全文获取类型
收费全文 | 18338篇 |
免费 | 3849篇 |
国内免费 | 4513篇 |
专业分类
测绘学 | 1139篇 |
大气科学 | 2935篇 |
地球物理 | 4684篇 |
地质学 | 10729篇 |
海洋学 | 2961篇 |
天文学 | 191篇 |
综合类 | 1294篇 |
自然地理 | 2767篇 |
出版年
2024年 | 117篇 |
2023年 | 255篇 |
2022年 | 552篇 |
2021年 | 852篇 |
2020年 | 782篇 |
2019年 | 944篇 |
2018年 | 785篇 |
2017年 | 828篇 |
2016年 | 815篇 |
2015年 | 939篇 |
2014年 | 1181篇 |
2013年 | 1430篇 |
2012年 | 1185篇 |
2011年 | 1252篇 |
2010年 | 1151篇 |
2009年 | 1168篇 |
2008年 | 1204篇 |
2007年 | 1275篇 |
2006年 | 1308篇 |
2005年 | 1122篇 |
2004年 | 1041篇 |
2003年 | 916篇 |
2002年 | 818篇 |
2001年 | 730篇 |
2000年 | 644篇 |
1999年 | 556篇 |
1998年 | 521篇 |
1997年 | 451篇 |
1996年 | 358篇 |
1995年 | 313篇 |
1994年 | 293篇 |
1993年 | 228篇 |
1992年 | 159篇 |
1991年 | 123篇 |
1990年 | 85篇 |
1989年 | 96篇 |
1988年 | 58篇 |
1987年 | 43篇 |
1986年 | 27篇 |
1985年 | 30篇 |
1984年 | 15篇 |
1983年 | 12篇 |
1982年 | 3篇 |
1981年 | 9篇 |
1980年 | 8篇 |
1979年 | 2篇 |
1978年 | 9篇 |
1973年 | 1篇 |
1971年 | 4篇 |
1954年 | 2篇 |
排序方式: 共有10000条查询结果,搜索用时 312 毫秒
41.
鱼卡煤田大煤沟组高分辨率层序地层特征及聚煤规律 总被引:1,自引:0,他引:1
利用高分辨率层序地层学理论和技术方法,综合地震、测井、岩芯和煤质资料对柴达木盆地北缘鱼卡煤田中下侏罗统大煤沟组地层河湖三角洲沉积体系进行高分辨率层序地层学分析,从中识别出短期、中期、长期3个级别的基准面旋回层序,划分出向上"变深"非对称型、向上"变浅"非对称型及对称型3种短期基准面旋回基本结构类型。依据等时地层对比法则进行不同级次基准面旋回层序的联井对比,建立了大煤沟组的等时地层格架,在等时地层格架内进行了煤层对比,并在等时地层格架内分析了聚煤规律。 相似文献
42.
碱消解-高效液相色谱-电感耦合等离子体质谱法测定生物样品中的甲基汞和乙基汞 总被引:7,自引:2,他引:5
建立了碱消解-高效液相色谱-电感耦合等离子体质谱联用系统测定生物样品中甲基汞(MeHg)与乙基汞(EtHg)的分析方法。为提高灵敏度,选用微流量的PFA雾化器,在优化的检测条件下,MeHg及EtHg检出限可达到0.036μg/L和0.03μg/L;线性范围达到4个数量级,两条工作曲线线性相关系数为1。对1.78μg/L MeHg、1.65μg/L EtHg的混合标准溶液重复测定7次,色谱峰面积的相对标准偏差(RSD)分别为1.79%和1.44%。对标准物质BCR 464(金枪鱼)的分析结果表明,测定值与标准值基本吻合,但略低于标准值;甲基汞和乙基汞的加标回收率分别为85.9%和84.5%。高效液相色谱与质谱联用技术的高灵敏度和低检出限能够满足生物样品中汞形态定量分析的要求。 相似文献
43.
A modified DRASTIC model for Siting Confined Animal Feeding Operations in Williams County,Ohio, USA 总被引:1,自引:0,他引:1
Three of DRASTIC’s parameters (Depth to Water, Soil Media, and Topography) were modified and another parameter was added (land
use/land cover) to the model to determine the potential impact on groundwater from Confined Animal Feeding Operations (CAFO)
manure lagoon settings and manure application as fertilizer. Williams County is a mostly agricultural county located in northwest
Ohio, USA. It currently has three CAFOs, all dairy, with the possibility of the construction of a multi-million chicken egg
CAFO in the near future. A Geographic Information System (GIS) was utilized to modify the Ohio Department of Natural Resources
(ODNR) DRASTIC map for the county to fully assess the county-wide pollution potential of CAFOs. The CAFO DRASTIC map indicates
that almost half of Williams County has elevated groundwater pollution potential. The rest of the county, primarily the southeast
corner, has lower CAFO groundwater pollution potential. Future CAFO development within the county should focus on the southeastern
portion of the county where the groundwater table is deeper, and the aquifer is composed of shale substrate with low hydraulic
conductivity. The CAFO DRASTIC results are intended to be used as a screening tool and are not to replace site-specific hydrogeologic
investigations. 相似文献
44.
