 |
1993年 | 18篇 |
1992年 | 24篇 |
1991年 | 13篇 |
1990年 | 18篇 |
1989年 | 13篇 |
1988年 | 13篇 |
1987年 | 18篇 |
1986年 | 13篇 |
1985年 | 18篇 |
1984年 | 41篇 |
1983年 | 23篇 |
1982年 | 25篇 |
1981年 | 25篇 |
1980年 | 26篇 |
1979年 | 24篇 |
1978年 | 35篇 |
1977年 | 26篇 |
1976年 | 22篇 |
1975年 | 19篇 |
1974年 | 17篇 |
1973年 | 18篇 |
1971年 | 10篇 |
排序方式: 共有1662条查询结果,搜索用时 15 毫秒
31.
An improved Ewing heat probe frame 总被引:3,自引:0,他引:3
The standard Ewing heat probe frame has been improved through the design of a new takedown model whose parts are held together by lynch pins. When a 600 lb (272 kg) unit of this type is taken apart, the heaviest single piece of the dismantled unit weighs only 75 lb (34 kg).This makes it feasible for one person to handle, transport, and assemble the new unit. A central, main support member of a heat probe using this frame can be suspended over the side of a research vessel while the heavier temperature recorder and ballast weights are attached by means of the quickaction lynch pins. This makes it possible to use this device on relatively small vessels which are not designed for coring work, having a minimum of free deck space and without launching platforms or hydraulic A-frame booms. 相似文献
32.
An experimental set-up is developed and proved to be effective for laboratory study of an underwater towed system. The experimental technique gives a practical method for monitoring the kinematic and dynamic performance of an underwater towed system in a ship towing tank. Both the theoretical and experimental results in the investigation indicate that the hydrodynamic response of a towed vehicle to the wave induced motion of a towing ship can be significantly reduced by applying a two-part tow method. A comparison of the numerical and experimental results in the investigation demonstrates that the numerical simulation results are close to the experimental data, overall agreement between experimental and theoretical results is satisfactory. The results qualitatively verify the mathematical model of a two-part underwater towed system proposed by Wu and Chwang [Wu, J., Chwang, A.T., 2000. A hydrodynamic model of a two-part underwater towed system. Ocean Engineering 27 (5), 455–472]. 相似文献
33.
34.
35.
It is suggested that Bouvet Island is the surface manifestation of a mantle plume which has resulted in the creation of a chain of seamounts in the South Atlantic and a general shoaling of the region.The strike of two newly defined large fracture zones borth and south of the Bouvet Island pedestal have been utilized to determine a pole of rotation at 12.5°S, 12.5°W for the Africa-Antarctica motion. A pole at 75°S, 13°E has been calculated for the South America-Antarctica motion. At the triple junction the South America-Africa relative motion is 3.3 cm yr-1 (whole rate) at 075°. The Africa-Antarctica motion is 1.7 cm yr-1 at 065° and the South America-Antarctica motion is 1.6 cm yr-1 at 085. 相似文献
36.
The linearized theory for the parallel propagation of magnetoacoustic-gravity surface waves is developed for an interface of a horizontal magnetic field above a field-free medium. The media either side of the interface are taken to be isothermal. The dispersion relation is obtained for the case of a constant Alfvén speed. In the absence of gravity the interface may support one or two surface modes, determined by the relative temperatures and magnetism of the two media. The effect of gravity on the modes is examined and dispersion diagrams and eigenfunctions are given. In the usual - k
x diagnostic diagram, the domain of evanescence is shown to be divided into two distinct regions determining whether a given mode will have a decaying or growing vertical velocity component. In the absence of a magnetic field the transcendental dispersion relation may be rewritten as a polynomial. This polynomial possesses two acceptable solutions only one of which may exist in any given circumstances (depending on the ratio of the densities). If the gas density within the field exceeds that in the field-free medium, then the f-mode may propagate. The f-mode exists in a restricted band of horizontal wavenumber and only when the field-free medium is warmer than the magnetic atmosphere. An analytical form for the wave speed of the f-mode is obtained for small values of the Alfvén speed. It is shown that the f-mode is related to the fast magnetoacoustic surface wave, merging into that mode at short wavelengths. 相似文献
37.
An in situ method of estimating the effective diffusion coefficient for a chemical constituent that diffuses into the primary porosity of a rock is developed by abruptly changing the concentration of the dissolved constituent in a borehole in contact with the rock matrix and monitoring the time-varying concentration. The experiment was conducted in a borehole completed in mudstone on the campus of the University of the Free State in Bloemfontein, South Africa. Numerous tracer tests were conducted at this site, which left a residual concentration of sodium chloride in boreholes that diffused into the rock matrix over a period of years. Fresh water was introduced into a borehole in contact with the mudstone, and the time-varying increase of chloride was observed by monitoring the electrical conductivity (EC) at various depths in the borehole. Estimates of the effective diffusion coefficient were obtained by interpreting measurements of EC over 34 d. The effective diffusion coefficient at a depth of 36 m was approximately 7.8×10?6 m2/d, but was sensitive to the assumed matrix porosity. The formation factor and mass flux for the mudstone were also estimated from the experiment. 相似文献
38.
