全文获取类型
收费全文 | 3311篇 |
免费 | 83篇 |
国内免费 | 52篇 |
专业分类
测绘学 | 319篇 |
大气科学 | 279篇 |
地球物理 | 597篇 |
地质学 | 1464篇 |
海洋学 | 152篇 |
天文学 | 505篇 |
综合类 | 52篇 |
自然地理 | 78篇 |
出版年
2023年 | 18篇 |
2022年 | 58篇 |
2021年 | 75篇 |
2020年 | 72篇 |
2019年 | 71篇 |
2018年 | 284篇 |
2017年 | 251篇 |
2016年 | 249篇 |
2015年 | 145篇 |
2014年 | 205篇 |
2013年 | 284篇 |
2012年 | 180篇 |
2011年 | 174篇 |
2010年 | 149篇 |
2009年 | 156篇 |
2008年 | 135篇 |
2007年 | 95篇 |
2006年 | 78篇 |
2005年 | 57篇 |
2004年 | 52篇 |
2003年 | 44篇 |
2002年 | 35篇 |
2001年 | 26篇 |
2000年 | 31篇 |
1999年 | 29篇 |
1998年 | 21篇 |
1997年 | 24篇 |
1996年 | 21篇 |
1995年 | 14篇 |
1994年 | 28篇 |
1993年 | 26篇 |
1992年 | 12篇 |
1991年 | 29篇 |
1990年 | 19篇 |
1989年 | 18篇 |
1988年 | 13篇 |
1987年 | 27篇 |
1986年 | 22篇 |
1985年 | 24篇 |
1984年 | 18篇 |
1983年 | 10篇 |
1982年 | 11篇 |
1981年 | 10篇 |
1980年 | 21篇 |
1979年 | 21篇 |
1978年 | 13篇 |
1975年 | 10篇 |
1974年 | 12篇 |
1972年 | 11篇 |
1971年 | 14篇 |
排序方式: 共有3446条查询结果,搜索用时 15 毫秒
221.
Monthly mean anomaly fields of various parameters like sea surface temperature, air temperature, wind stress, effective radiation
at the surface, heat gain over the ocean and the total heat loss between a good and bad monsoon composite and the evaporation
rates over the Arabian Sea and southern hemisphere have been studied over the tropical Indian Ocean. The mean rates of evaporation
on a seasonal scale over the Arabian Sea during a good and bad monsoon composites were equal (about 2·48 × 1010 tons/day). The evaporation rates over the southern hemisphere were greater during all the months. The mean evaporation rates
over the southern hemisphere on a seasonal scale for the good and bad monsoon composites were 4·4 × 1010 and 4·6 × 1010 tons/day respectively. The maximum evaporation rates over the southern hemisphere were observed in August. The anomalies
of wind stress, effective radiation at the surface and the heat gain over the ocean also exhibit large variations in August,
as compared to other monsoon months. 相似文献
222.
Dispersion of Rayleigh-type surface wave is studied in a homogeneous transversely isotropic elastic layer overlying a nondissipative liquid-saturated porous solid half-space and lying under a uniform layer of homogeneous liquid. The frequency equation in the form of ninth-order determinant is obtained.Special cases have been deduced by reducing the depth of the layers to zero and by changing the transverse isotropic layer to an isotropic layer. Dispersion curves for the phase velocity have been plotted for a particular model. 相似文献
223.
S. Kumar 《洁净——土壤、空气、水》1990,18(6):683-690
In three drinking water sources i. e. river, well and filter water, there were detected 50 taxa of animals: 10 amoebae and 2 ciliates (protozoa), 24 rotifera, 5 cladocerans, and 9 copepods. Chief components of the rotifers population were Brachionus and Keratella, although K. cochlearis is the most common form at all the sites. The presence of Entamoeba histolytica in well water was indicative of faecal contamination which may become a causative agent of amoebiasis in human beings. 相似文献
224.
The man made reservoir, G. B. Pant Sagar at the dam site (24° 15′ N Lat) in the South Mirzapur district of eastern U. P. was investigated in terms of water quality, phytoplankton diversity and production at monthly intervals between January 1987 and December 1987. The reservoir water showed low to moderate dissolved oxygen (4.3 … 7.5 mg/l) and quite low nutrient contents (0.01-0.2 mg l?1). Phytoplankton diversity narrowly fluctuated (3.17-3.74) and mean annual production (149.7 g/m3 · aC) showed a meso- to eutrophic condition. A significant positive correlation was obtained between gross production and phytoplankton density (r = 0.79; P < 0.001) Multiple regression equation (GPP = 11.292 + 0.043 BAC + 0.075 CHL + 0.073 CYN) indicates that Chlorophyta contributed more to gross production due to the presence of a higher number of individuals. 相似文献
225.
