全文获取类型
| 收费全文 | 615篇 |
| 免费 | 104篇 |
| 国内免费 | 75篇 |
专业分类
| 测绘学 | 22篇 |
| 大气科学 | 19篇 |
| 地球物理 | 204篇 |
| 地质学 | 208篇 |
| 海洋学 | 45篇 |
| 天文学 | 105篇 |
| 综合类 | 16篇 |
| 自然地理 | 175篇 |
出版年
| 2023年 | 6篇 |
| 2022年 | 1篇 |
| 2021年 | 7篇 |
| 2020年 | 5篇 |
| 2019年 | 10篇 |
| 2018年 | 9篇 |
| 2017年 | 14篇 |
| 2016年 | 16篇 |
| 2015年 | 21篇 |
| 2014年 | 23篇 |
| 2013年 | 26篇 |
| 2012年 | 19篇 |
| 2011年 | 24篇 |
| 2010年 | 15篇 |
| 2009年 | 27篇 |
| 2008年 | 31篇 |
| 2007年 | 36篇 |
| 2006年 | 46篇 |
| 2005年 | 35篇 |
| 2004年 | 26篇 |
| 2003年 | 28篇 |
| 2002年 | 44篇 |
| 2001年 | 33篇 |
| 2000年 | 36篇 |
| 1999年 | 34篇 |
| 1998年 | 38篇 |
| 1997年 | 23篇 |
| 1996年 | 23篇 |
| 1995年 | 18篇 |
| 1994年 | 18篇 |
| 1993年 | 18篇 |
| 1992年 | 17篇 |
| 1991年 | 10篇 |
| 1990年 | 17篇 |
| 1989年 | 22篇 |
| 1988年 | 6篇 |
| 1987年 | 8篇 |
| 1986年 | 3篇 |
| 1984年 | 1篇 |
排序方式： 共有794条查询结果，搜索用时 46 毫秒
91.
How-Wei Chen 《Geophysical Journal International》1998,133(2):363-378
This paper presents a geometrically based algorithm for computing synthetic seismograms for energy transmitted through a 3-D velocity distribution. 3-D ray tracing is performed to compute the traveltimes and geometrical spreading (amplitude). The formulations of both kinematic and dynamic ray-tracing systems are presented. The two-point ray-tracing problem is solved by systematically updating the initial conditions and adjusting the ray direction until the ray intersects the specified endpoint. The amount of adjustment required depends on the derivatives of the position with respect to the given starting angles between consecutive rays. The algorithm uses derivatives to define the steepest-descent direction and to update the initial directions. The convergence rate depends on the complexity of the model.
Test seismograms compare favourably with those from a 2-D asymptotic ray theory algorithm and a 3-D Gaussian-beam algorithm. The algorithm is flexible in modelling arbitrary source and recorder geometries for various smoothly varying 3-D velocity distributions. The algorithm is further tested by simulating surface-to-tunnel vibroseis field data. Shear waves as well as compressional waves may be approximately included. Application of the algorithm to a data set from the Rainier Mesa of the Nevada Test Site produced a good fit to the transmitted (first arrival) traveltimes and amplitudes, with approximately 15 per cent variation in the local 3-D velocity. 相似文献
Test seismograms compare favourably with those from a 2-D asymptotic ray theory algorithm and a 3-D Gaussian-beam algorithm. The algorithm is flexible in modelling arbitrary source and recorder geometries for various smoothly varying 3-D velocity distributions. The algorithm is further tested by simulating surface-to-tunnel vibroseis field data. Shear waves as well as compressional waves may be approximately included. Application of the algorithm to a data set from the Rainier Mesa of the Nevada Test Site produced a good fit to the transmitted (first arrival) traveltimes and amplitudes, with approximately 15 per cent variation in the local 3-D velocity. 相似文献
92.
93.
ＲａｙｅｑｕａｔｉｏｎｍｉｇｒａｔｉｏｎｏｆｗｉｄｅａｎｇｌｅｒｅｆｌｅｃｔｉｏｎｓｉｎＤａｂｉｅｏｒｏｇｅｎｉｃｚｏｎｅＸＵ－ＹＡＯＺＨＥＮＧ１）（郑需要）ＣＨＵＮ－ＹＯＮＧＷＡＮＧ２）（王椿镛）ＸＩＡＯ－ＬＩＮＧＬＡＩ１）（赖晓玲）ＸＩＡＮ... 相似文献
94.
