全文获取类型
收费全文 | 3263篇 |
免费 | 93篇 |
国内免费 | 92篇 |
专业分类
测绘学 | 70篇 |
大气科学 | 402篇 |
地球物理 | 812篇 |
地质学 | 1006篇 |
海洋学 | 648篇 |
天文学 | 302篇 |
综合类 | 46篇 |
自然地理 | 162篇 |
出版年
2023年 | 12篇 |
2022年 | 22篇 |
2021年 | 48篇 |
2020年 | 47篇 |
2019年 | 63篇 |
2018年 | 134篇 |
2017年 | 124篇 |
2016年 | 139篇 |
2015年 | 87篇 |
2014年 | 171篇 |
2013年 | 220篇 |
2012年 | 140篇 |
2011年 | 203篇 |
2010年 | 190篇 |
2009年 | 190篇 |
2008年 | 164篇 |
2007年 | 176篇 |
2006年 | 146篇 |
2005年 | 122篇 |
2004年 | 105篇 |
2003年 | 96篇 |
2002年 | 98篇 |
2001年 | 76篇 |
2000年 | 79篇 |
1999年 | 55篇 |
1998年 | 45篇 |
1997年 | 41篇 |
1996年 | 25篇 |
1995年 | 37篇 |
1994年 | 19篇 |
1993年 | 17篇 |
1992年 | 21篇 |
1991年 | 18篇 |
1990年 | 18篇 |
1989年 | 13篇 |
1988年 | 14篇 |
1987年 | 24篇 |
1986年 | 15篇 |
1985年 | 15篇 |
1984年 | 28篇 |
1983年 | 31篇 |
1982年 | 22篇 |
1981年 | 17篇 |
1980年 | 25篇 |
1979年 | 12篇 |
1978年 | 7篇 |
1977年 | 16篇 |
1975年 | 15篇 |
1974年 | 10篇 |
1973年 | 7篇 |
排序方式: 共有3448条查询结果,搜索用时 15 毫秒
91.
Effect of agricultural land use on the chemistry of groundwater from basaltic aquifers, Jeju Island, South Korea 总被引:2,自引:0,他引:2
Dong-Chan Koh Kyung-Seok Ko Yongje Kim Seung-Gu Lee Ho-Wan Chang 《Hydrogeology Journal》2007,15(4):727-743
Geochemical processes were identified as controlling factors of groundwater chemistry, including chemical weathering, salinization from seawater and dry sea-salt deposition, nitrate contamination, and rainfall recharge. These geochemical processes were identified using principal component analysis of major element chemistry of groundwater from basaltic aquifers in Jeju Island, South Korea, a volcanic island with intense agricultural activities. The contribution of the geochemical processes to groundwater chemistry was quantified by a simple mass-balance approach. The geochemical effects due to seawater were considered based on Cl contributions, whereas the effects due to natural chemical weathering were based on alkalinity. Nitrogenous fertilizers, and especially the associated nitrification processes, appear to significantly affect groundwater chemistry. A strong correlation was observed between Na, Mg, Ca, SO4 and Cl, and nitrate concentrations in groundwater. Correspondingly, the total major cations, Cl, and SO4 in groundwater were assessed to estimate relative effect of N-fertilizer use on groundwater chemistry. Cl originates more from nitrate sources than from seawater, whereas SO4 originates mostly from rainwater. N-fertilizer use has shown the greatest effect on groundwater chemistry, particularly when nitrate concentrations exceed 6–7 mg/L NO3–N. Nitrate contamination significantly affects groundwater quality and 18% of groundwater samples have contamination-dominated chemistry. 相似文献
92.
The purpose of this study was to compare enzymatic biomarker activities in fish caged at two sites, Masan Bay (contaminated) and Haeguemgang (reference). In the present study, ethoxyresorufin O-deethylase (EROD), brain acetyl cholinesterase (bAChE), muscle acetyl cholinesterase (mAChE) and butyryl cholinesterase (mBChE) in caged rockfish (Sebastes schlegeli) were measured 0, 1, 3, 7, 14, 21 and 30 days after caging. The level of CYP1A mRNA and Protein expression was induced higher in Masan Bay at 1, 3, 7, 14 and 30 days after caging. EROD activity in the caged fish was significantly higher in Masan Bay than in Haeguemgang 3 and 7 days after caging, but not at 14 and 30 days after caging. bAChE activity was significantly inhibited at 7 and 14 days after caging in Masan Bay. However, mBChE activity was not significantly inhibited during the experiment. Taken together, the data suggest that the caged fish were exposed, at least transiently, to CYP1A inducers and ChE inhibitors, which is consistent with our previous observations. 相似文献
93.
