全文获取类型
收费全文 | 24462篇 |
免费 | 172篇 |
国内免费 | 919篇 |
专业分类
测绘学 | 1410篇 |
大气科学 | 1981篇 |
地球物理 | 4516篇 |
地质学 | 11600篇 |
海洋学 | 1004篇 |
天文学 | 1659篇 |
综合类 | 2161篇 |
自然地理 | 1222篇 |
出版年
2021年 | 1篇 |
2020年 | 1篇 |
2019年 | 3篇 |
2018年 | 4762篇 |
2017年 | 4037篇 |
2016年 | 2579篇 |
2015年 | 234篇 |
2014年 | 81篇 |
2013年 | 30篇 |
2012年 | 990篇 |
2011年 | 2729篇 |
2010年 | 2019篇 |
2009年 | 2315篇 |
2008年 | 1888篇 |
2007年 | 2366篇 |
2006年 | 54篇 |
2005年 | 199篇 |
2004年 | 406篇 |
2003年 | 414篇 |
2002年 | 252篇 |
2001年 | 49篇 |
2000年 | 51篇 |
1999年 | 13篇 |
1998年 | 22篇 |
1997年 | 1篇 |
1996年 | 2篇 |
1994年 | 1篇 |
1991年 | 1篇 |
1987年 | 1篇 |
1983年 | 1篇 |
1981年 | 21篇 |
1980年 | 19篇 |
1978年 | 2篇 |
1977年 | 2篇 |
1976年 | 6篇 |
1974年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
821.
He-Jun Chai Jun-Jie Wang Sheng-Chuan Tang 《Geotechnical and Geological Engineering》2009,27(3):315-323
The visualization of rock structure is a very useful and important technique in many aspects in geological and geotechnical
engineering such as investigating and evaluating the stability of a high rock slope. The conventional method used to visualize
rock structure is limited to plotting the orientation of individual discontinuities by using the Schmidt stereographic net.
The present paper shows a new technique to visualize the three-dimensional structure of rock masses. The fitting function
for three-dimensional rock structure was established based on data gathered from field survey. Two fitting methods by which
the discontinuous points gathered from a plane and discrete points from field survey were fitted respectively are suggested.
The technique to visualize the three-dimensional structure of rock masses includes many aspects such as projection transformation,
blanking and mutual cutting, are investigated. Based on the visualizing technique, the rock structure at the site of Xilupdu dam which is being built in Southwest of China was investigated. 相似文献
822.
Thomas A. Anderson Erick A. Bestland Lesja Soloninka Ilka Wallis Edward W. Banks Markus Pichler 《Hydrogeology Journal》2017,25(8):2435-2451
High and variable levels of salinity were investigated in an intermittent stream in a high-rainfall area (~800 mm/year) of the Mt. Lofty Ranges of South Australia. The groundwater system was found to have a local, upslope saline lens, referred to here as a groundwater salinity ‘hotspot’. Environmental tracer analyses (δ18O, δ2H, 87/86Sr, and major elements) of water from the intermittent stream, a nearby permanent stream, shallow and deep groundwater, and soil-water/runoff demonstrate seasonal groundwater input of very saline composition into the intermittent stream. This input results in large salinity increases of the stream water because the winter wet-season stream flow decreases during spring in this Mediterranean climate. Furthermore, strontium and water isotope analyses demonstrate: (1) the upslope-saline-groundwater zone (hotspot) mixes with the dominant groundwater system, (2) the intermittent-stream water is a mixture of soil-water/runoff and the upslope saline groundwater, and (3) the upslope-saline-groundwater zone results from the flushing of unsaturated-zone salts from the thick clayey regolith and soil which overlie the metamorphosed shale bedrock. The preferred theory on the origin of the upslope-saline-groundwater hotspot is land clearing of native deep-rooted woodland, followed by flushing of accumulated salts from the unsaturated zone due to increased recharge. This cause of elevated groundwater and surface-water salinity, if correct, could be widespread in Mt. Lofty Ranges areas, as well as other climatically and geologically similar areas with comparable hydrogeologic conditions. 相似文献
823.
