全文获取类型
| 收费全文 | 1548篇 |
| 免费 | 129篇 |
| 国内免费 | 167篇 |
专业分类
| 测绘学 | 172篇 |
| 大气科学 | 85篇 |
| 地球物理 | 291篇 |
| 地质学 | 114篇 |
| 海洋学 | 963篇 |
| 天文学 | 26篇 |
| 综合类 | 134篇 |
| 自然地理 | 59篇 |
出版年
| 2024年 | 4篇 |
| 2023年 | 24篇 |
| 2022年 | 29篇 |
| 2021年 | 49篇 |
| 2020年 | 58篇 |
| 2019年 | 46篇 |
| 2018年 | 44篇 |
| 2017年 | 76篇 |
| 2016年 | 51篇 |
| 2015年 | 51篇 |
| 2014年 | 75篇 |
| 2013年 | 143篇 |
| 2012年 | 57篇 |
| 2011年 | 78篇 |
| 2010年 | 74篇 |
| 2009年 | 86篇 |
| 2008年 | 96篇 |
| 2007年 | 101篇 |
| 2006年 | 86篇 |
| 2005年 | 82篇 |
| 2004年 | 69篇 |
| 2003年 | 56篇 |
| 2002年 | 52篇 |
| 2001年 | 42篇 |
| 2000年 | 47篇 |
| 1999年 | 47篇 |
| 1998年 | 34篇 |
| 1997年 | 33篇 |
| 1996年 | 33篇 |
| 1995年 | 21篇 |
| 1994年 | 20篇 |
| 1993年 | 20篇 |
| 1992年 | 18篇 |
| 1991年 | 8篇 |
| 1990年 | 11篇 |
| 1989年 | 1篇 |
| 1988年 | 3篇 |
| 1987年 | 1篇 |
| 1986年 | 1篇 |
| 1985年 | 3篇 |
| 1984年 | 5篇 |
| 1983年 | 2篇 |
| 1982年 | 2篇 |
| 1981年 | 3篇 |
| 1980年 | 2篇 |
排序方式： 共有1844条查询结果，搜索用时 0 毫秒
11.
This paper presents results of a small scale study that utilized particle-tracking techniques to evaluate transport of river
water through an alluvial aquifer in a bank infiltration testing site in El Paso, Texas, USA. The particle-tracking survey
was used to better define filtration parameters. Several simulations were generated to allow visualization of the effects
of well placement and pumping rate on flow paths, travel time, the size of the pumping influence zone, and proportion of river-derived
water and groundwater mixing in the pumping well. Simulations indicate that migration of river water into the aquifer is generally
slow. Most water does not arrive at the well by the end of an 18-day pumping period at 0.54 m3/min pumping rate for a well located 18 m from the river. Forty-four percent of the water pumped from the well was river water.
The models provided important information needed to design appropriate sampling schedules for bank filtration practices and
ensured meeting adequate soil-retention times. The pumping rate has more effect on river water travel time than the location
of the pumping well from the river. The examples presented in this paper indicate that operating the pumping well at a doubled
distance from the river increased the time required for the water to travel to the well, but did not greatly change the capture
zone. 相似文献
12.
13.
14.
15.
16.
针对带乘性噪声的一类非线性系统,给出了1种带单重渐消因子的强跟踪状态滤波算法。该算法将非线性系统线性化后,采用了线性最小方差估计方法来进行状态估计,通过运用正交原理和引入渐消因子,使得滤波效果具有强跟踪的优良性能。该算法扩展了卡尔曼滤波在带乘性噪声非线性系统状态估计中的应用范围。仿真结果表明了该算法的有效性。 相似文献
17.
首次使用266nm激光脉冲对泥鳅受精卵进行辐照,引起畸变。辐照分别对全卵、只对胚盘、只对卵黄三种方式进行。对实验结果的分析说明266nm激光对泥鳅受精卵的作用是光化学作用,而不是光致热效应,可能引起DNA分子结构—碱基的变化。核质关系实验证明了细胞质对细脆发育过程有控制作用。 相似文献
18.
Estuary/ocean exchange and tidal mixing in a Gulf of Maine Estuary: A Lagrangian modeling study 总被引：2，自引：0，他引：2
Ata Bilgili Jeffrey A. Proehl Daniel R. Lynch Keston W. Smith M. Robinson Swift 《Estuarine, Coastal and Shelf Science》2005,65(4):607-624
A Lagrangian particle method embedded within a 2-D finite element code, is used to study the transport and ocean–estuary exchange processes in the well-mixed Great Bay Estuarine System in New Hampshire, USA. The 2-D finite element model, driven by residual, semi-diurnal and diurnal tidal constituents, includes the effects of wetting and drying of estuarine mud flats through the use of a porous medium transport module. The particle method includes tidal advection, plus a random walk model in the horizontal that simulates sub-grid scale turbulent transport processes. Our approach involves instantaneous, massive [O(500,000)] particle releases that enable the quantification of ocean–estuary and inter-bay exchanges in a Markovian framework. The effects of the release time, spring–neap cycle, riverine discharge and diffusion strength on the intra-estuary and estuary–ocean exchange are also investigated.The results show a rather dynamic interaction between the ocean and the estuary with a fraction of the exiting particles being caught up in the Gulf of Maine Coastal Current and swept away. Three somewhat different estimates of estuarine residence time are calculated to provide complementary views of estuary flushing. Maps of residence time versus release location uncover a strong spatial dependency of residence time within the estuary that has very important ramifications for local water quality. Simulations with and without the turbulent random walk show that the combined effect of advective shear and turbulent diffusion is very effective at spreading particles throughout the estuary relatively quickly, even at low (1 m2/s) diffusivity. The results presented here show that a first-order Markov Chain approach has applicability and a high potential for improving our understanding of the mixing processes in estuaries. 相似文献
19.
20.
This paper addresses an important problem in ship control application—the robust stabilization of underactuated ships on a linear course with comfort. Specifically, we develop a multivariable controller to stabilize ocean surface ships without a sway actuator on a linear course and to reduce roll and pitch simultaneously. The controller adapts to unknown parameters of the ship and constant environmental disturbances induced by wave, ocean current and wind. It is also robust to time-varying environmental disturbances, time-varying change in ship parameters and other motions of the ship such as surge and heave. The roll and pitch can be made arbitrarily small while the heading angle and sway are kept to be in reasonably small bounds. The controller development is based on Lyapunov’s direct method and backstepping technique. A Lipschitz continuous projection algorithm is used to update the estimate of the unknown parameters to avoid the parameters’ drift due to time-varying environmental disturbances. Simulations on a full-scale catamaran illustrate the effectiveness of our proposed controller. 相似文献