全文获取类型
收费全文 | 8616篇 |
免费 | 340篇 |
国内免费 | 60篇 |
专业分类
测绘学 | 280篇 |
大气科学 | 951篇 |
地球物理 | 1989篇 |
地质学 | 3358篇 |
海洋学 | 502篇 |
天文学 | 1347篇 |
综合类 | 30篇 |
自然地理 | 559篇 |
出版年
2022年 | 43篇 |
2021年 | 118篇 |
2020年 | 117篇 |
2019年 | 123篇 |
2018年 | 258篇 |
2017年 | 278篇 |
2016年 | 344篇 |
2015年 | 246篇 |
2014年 | 355篇 |
2013年 | 473篇 |
2012年 | 276篇 |
2011年 | 402篇 |
2010年 | 355篇 |
2009年 | 442篇 |
2008年 | 340篇 |
2007年 | 289篇 |
2006年 | 264篇 |
2005年 | 246篇 |
2004年 | 229篇 |
2003年 | 215篇 |
2002年 | 240篇 |
2001年 | 155篇 |
2000年 | 152篇 |
1999年 | 137篇 |
1998年 | 154篇 |
1997年 | 126篇 |
1996年 | 113篇 |
1995年 | 122篇 |
1994年 | 124篇 |
1993年 | 94篇 |
1992年 | 77篇 |
1991年 | 69篇 |
1990年 | 78篇 |
1989年 | 76篇 |
1988年 | 76篇 |
1987年 | 80篇 |
1986年 | 66篇 |
1985年 | 99篇 |
1984年 | 110篇 |
1983年 | 104篇 |
1982年 | 96篇 |
1981年 | 86篇 |
1980年 | 84篇 |
1979年 | 74篇 |
1978年 | 97篇 |
1977年 | 67篇 |
1976年 | 63篇 |
1975年 | 74篇 |
1974年 | 84篇 |
1973年 | 79篇 |
排序方式: 共有9016条查询结果,搜索用时 31 毫秒
1.
2.
Himmerfjärd is a Swedish estuary bordering on the Baltic. The estuary lacks astronomical tides and its circulation is driven by winds and freshwater runoff. Because of a tertiary sewage treatment plant located at its inner end, the estuary is becoming increasingly eutrophic. A field study was carried out for a 78-day period in late summer and early fall of 1977 to determine rates of nutrient transport and to construct nutrient budgets. Since physical parameters (current velocity, temperature, salinity, winds and water level changes) were measured more frequently than nutrients (phosphate, nitrate, nitrite and ammonium) it was necessary to develop a suitable method to calculate nutrient flux time series and net nutrient fluxes. Over the study period, Himmerfjärd imported phosphorus and exported nitrogen. Direction of nutrient fluxes and changes in flux direction were consistent with the structure of the baroclinic currents. 相似文献
3.
Thomas M. Tharp 《Geophysical Journal International》1985,80(2):419-437
Summary. Finite element models for shallow subduction produce realistic behaviour for a wide variety of mechanical strength and density distributions. Characteristic displacements are found to occur even without a discrete low-strength megathrust if there is a high-density subducted plate to localize lithospheric compression. A high-density plate is itself unnecessary in the presence of a low-strength megathrust and regional compression.
Successful finite element models produce an outer arc at the top of the trench slope, and forearc basin with geometry characteristic of natural analogues. These structural features occur by upward inelastic bending of the lithospheric wedge overlying the megathrust. This mechanically unstable behaviour may dissipate significant energy and cause the megathrust to migrate continuously by accretion, tectonic erosion, or abandonment and reinitiation farther offshore. Upward bending in the overriding plate is promoted by low megathrust dip, low megathrust shear strength, and high horizontal compression in the overriding plate. 相似文献
Successful finite element models produce an outer arc at the top of the trench slope, and forearc basin with geometry characteristic of natural analogues. These structural features occur by upward inelastic bending of the lithospheric wedge overlying the megathrust. This mechanically unstable behaviour may dissipate significant energy and cause the megathrust to migrate continuously by accretion, tectonic erosion, or abandonment and reinitiation farther offshore. Upward bending in the overriding plate is promoted by low megathrust dip, low megathrust shear strength, and high horizontal compression in the overriding plate. 相似文献
4.
