全文获取类型
| 收费全文 | 358篇 |
| 免费 | 73篇 |
| 国内免费 | 56篇 |
专业分类
| 测绘学 | 44篇 |
| 大气科学 | 141篇 |
| 地球物理 | 124篇 |
| 地质学 | 78篇 |
| 海洋学 | 33篇 |
| 天文学 | 4篇 |
| 综合类 | 17篇 |
| 自然地理 | 46篇 |
出版年
| 2023年 | 7篇 |
| 2022年 | 12篇 |
| 2021年 | 6篇 |
| 2020年 | 11篇 |
| 2019年 | 12篇 |
| 2018年 | 16篇 |
| 2017年 | 26篇 |
| 2016年 | 12篇 |
| 2015年 | 16篇 |
| 2014年 | 27篇 |
| 2013年 | 19篇 |
| 2012年 | 19篇 |
| 2011年 | 18篇 |
| 2010年 | 17篇 |
| 2009年 | 24篇 |
| 2008年 | 32篇 |
| 2007年 | 13篇 |
| 2006年 | 22篇 |
| 2005年 | 18篇 |
| 2004年 | 14篇 |
| 2003年 | 20篇 |
| 2002年 | 10篇 |
| 2001年 | 18篇 |
| 2000年 | 15篇 |
| 1999年 | 11篇 |
| 1998年 | 10篇 |
| 1997年 | 8篇 |
| 1996年 | 8篇 |
| 1995年 | 8篇 |
| 1994年 | 7篇 |
| 1993年 | 10篇 |
| 1992年 | 3篇 |
| 1991年 | 3篇 |
| 1990年 | 3篇 |
| 1989年 | 2篇 |
| 1988年 | 6篇 |
| 1987年 | 1篇 |
| 1982年 | 1篇 |
| 1978年 | 1篇 |
| 1954年 | 1篇 |
排序方式： 共有487条查询结果，搜索用时 234 毫秒
91.
Snow chemistry on the glaciers of alpine regions is a good indicator of atmospheric environmental change.We examine snow chemistry in three snowpits at different altitudes on the Haxilegen Glacier No.51,in the Kuitun River source,Tian Shan,China,during July-September 2004 to 2007.We use correlation analysis,factor analysis and sea-salt tracing methods to examine the characteristics and sources of major ions and mineral dust particles in the snow.Results show that mineral dust particles and major ions in the snow pits vary seasonally.During the Asian dust period in springtime,the concentration of mineral dust particles and major ions deposited in snow is high,while the concentration is relatively low during the non-dust period of summer and autumn.This may be caused by dust storm activity in central Asia.The order of major ionic concentrations in the snow packs was determined to be Ca2+ > SO42-> NH4+ > NO3-> Cl-> Na+ > Mg2+ > K+.Ca2+ was the dominant cation;SO42- was the dominant anion.We find,with the exception of NO3-,that the variabilities of ionic concentrations are highly correlated.Results show that the glacier region was significantly affected by dust activity and anthropogenic source.The major ions,especially Na+,originate from dust sources of central Asia and from the Ocean,transported by the westerly winds. 相似文献
92.
The natural stress state in the lithosphere consists of the vertical load and Poisson ratio, and then additional horizontal compression and extension (denoted by ΔσH and ΔσT, respectively) are assumed to be superimposed upon this gravitational stress field. The resulting stress state is composed of the maximum, medium and minimum stresses denoted by σ1, σ2, and σ3, respectively. The stress ratio is given as Φ = (σ2 − σ3)/(σ1 − σ3). A linear relation is found between Φ or 1/Φ and the vertical load in wrench-faulting and extensional stress regimes, respectively. The slope and intercept of the linear relation result in the additional horizontal stresses and level of (average) paleo-surface, respectively. Stress ratio is also determinable by the stress tensor inversion of fault-slip data. The earliest tectonic event (T_1 Event) in the Cretaceous Gyeongsang Basin consists of coexisting E–W compression and N–S extensional faulting episodes. Plots of Φ or 1/Φ against the burial depth (or vertical load) display several linear trends: two clusters in extensional episodes, and two or three clusters in wrench-faulting episodes. Because ΔσH is assumed to be null or negligible in the extensional regime, ΔσT is determinable from the slopes of two linear clusters as being −2.5 to −4.0 MPa. In wrench-faulting episodes, the values of ΔσH are given to be 61.6–101.4 MPa by applying determined additional horizontal extensions. Determined levels of average paleo-surfaces and those of syndepostional structures illustrate that more than five wrench-faulting or extensional episodes have occurred during the T_1 Event, whose active age, consequently, ranges from the Barremian to the Coniacian. This supports that the coexisting coaxial faulting episodes with the same extension may correspond to the alternation of wrench-faulting and extensional episodes. 相似文献
93.