Thomas B. Boving Mark H. Stolt Janelle Augenstern Brian Brosnan 《Environmental Geology》2008,55(3):571-582
The control of polluted surface runoff and the assessment of possible impacts on groundwater is a concern at the local and
regional scale. On this background, a study investigates possible impacts of organic and inorganic pollutants (including bacteria)
originating from a permeable asphalt parking lot on the water quality immediately beneath it. The functioning of the permeable
pavement, including clogging and restricted vertical percolation, was also evaluated. Four nested sample ports (shallow and
deep) were installed below low- and high-traffic areas, including one port outside the parking lot. At least initially there
was a good hydraulic connection between the parking surface and the shallow sample ports. The presence of a geotextile layer
at the base of the parking lot structure, however, was identified in lab tests as one factor restricting vertical percolation
to the deeper ports. Clogging of the permeable surface was most pronounced in heavy traffic areas and below snow pile storage
areas. Corroborated by high electric conductivity and chloride measurements, sand brought in by cars during winter was the
principal cause for clogging. No bacteria or BOD were found in percolating water. Polycyclic aromatic hydrocarbons (PAH) were
present at concentrations near minimum detection limit. Nutrients (nitrate and phosphate) were being leached into the ground
via the permeable parking lot surface at annual flux rates of 0.45–0.84 g/m2/year. A multi-species tracer test demonstrated a retention capacity of the permeable parking lot structure of >90% for metals
and 27% for nutrients, respectively. 相似文献
45.
M. I. Holloway F. Bussy T. W. Vennemann 《Contributions to Mineralogy and Petrology》2008,155(1):111-121
Migmatites produced by low-pressure anatexis of basic dykes are found in a contact metamorphic aureole around a pyroxenite–gabbro
intrusion (PX2), on Fuerteventura. Dykes outside and inside the aureole record interaction with meteoric water, with low or
negative δ18O whole-rock values (+0.2 to −3.4‰), decreasing towards the contact. Recrystallised plagioclase, diopside, biotite and oxides,
from within the aureole, show a similar evolution with lowest δ18O values (−2.8, −4.2, −4.4 and −7.6‰, respectively) in the migmatite zone, close to the intrusion. Relict clinopyroxene phenocrysts
preserved in all dykes, retain typically magmatic δ18O values up to the anatectic zone, where the values are lower and more heterogeneous. Low δ18O values, decreasing towards the intrusion, can be ascribed to the advection of meteoric water during magma emplacement, with
increasing fluid/rock ratios (higher dyke intensities towards the intrusion acting as fluid-pathways) and higher temperatures
promoting increasing exchange during recrystallisation.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献
46.
Almeev Renat; Holtz Francois; Koepke Jurgen; Haase Karsten; Devey Colin 《Journal of Petrology》2008,49(1):25-45
Phase equilibria simulations were performed on naturally quenchedbasaltic glasses to determine crystallization conditions priorto eruption of magmas at the Mid-Atlantic Ridge (MAR) east ofAscension Island (7–11°S). The results indicate thatmid-ocean ridge basalt (MORB) magmas beneath different segmentsof the MAR have crystallized over a wide range of pressures(100–900 MPa). However, each segment seems to have a specificcrystallization history. Nearly isobaric crystallization conditions(100–300 MPa) were obtained for the geochemically enrichedMORB magmas of the central segments, whereas normal (N)-MORBmagmas of the bounding segments are characterized by polybariccrystallization conditions (200–900 MPa). In addition,our results demonstrate close to anhydrous crystallization conditionsof N-MORBs, whereas geochemically enriched MORBs were successfullymodeled in the presence of 0·4–1 wt% H2O in theparental melts. These estimates are in agreement with direct(Fourier transform IR) measurements of H2O abundances in basalticglasses and melt inclusions for selected samples. Water contentsdetermined in the parental melts are in the range 0·04–0·09and 0·30–0·55 wt% H2O for depleted and enrichedMORBs, respectively. Our results are in general agreement (within±200 MPa) with previous approaches used to evaluate pressureestimates in MORB. However, the determination of pre-eruptiveconditions of MORBs, including temperature and water contentin addition to pressure, requires the improvement of magma crystallizationmodels to simulate liquid lines of descent in the presence ofsmall amounts of water. KEY WORDS: MORB; Mid-Atlantic Ridge; depth of crystallization; water abundances; phase equilibria calculations; cotectic crystallization; pressure estimates; polybaric fractionation 相似文献
47.
