全文获取类型
| 收费全文 | 64637篇 |
| 免费 | 539篇 |
| 国内免费 | 185篇 |
专业分类
| 测绘学 | 1423篇 |
| 大气科学 | 4538篇 |
| 地球物理 | 12691篇 |
| 地质学 | 23180篇 |
| 海洋学 | 5647篇 |
| 天文学 | 13862篇 |
| 综合类 | 161篇 |
| 自然地理 | 3859篇 |
出版年
| 2022年 | 386篇 |
| 2021年 | 669篇 |
| 2020年 | 798篇 |
| 2019年 | 864篇 |
| 2018年 | 1593篇 |
| 2017年 | 1588篇 |
| 2016年 | 1782篇 |
| 2015年 | 974篇 |
| 2014年 | 1702篇 |
| 2013年 | 3186篇 |
| 2012年 | 1962篇 |
| 2011年 | 2649篇 |
| 2010年 | 2282篇 |
| 2009年 | 2925篇 |
| 2008年 | 2578篇 |
| 2007年 | 2628篇 |
| 2006年 | 2482篇 |
| 2005年 | 1765篇 |
| 2004年 | 1768篇 |
| 2003年 | 1737篇 |
| 2002年 | 1623篇 |
| 2001年 | 1454篇 |
| 2000年 | 1379篇 |
| 1999年 | 1162篇 |
| 1998年 | 1147篇 |
| 1997年 | 1208篇 |
| 1996年 | 992篇 |
| 1995年 | 985篇 |
| 1994年 | 905篇 |
| 1993年 | 778篇 |
| 1992年 | 751篇 |
| 1991年 | 731篇 |
| 1990年 | 778篇 |
| 1989年 | 697篇 |
| 1988年 | 671篇 |
| 1987年 | 722篇 |
| 1986年 | 714篇 |
| 1985年 | 882篇 |
| 1984年 | 903篇 |
| 1983年 | 930篇 |
| 1982年 | 861篇 |
| 1981年 | 777篇 |
| 1980年 | 776篇 |
| 1979年 | 694篇 |
| 1978年 | 637篇 |
| 1977年 | 646篇 |
| 1976年 | 587篇 |
| 1975年 | 579篇 |
| 1974年 | 573篇 |
| 1973年 | 587篇 |
排序方式： 共有10000条查询结果，搜索用时 831 毫秒
971.
This study was conducted to investigate the volcano-sedimentary sequence of Kurmut terrane in southeast Bayuda Desert and its continuation east of the Nile. The sequence is situated at the inferred transition between the Neoproterozoic juvenile Arabian–Nubian Shield and the old reworked Saharan Metacraton. Lithologically, the rocks are composed of intermediate to basic metavolcanics and volcanoclastics intercalated with tuffeous material and metapelite sediments that formed in cyclic graded bedding, indicating a turbid environment in the active orogenic zone. For the first time in the study area, intraformational conglomerates with cigar-shaped volcanic clasts within the low-grade metasediments suggest a high-energy depositional environment. Minerals and textural examination of rock assemblage indicate an anticlockwise P/T condition from amphibolite facies to retrograde greenschist facies conditions in a large high strain zone of distributed deformation. Geochemical results of metavolcanics indicate derivation from different mantle sources (island arc tholeite, calc-alkaline basalt, mid-oceanic ridge, and back-arc basalt). Within this sequence, field relation and microstructural analysis clearly show the best evidence of superimposition of sinistral shear of the NNE–SSW-trending Keraf Shear Zone on the early dextral NE–SW Nakasib Suture Zone. Consequently, based on lithological and structural observations with support of available geochemical data, the volcano-sedimentary sequences SE of Bayuda Desert and east of Nile are similar to their equivalent assemblage of juvenile Neoproterozoic arc lavas association of the Arabian–Nubian Shield. 相似文献
972.
Tephra shards for electron probe microanalysis are most efficiently extracted from peat using acid digestion, which removes organic material that hinders density separation methods. However, strong acids are known to alter glass chemical compositions, and several studies have examined how acid digestion affects rhyolitic volcanic glass. The focus on rhyolitic tephra in these studies leaves considerable uncertainty, as the dissolution rates of natural glasses (including tephra) are determined by the chemical composition and surface area/volume ratio, both of which vary in tephra deposits. Here, we use duplicate samples of basaltic, trachydacitic and rhyolitic tephra to examine physical and geochemical alteration following acid digestion. Scanning electron microscope imagery reveals no discernible degradation of glass surfaces, and electron probe microanalysis results from duplicate samples are statistically indistinguishable. These findings suggest the acid digestion protocol for organic peats does not significantly alter glass geochemistry regardless of shard morphologies or geochemical compositions. 相似文献
973.
During a large nuclear war, the atmosphere would be loaded with huge quantities of pollutants, which are produced by fires in urban and industrial centers, cultivated lands, forests and grasslands. Especially detrimental are the effects of light absorbing airborne particles. An analysis of the amounts of the various types of fuels which could burn in a nuclear war indicates that more than 1014 g of black smoke could be produced by fires started by the nuclear explosions. Due to this, the penetration of sunlight to the earth's surface would be reduced greatly over extended areas of the northern hemisphere, maybe even globally. This could temporarily cause extreme darkness in large areas in midlatitudes and reduce crop growth and biospheric productivity.This situation would last for several weeks and cause very anomalous meteorological conditions. Much solar radiation would be absorbed in the atmosphere instead of at the earth's surface. The land areas and lower atmosphere would, therefore, cool and the overlying atmosphere warm, creating strong vertical thermal stability in a highly polluted atmosphere. For extended periods and in large parts of the world, weather conditions would be abnormal. The resulting cold, probably freezing, temperatures at the ground would interfere severely with crop production during the growing season and cause extreme conditions for large sections of the biosphere. The combination of lack of sunlight, frost and other adverse meteorological conditions would add enormously to the already huge problems of the survivors. 相似文献
974.
