全文获取类型
收费全文 | 113405篇 |
免费 | 1588篇 |
国内免费 | 748篇 |
专业分类
测绘学 | 2745篇 |
大气科学 | 7357篇 |
地球物理 | 21541篇 |
地质学 | 42249篇 |
海洋学 | 9741篇 |
天文学 | 25897篇 |
综合类 | 409篇 |
自然地理 | 5802篇 |
出版年
2022年 | 757篇 |
2021年 | 1280篇 |
2020年 | 1362篇 |
2019年 | 1506篇 |
2018年 | 4563篇 |
2017年 | 4109篇 |
2016年 | 4093篇 |
2015年 | 1737篇 |
2014年 | 3256篇 |
2013年 | 5395篇 |
2012年 | 4001篇 |
2011年 | 5813篇 |
2010年 | 5136篇 |
2009年 | 6352篇 |
2008年 | 5471篇 |
2007年 | 5887篇 |
2006年 | 4313篇 |
2005年 | 3156篇 |
2004年 | 3072篇 |
2003年 | 2938篇 |
2002年 | 2847篇 |
2001年 | 2420篇 |
2000年 | 2319篇 |
1999年 | 1799篇 |
1998年 | 1851篇 |
1997年 | 1747篇 |
1996年 | 1469篇 |
1995年 | 1466篇 |
1994年 | 1253篇 |
1993年 | 1187篇 |
1992年 | 1125篇 |
1991年 | 1129篇 |
1990年 | 1136篇 |
1989年 | 995篇 |
1988年 | 908篇 |
1987年 | 1071篇 |
1986年 | 910篇 |
1985年 | 1148篇 |
1984年 | 1290篇 |
1983年 | 1237篇 |
1982年 | 1140篇 |
1981年 | 1074篇 |
1980年 | 1007篇 |
1979年 | 880篇 |
1978年 | 898篇 |
1977年 | 763篇 |
1976年 | 745篇 |
1975年 | 741篇 |
1974年 | 718篇 |
1973年 | 791篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
891.
892.
S. T. G. Raghukanth 《Natural Hazards》2008,46(1):1-13
In this article, analytical methods have been used to estimate ground motion during the 8 October 2005, Kashmir earthquake.
Peak ground acceleration (PGA) values at several stations in the epicentral region have been estimated by empirical analytical
source mechanism models. As an alternate analysis, PGA estimates have also been obtained using the stochastic finite fault
seismological model. The estimated PGAs are compared with that obtained from damage values. A PGA contour map in the near-source
region is provided. It is found that very near to the epicenter, PGA would have reached more than 1 g. It is demonstrated
that empirical analytical models can be effectively used to estimate ground motion due to rupture of active faults. 相似文献
893.
H. M. Hussein K. M. Abou Elenean I. A. Marzouk A. Peresan I. M. Korrat E. Abu El-Nader G. F. Panza M. N. El-Gabry 《Natural Hazards》2008,47(3):525-546
The aim of the present work is to compile and update a catalogue of the instrumentally recorded earthquakes in Egypt, with
uniform and homogeneous source parameters as required for the analysis of seismicity and seismic hazard assessment. This in
turn requires a detailed analysis and comparison of the properties of different available sources, including the distribution
of events with time, the magnitude completeness, and the scaling relations between different kinds of magnitude reported by
different agencies. The observational data cover the time interval 1900–2004 and an area between 22°–33.5° N and 25°–36° E.
The linear regressions between various magnitude types have been evaluated for different magnitude ranges. Using the best
linear relationship determined for each available pair of magnitudes, as well as those identified between the magnitudes and
the seismic moment, we convert the different magnitude types into moment magnitudes M
W, through a multi-step conversion process. Analysis of the catalogue completeness, based on the M
W thus estimated, allows us to identify two different time intervals with homogeneous properties. The first one (1900–1984)
appears to be complete for M
W ≥ 4.5, while the second one (1985–2004) can be considered complete for magnitudes M
W ≥ 3. 相似文献
894.
