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61.
62.
大口径望远镜受大气湍流的影响,光学分辨率远远小于其自身光路所决定的衍射极限。为了相应的自适应光学系统设计,首先有必要对大气波动进行仿真以提供环境数据。通常的大气波前仿真方法需要通过计算结构函数,得到功率谱函数,进而得到仿真波前,但该方法存在计算速度慢,中间变量存储空间大的问题,给大口径望远镜或者长时间仿真带来很大不便。介绍了一种可行的基于迭代分形法的波前仿真方法,复杂度达到O(N),可以大大提高波前仿真的速度。 相似文献
63.
64.
Ya.A. Ilyushin 《Planetary and Space Science》2009,57(12):1458-1466
The effect of the ionospheric scintillations on the performance of the dekameter wave band synthetic aperture radar (SAR) is considered. The numerical model for computer simulations, based on the multiple phase screen technique and accounting for diffraction, is formulated. The result of numerical simulations of the radar pulses is presented and analyzed. The role of the various features of the ionospheric disturbances and their influence on the radar pulse characteristic parameters is figured out and discussed. Both effects of anisotropy of the correlation function of plasma irregularities and their temporal variations are studied numerically. 相似文献
65.
任玉伟 《测绘与空间地理信息》2012,(3):53-55,59
测绘资料档案客户服务系统是测绘档案部门最直接服务于客户的操作平台。本文阐述了测绘资料档案客户服务系统的总体框架设计、功能模块设计、数据结构设计以及系统的实现,为各省的档案客户服务系统的开发提供了参考和对比。 相似文献
66.
Fragmentation of magma during Plinian volcanic eruptions 总被引:2,自引:0,他引:2
James E. Gardner Richard M. E. Thomas Claude Jaupart Steve Tait 《Bulletin of Volcanology》1996,58(2-3):144-162
The ratio of the volume of vesicles (gas) to that of glass (liquid) in pumice clasts (V
G
/V
L
) reflects the degassing and dynamic history experienced by a magma during an explosive eruption. V
G
/V
L
in pumices from a large number of Plinian eruption deposits is shown here to vary by two orders of magnitude, even between
pumices at a given level in a deposit. These variations in V
G
/V
L
do not correlate with crystallinity or initial water content of the magmas or their eruptive intensities, despite large ranges
in these variables. Gas volume ratios of pumices do, however, vary systematically with magma viscosity estimated at the point
of fragmentation, and we infer that pumices do not quench at the level of fragmentation but undergo some post-fragmentary
evolution. On the timescale of Plinian eruptions, pumices with viscosities <109 Pa s can expand after fragmentation, as long as their bubbles retain gas, at a rate inversely proportional to their viscosity.
Once the bubbles connect to form a permeable network and lose their gas, expansion halts and pumices with viscosities <105 Pa s can collapse under the action of surface tension. Textural evidence from bubble sizes and shapes in pumices indicates
that both expansion and collapse have taken place. The magnitudes of expansion and collapse, therefore, depend critically
on the timing of bubble connectivity relative to the final moment of quenching. We propose that bubbles in different pumices
become connected at different times throughout the time span between fragmentation and quenching. After accounting for these
effects, we derive new information on the fragmentation process from two characteristics of pumices. The most important is
a relatively constant minimum value of V
G
/V
L
of ∼1.78 (64 vol.% vesicularity) in all samples with viscosities >105 Pa s. This value is independent of magma composition and thus reflects a property of the eruptive mechanism. The other characteristic
is that highly expanded pumices (>85 vol.% vesicularities) are common, which argues against overpressure in bubbles as a mechanism
for fragmenting magma. We suggest that magma fragments when it reaches a vesicularity of ∼64 vol.%, but only if sheared sufficiently
strongly. The intensity of shear varies as a function of velocity in the conduit, which is related to overpressure in the
chamber, so that changes in overpressure with time are important in controlling the common progression from explosive to effusive
activity at volcanoes.
Received: 19 April 1995 / Accepted: 3 April 1996 相似文献
67.
邵武--河源地震带中段从1978年以来出现一次地震群体活动,属于该地震带第Ⅱ活动周期的第Ⅳ活动幕,现已转入这次活动幕的尾声,分析该区历史和现今地震活动特征及其应力状态,认为该区震源构造应力已整调、处于松驰状态。未来一段时间的地震活动将向起伏性减弱趋势发展。 相似文献
68.
Experiments on degassing of water-saturated granite melts with a pressure drop from 100 and 450 MPa to 40 and 120 MPa, respectively,
at temperatures close to feldspar liquidus (750–700 °C), were carried out to determine the modality of water exsolution and
vesicle formation at the liquidus temperature. Pressure-drop rates as small as approximately 100 bar/day were used. Uniform
space distributions of bubbles of exsolved water were obtained with starting glass containing a small fraction (≈0.5 vol.%)
of trapped air bubbles. Volume crystallization of feldspar was observed in degassed melts supplied with seeds. Bubble size
distributions (BSD) measured in granite glasses after degassing are presented. Data on vesicle characteristics (number, radius,
area, elongation) were acquired on images digitized with standard software, while the reconstruction of size distributions
was performed with the Schwartz-Saltikov "unfolding" procedure. Bubble size distributions of size classes in the range 5–1000 μm
were acquired with proper magnification and satisfactory statistical reliability of determined number densities. The BSDs
of the experimental samples are compared with the results of measurements of rapidly degassed products of Mt. Etna and Vulcano
Island. Many particular features of the bubble nucleation and growth can be distinguished in an individual BSD. However, the
general BSD of the whole data set, including natural ones, can be relatively well described with linear regression in bilogarithmic
coordinates. The slope of this regression is approximately 2.8±0.1. This dependence is in striking contrast with distributions
theoretically predicted with classical nucleation models based on homogeneous nucleation of vesicles. The theoretical distribution
requires the occurrence of strong maxima that are not observed in our experimental and natural samples, thus arguing for heterogeneous
nucleation mechanisms.
Received: 1 October 1998 / Accepted: 25 June 1999 相似文献
69.
70.