全文获取类型
收费全文 | 365篇 |
免费 | 119篇 |
国内免费 | 15篇 |
专业分类
测绘学 | 6篇 |
大气科学 | 30篇 |
地球物理 | 348篇 |
地质学 | 60篇 |
海洋学 | 11篇 |
天文学 | 28篇 |
综合类 | 1篇 |
自然地理 | 15篇 |
出版年
2022年 | 3篇 |
2021年 | 2篇 |
2020年 | 13篇 |
2019年 | 11篇 |
2018年 | 13篇 |
2017年 | 6篇 |
2016年 | 12篇 |
2015年 | 14篇 |
2014年 | 27篇 |
2013年 | 14篇 |
2012年 | 9篇 |
2011年 | 15篇 |
2010年 | 13篇 |
2009年 | 24篇 |
2008年 | 28篇 |
2007年 | 23篇 |
2006年 | 26篇 |
2005年 | 23篇 |
2004年 | 24篇 |
2003年 | 13篇 |
2002年 | 17篇 |
2001年 | 17篇 |
2000年 | 26篇 |
1999年 | 14篇 |
1998年 | 18篇 |
1997年 | 8篇 |
1996年 | 13篇 |
1995年 | 18篇 |
1994年 | 11篇 |
1993年 | 7篇 |
1992年 | 7篇 |
1991年 | 8篇 |
1990年 | 6篇 |
1989年 | 7篇 |
1988年 | 4篇 |
1987年 | 1篇 |
1986年 | 1篇 |
1984年 | 1篇 |
1980年 | 1篇 |
1978年 | 1篇 |
排序方式: 共有499条查询结果,搜索用时 31 毫秒
101.
本文回顾分析1987年3月 ̄1992年5月行肾脏CT扫描102例,其中诊断肾结石31,当时因CT扫描前准备方法问题,及开始经验不足造成4例无结石的病人误诊为结石。找到误诊石的原因是CT扫描前静脉试验及口服泛影葡胺造影剂造成,采取改进CT扫描前准备方法;(1)扫描前不行静脉试验(2)口服造影剂改为口服温水或牛奶,经上述改进方法,以后未出现造影剂造成肾结石误诊。 相似文献
102.
为了提高对面部和颅底骨折的认为。本组74例,男性63例,女性11例,年龄2-71岁,车祸外伤64例,拳击伤5例,铁棍击伤2例,从高处跌下3例,全部病例均采用轴位CT平扫。眶内壁骨折20例,其中爆裂骨的11例,直接骨折8例,1列同时受直接和间接外力作用;眶下壁骨折1例,为爆裂骨折;眶外后壁骨折1例,眶顶壁骨折9例,均为直接骨折;其他骨折33例;蝶窦内液平为颅底骨折的重要间接征象。CT检查可确定骨折类 相似文献
103.
吴一刚 《CT理论与应用研究》1997,6(3):19-25
若二维RF窄波反射函数是定义在地面一圆形区域内部的,并假定该圆域表面介质是弱反射介质,那么在给定时刻,接收发射器(如机载合成孔径雷达)所接收到的近回数据可视 过该圆域的一圆弧上的线积分值,Mumson,et,al。在假定圆域半径与接收发射器运动路径到该圆域中心的距离相比很小的情况,用直线上的线积分值来代替圆弧线上的线积分值,利用古典的Radon变换方法,给出了该圆域反射函数的成像模式。本文将克服这 相似文献
104.
本文对ACH方法做简化处理,先以一个地震为单元形成子矩阵,并将慢度扰动用一平均量来代替;在修正参数时,震源部分按常规进行,对慢度部分则利用代数重建技术将其分配到各个块体上.数值模拟结果表明,该方法能很好地反演震源位置和速度结构.运用新丰江遥测台网的数据进行研究,表明地震强活动区位于大坝附近,特别是在人字石断裂和高寨断裂的交汇处,震源集中沿罗坑-双下、葫芦凹-燕岩和黄竹蒿3条地震带,呈密集的高倾角分布,地震活动主要与北北西向构造和东西向构造有关,而与地表甚为发育的北东东向构造关系不大.速度图像中的低速区对应地表的破碎区,即地震带区域,也是该区重磁异常急剧变化的区域;而高速区内地震较少.上述特征同水的渗透作用有密切关系. 相似文献
105.
106.
