全文获取类型
收费全文 | 3441篇 |
免费 | 427篇 |
国内免费 | 477篇 |
专业分类
测绘学 | 286篇 |
大气科学 | 485篇 |
地球物理 | 858篇 |
地质学 | 1439篇 |
海洋学 | 495篇 |
天文学 | 311篇 |
综合类 | 134篇 |
自然地理 | 337篇 |
出版年
2024年 | 20篇 |
2023年 | 42篇 |
2022年 | 102篇 |
2021年 | 158篇 |
2020年 | 134篇 |
2019年 | 151篇 |
2018年 | 187篇 |
2017年 | 194篇 |
2016年 | 205篇 |
2015年 | 190篇 |
2014年 | 207篇 |
2013年 | 244篇 |
2012年 | 268篇 |
2011年 | 316篇 |
2010年 | 241篇 |
2009年 | 262篇 |
2008年 | 247篇 |
2007年 | 172篇 |
2006年 | 178篇 |
2005年 | 157篇 |
2004年 | 113篇 |
2003年 | 101篇 |
2002年 | 79篇 |
2001年 | 59篇 |
2000年 | 43篇 |
1999年 | 41篇 |
1998年 | 26篇 |
1997年 | 18篇 |
1996年 | 18篇 |
1995年 | 22篇 |
1994年 | 18篇 |
1993年 | 7篇 |
1992年 | 9篇 |
1991年 | 20篇 |
1990年 | 11篇 |
1989年 | 6篇 |
1988年 | 4篇 |
1987年 | 6篇 |
1986年 | 11篇 |
1985年 | 4篇 |
1984年 | 7篇 |
1983年 | 4篇 |
1981年 | 5篇 |
1980年 | 4篇 |
1979年 | 6篇 |
1977年 | 3篇 |
1975年 | 3篇 |
1974年 | 4篇 |
1971年 | 5篇 |
1970年 | 4篇 |
排序方式: 共有4345条查询结果,搜索用时 31 毫秒
991.
Intrinsic and specific vulnerability of groundwater in central Spain: the risk of nitrate pollution 总被引:5,自引:1,他引:4
The intrinsic vulnerability of groundwater in the Comunidad de Madrid (central Spain) was evaluated using the DRASTIC and GOD indexes. Groundwater vulnerability to nitrate pollution was also assessed using the composite DRASTIC (CD) and nitrate vulnerability (NV) indexes. The utility of these methods was tested by analyzing the spatial distribution of nitrate concentrations in the different aquifers located in the study area: the Tertiary Detrital Aquifer, the Moor Limestone Aquifer, the Cretaceous Limestone Aquifer and the Quaternary Aquifer. Vulnerability maps based on these four indexes showed very similar results, identifying the Quaternary Aquifer and the lower sub-unit of the Moor Limestone Aquifer as deposits subjected to a high risk of nitrate pollution due to intensive agriculture. As far as the spatial distribution of groundwater nitrate concentrations is concerned, the NV index showed the greatest statistical significance (p?<?0.01). This new type of multiplicative model offers greater accuracy in estimations of specific vulnerability with respect to the real impact of each type of land use. The results of this study provide a basis on which to guide the designation of nitrate vulnerable zones in the Comunidad de Madrid, in line with European Union Directive 91/676/EEC. 相似文献
992.
Pablo H. Alasino Juan A. Dahlquist Carmen Galindo Cesar Casquet Julio Saavedra 《International Journal of Earth Sciences》2010,99(5):1051-1065
The La Costa pluton in the Sierra de Velasco (NW Argentina) consists of S-type granitoids that can be grouped into three igneous
facies: the alkali-rich Santa Cruz facies (SCF, SiO2 ~67 wt%) distinguished by the presence of andalusite and Na- and Li-rich cordierite (Na2O = 1.55–1.77 wt% and Li2O = 0.14–0.66 wt%), the Anillaco facies (SiO2 ~74 wt%) with a significant proportion of Mn-rich garnet, and the Anjullón facies (SiO2 ~75 wt%) with abundant albitic plagioclase. The petrography, mineral chemistry and whole-rock geochemistry of the SCF are
compatible with magmatic crystallization of Na- and Li-rich cordierite, andalusite and muscovite from the peraluminous magma
under moderate P–T conditions (~1.9 kbar and ca. 735°C). The high Li content of cordierite in the SCF is unusual for granitic rocks of intermediate
composition. 相似文献
993.