Mineralogy and geochemistry of El Dorado epithermal gold deposit, El Sauce district, central-northern Chile 总被引:1,自引:0,他引:1
J. Carrillo-Rosúa S. Morales-Ruano D. Morata A. J. Boyce M. Belmar A. E. Fallick P. Fenoll Hach-Alí 《Mineralogy and Petrology》2008,92(3-4):341-360
Summary The El Dorado Au-Cu deposit is located in an extensive intra-caldera zone of hydrothermal alteration affecting Upper Cretaceous
andesites of the Los Elquinos Formation at La Serena (≈ 29°47′S Lat., 70°43′W Long., Chile). Quartz-sulfide veins of economic potential are hosted by N25W and N20E
fault structures associated with quartz-illite alteration (+supergene kaolinite). The main ore minerals in the deposit are
pyrite, chalcopyrite ± fahlore (As/(As + Sb): 0.06−0.98), with electrum, sphalerite, galena, bournonite-seligmanite (As/(As
+ Sb): 0.21−0.31), marcasite, pyrrhotite being accessory phases. Electrum, with an Ag content between 32 and 37 at.%, occurs
interstitial to pyrite aggregates or along pyrite fractures. Pyrite commonly exhibits chemical zonation with some zones up
to 1.96 at.% As. Electron probe microanalyses of pyrite indicate that As-rich zones do not exhibit detectable Au values. Fluid
inclusion microthermometry shows homogenization temperatures between 130 and 352 °C and salinities between 1.6 and 6.9 wt.%
NaCl eq. Isotope data for quartz, ankerite and phyllosilicates and estimated temperatures show that δ18O and δD for the hydrothermal fluids were between 3 and 10‰ and between −95 and −75‰, respectively. These results suggest
the mineralizing fluids were a mixture of meteoric and magmatic waters. An epithermal intermediate-sulfidation model is proposed
for the formation of the El Dorado deposit.
Author’s present address: J. Carrillo-Rosúa, Dpto. de Didáctica de las Ciencias Experimentales, Universidad de Granada, Campus de Cartuja, 18071, Granada, Spain 相似文献
895.
Summary Boron-doped mullites were synthesized using aluminium nitrate-nonahydrate, tetraethoxysilane and boric acid in a sol–gel process
with subsequent annealing at 950 and 1300 °C for five hours. Two different bulk compositions with constant Al2O3 contents (60 and 70 mol%, respectively) and varying SiO2 plus B2O3 contents were investigated. X-ray powder diffraction analyses yielded a linear decrease of the lattice parameters with increasing
bulk B2O3 content, which was interpreted as to be due to boron incorporation. Related to the increasing boron content, corresponding
infrared spectra revealed a slight and continuous shift for most of the absorption bands. These data show that mullite is
able to incorporate large amounts of boron into its structure (up to about 20 mol% B2O3 depending on the bulk composition of the starting materials). Infrared analyses suggest that boron is incorporated into the
mullite structure in form of planar three-fold coordinated BO3 groups.
Author’s address: A. Beran, Institut für Mineralogie und Kristallographie, Universit?t Wien-Geozentrum, Althanstra?e 14, 1090 Wien, Austria 相似文献
896.
M. Mejías J. Garcia-Orellana J. L. Plata M. Marina E. Garcia-Solsona B. Ballesteros P. Masqué J. López C. Fernández-Arrojo 《Environmental Geology》2008,54(3):521-536
A methodology for the characterization of deep carbonate aquifers has been developed and applied to El Maestrazgo Jurassic
aquifer in Castellón, Spain. Characterization of these aquifer formations, located at more than 300 m deep, consisted of a
previous phase of compilation, analysis and synthesis of the existing information about the area, followed by a coordinated
combination of different speciality studies: geology, stratigraphy, structural analysis, hydrogeology, hydrochemistry, geophysics
and remote sensing. Geological studies included geological mapping, definition of stratigraphical units and facies and structural
analysis. The aim of the hydrogeology study was to define aquifer formations, recharge area, aquifer points inventory and
groundwater flow directions for the establishment of piezometric and water quality observation nets. Special techniques were
applied, like thermal infrared aerial images and the evaluation of submarine groundwater discharge by means of natural radium
isotopes. Hydrochemical techniques, including majority elements characterization and stable isotopes (18O, 2H and 3H) determination, allowed classifying hydrochemical facies and establishing a renewal pattern for water within the system.
