全文获取类型
收费全文 | 848篇 |
免费 | 14篇 |
国内免费 | 10篇 |
专业分类
测绘学 | 50篇 |
大气科学 | 34篇 |
地球物理 | 223篇 |
地质学 | 301篇 |
海洋学 | 32篇 |
天文学 | 195篇 |
综合类 | 4篇 |
自然地理 | 33篇 |
出版年
2022年 | 9篇 |
2021年 | 14篇 |
2020年 | 11篇 |
2019年 | 15篇 |
2018年 | 33篇 |
2017年 | 34篇 |
2016年 | 41篇 |
2015年 | 21篇 |
2014年 | 31篇 |
2013年 | 40篇 |
2012年 | 22篇 |
2011年 | 33篇 |
2010年 | 28篇 |
2009年 | 32篇 |
2008年 | 37篇 |
2007年 | 32篇 |
2006年 | 25篇 |
2005年 | 20篇 |
2004年 | 11篇 |
2003年 | 16篇 |
2002年 | 14篇 |
2001年 | 17篇 |
2000年 | 16篇 |
1999年 | 15篇 |
1998年 | 19篇 |
1997年 | 11篇 |
1996年 | 10篇 |
1995年 | 14篇 |
1994年 | 16篇 |
1993年 | 11篇 |
1992年 | 7篇 |
1991年 | 18篇 |
1990年 | 7篇 |
1989年 | 10篇 |
1988年 | 8篇 |
1987年 | 8篇 |
1986年 | 8篇 |
1985年 | 11篇 |
1984年 | 9篇 |
1983年 | 8篇 |
1982年 | 19篇 |
1981年 | 8篇 |
1980年 | 12篇 |
1979年 | 11篇 |
1978年 | 11篇 |
1977年 | 7篇 |
1974年 | 9篇 |
1973年 | 9篇 |
1972年 | 12篇 |
1971年 | 10篇 |
排序方式: 共有872条查询结果,搜索用时 859 毫秒
221.
We present a catalog of high-speed streams, along with their solar sources for solar cycle 23. We study their distribution
during different years and different phases of solar cycle after classifying them into different groups based on their source(s),
duration, and speed. We also study the average plasma/field properties of streams after dividing them into suitable groups
on the basis of their source(s), duration and speed. It is expected that the catalog and statistical results presented in
this work will further stimulate the space weather and solar-terrestrial studies involving high-speed streams. 相似文献
222.
G. R. Adhikari A. I. Theresraj R. Balachander R. N. Gupta 《Fragblast: International Journal for Blasting and Fragmentation》2001,5(4):221-234
During construction of the Sardar Sarovar (Narmada) Hydro Electric Project in India, an unprecedented flood entered the power house cavern through draft tube tunnels and caused damage to underground structures. After this incident, these draft tubes were plugged with concrete/reinforced concrete to prevent future flooding. After heavy gates were erected at the exit end of the draft tube tunnels, these plugs had to be removed by drilling and blasting without causing any damage to the concrete lining and the ribs erected in the tunnels. Controlled blasting was designed and executed successfully for removal of the plugs in the same manner as for tunnel blasting. This paper describes the details of concrete plugs, details of drilling and blasting adopted, and some interesting observations in respect of drilling and blasting in the concrete plugs. A seismic method based on the measurement of P-wave velocity in the concrete lining before and after blasting was used to quantify the changes (damage) to concrete lining. 相似文献
223.
A reappraisal of polymetamorphism in the Eastern Ghats belt — A view from north of the Godavari rift
Evidence collated from different parts of the Eastern Ghats belt north of the Godavari rift (barring the “Western Charnockite
Zone” ) indicates that this sector evolved through a series of compressive structures (F1 to F3), with prolific migmatization in quartzofeldspathic and metapelitic gneisses synchronous with F1 shortening, as was the syn-F1 emplacement of profuse megacrystic K-feldspar-bearing granitoid bodies. Thereafter, melt productivity of the rocks (synchronous
withF
2– F3 folding) sharply decreased. Mineral parageneses stable in the S1, S2 and S3 fabrics indicate persistence of granulite facies conditions. P-T estimates on orthopyroxene + garnet + plagioclase + quartz
assemblages anchored to recrystallized mosaic that overgrow all penetrative fabric elements in mafic granulites, granitoids
and quartzofeldspathic gneisses are in the range of 900‡-950‡C and P≅ 8–9 kbar. This estimate is comparable to those retrieved
from sapphirine-bearing paragenesis in Mg-Al metapelites that appear to be diachronous in relation to the fabric elements,
and arguably disrupt the granoblastic mosaic. These facets in the northern sector of the orogenic belt are compatible with
either a single cycle of tectonic events (i.e., F1, F2 and F3 in continuum), or temporally-separate thermo-tectonic events, with the peak of earlier metamorphism (pre- to syn-F1) at lower temperature (in the granulite facies) in comparison to the record of high post-F3-Tmax values.
