全文获取类型
收费全文 | 81931篇 |
免费 | 5656篇 |
国内免费 | 6958篇 |
专业分类
测绘学 | 2676篇 |
大气科学 | 10120篇 |
地球物理 | 17758篇 |
地质学 | 31758篇 |
海洋学 | 7598篇 |
天文学 | 14805篇 |
综合类 | 3169篇 |
自然地理 | 6661篇 |
出版年
2023年 | 590篇 |
2022年 | 1340篇 |
2021年 | 1675篇 |
2020年 | 1497篇 |
2019年 | 1596篇 |
2018年 | 2510篇 |
2017年 | 2292篇 |
2016年 | 2843篇 |
2015年 | 2053篇 |
2014年 | 2982篇 |
2013年 | 4248篇 |
2012年 | 2633篇 |
2011年 | 3238篇 |
2010年 | 3154篇 |
2009年 | 3701篇 |
2008年 | 3345篇 |
2007年 | 3274篇 |
2006年 | 2926篇 |
2005年 | 2635篇 |
2004年 | 2490篇 |
2003年 | 2416篇 |
2002年 | 2284篇 |
2001年 | 2086篇 |
2000年 | 2242篇 |
1999年 | 2636篇 |
1998年 | 2296篇 |
1997年 | 2295篇 |
1996年 | 1965篇 |
1995年 | 1817篇 |
1994年 | 1700篇 |
1993年 | 1500篇 |
1992年 | 1311篇 |
1991年 | 1185篇 |
1990年 | 1164篇 |
1989年 | 1038篇 |
1988年 | 965篇 |
1987年 | 992篇 |
1986年 | 822篇 |
1985年 | 993篇 |
1984年 | 1070篇 |
1983年 | 1025篇 |
1982年 | 989篇 |
1981年 | 931篇 |
1980年 | 795篇 |
1979年 | 773篇 |
1978年 | 728篇 |
1977年 | 673篇 |
1976年 | 639篇 |
1974年 | 620篇 |
1973年 | 622篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
961.
962.
主应力轴旋转对剪切带形成的影响分析 总被引:2,自引:2,他引:2
软土的剪切带形成对于研究土的工程性质、土体稳定性评价有重要的意义。 采用可以考虑主应力轴旋转影响的土的弹塑性本构模型———关口-太田模型, 分析主应力轴旋转对剪切带形成的影响, 得出最可能发生局部剪切变形的方向同主应力轴的旋转角有关, 但主轴旋转对激发变形局部化失稳的影响不大的结论。 相似文献
963.
Evaluation of heavy metal contamination and implication of multiple sources from Hunchun basin, northeastern China 总被引:1,自引:0,他引:1
J.-W. Moon H.-S. Moon N.-C. Woo J.-S. Hahn J.-S. Won Y. Song X. Lin Y. Zhao 《Environmental Geology》2000,39(9):1039-1052
Present concentrations and distributions of heavy metals through profiles, surface soil, and stream sediment samples in the
Hunchun area, north-eastern China, were investigated to determine the elemental background values. This study also aims to
characterize potentially toxic materials such as pulverized fly ash (PFA) from power stations or ash and slag from coal used
domestically in urban areas, agrochemicals applied inappropriately, and urban sewage sludges from Hunchun City, as well as
to ascertain the possibility of natural enrichment through site characterization by mineralogical and geochemical investigation.
The distribution of contaminants in the alluvial soils (fluvisol) of this area has been influenced by several interacting
factors. The parent alluvial materials from weathered products of amphiboles have made coatings such as ferrihydrite, goethite,
and hematite. This natural inheritance factor is supported by the fact that the concentrations of weak acid-extractable (plant-available)
heavy metals are very low, except for Fe and Mn. However, in agricultural soils and adjacent stream sediments, an anthropogenic
input of Cd, Pb, Ni and Cr by agrochemicals is strongly suggested. Also, F contamination by coal combustion and the dissolution
of F-bearing minerals could cause some future problems. Wide distribution and significantly high concentrations of Cd, Fe,
Mn, and F in soils throughout the combination of pollutants originating from lithogenic and the anthropogenic sources pose
potential problems in utilizing water resources.
Received: 14 June 1999 · Accepted: 27 December 1999 相似文献
964.