Fethi Medjani Belkacem Aissani Sofiane Labar Mohamed Djidel Danielle Ducrot Antoine Masse C. Mei-Ling Hamilton 《Arabian Journal of Geosciences》2017,10(7):176
Supervised and unsupervised satellite image classifications have progressed greatly in recent years. However, discrimination difficulties still remain among classes that directly affecting data extraction and surface mapping accuracy. The Ouargla region in southeastern Algeria is intersected by wadis, where direct communication between the shallow groundwater table and these dry, overlying ephemeral stream beds exists. Underflowing groundwater exfiltrates into low-lying aeolian blowouts or endorheic basins forming oases, chotts, and sebkhas, commonly known as saline wetlands. These wetlands are becoming increasingly vulnerable to anthropogenic stress, resulting in significant water degradation. Wetland microclimates are very important to arid regions, as they promote oasis ecosystem sustainability and preservation. High water salinity in these ecosystems, however, directly affects flourishing habitat and undermines successful desert oasis development. The objective of this work is to choose the best classification method to identify saline wetlands by comparison between the different results of land use mapping within the Ouargla basin. Landsat ETM+ (2000) satellite imagery, using visual analysis with colored compositions, has identified various forms of saline wetlands in the Ouargla region desert environment in southeast Algeria. The results show that supervised classification is validated in the identification of Saharan saline wetlands, and that support vector machine (SVM) algorithm presents the best overall accuracy. 相似文献
824.
A Leaky-Conduit Model of Transient Flow in Karstic Aquifers 总被引:1,自引:0,他引:1
Karst Flow Model (KFM) simulates transient flow in an unconfined karstic aquifer having a well-developed conduit system. KFM
treats the springshed as a two-dimensional porous matrix containing a triangulated irregular network of leaky conduits. The
number and location of conduits can be specified arbitrarily, perhaps using field information as a guide, or generated automatically.
Conduit networks can be tree-like or braided. Rainwater that has infiltrated down from the surface leaks into the conduits
from the adjacent porous matrix at a rate dictated by Darcy’s law, then flows turbulently to the spring via the conduits.
KFM is calibrated using the known steady state; geometry and recharge determine the steady fluxes in the conduits, and the
head distribution determines conduit gradients and sizes. Spring flow can vary with time due to spatially and temporally variable
recharge and due to prescribed variations in the elevation of the spring. KFM is illustrated by four examples run on a test
aquifer consisting of 27 nodes, 42 elements, and 26 conduits. Three examples (drought, uniform rainstorm, storm-water input
to one element) are simulations, while the fourth uses data from a spring-basin flooding event. The qualitative fit between
the predicted and observed spring discharge in the fourth example provides support of the hypothesis that the dynamic behavior
of a karst conduit system is an emergent property of a self-organized system, largely independent of the locations and properties
of individual conduits. 相似文献
825.
Ryszard Kryza Jolanta Muszer Joanna Haydukiewicz Czesław August Marta Jurasik Nickolay Rodionov 《International Journal of Earth Sciences》2011,100(6):1227-1235
A Lower Carboniferous platform sedimentary sequence (the Paprotnia Beds) in the Bardo Unit of the central Sudetes (NE part
of the Bohemian Massif, SW Poland) is biostratigraphically well dated, based on rich macro- and micro-fossil evidence, as
Late Viséan (late Asbian, crenistria, Go III α zone). The beds contain several bentonite layers, one of which was dated using the U–Pb SHRIMP method on volcanic
zircons and yielded an age of 334 ± 3 Ma. This date fits well to the recently established chronostratigraphic limits of the
Viséan, and is consistent with the newest isotopic age constraints of 336.5–332 Ma for the Asbian boundaries. 相似文献
826.