It is suggested on the basis of the above evidence that the late Proterozoic/Pan-African granulites in the Eastern Ghats belt
north of the Godavari rift, are unlikely to be reworked equivalents of any older granulitic crust, such as the ∼1.6 Ga granulites
south of the rift. Instead, the temporally disparate sectors may represent different crustal segments with unconnected pre-amalgamation
tectonic history. However, if the ∼ 1.6 Ga granulites of the Western Charnockite Zone continue northwards across the rift,
as suggested by recent isotope data, there are serious doubts as to the validity of a north-south division within the Eastern
Ghats belt. 相似文献
224.
Ionospheric electron content (IEC) observed at Delhi (geographic co-ordinates: 28.63°N, 77.22°E; geomagnetic co-ordinates: 19.08°N, 148.91E; dip Latitude 24.8°N), India, for the period 1975/80 and 1986/89 belonging to an ascending phase of solar activity during first halves of solar cycles 21 and 22 respectively have been used to study the diurnal, seasonal, solar and magnetic activity variations. The diurnal variation of seasonal mean of IEC on quiet days shows a secondary peak comparable to the daytime peak in equinox and winter in high solar activity. IECmax (daytime maximum value of IEC, one per day) shows winter anomaly only during high solar activity at Delhi. Further, IECmax shows positive correlation with F10.7 up to about 200 flux units at equinox and 240 units both in winter and summer; for greater F10.7 values, IECmax is substantially constant in all the seasons. IECmax and magnetic activity (Ap) are found to be positively correlated in summer in high solar activity. Winter IECmax shows positive correlation with Ap in low solar activity and negative correlation in high solar activity in both the solar cycles. In equinox IECmax is independent of Ap in both solar cycles in low solar activity. A study of day-to-day variations in IECmax shows single day and alternate day abnormalities, semi-annual and annual variations controlled by the equatorial electrojet strength, and 27-day periodicity attributable to the solar rotation. 相似文献
225.
S. N. Rajaguru Avijit Gupta V. S. Kale Sheila Mishra R. K. Ganjoo L. L. Ely Yahouda Enzel V. R. Baker 《地球表面变化过程与地形》1995,20(5):407-421
The 1300 km long Narmada River flows along a structural lineament, alternating between constricting rocky gorges and rapids, and meandering wide alluvial reaches. Channel forms and processes were studied in a 120 km long section of an alluvial reach. Channel size, shape and bedforms in the Narmada River are related to very large floods which have occurred three times in this century. During such floods the entire 400 m wide channel is utilized and 10–15 m high cliffs on both sides operate as riverbanks. Normally, even the high flows of the south-western monsoon are insufficient to fill the whole channel, and hence their effects are limited to building of discontinuous floodplains between the cliffs and modifying bedforms and bars. A channel-in-channel topography is thus created. The very large floods are also responsible for erosion of the rocky stretches and building of point bars. The river meanders, but its movement is restricted because of (1) rocky gorges and scablands operating as anchor points at intervals, and (2) the presence of high alluvial cliffs which are topped on extremely rare occasions. In spite of being located in a tectonically active zone in a monsoon setting, it is the exceptional high-magnitude floods at irregular intervals which control the form and behaviour of the Narmada River. 相似文献
226.