Rio Tinto estuary (Spain): 5000 years of pollution 总被引:6,自引:0,他引:6
R.A. Davis Jr. A. T. Welty J. Borrego J. A. Morales J. G. Pendon J. G. Ryan 《Environmental Geology》2000,39(10):1107-1116
Mining of massive sulfide deposits in southwestern Spain extending back to the Copper and Bronze Ages has resulted in the
pollution of the Rio Tinto fluvial-estuarine complex, the site of Columbus' departure for the New World in 1492. Additional
sources of potential pollution include the large industrial complex at Huelva near the lower portion of the estuary. Extensive
analysis of surface sediment samples and cores has established that there are no geographic trends in the distribution of
the pollutants, which include Cu, Fe, Pb, Zn, Ti, Ba, Cr, V and Co. These data have, however, demonstrated that tidal flux
within the estuary carries phosphorus and perhaps other elements from the industrial complex at Huelva to the tidal limit
of the system, several kilometers upstream from the discharge site. Radiometric analysis of short cores shows that sedimentation
rates over at least the past couple of centuries have been about 0.3 cm/year. These data and that from a single deep core
demonstrate that the estuary was polluted from mining activity long before the large-scale operations began in the late nineteenth
century.
Received: 3 May 1999 · Accepted: 18 October 1999 相似文献
965.
J. Nuzzo M. Hyman M. W. Rowe M. N. Rao R. L. Palma J. Westrich 《Journal of Earth System Science》2000,109(1):89-94
We have measured the abundance and isotopic composition of xenon in petroleum samples from the Shell Bullwinkle Field off
the coast of Louisiana. We used an oxidation and purification procedure designed to insure complete extraction and clean up
of xenon from the petroleum.
The xenon isotopic composition was found to be similar to the atmospheric value for one petroleum sample. While the results
of the second sample suggest possible enrichment of the heavier isotopes, the errors associated with these excesses preclude
a definitive statement to that effect. No monoisotopic enrichment in129Xe was detected in either sample, the presence of which might have allowed us to deduce the petroleum age. Our results represent
only the second xenon measurement from petroleum, and the concentrations are within the range of values published in the earlier
report. 相似文献
966.
R. H. Meade N. N. Bobrovitskaya V. I. Babkin 《International Journal of Earth Sciences》2000,89(3):461-469
Of the world's great rivers, the Ob and Yenisey rank among the largest suppliers of fresh water and among the smallest suppliers of suspended sediment to the coastal ocean. Sediment in the middle reaches of the rivers is mobilized from bordering terraces and exchanged between channels and flood plains. Sediment in the lower reaches of these great rivers is deposited and stored (permanently, on a millennial time scale) in flood plains. Sediment discharges, already small under natural conditions, are diminished further by large manmade reservoirs that trap significant proportions of the moving solids. The long winter freeze and sudden spring breakup impose a peakedness in seasonal water runoff and sediment discharge that contrasts markedly with that in rivers of the tropics and more temperate climates. Very little sediment from the Ob and Yenisey rivers is being transported to the open waters of the Arctic Ocean under present conditions. 相似文献
967.
S. K. Chaulya R. S. Singh M. K. Chakraborty B. K. Srivastava 《Geotechnical and Geological Engineering》2000,18(3):193-207
An integrated study on biological stabilisation of a dump slope has indicated that biological reclamation with grass and tree species should be considered for long term stability of this coal mine dump in India. The grasses have greater soil binding capacity and help to control soil erosion and improve dump stability. Native grasses such as Bamboo (Dendrocalmus strictus) and Kashi (Saccharum spontaneum) are the important constituents of grass species which can stabilise the dump slopes. Field observation of growth performance of grasses have indicated that mean grass height, root depth and below-ground root biomass are 185 cm (±68), 45 cm (±5) and 467 g m–2 (±170), respectively after three years of grass growth on Mudidih overburden dump slope in India. The growth performance of tree species, namely Sisum (Dalbergia sisoo) and Subabool (Leucena lecocephala), in terms of height, diameter increment, below-ground biomass and root depth have shown mean values of 219 cm (±94), 48 mm (±6), 4.0 kg m–2 (±1.5) and 1 m (±0.1), respectively. This acts as biological fertility which helps in root proliferation and enhancement of dump stability. From the numerical modelling it is suggested that roots of these grass and tree species have significantly enhanced the factor of safety of dump from 1.4 to 1.8 and therefore have a positive role in maintaining long term stability. 相似文献
968.