Nathan S. Hall R. Wayne Litaker Elizabeth Fensin Jason E. Adolf Holly A. Bowers Allen R. Place Hans W. Paerl 《Estuaries and Coasts》2008,31(2):402-418
A dense bloom of the ichthyotoxic dinoflagellate Karlodinium veneficum was discovered in the Neuse River Estuary, North Carolina, on 19 October 2006 and was associated with four subsequent fish
kills. Microscopic, photopigment, DNA, and toxicological techniques confirmed bloom identity and toxicity. High-resolution
spatio-temporal data from ship-board and fixed automated sampling stations provided a unique opportunity to investigate the
environmental conditions that initiated, maintained, and terminated the K. veneficum bloom. Bloom initiation and growth were favored by high nutrient availability and reduced dispersal during the period of
declining riverine discharge after Tropical Storm Ernesto. K. veneficum out-competed other co-occurring dinoflagellates, perhaps because of the production of karlotoxins that are known to act as
grazing deterrents and to facilitate mixotrophic feeding. Once the bloom was established, small-scale hydrodynamic processes,
coupled with vertical migration, concentrated cells along a frontal convergence to high densities (>200,000 cells per milliliter).
By 26 October 2006, wind mixing and possible nutrient stress disrupted the bloom. Release of cell-bound toxins during the
bloom collapse likely accounted for the associated fish kill events where fish were reported as exhibiting typical symptoms
of karlotoxin poisoning. The dynamics of this bloom underscore the tight control of harmful algal blooms by meteorological
forcing, hydrology, and sediment nutrient input in this shallow lagoonal estuary. 相似文献
39.
Application of FLATModel, a 2D finite volume code, to debris flows in the northeastern part of the Iberian Peninsula 总被引:7,自引:3,他引:4
FLATModel is a two-dimensional shallow-water approximation code with corrections and modifications that create a simulation
tool adapted to debris-flows behaviour. FLATModel uses the finite volume method with the numerical implementation of the Godunov
scheme and includes correction terms regarding the effect of flow over high slopes and curvature. Additionally, the stop-and-go
phenomenon, the basal entrainment and a correction regarding the front inclination of the final deposit are incorporated into
FLATModel. In addition, different flow resistance laws were integrated in the numerical code including Bingham, Herschel–Bulkley
and Voellmy fluid model. Firstly, our numerical model was validated using analytical solutions of a dam-break scenario and
published data on a laboratory experiment. Secondly, three real events, which occurred in the northeastern part of the Iberian
Peninsula, were back-calculated. Although field observations of the three events are not very detailed, the back-analyses
revealed interesting patterns on the flow dynamics, and the numerical results generally showed good agreement with field data.
Comparing the different flow resistance laws, the Voellmy fluid model presents the best behaviour regarding both the flow
behaviour and the deposit characteristics. Preliminary simulation runs incorporating the effect of basal entrainment offered
satisfactory results, although the final volume is rather sensitive on the selected friction angle of channel-bed material.
The outcomes regarding the correction of the calculated front inclination of the final deposit showed that this implementation
strongly improves the simulation results and better represents steep fronts of final deposits. 相似文献
40.
Lutz Nasdala Wolfgang Hofmeister Nicholas Norberg James M. Martinson Fernando Corfu Wolfgang Dörr Sandra L. Kamo Allen K. Kennedy Andreas Kronz Peter W. Reiners Dirk Frei Jan Kosler Yusheng Wan Jens Götze Tobias Häger Alfred Kröner John W. Valley 《Geostandards and Geoanalytical Research》2008,32(3):247-265
We introduce and propose zircon M257 as a future reference material for the determination of zircon U‐Pb ages by means of secondary ion mass spectrometry. This light brownish, flawless, cut gemstone specimen from Sri Lanka weighed 5.14 g (25.7 carats). Zircon M257 has TIMS‐determined, mean isotopic ratios (2s uncertainties) of 0.09100 ± 0.00003 for 206pb/238U and 0.7392 ± 0.0003 for 207pb/235U. Its 206pb/238U age is 561.3 ± 0.3 Ma (unweighted mean, uncertainty quoted at the 95% confidence level); the U‐Pb system is concordant within uncertainty of decay constants. Zircon M257 contains ~ 840 μg g?1 U (Th/U ~ 0.27). The material exhibits remarkably low heterogeneity, with a virtual absence of any internal textures even in cathodoluminescence images. The uniform, moderate degree of radiation damage (estimated from the expansion of unit‐cell parameters, broadening of Raman spectral parameters and density) corresponds well, within the “Sri Lankan trends”, with actinide concentrations, U‐Pb age, and the calculated alpha fluence of 1.66 × 1018 g?1. This, and a (U+Th)/He age of 419 ± 9 Ma (2s), enables us to exclude any unusual thermal history or heat treatment, which could potentially have affected the retention of radiogenic Pb. The oxygen isotope ratio of this zircon is 13.9%o VSMOW suggesting a metamorphic genesis in a marble or calc‐silicate skarn. 相似文献