（１）利用大气对流层中部无辐散层的存在,垂直积分热力学方程,并对整个对流层的距平加热场作解析近似的描写,联立涡度方程,得到最非绝热地转的２５０ｈＰａ距平流函数方程。它的形式酷似正压涡度方程。（２）讨论了罗斯贝波列的谱平均相速和群速。证明了气候平均高度场的闭合中心,就是中高纬度大气低频振子。其两侧位相变化相反,它的能源主要来自ＣＩＳＫ凝结正反馈或海温的感热。（３）导出了低频相关射线公式,它表明低频射 相似文献
95.
96.
The structure of the cosmic ray electron halo of a starburst galaxy depends strongly on the nature of galactic wind and the
configuration of the magnetic field. We have investigated these dependencies by solving numerically the propagation of electrons
originating in starburst galaxies, most likely in supernova remnants. The calculations are made for several models for the
galactic winds and for the configuration of the magnetic fields for comparison with observations. Our simulation of a quasi-radio
halo reproduces both the extended structure of ∼9 kpc and the subtle hollow structure near the polar region of the radio halo
that are observed in the starburst galaxy NGC 253. These findings suggest the existence of strong galactic wind in NGC 253.
相似文献
97.
Some remarks on the Gaussian beam summation method 总被引：1，自引：0，他引：1
Summary. Recently, a method using superposition of Gaussian beams has been proposed for the solution of high-frequency wave problems. The method is a potentially useful approach when the more usual techniques of ray theory fail: it gives answers which are finite at caustics, computes a nonzero field in shadow zones, and exhibits critical angle phenomena, including head waves. Subsequent tests by several authors have been encouraging, although some reported solutions show an unexplained dependence on the 'free' complex parameter ε which specifies the initial widths and phases of the Gaussian beams.
We use methods of uniform asymptotic expansions to explain the behaviour of the Gaussian beam method. We show how it computes correctly the entire caustic boundary layer of a caustic of arbitrary complexity, and computes correctly in a region of critical reflection. However, the beam solution for head waves and in edge-diffracted shadow zones are shown to have the correct asymptotic form, but with governing parameters that are explicitly ε-dependent. We also explain the mechanism by which the beam solution degrades when there are strong lateral inhomogeneities. We compare numerically our predictions for some representative, model problems, with exact solutions obtained by other means. 相似文献
We use methods of uniform asymptotic expansions to explain the behaviour of the Gaussian beam method. We show how it computes correctly the entire caustic boundary layer of a caustic of arbitrary complexity, and computes correctly in a region of critical reflection. However, the beam solution for head waves and in edge-diffracted shadow zones are shown to have the correct asymptotic form, but with governing parameters that are explicitly ε-dependent. We also explain the mechanism by which the beam solution degrades when there are strong lateral inhomogeneities. We compare numerically our predictions for some representative, model problems, with exact solutions obtained by other means. 相似文献
98.
99.
Alison Bimler 《新西兰海洋与淡水研究杂志》2013,47(4):715-720
The mud whelk Cominella glandiformis (Reeve, 1874) was observed on a tidal flat “floating” upside down beneath the water surface. Study in the laboratory showed that, when C. glandiformis is “floating”, the foot is distended with blood to approximately twice its normal surface area. In the laboratory C. glandiformis “floated” only during daylight hours, and the incidence of “floating” increased markedly as specimens became more overcrowded. Possibly “floating” is a rapid dispersal mechanism in response to overcrowding; it may also assist respiration in the event of overcrowding producing anaerobic conditions. Its occurrence only during daylight hours suggests that a reaction to light may also be involved. 相似文献
100.
旅行时射线追踪的精度和效率一直是影响地震层析成像质量和效果的关键因素。这里在三维近地表速度结构层析成像应用中,结合旅行时插值算法与最短路径算法,提出了一种三维初至波射线追踪算法。通过对双线性旅行时插值算法进行改进,并运用判定条件与简化插值计算公式进行快速计算,减少了插值次数,降低了运算量,同时也保证了较高的射线精度,有效地解决了三维射线追踪算法的计算低效率,射线精度不高的问题。 相似文献