全文获取类型
收费全文 | 261500篇 |
免费 | 4282篇 |
国内免费 | 3300篇 |
专业分类
测绘学 | 6822篇 |
大气科学 | 19166篇 |
地球物理 | 54467篇 |
地质学 | 91114篇 |
海洋学 | 21713篇 |
天文学 | 56965篇 |
综合类 | 994篇 |
自然地理 | 17841篇 |
出版年
2021年 | 2220篇 |
2020年 | 2574篇 |
2019年 | 2830篇 |
2018年 | 3919篇 |
2017年 | 3738篇 |
2016年 | 5913篇 |
2015年 | 4175篇 |
2014年 | 6891篇 |
2013年 | 14179篇 |
2012年 | 6675篇 |
2011年 | 8219篇 |
2010年 | 7480篇 |
2009年 | 9969篇 |
2008年 | 8645篇 |
2007年 | 8256篇 |
2006年 | 9570篇 |
2005年 | 7812篇 |
2004年 | 7614篇 |
2003年 | 7123篇 |
2002年 | 6637篇 |
2001年 | 5920篇 |
2000年 | 5885篇 |
1999年 | 5187篇 |
1998年 | 5213篇 |
1997年 | 5016篇 |
1996年 | 4657篇 |
1995年 | 4399篇 |
1994年 | 4086篇 |
1993年 | 3831篇 |
1992年 | 3614篇 |
1991年 | 3584篇 |
1990年 | 3748篇 |
1989年 | 3505篇 |
1988年 | 3291篇 |
1987年 | 3837篇 |
1986年 | 3397篇 |
1985年 | 4216篇 |
1984年 | 4723篇 |
1983年 | 4396篇 |
1982年 | 4305篇 |
1981年 | 3921篇 |
1980年 | 3637篇 |
1979年 | 3504篇 |
1978年 | 3481篇 |
1977年 | 3272篇 |
1976年 | 3038篇 |
1975年 | 2955篇 |
1974年 | 2912篇 |
1973年 | 3071篇 |
1972年 | 2023篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
C. Ranz-Guerra 《Geo-Marine Letters》1991,11(3-4):172-173
Mud is a complex mixture of water and solid particles and acoustics can help on its control. The mud layer, as an acoustic
channel, is characterized by its propagation constant relating the wave frequency, the sound velocity, and the energy absorption.
If the input acoustic pulse is known, it is feasible to easily characterize the mud layer as a low pass filter (absorption);
if it is also possible to insonify the mud with sound of both low and high frequencies, the sound speed of the mud mixture
can be evaluated, and the propagation constant is then known. 相似文献
992.
993.
994.
995.
A comprehensive study of undisturbed, mostly silty clay samples, taken from large-diameter surface cores collected in the North Atlantic and its bordering seas, shows that measurement of the Atterberg limits, water content, specific gravity of grains and grain-size distribution can provide the basis for predicting many seabed geotechnical and geophysical properties which can lead to a quick assessment of environmental conditions. Thus, the sediment compression curve can be reconstructed, acoustic properties assessed, electrical and thermal resistivities deduced and an indication of the shearing forces, which have affected the sediment, inferred. 相似文献
996.
997.
998.
The nonlinear energy transfer through the wave spectrum is studied on the basis of the previously obtained explicit equation for matrix elements of a four-wave kinetic integral. The equation describes the evolution of a system of gravity waves at the surface of a sea of finite depth with a uniform distribution of broken ice over the sea surface. Particular attention is paid to the analytical part of the algorithm of the calculation of the kinetic integral. This part differs from the standard algorithm by a set of prominent features of the dispersion relation for wave oscillations in the ice-covered water. The kinetic integral for the system under consideration is calculated, and the results are compared with the results obtained for the ice-free water. 相似文献
999.
Video assessment of environmental impacts of salmon farms 总被引:1,自引:0,他引:1
1000.
This article presents a comparative study of the breathing resistance and clearing characteristics of several types of commercially available snorkels in an attempt to device quantitative means that enable the divers to select the best snorkel and the designers to see the possible directions and modifications necessary for improving the performance of such important life support devices.Two test stands have therefore been built, the first to measure the breathing resistance and the second to monitor the clearing efficiency and speed of some of the commonly used snorkels.The tested snorkels varied from the simple J-type to the sophisticated wrap-around snorkels with convoluted flexible hoses and flared ends.The results obtained show that large diameter snorkels of the wrap-around design with sweeping barrels and flared ends (as the AMF Swimmaster snorkel number 2S55) offer the least breathing resistance. As for the clearing characteristics snorkels with small diameters and of the simple J-type (as the Spanish Aquasub snorkel: The Canarias) or with large diameters and sweeping barrels (as the Power Tuned snorkel of U.S. divers) possess the best clearing figure of merit. 相似文献