Closed form analytic expressions for displacement and stresses at any point of either of the two homogeneous, isotropic, perfectly
elastic half-spaces in welded contact due to very long strike-slip dislocations are obtained. Both cases of vertical and horizontal
strike-slip dislocations are discussed in detail. Variation of the displacement with horizontal distance from the fault and
with vertical distance from the interface for a vertical strike-slip fault is studied numerically. 相似文献
226.
Summary Laboratory model test results are presented that determine the effectiveness of using layers of geogrids as reinforcement in sand to reduce the settlement of square surface foundations subjected to transient loading. The model tests were conducted with only one type of geogrid at one relative density of compaction of sand. The maximum intensity of the transient load applied always exceeded the static ultimate bearing capacity of the foundation when supported by unreinforced sand. The settlement reduction factors for various depths of reinforcement have been determined. 相似文献
227.
A comparison is made between the unsteady parallel fracture model using the double porosity approach and the anisotropic
aquifer-water table aquitard system. The presence of a moving water table in the weathered zone (unconfined block), influenced
by recharge from rainfall or other sources and leakage to the fracture zone, is discussed. The effect of fracture skin on
the block-fracture interface is studied for both double-porosity and aquifer-water table aquitard systems. Typical type curves
are presented. It is found that the effect of both fracture skin and water table is to lead to a flat drawdown behaviour for
a large time. The applicability of the present model for parameters estimation is illustrated on a field pumping test of 7
days duration.
Résumé: Une comparaison a étéétablie entre le modèle de fractures non constamment parallèles utilisant l'approche de la double porosité et une système composé d'un aquifère anisotrope et un bloc de faible perméabilité avec une surface libre. La présence d'une surface libre variable dans la zone d'altération (bloc non captif), inflluencée par la recharge (pluie ou autre) et par l'écoulement vers la zone fracturée, est discutée. L'effet d'une conche superficielle à l'interface bloc-fracture est étudié pour les deux systèmes, celui à double porosité et celui composé d'un aquifère et d'un bloc de faible perméabilité. Des combes typiques sont montrées. Les résultats montrent que les effets d'une conche superficielle et de la surface de la nappe conduisent à une evolution constante en rabattement pour un temps long. L'application du modèle pour l'estimation des paramètres est justifiée par les résultats d'un pompage d'essai de 7 jours.相似文献
228.
229.
Global sea-level pressure distribution has been analysed for the months of April and July for 5 years of contrasting situations of Indian summer monsoon, comprising of two drought years (1972 and 1974), a flood year (1975) and two normal monsoon years (1970 and 1973). Mean monthly sea-level pressure data at about 400 stations have been used in the study. Prominent features of pressure departures from long-term normals have also been noted. It is observed that the month of April shows more prominent contrasting features than July. In April, the high pressure centres over USSR and the North Pacific move considerably eastward during poor monsoon years, while a breakaway cell of Icelandic Low goes deep south. Both the high pressure areas over south Indian Ocean and Australia are stronger in good monsoon years. In July, the subtropical high pressure zone over the southern Indian Ocean is stronger and the Australian high is more eastward, in good monsoon years. 相似文献
230.
Umesh C. Das 《Geophysical Prospecting》1995,43(3):417-432
We consider the calculation of the electrical field quantities, electric potential and the vertical component of the total volume density of electric current, in a horizontally layered, piecewise homogeneous and arbitrarily anisotropic earth due to a system of direct current point sources. By applying Fourier transformation with respect to the horizontal space coordinates to the static field equations, the field quantities are obtained as the solutions of the system of transform-domain differential equations in the vertical (depth) coordinates. A recurrence scheme has been given to compute the tranform-domain field quantities at any depth. The corresponding space-domain quantities are then obtained by inverse Fast Fourier Transformation (FFT). A complete computer program has been developed for computing the electric potentials at any depth of the layered earth, which is composed of an arbitrary number of anisotropic layers with arbitrary conductivity tensors. By considering the point sources at different depths from the surface, equipotential contours on the surface of arbitrarily anisotropic layered earth models are given. 相似文献