U-Pb Zircon Geochronology with an Integrated LA-ICP-MS Microanalytical Workstation: Achievements in Precision and Accuracy 总被引:1,自引:0,他引:1
Luigi A. Solari Arturo Gómez-Tuena Juan Pablo Bernal Ofelia Pérez-Arvizu Martin Tanner 《Geostandards and Geoanalytical Research》2010,34(1):5-18
Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) is a microanalytical tool especially suitable for providing fast and precise U-Pb geochronological results on zircon grains. A new 193 nm excimer laser adapted to a micromachining workstation, equipped with a newly designed two-volume ablation cell and coupled with a quadrupole ICP-MS, is presented here. The system was tuned routinely to achieve sensitivities in the range of 3000 cps/μg g−1 for 238 U (< 2% RSD), with a 34 μm spot size, at 5 Hz and ∼ 8 J cm−2 , while ablating the NIST SRM 612 glass reference material. The system was capable of providing fast (< 1.5 minutes each analysis) and precise (generally < 1.5% 1s errors) 206 Pb/238 U zircon ages. The ages of widely used reference material zircons (Plesovice, 337 Ma; Temora, 416 Ma; R33, 418 Ma; Sri Lanka, 564 Ma; 91500, 1065 Ma) could be precisely matched, with an accuracy on isotopic ratios that ranged from ∼ 2 to ∼ 6%, depending on the homogeneity of the natural reference materials. 相似文献
994.
Simple monitoring method for precaution of landslides watching tilting and water contents on slopes surface 总被引:2,自引:2,他引:0
Taro Uchimura Ikuo Towhata Trinh Thi Lan Anh Jou Fukuda Carlos J. B. Bautista Lin Wang Ichiro Seko Taro Uchida Akira Matsuoka Yosuke Ito Yuichi Onda Sho Iwagami Min-Seok Kim Naoki Sakai 《Landslides》2010,7(3):351-357
A low-cost and simple monitoring method for early warning of landslides is proposed. To detect abnormal deformation of a slope, this method employs a tilt sensor in place of an extensometer on the slope surface. In order to examine the relevance of measuring rotation angle on a slope surface by tilt sensor, model tests were conducted, and rotation on the slope surface was observed together with slide displacement along the surface. The rotation data responded 30 min before failure in a model test, which could be useful as a signal for early warning. However, the behavior of rotation before failure varies from case to case, and thus, criteria to issue warning should be defined more carefully. For a model slope made of uniform loose sand, measurement of slide displacement along the slope surface is sensitive to failure at the toe, while the measurement of rotation on the slope surface is useful to detect the development of progressive failure upward along the slope. Wireless sensor units with microelectromechanical systems (MEMS) tilt sensor and volumetric water content sensor were also examined on a real slope in Kobe City, and a long-term monitoring was attempted. A simple but possible way to define the criteria of judgment to issue warning can be proposed based on combination of data obtained by the tilt sensors and volumetric water content sensors. 相似文献
995.
德州市浅层地下水水质演化 总被引:1,自引:0,他引:1
德州市属于典型的黄河下游冲积平原孔隙水水文地质区.该地区具有咸、淡两种水体共存的独特水环境特征,淡水天然的水环境条件非常敏感和脆弱.由于持续开采地下水,使该区域地下水的水动力场和水化学场发生了较大变化.根据德州市1996-2007年的地下水水质监测数据,论述了浅层地下水的水质特征,分析研究了浅层地下水的水质演化.结果表明:在开采条件和自然条件的共同作用下,浅层地下水水质不断演化,其中变化比较明显的水质指标是总硬度、矿化度和Cl-等.在区内齐河县、夏津县的苏留庄等一带,水动力场的变化导致地下水的矿化作用减弱,浅层地下水的矿化度、总硬度有所降低,水质趋向淡化.在宁津县、禹城市的张庄乡等一带,水动力场的变化导致地下水的矿化作用增强,浅层地下水的矿化度、总硬度明显升高,水质持续咸化.针对研究区地下水水质的演化趋势,提出了使微咸水逐渐向淡水方向演化的水质调控措施. 相似文献
996.
共存镉/铜对沉积物(生物膜)中主要组分吸附阿特拉津的影响 总被引:1,自引:0,他引:1
采用选择性萃取分离技术对沉积物(生物膜)中非残渣态组分(铁、锰氧化物和有机质)进行分离,研究阿特拉津(atrazine,AT)-Cd-Cu系中共存重金属对选择性萃取前后沉积物(生物膜)吸附AT的影响.研究结果表明:共存体系中,沉积物(生物膜)中铁氧化物对AT的吸附贡献最大,其次是有机质,而锰氧化物则抑制AT的吸附;共存镉促进了AT在沉积物(生物膜)原样及各非残渣态组分上的吸附,镉质量浓度为0.2 mg/L时,促进作用最显著,各组分对阿特拉津的吸附贡献增加了3.20%~32.31%(沉积物)和1.61%~17.10%(生物膜),但随镉质量浓度的增加,其促进作用减弱;共存铜抑制沉积物(生物膜)原样及铁氧化物、有机质对AT的吸附,铜质量浓度为75.0 mg/L时,其抑制作用最强,铁氧化物和有机质对阿特拉津的吸附贡献分别降低了29.21%、22.36%(沉积物)和9.46%、77.27%(生物膜),但同时减弱了锰氧化物对AT吸附的抑制作用,其吸附贡献分别提高了28.98%(沉积物)和17.46%(生物膜). 相似文献
997.