Geophysics was useful in determining the aquifer geometry, the features of the basement and the petrophysical characteristics
of the geological formations. Preliminary results show an important tectonic complexity and the possibilities for groundwater
uses in the area of study. 相似文献
897.
Implementation of reconstructed geomorphologic units in landslide susceptibility mapping: the Melen Gorge (NW Turkey) 总被引:6,自引:2,他引:4
In the international literature, although considerable amount of publications on the landslide susceptibility mapping exist,
geomorphology as a conditioning factor is still used in limited number of studies. Considering this factor, the purpose of
this article paper is to implement the geomorphologic parameters derived by reconstructed topography in landslide susceptibility
mapping. According to the method employed in this study, terrain is generalized by the contours passed through the convex
slopes of the valleys that were formed by fluvial erosion. Therefore, slope conditions before landsliding can be obtained.
The reconstructed morphometric and geomorphologic units are taken into account as a conditioning parameter when assessing
landslide susceptibility. Two different data, one of which is obtained from the reconstructed DEM, have been employed to produce
two landslide susceptibility maps. The binary logistic regression is used to develop landslide susceptibility maps for the
Melen Gorge in the Northwestern part of Turkey. Due to the high correct classification percentages and spatial effectiveness
of the maps, the landslide susceptibility map comprised the reconstructed morphometric parameters exhibits a better performance
than the other. Five different datasets are selected randomly to apply proper sampling strategy for training. As a consequence
of the analyses, the most proper outcomes are obtained from the dataset of the reconstructed topographical parameters and
geomorphologic units, and lithological variables that are implemented together. Correct classification percentage and root
mean square error (RMSE) values of the validation dataset are calculated as 86.28% and 0.35, respectively. Prediction capacity
of the different datasets reveal that the landslide susceptibility map obtained from the reconstructed parameters has a higher
prediction capacity than the other. Moreover, the landslide susceptibility map obtained from the reconstructed parameters
produces logical results. 相似文献
898.
899.
K. Vinod kumar R. C. Lakhera Tapas R. Martha R. S. Chatterjee A. Bhattacharya 《Environmental Geology》2008,55(4):789-799
Mass movements such as landslides in mountainous terrains are natural degradation processes and one of the most important
landscape-building factors. Varunawat Parbat overlooking Uttarkashi town witnessed a series of landslides on 23 September
2003 and the debris slides and rock falls continued for 2 weeks. This landslide complex was triggered due to the incessant
rainfall prior to the event, and its occurrence led to the blockage of the pilgrim route to Gangotri (source of the Ganges
river) and evacuation of thousands of people to safer places. Though there was no loss of lives due to timely evacuation,
heavy losses to the property were reported. High-resolution stereoscopic earth observation data were acquired after the incidence
to study the landslide in detail with emphasis on the cause of the landslide and mode of failure. Areas along the road and
below the Varunawat foothill region are mapped for landslide risk. It was found that the foothill region of the Varunawat
Parbat was highly disturbed by man-made activities and houses are dangerously located below steep slopes. The potential zones
for landslides along with the existing active and old landslides are mapped. These areas are critical and their treatment
with priority is required in order to minimise further landslide occurrences. 相似文献
900.
Bal Krishna Maheshwari Deepak A. Gunjagi 《Geotechnical and Geological Engineering》2008,26(1):101-107
Geotextile filters are rapidly replacing graded granular filters as the standard of practice in geotechnical design. The objective
of the present study is to predict the behavior of the geotextile filter for the locally (Roorkee—India) available soils using
three different geotextiles (two nonwoven and one woven). Nonwoven geotextiles are commonly used in filtration applications.
This paper evaluates the long-term performance of two needle-punched nonwoven geotextiles and one woven geotextile. Filtration
tests on soil–geotextile filter systems were conducted in the laboratory in order to evaluate the filtration and clogging
behavior. Laboratory test program, include evaluation of performance of two different nonwoven and one woven geotextile filters
in combination with locally available Solani River Sand and Clayey Soil. The paper describes the concept and details of the
Fine Fraction Filtration (F3) test and presents data on three different geotextiles, which were evaluated using three different soil types. It is deduced
that in all filtration applications, the non-woven geotextile would perform better than the woven geotextile. 相似文献