全文获取类型
收费全文 | 42074篇 |
免费 | 597篇 |
国内免费 | 1059篇 |
专业分类
测绘学 | 1856篇 |
大气科学 | 3179篇 |
地球物理 | 8162篇 |
地质学 | 17998篇 |
海洋学 | 2713篇 |
天文学 | 5296篇 |
综合类 | 2225篇 |
自然地理 | 2301篇 |
出版年
2022年 | 175篇 |
2021年 | 285篇 |
2020年 | 282篇 |
2019年 | 350篇 |
2018年 | 5364篇 |
2017年 | 4550篇 |
2016年 | 3223篇 |
2015年 | 590篇 |
2014年 | 688篇 |
2013年 | 1040篇 |
2012年 | 1682篇 |
2011年 | 3539篇 |
2010年 | 2814篇 |
2009年 | 3275篇 |
2008年 | 2753篇 |
2007年 | 3145篇 |
2006年 | 816篇 |
2005年 | 781篇 |
2004年 | 984篇 |
2003年 | 900篇 |
2002年 | 740篇 |
2001年 | 412篇 |
2000年 | 447篇 |
1999年 | 287篇 |
1998年 | 320篇 |
1997年 | 255篇 |
1996年 | 201篇 |
1995年 | 216篇 |
1994年 | 208篇 |
1993年 | 171篇 |
1992年 | 162篇 |
1991年 | 168篇 |
1990年 | 154篇 |
1989年 | 120篇 |
1988年 | 137篇 |
1987年 | 146篇 |
1986年 | 137篇 |
1985年 | 174篇 |
1984年 | 159篇 |
1983年 | 180篇 |
1982年 | 143篇 |
1981年 | 167篇 |
1980年 | 170篇 |
1979年 | 151篇 |
1978年 | 138篇 |
1977年 | 112篇 |
1976年 | 93篇 |
1975年 | 89篇 |
1974年 | 99篇 |
1973年 | 94篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
981.
Chemistry of major and minor elements, 87Sr/86Sr, δD, δ18O and δ34S of brines were measured from Tertiary strata and Quaternary salt lakes in the western Qaidam Basin. The water chemistry
data show that all oilfield brines are CaCl2 type. They were enriched in Ca2+, B3+, Li+, Sr2+, Br−, and were depleted in Mg2+, SO4
2−, which indicated that these brines had the characteristics of deeply circulated water. The relationship between δD and δ18O shows that all data of these brines decline towards the Global Meteoric Water Line (GWL) and Qaidam Meteoric Water Line
(QWL), and that the intersection between oilfield brines and Meteoric Water Lines was close to the local spring and fresh
water in the piedmont in the western Qaidam Basin. The results suggest that oilfield brines has initially originated from
meteoric water, and then might be affected by water-rock metamorphose, because most oilfield brines distribute in the range
of metamorphosing water. The 87Sr/86Sr values of most oilfield brines range from 0.71121 to 0.71194, and was less than that in salt lake water (>0.712), but close
to that of halite in the study area. These imply that salt dissolution occurred in the process of migration. In addition,
all oilfield brines have obviously much positive δ34S values (ranging from 26.46‰ to 54.57‰) than that of salt lake brines, which was caused by bacterial sulfate reduction resulting
in positive shift of δ34S value and depleteed SO4
2− in oilfield brines. Combined with water chemical data and δD, δ18O, 87Sr/86Sr, δ34S values, we concluded that oilfield brines mainly originate from the deeply circulated meteoric waters, and then are affected
by salt dissolution, water-rock metamorphose, sulfate reduction and dolomitization during the process of migration. These
processes alter the chemical compositions of oilfield brines and accumulate rich elements (such as B, Li, Sr, Br, K and so
on) for sustainable utilization of salt lake resources in the Qaidam Basin. 相似文献
982.