998.
Raimundo Jiménez Ballesta Paz Conde Bueno Juan A. Martin Rubi Rosario Garcia Giménez 《Central European Journal of Geosciences》2010,2(4):441-454
To evaluate trace element soil contamination, geochemical baseline contents and reference values need to be established. Pedo-geochemical baseline levels of trace elements in 72 soil samples of 24 soil profiles from the Mediterranean, Castilla La Mancha, are assessed and soil quality reference values are calculated. Reference value contents (in mg kg?1) were: Sc 50.8; V 123.2; Cr 113.4; Co 20.8; Ni 42.6; Cu 27.0; Zn 86.5; Ga 26.7; Ge 1.3; As 16.7; Se 1.4; Br 20.1; Rb 234.7; Sr 1868.4; Y 38.3; Zr 413.1; Nb 18.7; Mo 2.0; Ag 7.8; Cd 4.4; Sn 8.7; Sb 5.7; I 25.4; Cs 14.2; Ba 1049.3; La 348.4; Ce 97.9; Nd 40.1; Sm 10.7; Yb 4.2; Hf 10.0; Ta 4.0; W 5.5; Tl 2.3; Pb 44.2; Bi 2.2; Th 21.6; U 10.3. The contents obtained for some elements are below or close to the detection limit: Co, Ge, Se, Mo, Ag, Cd, Sb, Yb, Hf, Ta, W, Tl and Bi. The element content ranges (the maximum value minus the minimum value) are: Sc 55.0, V 196.0, Cr 346.0, Co 64.4, Ni 188.7, Cu 49.5, Zn 102.3, Ga 28.7, Ge 1.5, As 26.4, Se 0.9, Br 33.0 Rb 432.7, Sr 3372.6, Y 39.8, Zr 523.2, Nb 59.7, Mo 3.9, Ag 10.1, Cd 1.8, Sn 75.2, Sb 9.9, I 68.0, Cs 17.6, Ba 1394.9, La 51.3, Ce 93.5, Nd 52.5, Sm 11.2, Yb 4.2, Hf 11.3, Ta 6.3, W 5.2, Tl 2.1, Pb 96.4, Bi 3.0, Th 24.4, U 16.4 (in mg kg?1). The spatial distribution of the elements was affected mainly by the nature of the bedrock and by pedological processes. The upper limit of expected background variation for each trace element in the soil is documented, as is its range as a criterion for evaluating which sites may require decontamination. 相似文献
999.
Carlos Frajuca Fabio da Silva Bortoli Natan Vanelli Garcia Nadja Simao Magalhaes 《Astronomische Nachrichten》2024,345(2-3):e230177
An experimental set up was proposed to determine the speed of gravitational signals traveling in air or in some other medium. It involves two vibrating masses—the emitters, which will be the sources of periodic tidal gravitational signals—and one sapphire-made mass that will act as a detector, positioned between the two emitters. The detector is planned to be suspended in vacuum and cooled down to 4.2 K, and its vibrational amplitude should be measured by a microwave signal (with ultra-low phase-noise) that is expected to resonate with the whispering gallery modes inside the detector. The mechanical and electrical quality factors of sapphire are quite high, yielding a very narrow detection band that reduces the detector sensitivity while amplifying the phase difference of the emitters' signals. The frequencies of the normal modes of the detector were previously determined using a finite element program. In this work, these frequencies are applied to the calculation of a first estimate of the sensitivity of the experiment. 相似文献
1000.
Anabela Carvalho Mike D. Flannigan Kim A. Logan Lynn M. Gowman Ana Isabel Miranda Carlos Borrego 《Climatic change》2010,98(1-2):177-197
In this study, we investigated the impact of future climate change on fire activity in 12 districts across Portugal. Using historical relationships and the HIRHAM (High Resolution Hamburg Model) 12 and 25 km climate simulations, we assessed the fire weather and subsequent fire activity under a 2 × CO2 scenario. We found that the fire activity prediction was not affected by the spatial resolution of the climate model used (12 vs. 25 km). Future area burned is predicted to increase 478% for Portugal as a whole, which equates to an increase from 1.4% to 7.8% of the available burnable area burning annually. Fire occurrence will also see a dramatic increase (279%) for all of Portugal. There is significant spatial variation within these results; the north and central districts of the country generally will see larger increases in fire activity. 相似文献