全文获取类型
收费全文 | 9629篇 |
免费 | 1344篇 |
国内免费 | 2493篇 |
专业分类
测绘学 | 385篇 |
大气科学 | 847篇 |
地球物理 | 3287篇 |
地质学 | 6284篇 |
海洋学 | 795篇 |
天文学 | 760篇 |
综合类 | 688篇 |
自然地理 | 420篇 |
出版年
2022年 | 48篇 |
2021年 | 110篇 |
2020年 | 115篇 |
2019年 | 76篇 |
2018年 | 227篇 |
2017年 | 217篇 |
2016年 | 327篇 |
2015年 | 264篇 |
2014年 | 281篇 |
2013年 | 383篇 |
2012年 | 300篇 |
2011年 | 755篇 |
2010年 | 522篇 |
2009年 | 659篇 |
2008年 | 323篇 |
2007年 | 455篇 |
2006年 | 663篇 |
2005年 | 673篇 |
2004年 | 1264篇 |
2003年 | 829篇 |
2002年 | 589篇 |
2001年 | 484篇 |
2000年 | 310篇 |
1999年 | 297篇 |
1998年 | 368篇 |
1997年 | 321篇 |
1996年 | 208篇 |
1995年 | 169篇 |
1994年 | 161篇 |
1993年 | 350篇 |
1992年 | 323篇 |
1991年 | 171篇 |
1990年 | 93篇 |
1989年 | 114篇 |
1988年 | 51篇 |
1987年 | 89篇 |
1986年 | 60篇 |
1985年 | 30篇 |
1984年 | 35篇 |
1983年 | 28篇 |
1982年 | 37篇 |
1981年 | 29篇 |
1980年 | 27篇 |
1979年 | 36篇 |
1978年 | 34篇 |
1977年 | 42篇 |
1976年 | 36篇 |
1975年 | 27篇 |
1973年 | 30篇 |
1971年 | 38篇 |
排序方式: 共有10000条查询结果,搜索用时 46 毫秒
831.
1989年,张俊峰发表了山东山旺盆地中新世(N2)2个蚂蚁新属新种:Heteromyrmex atopogaster Zhang,1989和Tolylasius inflatus Zhang,1989。经检验,前一个属是基于不实的观察结果,新属、种应予废除,作为未定属种归人蚁亚科(Formicinae);后一个属的建属根据都非建属的根据,其他的特征系毛蚁属(Lasius)的属征范围,没有建属条件,属应予废,保留其种,转移到毛蚁属。 相似文献
832.
东昆仑活动断裂托索湖——玛曲以东肯定那一带, 可据阿尼玛卿玛积峰为界再分为花石峡段和玛沁段两个在几何上不连续的段落. 两段在表征断层全新世活动特征的古地震事件方面有明显差异, 花石峡段的地震活动性明显高于玛沁段的地震活动性. 古地震研究表明, 花石峡段上3次强震活动相邻两次地震发生的时间间隔分别约为500 a和640 a, 玛沁段上最近两次古地震事件间大致有1 000 a左右的时间间隔. 根据断层平均滑动速率计算的花石峡段7.5级地震的平均复发间隔为411~608 a, 相对应的同震平均水平位错约为(5.75plusmn;0.57)m. 虽然玛沁段的地震活动性较弱, 但由于该段上最近一次地震事件离现在较为久远, 已经积累的应变能应该使我们对其未来地震危险性的分析有足够重视. 相似文献
833.
Jan Šafanda Philipp Heidinger Helmut Wilhelm Vladimír Čermák 《Hydrogeology Journal》2007,15(2):423-428
As part of the International Continental Scientific Drilling Program (ICDP), the 1.5-km-deep borehole Yaxcopoil-1, located in the Chixculub meteor impact structure in Mexico, has undergone further study after drilling operations ceased. Temperature logs were repeated ten times at intervals 0.3–0.8, 15, 24 and 34 months after borehole shut-in. The logs bear a distinct signature of transient heat transfer by groundwater flow manifested by a gradual distortion of the linear temperature profile when a cold wave of 0.8–1.6°C amplitude was detected propagating downward from 145 to 312 m at a rate of 4–6 m/month. To understand the nature of this moving anomaly, a 20-day monitoring of the cold wave was carried out at a depth of 307 m that showed further cooling of 0.6°C during the first 16 days of the passage followed by temperature stabilisation. As an explanation of this unusual phenomenon, a theory is proposed, whereby the drilling mud has accumulated within the overlying and cooler highly porous and permeable karstic rocks during the drilling and migrates downward. The observed migration rate suggests a permeability higher than 10?11 m2. This indicates a high vulnerability to contamination of the only freshwater aquifer in the Yucatan region. 相似文献
834.