Debasis Sen Gupta 《Journal of Earth System Science》1995,104(3):407-417
Recrystallized grain size was measured from quartzite mylonite specimens collected from parts of Singhbhum shear zone in eastern
India. The specimens were collected along five traverses (Mushabani, Pathargora, Surda, Rakha and Jadugoda) across the elongation
of the shear zone. The sheared quartzites range from protomylonite through mylonite to ultramylonite. The microstructural
studies of the specimens reflect that dynamic recrystallization was the main deformation process. Estimation of flow stresses
were derived from these specimens using empirical equations relating to flow stress and recrystallized grain size. The calculated
stresses range from 12–28 MPa (Mercieret al 1977), 23–49 MPa (Twiss 1977), 20–68 MPa (Christie and Ord 1980), considering all the traverses. The results show that these
values can only be used semiquantitatively. 相似文献
227.
The paper presents an efficient finite difference based 2D-inversion algorithm, EM2INV, for geoelectromagnetic data. The special
features of the algorithm are
The algorithm is tested rigorously by setting up exercises of diverse nature and of practical significance. The stability
of the algorithm is established by inverting the synthetic response corrupted with Gaussian noise. The inversion experiments
are aimed at studying
It has been observed that the Magneto-telluric data deciphers better the vertical position of the target and Geomagnetic Depth
Sounding data deciphers the horizontal variations in a better way. The conductive and resistive bodies are better resolved
by inversion of E- and B-polarization data respectively. The results of multi-frequency inversion imply that the increase
in the number of frequencies does not necessarily enhance the inversion quality especially when the spread of observation
points is sufficiently large to sense the target. The study of a minimum number of observation points highlights the importance
of single point inversion that furnishes useful information about the inhomogeneity. 相似文献
– | • optimal grid generation based on grid design thumb rules, |
– | • finite domain boundary conditions, |
– | • interpolation matrix that permits generation of response at observation points different from grid points, |
– | • Gaussian elimination forward matrix solver, that enables reuse of already decomposed coefficient matrix, |
– | • super-block notion that reduces the number of blocks with unknown resistivities and, in turn, the size of Jacobian matrix and |
– | • bi-conjugate gradient matrix solver for inverse problem which circumvents the need of explicit Jacobian matrix computation. |
– | • relative performance of response functions, |
– | • inversion quality of E- and B-polarization data, |
– | • efficacy of single and multi-frequency data inversion, |
– | • minimum number of frequencies and observation points needed for successful data inversion. |
228.
The system diopside-nepheline-leucite, representing a join in the undersaturated part of the system nepheline (Ne)-kalsilite (Ks)-CaO-MgO-SiO2, has been investigated at atmospheric pressure. The system is pseudoternary and cuts the primary phase volumes of forsterite solid solution (Foss), diopside solid solution (Diss), nepheline solid solution (Ness), carnegieite solid solution (Cgss), and leucite solid solution (Lcss). Melilite (Mel) occurs as a subliquidus phase. The phase diagram has two four-phase points: 1. one at 1275±5° C and Di60Ne8Lc32 where liquid coexists with Foss, Diss and Lcss, corresponding to olivine (Ol) leucitite; 2. the other at 1194±5° C and Di27.5Ne29.5Lc43 where Ness, Foss and Lcss coexist with liquid, corresponding to Ol-Ne italite. With decreasing temperature, liquid moves from point (1) to a five-phase assemblage (3) where liquid is in equilibrium with Foss, Diss, Mel and Lcss (1258±5°C), which is representative of Ol-Mel-leucitite. From point (2) liquid moves to a second five-phase assemblage (4), where Foss, Mel, Ness, Lcss and liquid are in equilibrium (1175±5°C, corresponding to a Lc-Ne katungite. The assemblage Foss+Ness+Diss+Mel+Lcss+ liquid, is reached between 1168° and 1100° C and corresponds to Ol-Mel-Ne leucitite. Foss reacts with liquid and disappears. Near the point (1) it disappears at 1135±10° C, whereas near the point (2) it reacts out at 1060±10° C. Near the join Di-Ne it disappears at 950±10° C. The final assemblage in the system is representative of Mel-Ne leucitite.Presented at the symposium Recent Advances in the Studies of Rocks and Minerals at High Pressures and Temperatures held in Montreal, 1972. Jointly sponsored by the International Mineralogical Association and the Commission on Experimental Petrologie. 相似文献
229.
230.