Rainfall, throughfall, stemflow and spring were studied in a secondary forest during a wet season from April to August in
2006. Some of the chemicals in throughfall, stemflow and spring were increased in contrast with incident rainfall. Specifically,
Cl−, HCO3
−, Na+ and Ca2+ were leached negatively in throughfall, but K+ and Mg2+ were leached positively. In stemflow, Cl− and Na+ were leached negatively, the others were leached positively and their concentrations were higher than those in throughfall.
Total carbon, organic carbon and inorganic carbon in throughfall and stemflow were increased as rainfall went through the
secondary forest. The concentration of free CO2 in rainfall was lower than both, throughfall and stemflow; the relationship between total acidity and free CO2 was linear. pH of throughfall and stemflow , such as maximum, minimum and mean, were lower than that of rainfall and the
extent of pH in spring was changed minimally. We came to a conclusion that rainfall via the secondary forest can lead to further
erosion, accelerate the biogeochemical cycle in epikarst zone, enhance the effective state of alkali elements in the soil,
supply vegetation with more nutrients and advance vegetation’s growth and succession, which are reasonably sufficient to form
a stable karst ecosystem. 相似文献
983.
Flow slides in municipal solid waste (MSW) landfills are common geoenvironmental issues in the urban environment and can pose a serious threat to the surrounding population and infrastructure. Prediction of the maximum run-out distance of flow slides in MSW landfills is therefore an essential part of hazard and risk assessment in engineering design. Based on the framework for simple analysis originally developed by Hungr (1995), we propose a simplified analytical model for calculating dam breaks in a plastic fluid along a single inclined base. In the proposed model, a quarter-elliptical shape is used to describe the approximate configuration of the flow slide. Following this step, the physical laws relating to the conservation of mass and energy are used to calculate the potential flow. Of additional note is a boundary condition in mathematics relating to this simplified analytical model, which is also reported in this study. Taking the obvious mobility characteristics of the MSW at point of failure into consideration, a three-phase simplified model along double inclined bases has been further developed for run-out prediction of the flow slide in MSW landfill. The proposed three-phase model is then applied to estimate the maximum run-out distance of two typical flow failures of landfills located in Sarajevo and Bandung, which demonstrate the capability of the proposed simplified analytical model for use in hazard assessments of landfills. 相似文献
984.
Successional stages of biological soil crusts and their microstructure variability in Shapotou region (China) 总被引:5,自引:1,他引:5
In order to investigate succession of biological soil crusts (BSCs) and their microstructure variability, we conducted this
work in Shapotou revegetation region at the southeast edge of Tengger Deser. The results showed that BSCs generally succeeded
as a pathway of “Algae crusts, algae–lichen crusts, lichen crusts, lichen–moss crusts and moss crusts”. Occasionally mosses
directly occurred on algae crusts, and BSCs succeeded from algae crusts to moss crusts. Crust vertical stratification was
a common phenomenon, from top to bottom an inorganic layer, algae-dense layer and algae-sparse layer were divided in algae
crusts; a thallus layer, rhizoid layer and sub-rhizoid layer in lichen crusts; a “stem-leaf” layer, rhizoid layer and sub-rhizoid
layer in moss crusts, respectively. The main crust binding organisms varied from filamental cyanobacteria (dominated by Microcoleus) in algae crusts to lichen rhizoids, free-living cyanobacterial filaments and fungal hyphaes in lichen crusts, and to moss
rhizoids and fungal hyphaes in moss crusts. The dominant phototrophic organisms varied from Microcoleus (algae) in algae crusts to Collema (lichens) in lichen crusts, and to Bryum (or Didymodon and Tortula; mosses) in moss crusts. Total phototrophic biomass increased while the free-living algal biomass decreased with the succession
of BSCs. In addition, exopolysaccharides and fine particles accumulated in the course of development and succession of BSCs,
all of which lead to a gradual increase in crust thickness and porosity, while decrease in the bulk density. 相似文献
985.