In the upper Chesapeake Bay (Maryland, U.S.A.) field surveys were conducted at 18 multiple longshore sand bar sites. The multiple bar systems were found in water depths less than approximately 2 m (mean sea level), and exhibited mild bottom slopes of 0·0052 or less. The number of bars composing each system ranged from four to 17 and the spacing between the crests typically increased in the offshore direction, ranging from 12 to 70 m. Bar height also typically increased with distance offshore and ranged from 0·03 to 0·61 m. A grain size analysis of crest and trough sediment did not reveal any significant differences and the sediment was categorized as ‘fine sand’. A review of the literature data indicated that the Chesapeake Bay multiple bars possessed similar characteristics to those found in Gelding Bay (Baltic Sea); similarities in fetch, wave height and tidal range between the two bays may account for this finding. The surf-scaling parameter indicated that the multiple bar systems were extremely dissipative with regard to wave energy, and wave height appeared to be an important factor in controlling bar spacing and bar height. A multiple wave break point hypothesis was discussed as a possible mechanism for the formation of Chesapeake Bay multiple longshore bars, and limited observational evidence appeared to support such a mechanism. 相似文献
5.
6.
Gösta Gahm 《Astrophysics and Space Science》2006,304(1-4):149-152
The number of confirmed and suspected close T Tauri binaries (period days) is increasing. We discuss some systems with enhanced emission line activity and periodic line profile changes. Non-axisymmetric flows of plasma in the region between the circumbinary disk and the stars can be generated through the influence of the secondary component. Such enhanced activity is found around binaries with eccentric as well as circular orbits. We discuss our observations of the T Tauri stars RW Aurigae A and RU Lupi, which may host very close brown dwarf companions. Model simulations indicate that non-axisymmetric flows are generated around close binaries with circumbinary disks, also in systems with circular orbits. 相似文献
7.
Anita Enmark Thomas Berkefeld Oskar von der Lühe Torben Andersen 《Experimental Astronomy》2006,21(2):87-99
A simulation model of the adaptive optics of the German Vacuum Tower Telescope (VTT), Observatorio del Teide, Tenerife, is
presented. The model uses modules from the integrated model of the Euro50 extremely large telescope, and includes submodels
of a Shack-Hartmann wavefront sensor, a de-formable mirror, a tip-tilt mirror, high-voltage amplifier low-pass filters, a
reconstructor and a controller. We investigate the impact on the closed loop bandwidth of changes in controller configuration
and certain system parameters, such as low pass filter bandwidth and camera integration and readout time. Control strategies
were tested on simple models before implementation on the full VTT model. Using the models, different control strategies are
compared. 相似文献
8.
Joachim Köppen 《Astrophysics and Space Science》2003,284(2):837-840
Representative results from a comparison of the chemical evolution of spherical collapse models without and with a intercloud
medium are presented. The hot metal-rich gas distributes quickly the metals produced in supernovae throughout the galaxy,
thus leading to a more homogeneous chemical evolution and flatter metallicity gradients in the gas and the stars. The stellar
population is somewhat less concentrated towards the centre. The strong outflow results in a substantial loss of metals from
the galaxy to its surroundings, and a lower effective yield in the galaxy.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
9.
We present warm dark matter (WDM) as a possible solution to the missing satellites and angular momentum problem in galaxy
formation and introduce improved initial conditions for numerical simulations of WDM models, which avoid the formation of
unphysical haloes found in earlier simulations. There is a hint, that because of that the mass function of satellite haloes
has been overestimated so far, pointing to higher values for the WDM particle mass.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
10.
This paper proposes a multi‐level parallelized substructuring–frontal combined algorithm for the analysis of the problem of thermo/hydraulic/mechanical behaviour of unsaturated soil. Temperature, displacement, pore water pressure and pore air pressure are treated as the primary variables in a non‐linear analysis. Details are given firstly of the substructuring–frontal combined approach. The incorporation of the algorithm in a multi‐level parallel strategy is then discussed. The parallel processing can thus be carried out at different substructural levels. The method thus developed impacts, in a positive way, on both computer storage requirement and execution time. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献