全文获取类型
收费全文 | 22630篇 |
免费 | 4332篇 |
国内免费 | 5870篇 |
专业分类
测绘学 | 2188篇 |
大气科学 | 4221篇 |
地球物理 | 5485篇 |
地质学 | 11858篇 |
海洋学 | 3204篇 |
天文学 | 846篇 |
综合类 | 2177篇 |
自然地理 | 2853篇 |
出版年
2024年 | 197篇 |
2023年 | 560篇 |
2022年 | 1216篇 |
2021年 | 1442篇 |
2020年 | 1244篇 |
2019年 | 1357篇 |
2018年 | 1446篇 |
2017年 | 1289篇 |
2016年 | 1372篇 |
2015年 | 1296篇 |
2014年 | 1517篇 |
2013年 | 1552篇 |
2012年 | 1509篇 |
2011年 | 1510篇 |
2010年 | 1482篇 |
2009年 | 1417篇 |
2008年 | 1295篇 |
2007年 | 1192篇 |
2006年 | 1007篇 |
2005年 | 887篇 |
2004年 | 670篇 |
2003年 | 604篇 |
2002年 | 582篇 |
2001年 | 621篇 |
2000年 | 607篇 |
1999年 | 742篇 |
1998年 | 592篇 |
1997年 | 542篇 |
1996年 | 530篇 |
1995年 | 416篇 |
1994年 | 409篇 |
1993年 | 349篇 |
1992年 | 312篇 |
1991年 | 201篇 |
1990年 | 167篇 |
1989年 | 140篇 |
1988年 | 107篇 |
1987年 | 82篇 |
1986年 | 67篇 |
1985年 | 57篇 |
1984年 | 44篇 |
1983年 | 39篇 |
1982年 | 36篇 |
1981年 | 14篇 |
1980年 | 19篇 |
1979年 | 22篇 |
1978年 | 17篇 |
1977年 | 9篇 |
1976年 | 9篇 |
1958年 | 9篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
51.
—?This paper reports results from two recent monitoring experiments in Wyoming. Broadband seismic recordings of kiloton class delay-fired cast blasts and instantaneous calibration shots in the Black Thunder coal mine were made at four azimuths at ranges from 1° to 2°. The primary focus of this experiment was to observe and to explain low-frequency signals that can be seen at all azimuths and should routinely propagate above noise to mid-regional distances where most events will be recorded by International Monitoring System (IMS) stations.¶The recordings clearly demonstrate that large millisecond delay-fired cast blasts routinely produce seismic signals that have significant spectral modulations below 10?Hz. These modulations are independent of time, the azimuth from the source and the orientation of the sensor. Low-frequency modulations below 5?Hz are seen beyond 9°. The modulations are not due to resonance as they are not produced by the calibration shots. Linear elastic modeling of the blasts that is guided by mine-blast reports fails to reproduce the fine detail of these modulations but clearly indicates that the enhanced “spectral roughness” is due to long interrow delays and source finiteness. The mismatch between the data and the synthetics is likely due to source processes, such as nonlinear interactions between shots, that are poorly understood and to other effects, such as variations of shot time and yield from planned values, that are known to be omnipresent but cannot be described accurately. A variant of the Automated Time-Frequency Discriminant (Hedlin, 1998b), which uses low-frequency spectral modulations, effectively separates these events from the calibration shots.¶The experiment also provided evidence that kiloton class cast blasts consistently yield energetic 2–10 second surface waves. The surface waves are strongly dependent on azimuth but are seen beyond 9°. Physical modeling of these events indicates that the surface waves are due mainly to the extended source duration and to a lesser extent to the slap-down of spalled material. The directionality is largely a path effect. A discriminant that is based on the partitioning of energy between surface and body waves routinely separates these events from the calibration shots.¶The Powder River Basin has essentially no natural seismic activity. How these mining events compare to earthquake observations remains to be determined. 相似文献
52.
53.
54.
岫岩-海城5.4级地震前小震震源机制解与记录特征分析 总被引:4,自引:2,他引:4
采用Pn、Pg初始波初动符号,利用乌尔夫网上半球投影,用作图方法求解了岫岩-海城震区(1999年1月-1999年11月29日)主震前辽宁数字地震台网记录(ML≥2.5)的41个小震的震源机制参数。结合前震记录的某些特征,对主震前应力方向的时空变化,震源错动性质进行分析和讨论。 相似文献
55.
The roughness length for momentum (z0m), zero-plane displacementheight (d), and roughness length for heat (z0h) are importantparameters used to estimate land-atmosphere energy exchange. Although many different approaches have been developed to parameterizemomentum and heat transfer, existing parameterizations generally utilizehighly simplified representations of vegetation structure. Further, a mismatch exists between the treatments used for momentum and heat exchange and those used for radiative energy exchanges. In this paper, parameterizations are developed to estimate z0m, d, and z0h for forested regimes using information related to tree crown density and structure. The parameterizations provide realistic representationfor the vertical distribution of foliage within canopies, and include explicit treatment for the effects of the canopy roughness sublayer and leaf drag on momentum exchange. The proposed parameterizationsare able to realistically account for site-to-site differences in roughness lengths that arise from canopy structural properties.Comparisons between model predictions and field measurements show good agreement, suggesting that the proposed parameterizations capture the most important factors influencing turbulent exchange of momentumand heat over forests. 相似文献
56.
对中国大陆已发生的多次7级以上强震震中区气温、气压资料进行了研究,结果表明:在大震前(30天之内),气温-气压函数的平衡状态被破坏,其相关系数(代数值)不断增大,“正弓”形可作为短临异常的标志,同时对其成因也进行了初步探讨。 相似文献
57.
58.
用Pg波走时重建华北地区结晶基底速度及时间项图像 总被引:3,自引:0,他引:3
应用层析成象以投影方法,对华北地区28条深地震剖剖面地震测深剖面Pg波走时进行处理,作出该区结晶基底速度及时间项成像。并作了方法的数值模型合成数据的敏感性分析,证明所采用有效性及成象结果的可靠性。计算中应用LSQR算法求解大型稀疏超定方程。并采用只存储矩阵非零元素技术,使存储量减小到原矩阵百分之一,同时零元素不参数加计算,提高了运算速度。华北地区结昌基底速度为5.9-6.35km/s,时间最大为2 相似文献
59.
应用人工神经网络的方法,利用30次强震震后1天和2天内的地震资料作为学习样本,对广西及其邻区发生的4次地震的震型作了早期预测判定,结果表明应用效果较好,正确率达75%。该方法值得进一步研究。 相似文献
60.
921型超低频有源伺服式速度,位移计 总被引:1,自引:2,他引:1
本介绍一种在很小体积的往复式摆上采用有源伺服技术实现超低频大幅值测量用的速度计、位移计,它可在多个频带测量速度和位移。最低频率可达0.04Hz,最大可测位移为±15m。它可用于海浪浪高、抽油机行程、高柔结构的超低频大位移测量。该仪器于1992年获得国家专利,并获得中国专利博览会银奖。 相似文献