Javier Palarea-Albaladejo Josep A. Martín-Fernández Juan Gómez-García 《Mathematical Geology》2007,39(7):625-645
In this work, a parametric approach for replacing data below the detection limit, also known as rounded zeros, in compositional
data sets is proposed. Compositional rounded zeros correspond to small proportions of some whole that cannot be reliably detected
by the analytical instruments under given operating conditions. This kind of zeros appear frequently in the data collection
process in geosciences. They must be treated in an adequate way before some multivariate analysis can be applied. Our procedure
results from a modification of the Expectation-Maximization (EM) algorithm and is based on the additive log-ratio transformation.
Its coherence with the nature of compositional data and with basic operations in the simplex sample space is checked. Using
real data sets, we find that this approach improves other parametric and non-parametric techniques for compositional rounded
zeros. 相似文献
835.
Installation of a vertical slurry wall around an Italian quarry lake: complications arising and simulation of the effects on groundwater flow 总被引:1,自引:0,他引:1
Slurry walls are non-structural barriers that are constructed underground to impede groundwater flow or manage groundwater
control problems. The study area is in the Piemonte plain (Italy), close to the River Po. Quarrying works carried out below
the piezometric surface created two big quarry lakes. The local groundwater system is characterized by a lower semi-confined
aquifer, which is overlain by a semi-permeable bed of clayey peat (aquitard) and an upper unconfined aquifer. Locally, the
peat fades away and the granulometry of this horizon becomes silty sandy. A planned enlargement of the quarry will increase
the size and depth of the quarry lakes. So the aquitard bed between the two aquifers will be damaged, creating a mixing rate
of groundwater. Such a procedure would not be compatible with the presence of two municipal wells upstream from the quarries.
Consequently, the installation of a vertical diaphragm (slurry wall) is recommended to separate the aquifers and to act as
a filter for the groundwater flowing from the unconfined to the semi-confined aquifer. To predict the consequences caused
by the installation of the vertical diaphragm separating the unconfined aquifer and the semi-confined one, a specifically
adjusted finite-difference model was used. The model showed a maximum rising of the water table equal to 12 cm, just upstream
of the diaphragm and for a distance of about 100 m, and a maximum lowering of 2 cm just downstream of the diaphragm. However,
the slurry wall would not cause any change in the piezometric head in the area where there are municipal wells and, hence,
will not have any negative effect on the functionality of the municipal wells. Moreover, the migration of water from the unconfined
aquifer through the vertical diaphragm will stimulate a series of attenuation and auto-depuration processes of eventual contaminants.
These processes are due to the higher crossing time that the groundwater flow takes to go through the vertical barrier (t
a = 96.5 days, whereas for the horizontal semi-permeable layer t
a = 9.6 days). So, the vertical diaphragm can be a resolutive element, representing a mediation and separation factor between
the unconfined and the semi-confined aquifers along the border of the quarrying areas, and a protective barrier for the water
quality of the quarry lake and the semi-confined aquifer. 相似文献
836.
María Gabriela García Ondra Sracek Diego Sebastián Fernández Margarita del Valle Hidalgo 《Environmental Geology》2007,52(7):1261-1275
The concentration of arsenic measured in groundwater from three aquifers in the study area located in the Eastern Tucuman
province, Argentina, mostly depends on the lithology, but the spatial and temporal variations of concentrations seem to be
also controlled by pH changes, climatic factors, and human perturbations. The highest concentrations of As (more than 1,000 μg
L−1) were found in the shallow aquifer, made of As-rich loess, while the lowest concentrations were measured in the deep confined
aquifer, consisting of alternating layers of alluvial sands/gravels and clays. Intermediate values were measured in the semiconfined
aquifer made of the fluvial sediments deposited in the Salí River valley, that alternate in the upper part of the sedimentary
sequence with layers of loess. Because most of As in the loess is considered to be adsorbed onto Fe-oxyhydroxide coatings,
the increase of pH in the flow direction (west-east) leads to increasing arsenic concentrations towards the eastern border
of the study area. The decomposition of organic wastes poured into the Salí River or associated with local and diffuse sources
of contamination in the eastern part of the study area depletes dissolved oxygen, which leads to the reductive dissolution
of Fe and Mn oxyhydroxides, and to the subsequent release of the adsorbed and co-precipitated As. This process mainly affects
shallow groundwater and the upper part of the semiconfined aquifer. Geochemical and hydrological data also suggest that rising
water table levels at the end of the wet season may also lead to reductive dissolution of As-rich Fe oxyhydroxides in the
shallow aquifer. 相似文献
837.