94.
95.
用解析分析、时域有限差分、时-频分析的方法,以地面中心回线装置和阶跃脉冲激励源为例,分析讨论了瞬变电磁测深法的勘探深度问题,以便为野外勘探工作设计提供依据,达到预期的探测目的.解析计算证实了瞬变场在地下以有限速度传播,数值模拟表示出了准静态条件下瞬变场的反射.研究结果表明,由于时间域电磁场遵循因果律,瞬变电磁法的探测深度主要由观测时间决定. 瞬变电磁场的初始传播速度与大地电阻率无关,继后在大地色散作用下,阶跃脉冲前沿逐渐变得平缓,各频率分量的传播速度与电阻率有关,在低阻地层中探测同样的深度需要较长的观测时间. 最大探测深度是在给定时间内电磁波往返地下某一深度的单程距离,最小探测深度受仪器性能的限制,但是埋藏较浅的异常体也有可能在晚时段被观测到.从时-频密度谱中可得到瞬变电磁场信号时间与频率的关系. 相似文献
96.
用双差地震定位法对2005年9月23日克孜尔震群进行重新定位。 从平面上, 重新定位地震集中分布在一个长约14.5 km, 宽约 9.0 km的长方形内, 其长轴为N30°W向, 与克孜尔断裂近乎垂直。 从震源深度来看, 重新定位地震的震源深度全部分布在21 km以内, 集中分布在10~19 km, 平均深度为13.6 km; 震群中绝大部分小震发生在沉积层内, 而震级较大地震基本发生在结晶地壳的上地壳内。 其剖面图显示, 这次震群是从结晶地壳开始沿着N30°W方向向上破裂至沉积层。 根据震区附近断裂性质和该区历史小震震中分布分析认为, 克孜尔水库库区附近可能存在两条共轭断裂, 右翼断裂可能是这次震群的发震构造。 相似文献
97.
Sujay V. Kumar Rolf H. Reichle Christa D. Peters-Lidard Randal D. Koster Xiwu Zhan Wade T. Crow John B. Eylander Paul R. Houser 《Advances in water resources》2008
The Land Information System (LIS) is an established land surface modeling framework that integrates various community land surface models, ground measurements, satellite-based observations, high performance computing and data management tools. The use of advanced software engineering principles in LIS allows interoperability of individual system components and thus enables assessment and prediction of hydrologic conditions at various spatial and temporal scales. In this work, we describe a sequential data assimilation extension of LIS that incorporates multiple observational sources, land surface models and assimilation algorithms. These capabilities are demonstrated here in a suite of experiments that use the ensemble Kalman filter (EnKF) and assimilation through direct insertion. In a soil moisture experiment, we discuss the impact of differences in modeling approaches on assimilation performance. Provided careful choice of model error parameters, we find that two entirely different hydrological modeling approaches offer comparable assimilation results. In a snow assimilation experiment, we investigate the relative merits of assimilating different types of observations (snow cover area and snow water equivalent). The experiments show that data assimilation enhancements in LIS are uniquely suited to compare the assimilation of various data types into different land surface models within a single framework. The high performance infrastructure provides adequate support for efficient data assimilation integrations of high computational granularity. 相似文献
98.
Using reflectance values from the seven MODIS “land” bands with 250 or 500 m resolution, along with a corresponding cloud product, we estimate the fraction of each 500 m pixel that snow covers, along with the albedo of that snow. The daily products have data gaps and errors because of cloud cover and sensor viewing geometry. Rather than make users interpolate and filter these patchy daily maps without completely understanding the retrieval algorithm and instrument properties, we use the daily time series to improve the estimate of the measured snow properties for a particular day. We use a combination of noise filtering, snow/cloud discrimination, and interpolation and smoothing to produce our best estimate of the daily snow cover and albedo. We consider two modes: one is the “predictive” mode, whereby we estimate the snow-covered area and albedo on that day using only the data up to that day; the other is the “retrospective” mode, whereby we reconstruct the history of the snow properties for a previous period. 相似文献
99.
100.