Juan C. Tokeshi Madan B. Karkee Yoshihiro Sugimura 《Soil Dynamics and Earthquake Engineering》2006,26(2-4):163-174
The paper utilizes previously developed microtremor simulation technique to evaluate the reliability of Rayleigh wave dispersion curve estimated by f–k spectral analysis of microtremor array measurement. The simulated microtremors are obtained for a fictitious (TEST) site. Attempt is also made to obtain the dispersion curve for two real sites (OHDATE and SKC) by inverse analysis of the microtremor array measurement using f–k spectra method. The estimated dispersion curve from simulated microtremors (TEST site) compares well with the theoretical dispersion curve, demonstrating the reliability of f–k spectra method and indicating that the estimated dispersion curve from microtremor measurements could be adequately used as the target for inverse analysis purposes. It is also demonstrated that the dispersion curve from microtremor measurements can be utilized to estimate the soil profile at OHDATE and SKC sites by inverse analysis. Results show that the theoretical dispersion curve of the fundamental mode of Rayleigh wave after the end of inverse analysis are in good agreement with the dispersion curve obtained by f–k spectral analysis of microtremor array measurement. 相似文献
975.
Geomagnetism and Aeronomy - Abstract—The main subject of the study is an analysis of time series of the width of annual rings of spruce and pine (D) that were obtained during the... 相似文献
976.
Man's engineering activities are concentrated on the uppermost part of the earth's crust which is called engineering-geologic zone. This zone is characterized by a significant spatialtemporal variation of the physical properties status of rocks, and saturating waters. This variation determines the specificity of geophysical and, particularly, geoelectrical investigations. Planning of geoelectric investigations in the engineering-geologic zone and their subsequent interpretation requires a priori) geologic-geophysical information on the main peculiarities of the engineering-geologic and hydrogeologic conditions in the region under investigation. This information serves as a basis for the creation of an initial geoelectric model of the section. Following field investigations the model is used in interpretation. Formalization of this a priori) model can be achieved by the solution of direct geoelectric problems. An additional geologic-geophysical information realized in the model of the medium allows to diminish the effect of the “principle of equivalence” by introducing flexible limitations in the section's parameters. Further geophysical observations as well as the correlations between geophysical and engineering-geologic parameters of the section permit the following step in the specification of the geolectric model and its approximation to the real medium. Next correction of this model is made upon accumulation of additional information. The solution of inverse problems with the utilization of computer programs permits specification of the model in the general iterational cycle of interpretation. 相似文献
977.
Three-dimensional Layer-integrated Modelling of Estuarine Flows with Flooding and Drying 总被引：1，自引：0，他引：1
Details are given of the refinement and application of a thee-dimensional (3-D) layer-integrated numerical model of tidal circulation, with the aim of simulating severe tidal conditions for practical engineering applications. The mode splitting strategy has been used in the model. A set of depth-integrated 2-D equations are first solved to give the pressure gradient, and the layer-integrated 3-D equations are then solved to obtain the vertical distributions of the flow velocities. Attention has been given to maintaining consistency of the physical quantities derived from the 2-D and 3-D equations. A TWO=layer mixing length turbulence model for the vertical shear stress distribution has been included in the model. Emphasis has been focused on applying the model to a real estuary, which is geometrically complicated and has large tidal ranges giving rise to extensive flooding and drying. The model has been applied to three examples, including: wind-driven flow in a rectangular lake, tidal circulation in a model rectangular harbour, and tidal circulation in a large estuary. Favourable results have been obtained for both the simple and complex flow beds. 相似文献
978.
S.B. Rakesh Kumar Singh Mohan L. Gupta G.V. Rao 《Journal of Volcanology and Geothermal Research》1982,13(3-4)
Direct current resistivity surveys and shallow temperature measurements were carried out for geothermal exploration in a part of Parvati valley, goethermal field, Himachal Pradesh, India. At a few places, the Schlumberger soundings pointed to the presence of a relatively low-resistivity shallow layer, which probably represents fractured and jointed quartzite, saturated with hot/cold water. Wenner resistivity profiles indicate the presence of some possible shallow subsurface lateral hot water channels across the valley at Manikaran. Shallow temperature measurements show a good subsurface thermal anomaly near the confluence of the rivers Brahmaganga and Parvati. The results of the survey, together with other available geodata, suggest that an anomalous heat source does not lie beneath the study area.It is postulated that the meteoric water, originating at high elevations after heating as a result of circulation at depth, emerges at the surface in the Parvati valley as hot springs, after mixing in various proportions with near surface cold waters. 相似文献
979.
A self-similar solution to the problem of the implosion of a cylindrical shock wave in the presence of a magnetic field has been investigated. A strong shock wave in a cylindrically-symmetric flow travels to the axis of symmetry through a gas of uniform initial density 0 and zero-pressure. A comparative study has been made between the results obtained in ordinary gasdynamics and magnetogasdynamics with transverse and axial components of the magnetic field. The value of similarity exponent has been assigned from that found in the paper of Whitham (1958). 相似文献
980.
The problem of the diversity of the geochemical types of carbon dioxide waters (CDW) in petrografically and mineralogically uniform crystalline rock masses is solved with allowance made for the effect of different boundary conditions (physicochemical parameters) on the geochemical effect of interaction in the rock–water system. The formation of the entire geochemical spectrum of CDW in crystalline rock masses is shown to be explicable on the basis of a model of interaction in granite–water systems at different mass ratios of reacting rock (S) and water (L), different temperatures T, and equilibrium concentrations of dissolved CO2 (P
CO2). 相似文献