Oscar Escolero Luis E. Marin Eloisa Domínguez-Mariani Sandra Torres-Onofre 《Environmental Geology》2007,51(5):719-723
A hydraulic analysis of the interface between freshwater–saltwater behavior was done in the Merida Yucatan zone, two machines
that constantly register the groundwater levels were installed, and three electric conductivity logs were taken from wells.
When comparing the measured results with the ones obtained using theoretical equations developed to calculate the freshwater–saltwater
interface position, it was proved that in some cases these equations can be applied, and in others not. Two effects that rule
the behavior of karst aquifers in extraordinary conditions were found. 相似文献
838.
I. Camilo Higuera-Díaz Philip J. Carpenter Michael D. Thompson 《Environmental Geology》2007,53(4):805-812
Karst aquifers are highly susceptible to contamination, with numerous points of entry for contaminants through recharge features
such as sinkholes, swallow holes and solutionally enlarged fractures. These recharge features may be filled or obscured at
the surface, requiring the use of geophysical or remote sensing techniques for their identification. This study uses seismic
refraction data collected at the Ft. Campbell Army Airfield (CAAF), Kentucky, USA, to test the hypothesis that refraction
tomography is a useful tool for imaging bedrock depressions beneath thick overburden (greater than 20 m of unconsolidated
sediment). Southeast of the main taxiway of CAAF seismic velocity tomograms imaged a bedrock low, possibly a closed depression,
at a depth of 25 m that had been earlier identified through delay-time analysis of the same refraction data. Tomography suggests
the bedrock low is about 250-m wide by 10-m deep at its widest point. High rates of contaminant vapor extraction over the
western extension of this feature suggest a high concentration of contaminants above, and within, this filled bedrock low,
the base of which may contain solutionally enlarged fractures (i.e. karst conduits) that could funnel these contaminants to
the upper or lower bedrock aquifers. This study thus demonstrates the viability of seismic refraction tomography as a tool
for identification of filled sinkholes and bedrock depressions in karst areas. 相似文献
839.
Ecological shelter zones reconstruction is an ecosystem restoration and conserva-tion project aimed to the ecological safety of nations,regions and basins. Reconstruction of ecological shelter zones of the upper reaches of the Yangtze River became one of the most important tasks of Western Development strategy. This article,taking Zhaotong as an exam-ple,studies the functional regionalization of ecological shelter zones. The study supplies a case for functional regionalization of small and medium regions whose main tasks are envi-ronment reservation. With the guidance of theories of functional regionalization,and based on the analysis of Zhaotong's natural,ecological and socioeconomic factors,the paper suggests five principles for factors selection. These principles include: (1) reversing order evaluation; (2) selecting main factors; (3) keeping the integrality of administrative regions of towns; and (4) making the products acceptable by local government. To analyze spatial status of selected factors,LUCC data in 2002,1:50,000 relief maps and town-unit socioeconomic statistical data in 2004 are used. RS and GIS tools are also applied to melt traditional and modern geo-graphical methods. This would be useful to functional regionalization research in mountain-ous areas. As a conclusion,the leading functional regions of ecological conservation or economic development are suggested,respectively. Zhaotong city is divided into two-level functional regions. The first-level includes three leading functional regions and they will lead developing direction of sub-regions. The second-level includes eight sub-regions,which are policy implemented regions,and will supply guidance to Zhaotong's ecological shelter zones reconstruction. 相似文献
840.
North End Lake is a polluted and eutrophic freshwater system located in Port Elizabeth, South Africa. Since the lake is expected
to be used for recreational/tourist purposes by 2010, a rehabilitation program will have to be designed. For this reason,
we retrieved a sediment core from the central region of the lake to decipher the effect of historical human impacts on the
water body. Pre-disturbance paleolimnological inferences indicate that the lake was likely mesotrophic. After ∼1831, when
sheep farming activities were undertaken in the catchment, increases in trophic state and changes in sediment composition
were observed. After ∼1937, increases in trace metal levels, organic matter, spheroidal carbonaceous particles (SCP) and changes
in sediment composition were recorded. The system became eutrophic as indicated by the dominance of the diatom Actinocyclus normanii, a cosmopolitan species often observed in systems where water quality has been dramatically degraded. The conditions worsened
after 1986 because of the construction of a storm-water retention system, which intentionally channeled storm-water runoff
into the lake. Because of this, extremely high values of fecal coliforms (i.e. 2 × 106 every 100 ml) have been measured in the water column. The paleolimnological information identified the sharp increase in
organic content in the uppermost section of the core, and this could be correlated to the operation of the storm-water retention
system. Therefore, as an immediate management measure, we suggest that the storm-water retention system should either no longer
be utilized, or the storm-water runoff should be treated before disposal into the lake. In addition, an effective sewage system
has to be constructed. 相似文献