全文获取类型
收费全文 | 3246篇 |
免费 | 121篇 |
国内免费 | 19篇 |
专业分类
测绘学 | 120篇 |
大气科学 | 352篇 |
地球物理 | 707篇 |
地质学 | 1008篇 |
海洋学 | 351篇 |
天文学 | 573篇 |
综合类 | 7篇 |
自然地理 | 268篇 |
出版年
2023年 | 9篇 |
2022年 | 11篇 |
2021年 | 38篇 |
2020年 | 51篇 |
2019年 | 49篇 |
2018年 | 95篇 |
2017年 | 66篇 |
2016年 | 129篇 |
2015年 | 71篇 |
2014年 | 106篇 |
2013年 | 165篇 |
2012年 | 142篇 |
2011年 | 184篇 |
2010年 | 154篇 |
2009年 | 228篇 |
2008年 | 199篇 |
2007年 | 177篇 |
2006年 | 143篇 |
2005年 | 131篇 |
2004年 | 127篇 |
2003年 | 123篇 |
2002年 | 111篇 |
2001年 | 89篇 |
2000年 | 93篇 |
1999年 | 75篇 |
1998年 | 91篇 |
1997年 | 49篇 |
1996年 | 47篇 |
1995年 | 34篇 |
1994年 | 25篇 |
1993年 | 34篇 |
1992年 | 23篇 |
1991年 | 38篇 |
1990年 | 13篇 |
1989年 | 17篇 |
1987年 | 14篇 |
1986年 | 10篇 |
1985年 | 19篇 |
1984年 | 25篇 |
1983年 | 19篇 |
1982年 | 12篇 |
1981年 | 6篇 |
1980年 | 8篇 |
1979年 | 8篇 |
1978年 | 10篇 |
1977年 | 11篇 |
1976年 | 13篇 |
1975年 | 13篇 |
1973年 | 9篇 |
1971年 | 7篇 |
排序方式: 共有3386条查询结果,搜索用时 359 毫秒
941.
Annual precipitation, July and January temperatures were reconstructed from a continuous Holocene pollen sequence from the
Middle Atlas, Morocco, using the best modern analogues method. The reconstructions show a clear difference between the early
and late Holocene: from ∼10 ka to ∼6.5 ka the climate was drier and warmer than during the period since 6.5 ka. The average
value of annual precipitation was ∼870 mm until 6.5 ka, then rose to ∼940 mm. Between 10 ka and 6.5 ka January and July temperatures
were about 4 °C higher than the present. Both temperatures show a marked decrease between 7 ka and 6 ka. After 6.5 ka July
and January temperatures fluctuated between 21 and 23 °C, and 2.5 and 5 °C respectively. January temperatures show a period
of intermediate values (∼3.5 °C) between 4 ka and 5.5 ka. The reconstructed climate values generally match palaeolimnological
data from the same core, which show five intervals of low lake level during the Holocene. They are also consistent with regional-scale
COHMAP simulated palaeoclimate that shows contrasting patterns of rainfall variation between the northwesternmost part of
Africa and the intertropical band.
Received: 7 July 1997 / Accepted: 28 May 1998 相似文献
942.
Conventional finite-difference modelling algorithms for seismic forward modelling are based on a time-stepping scheme with a constant (global) time step. Large contrasts in the velocity model or in the spatial sampling rate cause oversampling in time for some regions of the model. The use of locally adjustable time steps can save large amounts of computation time for certain modelling configurations. The computation of spatial derivatives across the transition zone between regions of the model with different temporal sampling requires the definition of the wavefield at corresponding time levels on both sides of the transition zone. This condition can be obtained by extrapolation in time, which is inaccurate, or by multiple time integration in the transition zone. The error in the latter solution is of the same order as the conventional time-stepping scheme because both methods are based on the same iteration formula. The technique of multiple time integration simply requires the use of different sizes of time step. It is applicable only for certain factors of variation of the time step. 相似文献
943.
In the Rogaland–Vest Agder terrain of the Sveconorwegian Province of SW Norway, two main Sveconorwegian metamorphic phases
are reported: a phase of regional metamorphism linked to orogenic thickening (M1) and a phase of low-pressure thermal metamorphism
associated with the intrusion of the 931 ± 2 Ma anorthosite-charnockite Rogaland igneous complex (M2). Phase M1 reached granulite
facies to the west of the terrane and M2 culminated locally at 800–850 °C with the formation of dry osumilite-bearing mineral
associations. Monazite and titanite U-Pb geochronology was conducted on 17 amphibolite- to granulite-facies orthogneiss samples,
mainly from a suite of 1050 +2/−8 Ma calc-alkaline augen gneisses, the Feda suite. In these rocks, prograde negatively discordant monazite crystallized during
breakdown of allanite and titanite in upper amphibolite facies at 1012–1006 Ma. In the Feda suite and other charnockitic gneisses,
concordant to slightly discordant monazite at 1024–997 Ma probably reflects breakdown of biotite during granulite-facies M1
metamorphism. A spread of monazite ages down to 970 Ma in biotite ± hornblende samples possibly corresponds to the waning
stage of this first event. In the Feda suite, a well defined monazite growth episode at 930–925 Ma in the amphibolite-facies
domain corresponds to major clinopyroxene formation at the expense of hornblende during M2. Growth or resetting of monazite
was extremely limited during this phase in the granulite-facies domain, up to the direct vicinity of the anorthosite complex.