Adsorption characteristics of perchloroethylene in natural sandy materials with low organic carbon content 总被引:2,自引:0,他引:2
Laboratory batch tests were conducted to investigate the sorption isotherms and sorption kinetics of the chlorinated hydrocarbon
perchloroethylene (PCE) in five natural sandy materials with an organic carbon content (f
oc) in the range 0.080–0.540%. The amended non-linear dual-mode model can describe the sorption isotherms in materials with
f
oc in the range 0.080–0.090%. For a sample with a much higher f
oc of 0.54%, the absorption isotherm was found to fit a linear model. These results may indicate that organic carbon is not
the main factor influencing the sorption isotherm. The sorption kinetics of PCE in samples with f
oc in the range 0.080–0.090% are not first-order and are different from those observed in the samples with higher f
oc. The sorption process in the materials with lower f
oc involves fast sorption, fast desorption and an equilibrium stage. The results may imply that the factors affecting sorption
kinetics of PCE in low f
oc media are pore filling and capillary condensation rather than organic carbon content. 相似文献
986.
Heavy metal contamination of roadside topsoil in Osogbo,Nigeria: its relationship to traffic density and proximity to highways 总被引:12,自引:0,他引:12
The levels of lead, cadmium, copper, nickel and zinc were determined in the roadside topsoil in Osogbo, Nigeria, with the view to determining the effect of traffic density and vehicular contribution to the soil heavy metal burden. The levels of the metals at the high density roads were significantly higher than the corresponding levels at the medium and low traffic density roads. The average levels of Pb, Cd, Cu, Ni, and Zn in all road locations at a distance of 5 m from the roads were 68.74ᆶ.82, 0.60ǂ.31, 21.19ᆟ.34, 8.38DŽ.40, and 42.45ᆥ.68 mg/kg, respectively. Pb, Cd, and Cu were of average levels of 92.07ᆩ.25, 0.76ǂ.35 and 27.69 mg/kg, respectively at a distance of 5 m from the road at high traffic density roads, while the levels of Ni and Zn averaged 9.65DŽ.61 and 56.27ᆟ.58 mg/kg, respectively. There was a rapid decrease in the level of the metals with distance, with the metal levels at a distance of 50 m from the road almost reaching the natural background levels of the metals at the control sites. The levels of the metals were also determined at the four major motor parks and at the seven mechanic workshop settlements. The levels of the metals at the motor parks and mechanic workshops were far above the levels at the control sites. The levels of Pb, Cd, Cu, Ni, and Zn at the motor parks were 519ᇝ.0, 3.6ǂ.8, 37.9ᆠ.5, 17.3dž.6 and 71.9ᆟ.3 mg/kg, respectively, with the levels of Pb, Cd, Cu, Ni, and Zn at the mechanic workshops averaging 729.57뀶.93, 4.59ǃ.01, 116.6ᆮ.8, 30.21Nj.40, and 88.74ᆰ.11 mg/kg, respectively. 相似文献
987.
Land subsidence caused by groundwater exploitation in Suzhou City,China 总被引:10,自引:2,他引:10
Suzhou City, located at the lower reaches of the Yangtze River in southeastern Jiangsu Province, is one of the few cities
in China which suffer from severe ground settlement. A research project was carried out to investigate this problem. Geological
and hydrogeological studies show that there is a multi-layered aquifer system with three distinct, soft mud layers of marine
and lagoonal origins. An examination of historical records of groundwater extraction, water levels, and ground settlement
shows that the ground subsidence is associated with the continuously increasing groundwater extraction in the deep, confined
aquifer. It is believed that the consolidation of the soft mud layers, especially the third layer which is thick and close
to the main pumped aquifer, contributes to the ground settlement. A three-dimensional finite difference numerical model representing
the multi-layered aquifer system was developed to study the ground settlement in response to groundwater extraction. By calibrating
the model with both the measured groundwater level and ground settlement, the aquifer parameters were estimated. The model
outputs fit reasonably well with the observed results, which indicates that the numerical model can reproduce the dynamic
processes of both groundwater flow and soil consolidation. The hydraulic conductivity of the third mud layer near the center
of the ground settlement has been reduced by over 30% in the last 14 years. The gradual deterioration in the hydraulic conductivity
of the mud may have significant adverse effect on the sustainable groundwater resource of the deep confined aquifer, since
the recharge from the shallow aquifers through the mud layer is the only source of water to the deep aquifer. An analysis
of the spatial distributions of groundwater drawdown and ground settlement shows that the area with maximum drawdown is not
necessarily the area with maximum ground settlement due to the occurrence of the soft mud layer. A simple reallocation in
pumping rates on the basis of the spatial distribution of the thick mud layer could significantly reduce the ground settlement.
