全文获取类型
收费全文 | 14117篇 |
免费 | 3053篇 |
国内免费 | 1394篇 |
专业分类
测绘学 | 802篇 |
大气科学 | 515篇 |
地球物理 | 9037篇 |
地质学 | 4353篇 |
海洋学 | 1106篇 |
天文学 | 254篇 |
综合类 | 899篇 |
自然地理 | 1598篇 |
出版年
2024年 | 28篇 |
2023年 | 116篇 |
2022年 | 356篇 |
2021年 | 490篇 |
2020年 | 566篇 |
2019年 | 653篇 |
2018年 | 501篇 |
2017年 | 548篇 |
2016年 | 504篇 |
2015年 | 569篇 |
2014年 | 704篇 |
2013年 | 798篇 |
2012年 | 813篇 |
2011年 | 845篇 |
2010年 | 686篇 |
2009年 | 778篇 |
2008年 | 838篇 |
2007年 | 963篇 |
2006年 | 964篇 |
2005年 | 804篇 |
2004年 | 789篇 |
2003年 | 704篇 |
2002年 | 618篇 |
2001年 | 438篇 |
2000年 | 474篇 |
1999年 | 427篇 |
1998年 | 424篇 |
1997年 | 364篇 |
1996年 | 375篇 |
1995年 | 283篇 |
1994年 | 261篇 |
1993年 | 231篇 |
1992年 | 155篇 |
1991年 | 107篇 |
1990年 | 82篇 |
1989年 | 77篇 |
1988年 | 61篇 |
1987年 | 49篇 |
1986年 | 32篇 |
1985年 | 18篇 |
1984年 | 8篇 |
1983年 | 10篇 |
1982年 | 6篇 |
1981年 | 7篇 |
1980年 | 5篇 |
1979年 | 18篇 |
1976年 | 3篇 |
1973年 | 1篇 |
1971年 | 1篇 |
1954年 | 11篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
992.
Wavepath traveltime tomography 总被引:1,自引:1,他引:1
The elastic-wave equation is used to construct sensitivity kernels relating perturbations in elastic parameters to traveltime deviations. Computation of the functions requires a correlation of the forward-propagating seismic wavefield with a backward propagation of the residual wavefield. The computation of the wavefields is accomplished using a finite difference algorithm and is efficiently executed on a CM-2 parallel processor. The source and receiver locations have maximum sensitivity to velocity structure. The sensitivity kernels or wavepaths are well suited for transmission traveltime inversion such as cross-borehole tomography and vertical seismic profiling. Conventional ray tomography and wavepath tomography are applied to a set of P -wave arrival times, from a cross-borehole experiment at Kesterson, California. Because the wavepaths have increased sensitivity near the source and receiver there are differences in resolution of the velocity structure. Both techniques recover the same relative variations in velocity where the coverage is adequate. The wavepath solution is more laterally continuous and the dominant variation is vertical, as is expected for the layered sediments in this region. 相似文献
993.
We invert ISC PcP and PKP absolute and differential traveltimes in an attempt to infer the long-wavelength topography of the core-mantle boundary (CMB). The data selection and processing methods are described and evaluated. These travel-time data are very noisy and the geographic distribution of the data is highly non-uniform, inhibiting reliable inference of CMB topography. Spatial averaging enhances the coherent component of the residual variance (related to heterogeneity), however, the random component of the variance is much larger than the coherent component. We show that for PcP data the coherent signal due to mantle heterogeneity overshadows that arising from the CMB, and that the effects of mantle heterogeneity are mapped into our inferred CMB solutions. The PcP data are not correlated across the spatial averaging bins and seem to have a strong bias due to small-scale structure and/or noise. The non-uniform geographic sampling of the data plays a role in the mapping of mantle heterogeneity onto the CMB. Spatial patterns of CMB models inferred from different phases do not agree. Amplitudes of seismically inferred CMB undulations vary greatly. The sensitivity of inferred CMB models to the processing, spatial averaging procedure, and inversion techniques are investigated. Topographic amplitudes increase strongly with increasing input residual variance. The power spectrum of inferred topography indicates that there are unmodelled heterogeneities that must be described with spherical harmonics of degree 6 and higher. Based on this work, we conclude that reliable inference of long-wavelength CMB topography is not likely with the current ISC data set or with a spherical harmonic expansion truncated to degree and order 6. 相似文献
994.
995.
本文讨论了适合于SPOT图象的用光束法平差解算外方位元素的数学模型;介绍了在国产解析测图仪APS—1上进行SOPT图象立体测图的方法;研究了SPOT投影方式的实时循环程序LOOP及SPOT图象的测图软件包。 相似文献
996.
福州盆地及其周围地区地壳深部结构与构造的初步研究 总被引:20,自引:4,他引:20
1986-1988年,福建省地震局在福州盆地及其周围地区完成了五条地震测深剖面:宁德-永春剖面;洪懒-宁德剖面;莲峰-福州(尚干)剖面;南平-永泰-平潭非纵剖面;并在宁德-古田-嵩口首次试验扇形剖面观测系统,结果表明,扇形剖面对确定断裂是有效的. 根据对地震波走时的正、反演计算,用理论地震图和射线追踪方法进一步修改和完善,得到了福州盆地及其周围地区地壳深部为高、低速相间的速度分布特征.地壳介质速度为6.40km/s,在中地壳普遍发育一层速度为5.80-5.90 km/s、厚度约5.0 km的低速层.Moho面深度30-33 km,福州盆地为一Moho面隆起区,隆起幅度达3.0 km.北西向断裂发育,其中,闽江断裂在纵剖面和扇形剖面均有明显的反映,并切割到Moho面2.0-3.0 km. 反射波谱频方法计算得到福州盆地地壳Q值仅120-150,比邻近地区明显偏低,中地壳低速层Q值为40-80,是一典型的低速-高导-低Q(高衰减)结构层,此层上界面埋深14-15 km,与居里面埋深较一致.这些结果为福州地区的地热资源远景预测和开发提供了地壳深部结构和某些地球物理场背景. 相似文献
997.
998.
Damage caused by the earthquake of 7 December 1986 (M = 5.7) and its aftershocks in low-storey residential buildings in the town of Strazhitza, Bulgaria, situated in the epicentral zone are systematized. A scale of damages is compiled for two types of buildings which is coordinated with the MSK-64 scale. The territorial distribution of the seismic intensity is compared with the results of the detailed engineering-geological study. A correlation between the Quarternary deposit thickness and the observed seismic intensity is obtained.Paper presented at the 21st general assembly of the European Seismological Commission held in Sofia, 1988. 相似文献
999.
作者详细推导了N个薄层介质组合时所引起的视各向异性介质的弹性参数,扩展了J.E.white先生在《Underground Sound》一书中关于双层介质组合的视各向异性介质的弹性参数应用范围,并计算了2个薄层各向同性的弹性介质组合时视各向异性介质中地震波速度随波动传播方向而变化的关系曲线,说明了薄层组合确实可以引起介质的视各向异性。 相似文献
1000.
几十年来,人们对地震波在真实介质中传播发生的衰减进行了理论与实践两方面研究。文中综述了前人工作,包括机制与测试工作,同时给以评论,指出对于传播距离与频率之间关系研究不够,并且对实测结果作物理解释较少。作者用VSP技术进行了不同距离上衰减与频率之间关系的研究,提出了反射补偿的具体方法,采用了多项式拟合技术,得到了近抛物线型衰减的结果。然后,利用波前压力与振动的固有频率思想对测试结果进行了讨论,得出一些有益的结论。 相似文献