全文获取类型
收费全文 | 6185篇 |
免费 | 1163篇 |
国内免费 | 1633篇 |
专业分类
测绘学 | 757篇 |
大气科学 | 542篇 |
地球物理 | 2036篇 |
地质学 | 3815篇 |
海洋学 | 574篇 |
天文学 | 236篇 |
综合类 | 509篇 |
自然地理 | 512篇 |
出版年
2024年 | 52篇 |
2023年 | 132篇 |
2022年 | 200篇 |
2021年 | 300篇 |
2020年 | 339篇 |
2019年 | 330篇 |
2018年 | 294篇 |
2017年 | 357篇 |
2016年 | 370篇 |
2015年 | 354篇 |
2014年 | 403篇 |
2013年 | 473篇 |
2012年 | 411篇 |
2011年 | 380篇 |
2010年 | 341篇 |
2009年 | 341篇 |
2008年 | 412篇 |
2007年 | 395篇 |
2006年 | 372篇 |
2005年 | 312篇 |
2004年 | 261篇 |
2003年 | 256篇 |
2002年 | 237篇 |
2001年 | 176篇 |
2000年 | 197篇 |
1999年 | 188篇 |
1998年 | 180篇 |
1997年 | 169篇 |
1996年 | 136篇 |
1995年 | 130篇 |
1994年 | 94篇 |
1993年 | 85篇 |
1992年 | 69篇 |
1991年 | 51篇 |
1990年 | 53篇 |
1989年 | 36篇 |
1988年 | 22篇 |
1987年 | 24篇 |
1986年 | 6篇 |
1985年 | 10篇 |
1984年 | 5篇 |
1983年 | 6篇 |
1982年 | 5篇 |
1981年 | 2篇 |
1980年 | 3篇 |
1979年 | 2篇 |
1976年 | 3篇 |
1973年 | 1篇 |
1971年 | 1篇 |
1954年 | 4篇 |
排序方式: 共有8981条查询结果,搜索用时 15 毫秒
111.
112.
113.
114.
115.
根据统计分布与信息熵理论,定义了震级信息熵Hm和地震间隔时间信息熵Ht,并推导了它们的计算公式。通过时空扫描和计算,发现强震前1~3年Hm和Ht出现低值异常,与地震有较好的对应关系,可以作为一组中期或中短期预测指标。 相似文献
116.
117.
Introduction The Western Kunlun Mountain Pass M=8.1 earthquake occurred on November 14, 2001 is the other M=8 earthquake occurred 50 years after Dangxiong, Tibet M=8.0 earthquake in Chinese mainland. The earthquake has caused the attention of the seismologists in the following aspects: 1) The fracture length is more than 400 km, which is far away from the estimated length by the statistic empirical function between the magnitude and the fracture length (WANG, et al, 2002); 2) The aftersh… 相似文献
118.
George F. Risk T. Grant Caldwell Hugh M. Bibby 《Journal of Volcanology and Geothermal Research》2003,127(1-2):33-54
Experimental measurements in the Ngatamariki geothermal field, North Island, New Zealand were made to test the applicability of the time domain electromagnetic method for detailed investigation of the resistivity structure within a geothermal field. Low-frequency square wave signals were transmitted through three grounded bipole current sources sited about 8 km from the measurement lines. Despite high levels of electrical noise, transient electric field vectors could be determined reliably for times between 0.02 and 3.3 s after each step in the source current. Instantaneous apparent resistivity tensors were then calculated. Apparent resistivity pseudosections along the two measurement lines show smooth variations of resistivity from site to site. Over most of the field the images consistently show a three-layer resistivity structure with a conductive middle layer (3–10 Ωm) representing the conductive upper part of the thermal reservoir. A deep-seated region of low resistivity in the northwest of the field may indicate a conductive structure at about 1 km associated with a deeper diorite intrusion. Measurements sited closer than about 100 m to drillholes appear to have been disturbed by metallic casing in the holes. A change in resistivity structure in the east of the field may indicate a major geological or hydrothermal boundary. 相似文献
119.
A Bayesian inference approach is introduced to identify soil degradation behaviours at four downhole array sites. The approach of inference is based on a parametric time‐varying infinite impulse response filter model. The approach is shown to be adaptive to the changes of filter parameters and noise amplitudes. Four sites, including the Lotung (Taiwan), Chiba (Japan), Garner Valley (California), and Treasure Island (California) sites with downhole seismic arrays are analysed. Our results show two major types of soil degradation behaviour: the well‐known strain‐dependent softening, and reduction in stiffness that is not instantaneously recoverable. It is also found that both types of soil degradation are more pronounced in sandy soils than in clayey soils. The mechanism for the second type of soil degradation is not yet clear to the authors and suggested to be further studied. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
120.
A predictive instantaneous optimal control (PIOC) algorithm is proposed for controlling the seismic responses of elastic structures. This algorithm compensates for the time delay that happens in practical control applications by predicting the structural response over a period that equals the time delay, and by substituting the predicted response in the instantaneous optimal control (IOC) algorithm. The unique feature of this proposed PIOC algorithm is that it is simple and at the same time compensates for the time delay very effectively. Numerical examples of single degree of freedom structures are presented to compare the performance of PIOC and IOC systems for various time delay magnitudes. Results show that a time delay always causes degradation of control efficiency, but PIOC can greatly reduce this degradation compared to IOC. The effects of the structure's natural periods and the choice of control gains on the degradation induced by the time delay are also analyzed. Results show that shorter natural periods and larger control gains are both more sensitive and more serious to the degradation of control efficiency. Finally, a practical application of PIOC is performed on a six‐story moment‐resisting steel frame. It is demonstrated that PIOC contributes significantly to maintain stability in multiple degree of freedom structures, and at the same time PIOC has a satisfactory control performance. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献