Spherical aggregates of orthopyroxene are reported from some parts of the Bushveld Complex in a variety of host rocks.Detailed mapping has shown that these spherical aggregates, comprising pyroxenite spheroids in a quartz-norite matrix, are contact phenomena and not stratigraphic markers. Orthopyroxene, biotite and amphibole are enriched in spheroids relative to matrix; their mineral chemistry showing a fairly constant orthopyroxene and plagioclase composition through the spheroids and into the matrix, indicating in-situ formation.Bulk chemistry shows spheroid to matrix tie-lines orthogonal to those generally accepted for silicate liquid immiscibility, but other chemical information is consistent with the occurrence of immiscibility.The formation of the aggregates may be related to the industrial process of spherical agglomeration, by which spheroids are formed by the introduction of an immiscible “bridging liquid” to the melt — probably derived from the floor rocks in this case. The mechanism accounts for the field relationships, petrography and chemistry of the aggregate-matrix system. The petrology of the process equates with a special case of silicate liquid immiscibility induced by local contamination and ageing of the original magma.A similar “bridging liquid” mechanism could also account for the formation of the so-called “boulder bed” beneath the Merensky Reef. 相似文献
94.
95.
Camille Cordes Larry Atkinson Richard Lee Jack Blanton 《Marine pollution bulletin》1980,11(11):315-317
Following the Ixtoc I oil rig blowout in Campeche Bay we hypothesized that resulting tarballs should eventually appear in the Gulf Stream off Gerogia and that because of dynamic barriers in the innershelf little would reach nearshore areas. To test these hypotheses, surface tows to collect floating tar were taken off the coasts of Georgia and Florida in October and December, 1979.No tar was found within 40 km of the shore. All samples more than 40 km offshore contained some tar. The mean concentration was 0.82 mg m?2 with a range of 0.01–5.6 mg m?2. Closely spaced sampling showed extreme variation but trends were consistent. Perylene was the most abundant compound in the tarballs. 相似文献
96.
Aqueous extracts of two petroleum oils, No. 2 fuel oil and Southern Louisiana crude, were tested on two amphipods, Gammarus muccronatus and Amphithoe valida, for survival. The oils were toxic at concentrations of 0.8 ppm (fuel oil) and 2.4 ppm (S. Louisiana crude). Mortalities increased with the concentration and length of exposure. Few or no young were produced at these and higher concentrations (breeding adults were decreasing rapidly in numbers). The amphipods are more sensitive to aqueous extracts of these oils than benthic polychaetes and shrimp, for which data are available. 相似文献
97.
The voltage induced in a horizontal loop on a layered ground has been calculated for the case where the loop is excited by a step current and measurements are made during the off-cycle. The expressions derived for a uniform ground show that for large time t the induced voltage E(t) is approximately given by E(t)?— (Ibαμ/20t) (σμ2/t)3/2 where σ is the conductivity of the ground, μ the permeability, b the loop radius, and I the amplitude of the current step. For small times the corresponding result is E(t)?—Ibμ/2t. When the ground is composed of a number of layers a numerical procedure for calculating the induced voltage is described. The calculated responses of various multilayered structures show that at short times the induced voltage is asymptotic to that produced in the case of a uniform ground of conductivity equal to the top layer. Interference effects in the top layer can lead to anomalous decay curves which may result in the underestimation of the conductivity of a buried layer. 相似文献
98.
Chang Hwan Lee Tae-Woong Kim Gunhui Chung Minha Choi Chulsang Yoo 《Stochastic Environmental Research and Risk Assessment (SERRA)》2010,24(3):389-397
Bivariate distributions have been recently employed in hydrologic frequency analysis to analyze the joint probabilistic characteristics
of multivariate storm events. This study aims to derive practical solutions of application for the bivariate distribution
to estimate design rainfalls corresponding to the desired return periods. Using the Gumbel mixed model, this study constructed
rainfall–frequency curves at sample stations in Korea which provide joint relationships between amount, duration, and frequency
of storm events. Based on comparisons and analyses of the rainfall–frequency curves derived from univariate and bivariate
storm frequency analyses, this study found that conditional frequency analysis provides more appropriate estimates of design
rainfalls as it more accurately represents the natural relationship between storm properties than the conventional univariate
storm frequency analysis. 相似文献
99.
100.
David R. Geist Mark C. Joy David R. Lee Tom Gonser 《Ground Water Monitoring & Remediation》1998,18(1):78-82
An impact drive point method is described for emplacing piezometers in a cobble river bottom where this has previously been difficult without the use of drilling rigs. To force the drive point piezometers through coble, the vibrational impact of an air-powered hammer was carried directly to the drive point by the use of an internal drive rod. After insertion to depth, the drive rod was removed from the lower portion of the piezometer and a standpipe was added to extend the piezometer above the river level. Piezometers installed in this way have permitted water quality analysis and dynamic measurement of vertical potentials in cobble sediments ranging in size from 2.5 to >30 cm and the method has been successfully used in the Columbia River, USA, and Töss River, Switzerland. This innovative method provides information on the hydrodynamics of pore water in highly permeable, cobble deposits that are common in high-energy river and lake bottoms. Piezometers installed using the internal drive rod method facilitate the assessment of the temporal and spatial dynamics of recharge and discharge at the ground water/surface water interface and analyses of the ecological connectivity between the hyporheic zone and surface water of rivers and streams. This information will lead to improved management decisions related to our nation's ground water and surface water supplies. 相似文献