The M2 event was shortly followed by cooling through ca. 610 °C as indicated by tightly grouped U-Pb ages of accessory titanite
and titanite relict inclusions at 918 ± 2 Ma over the entire region. A last generation of U-poor monazite formed during regional
cooling below 610 °C, in hornblende-rich samples at 912–904 Ma. This study suggests: (1) that monazite formed during the prograde
path of high-grade metamorphism may be preserved; (2) that monazite ages reflect primary or secondary growth of monazite linked
to metamorphic reactions involving redistribution of REEs and Th, and/or fluid mobilisation; (3) that the U-Pb system in monazite
is not affected by thermal events up to 800–850 °C, provided that conditions were dry during metamorphism.
Received: 9 January 1997 / Accepted: 15 April 1998 相似文献
944.
Fans formed under subaerial terminoglacial (previously called ice-contact) conditions have several characteristics that differ from those formed under other conditions. Twenty-five such fans in NE Poland were investigated to model the dominant genetic processes involved. These fans show, as do other types, a proximal, a middle and a distal environment. The present study deals with the proximal environment. The fans date from the last, i.e. Weichselian or Vistulian, glacial. The proximal terminoglacial fan comprises abundant gravelly sediments, resembling the glacial deposits from which they were derived. Three facies, each subdivided into two subfacies, can be distinguished; these are dominated by mass flows, unchannelised flows, and stream (= channelised) flows, respectively. The characteristics of the facies are described and illustrated. It is concluded that the irregular supply of water by the nearby ice masses dominates the sedimentary processes. 相似文献
945.
The spontaneous combustion of coal causes widespread underground coal fires in several countries, amongst which is China. These coal fires cause serious environmental, economic and safety problems. In northern China, the coal fires occur within a wide region stretching 5000 km east-west and 750 km north-south. Remote sensing therefore provides an ideal tool for monitoring this environmental hazard over such a large and remote area. As part of a research project to detect, measure, monitor and extinguish these coal fires, this paper describes a remote-sensing-based multi-sensor data-fusion methodology for detecting the underground fires. The methodology is based on fusing a variety of satellite-based image types (optical, thermal, microwave) together with airborne data (optical and thermal infrared) and ancillary data sources such as geological and topographic maps. The results of the remote-sensing data fusion are presented, using pixel-based, feature-based and decision-based fusion approaches. 相似文献
946.
Geologie en Mijnbouw - 相似文献
947.
Imaging spectrometry for geological remote sensing 总被引:2,自引:0,他引:2
Without use of imaging spectrometry, imaging of the Earth's surface from aircraft and from spacecraft is hampered by the low spectral resolution and limited number of spectral bands, typically less than 10 bands of 100 to 200 nm width. Imaging spectrometry in remote sensing concerns the acquisition of image data in many narrow (< 40 nm wide) contiguous spectral bands with the ultimate goal of producing detailed spectral reflectance curves for each pixel in the image. Many minerals and rocks have unique spectral signatures with characteristic absorption features that are 20 to 40 nm wide. Imaging spectrometers allow to depict these narrow features and thus map surface mineralogy based on spectral image characterization. This paper gives a review of imaging spectrometry and addresses the following topics: airborne and spaceborne systems available, spectral and geometric data pre-processing, atmospheric correction, techniques for thematic data analysis, and applications in the field of geological remote sensing. In the final section a case study is described where imaging spectrometer data is used for mapping surface mineralogy in a hydrothermal alteration system, thus guiding gold exploration. 相似文献
948.
Removal of LNAPL (oil) from an aquifer is described using a multiphase flow model. At the well boundary seepage face conditions are imposed. These conditions are implemented in a numerical model and withdrawal in a twodimensional domain is simulated for two different geometries of the oil lens and for varied values of the physical parameters. Assuming vertical equilibrium, the oil flow equation is reduced by vertical integration. The well boundary condition is approximated by imposing zero oil lens thickness. Similarity solutions of the reduced equations for the two geometries show good agreement with the numerical results in most cases. 相似文献
949.
950.
Modelling is an indispensable tool in geochemical engineering in predicting the outcome of our intended interferences in geochemical systems. Because such systems are highly complex, investigation by means of designed experiment can only apply to subsystems and processes. Models too can only capture a partial, simplified image of the true system. Conceptual models play a vital role. Advantages of the use of models are, a.o., better testing of our understanding of geochemical processes, better formulation of this understanding, and better prediction of the outcome of our conjectures. Examples from hydrogeochemistry are discussed to demonstrate this. 相似文献