全文获取类型
收费全文 | 55093篇 |
免费 | 594篇 |
国内免费 | 1279篇 |
专业分类
测绘学 | 2498篇 |
大气科学 | 4342篇 |
地球物理 | 10592篇 |
地质学 | 22072篇 |
海洋学 | 3468篇 |
天文学 | 9104篇 |
综合类 | 2284篇 |
自然地理 | 2606篇 |
出版年
2021年 | 279篇 |
2020年 | 277篇 |
2019年 | 325篇 |
2018年 | 5349篇 |
2017年 | 4596篇 |
2016年 | 3504篇 |
2015年 | 813篇 |
2014年 | 1033篇 |
2013年 | 1701篇 |
2012年 | 1954篇 |
2011年 | 3821篇 |
2010年 | 2971篇 |
2009年 | 3561篇 |
2008年 | 2992篇 |
2007年 | 3405篇 |
2006年 | 1229篇 |
2005年 | 1170篇 |
2004年 | 1252篇 |
2003年 | 1253篇 |
2002年 | 1093篇 |
2001年 | 816篇 |
2000年 | 793篇 |
1999年 | 685篇 |
1998年 | 638篇 |
1997年 | 629篇 |
1996年 | 568篇 |
1995年 | 534篇 |
1994年 | 506篇 |
1993年 | 428篇 |
1992年 | 384篇 |
1991年 | 411篇 |
1990年 | 411篇 |
1989年 | 387篇 |
1988年 | 359篇 |
1987年 | 432篇 |
1986年 | 361篇 |
1985年 | 444篇 |
1984年 | 478篇 |
1983年 | 462篇 |
1982年 | 444篇 |
1981年 | 375篇 |
1980年 | 377篇 |
1979年 | 311篇 |
1978年 | 306篇 |
1977年 | 294篇 |
1976年 | 264篇 |
1975年 | 254篇 |
1974年 | 281篇 |
1973年 | 305篇 |
1972年 | 194篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
111.
A formal derivation and numerical modelling of the improved Boussinesq equations for varying depth 总被引:1,自引:0,他引:1
A formal derivation of the improved Boussinesq equations of Madsen and Sørens (1992) is presented to provide the correct forms of the depth-gradient related terms. Linear shoaling characteristics of the new equations are investigated by the method of Madsen and Sørensen (1992) and by the energy flux concept separately and found to agree perfectly, whereas these approaches give conflicting results for the equations derived by Madsen and Sørensen (1992). Furthermore, Nwogu's (1993) modified Boussinesq model is found to produce a linear shoaling-gradient identical with the present work. Numerical modelling of the derived equations for directional waves is carried out by three-time-level finite-difference approximations. A higher-order radiation condition is implemented for effective absorption of the outgoing waves. Several test cases are included to demonstrate the performance of the model. 相似文献
112.
Wave records at seven different locations within a groin field have been analysed by both statistical and spectral approaches to study the general wave climate. The wave heights and periods from the wave records were obtained by both upcross and downcross methods for the statistical approach. The variation of different wave height and wave period parameters with respect to the time of measurement at a particular location and its variation at different locations within the groin field at a particular instant of time are presented and discussed in detail in this paper. It is generally found that the wave heights follow the Weibull distribution. 相似文献
113.
Detection of buried targets using a synthetic aperture sonar 总被引:1,自引:0,他引:1
Piper J.E. Commander K.W. Thorsos E.I. Williams K.L. 《Oceanic Engineering, IEEE Journal of》2002,27(3):495-504
This paper presents observations of buried target detections made using a 20-kHz synthetic aperture sonar. At grazing angles below the critical angle, surprisingly high signal-to-noise detections were made of cylindrical targets buried at depths of 15 and 50 cm. During a separate set of measurements, buried spheres were clearly seen at steep grazing angles, but were generally not seen below the critical angle. Since scattering from wave-generated sand ripples may contribute to detections at grazing angles below critical angle, the information available on the ripple fields is discussed and used in acoustic backscatter simulations for the buried spheres. Lack of information on the ripple height precludes a definitive explanation for the absence of buried sphere detections at subcritical grazing angles. 相似文献
114.
115.
K. T. Pickering 《Geo-Marine Letters》1982,2(1-2):41-46
Deep-water siliciclastic systems are classified primarily on their shape as: submarine fans with well developed or poorly
developed morphology, slope drapes, for example, over relatively stable basin margins, fault-scarp aprons, canyons and large
channels, under-supplied sheet systems such as abyssal plains, non-fan ponded systems such as over-supplied perched basins,
and fan deltas. Collectively, or separately, these systems may form sedimentary basin fills that can be over or under-supplied
with respect to the sediment input although most systems will tend toward over-supply/overflow with time. Finally, the sum
total of the siliciclastic systems and basins can be used to define the tectonic milieux such as passive, strike-slip and
convergent margins. 相似文献
116.
A. A. Bezborodov S. L. Gokchen V. N. Eremeev I. T. Konuk A. K. Saidam E. M. Tirasin 《Physical Oceanography》1991,2(3):227-231
The hydrological and hydrochemical structures of the upper 300 m water column of the Black Sea in autumn 1988 have been studied. Regularities in the distribution of the physico-chemical characteristics in the zone of interaction between aerobic and anaerobic waters as well as the topography of the H2S zone boundary and its connection with a certain density gradient have been found.Translated by Mikhail M. Trufanov. 相似文献
117.
118.
K. H. Kwik 《Ocean Engineering》1989,16(5-6)
The moment at which in a two-ship encounter a collision avoidance manoeuvre should be initiated in order to achieve a specific distance at the closest point of approach has been calculated. The calculations were based on the kinematics of encounter and simplified equations of motion. Mathematical equations which relate the maneouvring distance, the turning direction and the extent of course change for any desired passing distance and any given turning rate are determined. Knowledge of these quantities is essential for the safe conduct of the ship with automatic, computer-assisted navigation as well as with ordinary, manual navigation if the navigator no longer contents himself with estimates but wishes to know precisely in advance the outcome of any specific manoeuvre he chooses. 相似文献
119.
120.