用C波段双线偏振雷达研究冰雹云 总被引:18,自引:1,他引:18
本文计算了不同相态、不同形状的降水粒子对5.6cm,10.7cm雷达波的散射特性,并利用降水粒子散射特性及云雨雷达回波的Z_(DR)特征并结合地面降雹情况,分析并解释了1990年8月9日和8月30日甘肃省平凉两次雹暴过程的雷达RHI资料.我们推测:Z_(DR)值为负值的区域相应于降雹区,ZDR负值与大扁冰雹和小锥状冰雹关系密切;而雨区Z_(DR)具有较大正值,雨区大于5dB的Z_(DR)由直径D>0.5cm的雨滴所致.利用冰雹区和雨区Z_(DR)信息的不同可能识别降水粒子的相态.C波段双线偏振雷达和10cm波长雷达相比有独到之处:雨区Z_(DR)值较大,而冰雹区Z_(DR)值又较小,这更有利于研究云雨相态和空间结构.可以预期双线偏振雷达对防雹工作及云物理研究工作是有很大帮助的. 相似文献
107.
P. H. Stoker 《Astrophysics and Space Science》1995,230(1-2):405-413
The absorption of cosmic radio noise passing through the ionosphere may be described as a function of radio wave frequencyA(f
e
) f
e
-n
, with n 2.0 for spatially uniform precipitation of electrons and n < 2.0 for spatially nonuniform precipitation. Using multifrequency riometer recordings at SANAE, the following observations are reviewed: (1) The frequency distribution of the power index, n, obtained from 4 min averaged absorptions during 1983, shows a most probable value around n 1.5, indicating that mostly energetic electrons are precipitated spatially structured onto the upper atmosphere, as in optical aurora. (2) Multifrequency riometer recordings suggest that field-aligned ionospheric irregularities have scattered additional cosmic radio waves from the central region of the Galaxy into the fields of views of the riometer antennae during an auroral absorption event in the early morning hours of 27 July, 1982. With the power reflectivity by ionospheric irregularities inversely proportional to the fourth power of radio wave frequency, as required by the Bragg condition, an estimated 70% increase in the 20 MHz radio flux at 01:22 UT, at the strong absorption peak, can explain the strongly reduced absorption observed in 20 MHz relative to 30 and 51.4 MHz. (3) Gradual increases in absorptions observed at all three riometer frequencies from onset at 11:50 UT of the largest solar proton ground level enhancement on 29 September, 1989, until 18:00 UT, suggest diffusion of the much more intense low energy protons from the polar cap to the L=4.0 geomagnetic field shell and subsequent precipitation at SANAE due to the South Atlantic Geomagnetic Anomaly. (4) The flux of electron energy deposited per second at SANAE is closely related to geomagnetic activity, but has a lower maximum during the years 1971 and 1980 of solar polar magnetic reversals than in the years 1976 and 1986/87 of minimum solar activity. (5) A significant correlation has been found between the arrival of single-hop whistlers and 30 MHz riometer absorption events, using point statistics. The maximum absorption at 30 MHz was 0.04 dB with a delay of 3 ± 2 s relative to the whistler. 相似文献
108.
The non-thermal shielding effects on the inverse Compton scattering are investigated in astrophysical non-thermal Lorentzian plasmas. The inverse Compton power is obtained by the modified Compton scattering cross section in Lorentzian plasmas with the blackbody photon distribution. The total Compton power is also obtained by the Lorentzan distribution of plasmas. It is found that the influence of non-thermal character of the plasma suppresses the inverse Compton power in astrophysical Lorentzian plasmas. It is also found that the non-thermal effect on the inverse Compton power decreases with an increase of the temperature. In addition, the non-thermal effect on the total Compton power with Lorentzan plasmas increases in low-temperature photons and, however, decreases in intermediate-temperature photons with increasing Debye length. The variation of the total Compton power is also discussed. 相似文献
109.
M. D. Sharma 《Journal of Earth System Science》2001,110(3):205-213
Perturbations in elastic constants and density distinguish a volume inhomogeneity from its homogeneous surroundings. The equation
of motion for the first order scattering is studied in the perturbed medium. The scattered waves are generated by the interaction
between the primary waves and the inhomogeneity. First order scattering theory is modified to include the source term generating
the primary waves. The body force equivalent to the scattering source is presented in a convenient form involving the perturbations
in wave velocities and gradient of density perturbation.
A procedure is presented to study the scattering of a spherical pulse from a small inhomogeneity, in time domain. The size
of inhomogeneity is assumed small as compared to its distance from source and receiver. No restrictions are placed on the
positions of source, receiver and inhomogeneity. The dilatation and rotations are calculated for a pulse scattered from an
arbitrary point in a spherical volume. The aggregate of the scattered phases from all the points of the inhomogeneity, reaching
at a fixed receiver, gives the amount of scattering from the inhomogeneity. The interaction of bothP andS waves with inhomogeneity are considered. Dilatation and rotations for scattering are obtained as integral expressions over
the solid angle of inhomogeneity. These expressions are computed numerically, for hypothetical models. The effects of source
(unit force) orientations, velocity and density perturbations, and size of inhomogeneity, on the scattered phases, are discussed. 相似文献
110.