48.
The Samborombon Bay wetland is located on the west margin of the Rio de la Plata estuary, in the Province of Buenos Aires,
Argentina. This paper analyses the geological, geomorphologic, soil and vegetation characteristics of the southernmost sector
of this wetland and their influence on surface water and groundwater. The study area presents three hydrologic units: coastal
dunes, sand sheets and coastal plain. Coastal dunes and sand sheets are recharge zones of high permeability with well-drained,
non-saline soils, and a few surface water flows. Changes in the water table are related to rainfall. Groundwater in coastal
dunes is Ca–Mg–HCO3 to Na–HCO3, and of low salinity (590 mg/l). Groundwater in sand sheets is mainly Na–HCO3 with a salinity of about 1,020 mg/l. The coastal plain exhibits medium to low permeability sediments, with submerged saline
soils poorly drained. Groundwater is Na–Cl with a mean salinity of 16,502 mg/l. A surface hydrological network develops in
the coastal plain. Surface water levels near the shoreline are affected by tidal fluctuations; far from the shoreline water
accumulates because of poor drainage. Both sectors have Na–Cl water, but the former is more saline. Human intervention and
sea level rise may affect the wetland severely. 相似文献
49.
将稳定同位素效应引入CLM(Community Land Model),并对巴西马瑙斯站在平衡年的稳定水同位素的逐日变化进行模拟和分析.结果表明: 降水、水汽和地表径流中δ18O存在明显的季节变化,并与相应的水量存在显著的负相关关系,但凝结物中δ18O与地面凝结量存在显著的正相关关系,蒸发水汽中δ18O与蒸发量之间无显著的相关关系.受土壤贮水削峰功能的影响,表层土壤和根区水中δ18O的季节变化全无.植被层蒸发水汽中稳定同位素的丰度与大气的干湿程度存在密切联系: 当降水量少时,大气干燥,植被层的蒸发较少,植被蒸发中δ18O较高;当降水量较大时,空气湿润,植被层的蒸发量较大,蒸发中δ18O则较低.植被蒸腾中δ18O的变化与源区水体中δ18O的变化保持一致,尤其是与根区水中的δ18O.由于地下径流直接源自根区水的补充,因此,地下径流中δ18O等于根区水中的δ18O.模拟结果还显示,降水MWL (大气水线)的梯度项和常数项均比全球平均MWL略偏小.尽管主要来自降水的贡献,但地表径流和植被层水体的MWLs与降水MWL存在较大的差异,这一方面与两类水体在蒸发过程中的稳定同位素的富集作用有关,另一方面与CLM模拟的水量有关.大气水汽线与降水的MWL的梯度值相近,说明大气水汽与降水近似处于稳定同位素平衡状态.另外,模拟的地面的凝结线与植被层的凝结线均与全球大气水线相近,且具有非常高相关程度,说明CLM的模拟是合理的. 相似文献
50.
渤海新近系浅水三角洲沉积体系与大型油气田勘探 总被引:32,自引:8,他引:24
新近纪渤海具有构造稳定、沉降缓慢、湖泊水域范围大、内部无分割、地形平缓、坡度小等特点,广泛发育浅水三角洲,从而形成渤海海域新近系良好的储盖组合和优越的油气成藏条件。浅水三角洲砂岩储层由于受河流作用的控制,砂体具有明显的方向性,存在明显的分叉现象,与曲流河相比,砂体连通性较好,砂体等厚图呈典型的朵叶状。湖相厚层泥岩普遍发育,与浅水三角洲砂体组成多套垂向优质储盖组合。浅水三角洲发育于渤海新近纪至今处于生烃的峰期主力富烃凹陷的有利区带,新构造运动造成新近纪以来凹陷主力源岩生烃峰期与新构造运动的同步耦合,为渤海大型浅水三角洲成因的优质储层油气成藏创造了优越条件,可以形成大型油气田。因此,重视并充分挖掘渤海浅水三角洲储层类型油气藏的勘探潜力将是追求浅层优质油藏、进一步增储上产的方向所在。 相似文献