Groundwater targeting in a hard-rock terrain using fracture-pattern modeling, Niva River basin, Andhra Pradesh, India 总被引:1,自引:0,他引:1
In hard-rock terrain, due to the lack of primary porosity in the bedrock, joints, fault zones, and weathered zones are the
sources for groundwater occurrence and movement. To study the groundwater potential in the hard-rock terrain and drought-prone
area in the Niva River basin, southern Andhra Pradesh state, India, Landsat 5 photographic data were used to prepare an integrated
hydrogeomorphology map. Larsson's integrated deformation model was applied to identify the various fracture systems, to pinpoint
those younger tensile fracture sets that are the main groundwater reservoirs, and to understand the importance of fracture
density in groundwater prospecting. N35°–55°E fractures were identified as tensile and N35°–55°W fractures as both tensile
and shear in the study area. Apparently, these fractures are the youngest open fractures. Wherever N35°–55°E and N35°–55°W
fracture densities are high, weathered-zone thickness is greater, water-table fluctuations are small, and well yields are
high. Groundwater-potential zones were delineated and classified as very good, good to very good, moderate to good, and poor.
Electronic Publication 相似文献
969.
M. T. Krupenin A. V. Maslov I. D. Tsybulenko R. A. Nafikov V. G. Petrishcheva 《Lithology and Mineral Resources》2000,35(3):267-277
A property of swelling on heating to yield firm lightweight granules has first been revealed for lowcarbonaceous shales (LCS)
from the Riphean stratotype in the Bashkirian Megaanticlinorium, Southern Urals. These granules possess all technological
properties of “keramzite” or claydite (an artificial, expanded clayey material) and can successfully substitute this material.
Moreover, the procedure of their manufacture, analogous to the manufacture of “shungizite” (an artificial carbonaceous-clayey
material) from Lower Proterozoic clayey rocks of Karelia, is more economical than that for keramzit. Productive beds of the
homogeneous LCS make up extended bodies tens of meters thick. At the present time, they can be used as cover rocks during
the exploitation of the Bakal siderite deposits. The LCS represent fine-grained quartz-chlorite-sericite aggregates (with
variable relationships between the major components), bearing fine-dispersed shungite in amounts of 0.5–1.0%. High-quality
raw material of this kind is characterized by a fine-dispersed admixture of a mixed-layer clayey material bearing montmorillonite
and vermiculite. Geological, lithological, mineralogical, and chemical prognostic criteria have been developed for the shungizite
raw material. It has been concluded that this construction material, new for the Urals, is promising for industrial use 相似文献
970.
The Uchi subprovince of the Archean Superior Province is a series of greenstone belts extending 600 km east–west along the
southern margin of the North Caribou Terrane protocontinent. The 2.7 Ga Confederation tectonostratigraphic assemblage of the
Birch–Uchi greenstone belt, northwest Ontario, is dominated by volcanic suites of mafic, intermediate and felsic composition.
Tholeiitic basalts range compositionally from Mg# 59–26 evolving continuously to greater REE contents (La=2–19 ppm; Th/Lapm˜1), with small negative Nb anomalies. Primitive tholeiites are similar to modern intraoceanic arc basalts, whereas evolved
members extend to greater concentrations of Ti, Zr, V, Sc, and Y, and lower Ti/Zr, but higher Ti/Sc and Ti/V ratios characteristic
of back arc basalts. Calc-alkaline basalts to dacites are characterised by more fractionated REE (La/Ybn=1–8), high Th/Nbpm ratios and deeper negative Nb anomalies; they plot with modern oceanic arc basalts and some may qualify as high magnesium
andesites. The two suites are interpreted as a paired arc–back arc sequence. A third group of Nb-enriched basalts (NEB; Nb=9–18 ppm)
extend to extremely high TiO2, Ta, P2O5, Sc and V contents, with strongly fractionated REE and ratios of Nb/Ta and Zr/Hf greater than primitive mantle values whereas
Zr/Sm ratios are lower. The most abundant rhyolitic suite has extremely enriched but flat trace element patterns and is interpreted
as strongly fractionated tholeiitic basalt liquids. A second group are compositionally similar to Cenozoic adakites and Archean
high-Al, high-La/Ybn tonalites; they possess Yb ≤ 0.4 ppm, Y ≤ 6 ppm and Sc ≤ 8 ppm, with La/Ybn of 19–30 and Zr/Sm of 50–59. They are interpreted as melts of ocean lithosphere basaltic crust in a hot shallow subduction
zone. Adakites are associated with NEB in Cenozoic arcs where there is shallow subduction of young and/or hot ocean lithosphere,
often with oblique subduction. Slab melt adakites erupt, or metasomatise sub-arc mantle peridotite to generate an HFSE-enriched
source that subsequently melts during induced mantle convection. The Archean adakite–NEB association erupted during development
of the tholeiitic to calc-alkaline arc and its associated back arc. Their coexistence in the Confederation assemblage of the
Birch–Uchi greenstone belt implies convergent margin processes similar to those in Cenozoic arcs.
Received: 2 June 1999 / Accepted: 29 December 1999 相似文献