Prosanta Kumar Khan 《International Journal of Earth Sciences》2011,100(7):1749-1758
Present study addresses the role of major plate-driving forces, particularly the slab pull and slab resistive forces, for
the generation of 26 December 2004 M
w > 9.0 off Sumatra megathrust earthquake. Major controls on the plate-driving forces are normally visualized through age,
speed, and average dip of the slab during subduction. Wide variation in age, plate obliquity, stress obliquity, subduction
rate, dip angle, and flexing depth of the subducting oceanic lithosphere between Andaman and Sumatra thus allowed us for quantitative
evaluation of the slab pull (F
SP) and slab resistive (F
SR) forces in three well-defined sectors (I, II and III). Computed values of these forces in the three sectors: (1) F
SP = 1.29 × 1013 N/m, F
SR = 1.41 × 1013 N/m; sector I, (2) F
SP = 2.10 × 1013 N/m, F
SR = 1.13 × 1013 N/m; sector II, and (3) F
SP = 2.08 × 1013 N/m, F
SR = 2.72 × 1013 N/m; sector III clearly suggest a spatial variation of stress regime in the subducting oceanic lithosphere. Excess F
SR in sectors I and III are interpreted as the causative forces behind the triggering of major seismic energy bursts near Sumatra
and Andaman on 26 December 2004. A gap of minimum seismic energy burst near Great Nicobar possibly was controlled by the excess
of F
SP in sector II. This study further advocates that the cyclic stress, resulted from unbalanced component of slab resistive force,
had a definite control on the occurrence of 2004 off Sumatra megathrust earthquake around the flexing zone of the subducting
lithosphere. 相似文献
827.
Nataša Ravbar Juan Antonio Barberá Metka Petrič Janja Kogovšek Bartolomé Andreo 《Environmental Earth Sciences》2012,65(8):2259-2272
In this study a multi-tracer test with fluorescent tracers was combined with time series analyses of natural tracers to characterize
the dynamics of the solute transport through different recharge pathways and to study hydraulic behaviour of a binary karst
system under low-flow conditions. Fluorescent tracer testing included the introduction of uranine, amidorhodamine G, or naphthionate
at three injection points. Sampling and monitoring took place at two karst springs (Malenščica, Unica) and at two underground
rivers (Pivka, Rak) recharging the Unica River at the Polje of Planina, SW Slovenia. Other monitored parameters included precipitation,
spring or underground river discharge, water temperature, and electrical conductivity. Water samples were collected and analyzed
for total organic carbon, Mg2+, SO4
2−, and NO3
− in the laboratory. In the study area, results of the tracer test suggest that contaminant transport in karst may be retarded
for several weeks during low-flow conditions followed by increases in contaminant concentrations after subsequent rainfall
events. Based on interpretation of tracer concentration breakthrough curves, low apparent dominant flow velocities (i.e.,
between 5.8 and 22.8 m/h through the well developed karst conduits, and 3.6 m/h through the prevailing vadose zone with a
dominant influence of a diffuse recharge) were obtained. Together with analyses of hydro-chemographs the artificial tracing
identified different origins of water recharging the studied aquifer. During prolonged low-water conditions the Malenščica
spring is mainly recharged from the karst aquifer and the Unica spring by the sinking Pivka River. After more intensive rainfall
events allogenic recharge from Cerknica prevails in the Malenščica spring, while the Unica spring drains mainly the allogenic
water from the Pivka Valley. These findings of alternating hydraulic connections and drainage areas due to respective hydrological
conditions are important and should be considered when monitoring water quality, implementing groundwater protection measures,
and optimizing future water exploitation. 相似文献
828.
Samples from diamondiferous pipes in the Mengyin and Fuxian regions were investigated. The chemical compositions of Cr spinels
in kimberlites of China were found to be similar to those in kimberlites of the Arkhangelsk province in Russia. A long and
complex evolution that was individual for each pipe was demonstrated. The kimberlites of the Shandong Province proved to be
rich in high-Cr chromites. This means that the kimberlites formed at large depths in the field of diamond thermodynamic stability.
