全文获取类型
收费全文 | 43752篇 |
免费 | 4042篇 |
国内免费 | 5623篇 |
专业分类
测绘学 | 2355篇 |
大气科学 | 6026篇 |
地球物理 | 9505篇 |
地质学 | 21769篇 |
海洋学 | 3394篇 |
天文学 | 2528篇 |
综合类 | 4208篇 |
自然地理 | 3632篇 |
出版年
2024年 | 98篇 |
2023年 | 402篇 |
2022年 | 918篇 |
2021年 | 1080篇 |
2020年 | 907篇 |
2019年 | 1001篇 |
2018年 | 5562篇 |
2017年 | 4786篇 |
2016年 | 3569篇 |
2015年 | 1162篇 |
2014年 | 1352篇 |
2013年 | 1248篇 |
2012年 | 2072篇 |
2011年 | 3804篇 |
2010年 | 3036篇 |
2009年 | 3329篇 |
2008年 | 2754篇 |
2007年 | 3177篇 |
2006年 | 809篇 |
2005年 | 991篇 |
2004年 | 936篇 |
2003年 | 993篇 |
2002年 | 846篇 |
2001年 | 610篇 |
2000年 | 714篇 |
1999年 | 1014篇 |
1998年 | 829篇 |
1997年 | 885篇 |
1996年 | 817篇 |
1995年 | 686篇 |
1994年 | 564篇 |
1993年 | 514篇 |
1992年 | 395篇 |
1991年 | 292篇 |
1990年 | 234篇 |
1989年 | 183篇 |
1988年 | 174篇 |
1987年 | 126篇 |
1986年 | 107篇 |
1985年 | 76篇 |
1984年 | 53篇 |
1983年 | 46篇 |
1982年 | 37篇 |
1981年 | 49篇 |
1980年 | 60篇 |
1979年 | 28篇 |
1978年 | 15篇 |
1976年 | 17篇 |
1975年 | 14篇 |
1958年 | 18篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
61.
The coal seams of Sawang Colliery, East Bokaro Coalfields are bituminous to sub-bituminous in nature and categorized as high
gaseous seams (degree II to degree III level). These seams have the potential for coal bed methane (CBM) and their maturity
increases with increasing depth, as a result of enhanced pressure-temperature conditions in the underground. The vitrinite
maceral group composition of the investigated coal seams ranges from 62.50–83.15%, whereas the inertinite content varies from
14.93–36.81%. The liptinite content varies from 0.66% to 3.09%. The maximum micro-pores are confined within the vitrinite
group of macerals. The coal seams exhibit vitrinite reflectance values (Ro% calculated) from 0.94% (sample CG-97) to 1.21%
(sample CG-119).
Proximate analyses of the investigated coal samples reveal that the moisture content (M%) ranges from 1.28% to 2.98%, whereas, volatile matter (VM%) content is placed in the range of 27.01% to 33.86%. The ash
content (A%) ranges from 10.92% to 30.01%. Fixed carbon (FC%) content varies from 41.53% to 55.93%. Fuel ratio variation shows a restricted
range from 1.53 to 1.97. All the coal samples were found to be strongly caking and forming coke buttons.
The present study is based on the adsorption isotherm experiments carried out under controlled P-T conditions for determination of actual gas adsorption capacity of the coal seams. This analysis shows that the maximum methane
gas adsorbed in the coal sample CG-81 is 17 m3/t (Std. daf), at maximum pressure of 5.92 MPa and experimental temperature of 30°C. The calculated Langmuir regression parameters
PL and VL range from 2.49 to 3.75 MPa and 22.94 to 26.88 m3/t (Std. daf), respectively. 相似文献
62.
Singh et al (2005) examined the potential of the ANN and neuro-fuzzy systems application for the prediction of dynamic constant of rockmass.
However, the model proposed by them has some drawbacks according to fuzzy logic principles. This discussion will focus on
the main fuzzy logic principles which authors and potential readers should take into consideration. 相似文献
63.
钱家店凹陷中的含矿层位主要为上白垩统姚家组,前人认为其中的红色砂岩为原生成因,但大量的证据证实砂岩原生应以灰色为主,红色砂岩为后生氧化蚀变造成,并控制着钱家店铀矿床铀矿化的产出.在此基础上建立该矿床的后生蚀变分带,依次为红色蚀变带、黄色蚀变带、灰白色蚀变带、过渡带、原生带,红色蚀变带为主要的氧化带,铀矿化主要集中在过渡带.平面上铀矿化主要分布于层间氧化带前锋线附近的位置,并在氧化舌状体的前端和两个氧化舌状体之间存在铀的富集.文章最后还建立了该矿床的成矿模式,并对区域上层间氧化带的展布进行了讨论. 相似文献
64.
Delineation of groundwater protection zones by the backward particle tracking method: theoretical background and GIS-based stochastic analysis 总被引:1,自引:0,他引:1
Konstadinos N. Moutsopoulos Alexandra Gemitzi Vassilios A. Tsihrintzis 《Environmental Geology》2008,54(5):1081-1090
The backward particle tracking method, an effective and powerful tool that can be used to delineate groundwater protection
zones, is presented. The theoretical background and insights on the applicability of this method are provided. Moreover, the
present work enriches the backward particle tracking method with an uncertainty analysis concerning the porosity values, applying
a Monte Carlo (MC) approach, coupled with the use of geographical information systems (GIS). As an application example, a
wellfield in the Komotini area, Greece, is investigated. The present study may serve as a potential guideline for wellfield
delineation, particularly in areas like Greece where lack of data related to the hydrogeological system is often a problem. 相似文献
65.