Electronic Publication 相似文献
988.
Due to the existence of fragile karst geo-ecological environments, such as environments with extremely poor soil cover, low
soil-forming velocity, and fragmentized terrain and physiognomy, as well as inappropriate and intensive land use, soil erosion
is a serious problem in Guizhou Province, which is located in the centre of the karst areas of southwestern China; evaluation
of soil loss and spatial distribution for conservation planning is urgently needed. This study integrated the revised universal
soil loss equation (RUSLE) with a GIS to assess soil loss and identify risk erosion areas in the Maotiao River watershed of
Guizhou. Current land use/cover and management practices were evaluated to determine their effects on average annual soil
loss and future soil conservation practices were discussed. Data used to generate the RUSLE factors included a Landsat Thematic
Mapper image (land cover), digitized topographic and soil maps, and precipitation data. The results of the study compare well
with the other studies and local data, and provide useful information for decision makers and planners to take appropriate
land management measures in the area. It thus indicates the RUSLE–GIS model is a useful tool for evaluating and mapping soil
erosion quantitatively and spatially at a larger watershed scale in Guizhou. 相似文献
989.
The most fundamental character of lunar soil is its high concentrations of solar-wind-implanted elements, and the concentrations and behavior of the noble gases He, Ne, Ar, and Xe, which provide unique and extensive information about a broad range of fundamental problems. In this paper, the authors studied the forming mechanism of lunar regolith, and proposed that most of the noble gases in lunar regolith come from the solar wind. Meteoroid bombardment controls the maturity of lunar soil, with the degree of maturation decreasing with grain size; the concentrations of the noble gases would be of slight variation with the depth of lunar soil but tend to decrease with grain size. In addition, the concentrations of noble gases in lunar soil also show a close relationship with its mineral and chemical compositions. The utilization prospects of the noble gas ^3He in lunar regolith will be further discussed. 相似文献
990.
Estimating the settling velocity of bioclastic sediment using common grain‐size analysis techniques 下载免费PDF全文
Michael V. W. Cuttler Ryan J. Lowe James L. Falter Daniel Buscombe 《Sedimentology》2017,64(4):987-1004
Most techniques for estimating settling velocities of natural particles have been developed for siliciclastic sediments. Therefore, to understand how these techniques apply to bioclastic environments, measured settling velocities of bioclastic sedimentary deposits sampled from a nearshore fringing reef in Western Australia were compared with settling velocities calculated using results from several common grain‐size analysis techniques (sieve, laser diffraction and image analysis) and established models. The effects of sediment density and shape were also examined using a range of density values and three different models of settling velocity. Sediment density was found to have a significant effect on calculated settling velocity, causing a range in normalized root‐mean‐square error of up to 28%, depending upon settling velocity model and grain‐size method. Accounting for particle shape reduced errors in predicted settling velocity by 3% to 6% and removed any velocity‐dependent bias, which is particularly important for the fastest settling fractions. When shape was accounted for and measured density was used, normalized root‐mean‐square errors were 4%, 10% and 18% for laser diffraction, sieve and image analysis, respectively. The results of this study show that established models of settling velocity that account for particle shape can be used to estimate settling velocity of irregularly shaped, sand‐sized bioclastic sediments from sieve, laser diffraction, or image analysis‐derived measures of grain size with a limited amount of error. Collectively, these findings will allow for grain‐size data measured with different methods to be accurately converted to settling velocity for comparison. This will facilitate greater understanding of the hydraulic properties of bioclastic sediment which can help to increase our general knowledge of sediment dynamics in these environments. 相似文献