Variations in the redox conditions were noted. They manifested themselves as a wide range of fluctuations of the chemical
composition of microcrystalline spinels, up to formation to Ti-magnetite and magnetite. 相似文献
829.
O. Bourgeois M. Ford M. Diraison C. Le Carlier de Veslud M. Gerbault R. Pik N. Ruby S. Bonnet 《International Journal of Earth Sciences》2007,96(6):1003-1031
The development of the Alpine mountain belt has been governed by the convergence of the African and European plates since
the Late Cretaceous. During the Cenozoic, this orogeny was accompanied with two major kinds of intraplate deformation in the
NW-European foreland: (1) the European Cenozoic Rift System (ECRIS), a left-lateral transtensional wrench zone striking NNE-SSW
between the western Mediterranean Sea and the Bohemian Massif; (2) long-wavelength lithospheric folds striking NE and located
between the Alpine front and the North Sea. The present-day geometry of the European crust comprises the signatures of these
two events superimposed on all preceding ones. In order to better define the processes and causes of each event, we identify
and separate their respective geometrical signatures on depth maps of the pre-Mesozoic basement and of the Moho. We derive
the respective timing of rifting and folding from sedimentary accumulation curves computed for selected locations of the Upper
Rhine Graben. From this geometrical and chronological separation, we infer that the ECRIS developed mostly from 37 to 17 Ma,
in response to north-directed impingement of Adria into the European plate. Lithospheric folds developed between 17 and 0 Ma,
after the azimuth of relative displacement between Adria and Europe turned counter-clockwise to NW–SE. The geometry of these
folds (wavelength = 270 km; amplitude = 1,500 m) is consistent with the geometry, as predicted by analogue and numerical models,
of buckle folds produced by horizontal shortening of the whole lithosphere. The development of the folds resulted in ca. 1,000 m of rock uplift along the hinge lines of the anticlines (Burgundy–Swabian Jura and Normandy–Vogelsberg) and ca. 500 m of rock subsidence along the hinge line of the intervening syncline (Sologne–Franconian Basin). The grabens of the
ECRIS were tilted by the development of the folds, and their rift-related sedimentary infill was reduced on anticlines, while
sedimentary accumulation was enhanced in synclines. We interpret the occurrence of Miocene volcanic activity and of topographic
highs, and the basement and Moho configurations in the Vosges–Black Forest area and in the Rhenish Massif as interference
patterns between linear lithospheric anticlines and linear grabens, rather than as signatures of asthenospheric plumes.
相似文献
O. BourgeoisEmail: |
830.
Pengfei Si Jordan Aaron Scott McDougall Ji Lu Xiping Yu Nicholas J. Roberts John J. Clague 《Landslides》2018,15(4):711-726
This paper proposes and demonstrates a two-layer depth-averaged model with non-hydrostatic pressure correction to simulate landslide-generated waves. Landslide (lower layer) and water (upper layer) motions are governed by the general shallow water equations derived from mass and momentum conservation laws. The landslide motion and wave generation/propagation are separately formulated, but they form a coupled system. Our model combines some features of the landslide analysis model DAN3D and the tsunami analysis model COMCOT and adds a non-hydrostatic pressure correction. We use the new model to simulate a 2007 rock avalanche-generated wave event at Chehalis Lake, British Columbia, Canada. The model results match both the observed distribution of the rock avalanche deposit in the lake and the wave run-up trimline along the shoreline. Sensitivity analyses demonstrate the importance of accounting for the non-hydrostatic dynamic pressure at the landslide-water interface, as well as the influence of the internal strength of the landslide on the size of the generated waves. Finally, we compare the numerical results of landslide-generated waves simulated with frictional and Voellmy rheologies. Similar maximum wave run-ups can be obtained using the two different rheologies, but the frictional model better reproduces the known limit of the rock avalanche deposit and is thus considered to yield the best overall results in this particular case. 相似文献