Anne Marie de Grosbois 《Environmental Geology》2008,54(5):1121-1126
66.
The largest karst cave discovered in a tunnel during motorway construction in Slovenia’s Classical Karst (Kras) 总被引:1,自引:0,他引:1
Karst researchers of the Karst Research Institute ZRC SAZU are regularly consulted during the planning of Slovenia’s motorways
and invited to observe and monitor construction in the karst areas. More than 350 caves have been discovered in the course
of building 60 km of new motorways over the past decade. Access to the most important caves is preserved by concrete tubes
closed with metal covers at the roadside. The largest cave system in a tunnel, named LC-S647, is almost entirely preserved.
The protection of this cave will serve as a good example for the preservation of natural heritage in the future. Construction
work for the motorway uncovered a high degree of karstification. In particular, the discovery of this cave in the Kastelec
tunnel LC-S647 showed the existence of a major cave system in the geological and speleological past. A number of unconnected
passages have also been found. Caves discovered during highway construction have brought new knowledge about the cavernosity
and the geological history of this part of the karst. 相似文献
67.
A detailed characterization of the site is crucial to designing an efficient method of managing the risks associated with
tailings from abandoned mines. Therefore, samples collected from various depths within tailings in Guryong mine, Korea, were
analyzed for their chemical, physical and mineralogical characteristics. All samples of the Guryong tailings had acid-generating
potential. However, in the oxidation zone, the net acid generation (NAG) was low (30 kg H2SO4 t−1) although the acid neutralization potential (ANP) was less than zero. The ANP values in the unoxidation zone were higher
(> −56.0 kg CaCO3 t−1) than in the other zones. As a result, the amount of alkali ions that are needed to neutralize the acid needs to be considered.
In this experiment G3, G4 and G6 drill cores containing fine tailings particles near the unoxidation zone were observed to
contain calcite (CaCO3) with acid-neutralizing capacity. A low pH (2−4) in the oxidation zone of the tailings changed to a neutral pH in the unoxidation
zone of the tailings. These results suggest that the acid-neutralizing capacity of the tailings was controlled by particle
and mineral composition of tailings. 相似文献
68.
Daniel Bernard Jamal El Khattabi Emilie Lefevre Hani Serhal Sabine Bastin-Lacherez Isam Shahrour 《Environmental Geology》2008,53(5):1129-1138
In the north of France, high registers of nickel are sometimes recorded within the chalk aquifer. In a confined context, the
presence of pyrite in the covering clays or in the marcasite nodules encrusted in the clay may constitute a natural source
of trace metals. With an objective of sanitary control, the limits of chemical contents regulating the quality of water destined
for human consumption have been lowered by the European Framework Directive in the field of water policy (2000/60/EC). As
a result, nickel limits have been reduced from 50 to 20 μg/l. The analyses, carried out on three water catchment fields in
our area of study, were centred on variable parameters (Eh, O2(d), pH, Conductivity, T°), major elements (SO4, NO3) and metals (Fe, Ni, Mn, Co). The acquired data enabled us to identify from one hand, the conditions which are presented
within the site, special thanks to the evolution of nitrate and iron contents and on the other hand, the natural origin (geological)
of nickel for two of the three sites studied based essentially on the evaluation of the Nickel/Cobalt ratio. Thus, on the
first site, the evolution of nickel content and nitrate content showed the influence of the phenomenon of denitrification
on the re-mobilisation of the nickel. Whereas on the second site, a high variation of total iron content and oxygen dissolved
in solution highlighted a particular phenomenon of oxidation of the pyrite through molecular oxygen. Finally, the correlation
with the sulphates clearly showed behaviour of the nickel, once released, that was entirely dependent on the phenomenon of
adsorption on the iron and manganese hydroxides. 相似文献
69.
Mahmoud M. El Banna 《Environmental Geology》2008,54(6):1291-1299
Types, distribution, and origin of recent sand dunes between Rosetta and Idku, in the western sector of the Nile Delta, Egypt
were investigated. Sand samples from the dunes, beach, and seafloor were studied for grain size distribution and mineralogical
composition. It has been found that most of the dunes in the study area have been subjected to deterioration and removal due
to the construction of buildings and the International Coastal Highway. The remnant constitutes a damaged belt of foredunes
that extends from El Bouseily village to the west of Idku town. The dune’s origin is interpreted to be the result of coastal
drifting and the subsequent transport of sediments of the former Canopic Nile branch eastward by the predominant longshore
current and by aeolian processes. The blown sand grains accumulated to form a belt of coastal sand dunes of original longitudinal
and crescentic forms. Urbanization of the coast has severely altered the landscape. The study area is considered vulnerable
to the impacts of climate change and the expected rise in sea level. The outcome of potential sea level rise is serious; erosion
problems are expected to be exacerbated and vast areas from land and property would be lost. Thus, protection and preservation
the remaining dunes in the study area are vital requirements for shore protection. 相似文献
70.
N. Nur Ozyurt 《Environmental Geology》2008,54(4):731-736
Salinity and temperature variations in groundwater discharge from the Altug submarine karst cave have been observed at 28 m
below sea level for every 10 min between November 2004 and August 2005 to determine the drivers that govern the salinization.
Comparisons between temporal trends of salinity and temperature with those of precipitation, air pressure, sea level and wind
velocity revealed an apparent dominance of precipitation regime on the salinity and temperature variations. Spectral analyses
applied to observations showed that the air pressure and sea level oscillations are affected by sun and moon tides which do
not have an appreciable impact on the salinity and temperature variations. Annual rate of salinization in Altug cave seems
inversely related to the inland groundwater head so that the maximum and minimum fresh water contributions occur at mid-spring
and late-summer, respectively. 相似文献