全文获取类型
收费全文 | 1301篇 |
免费 | 44篇 |
国内免费 | 14篇 |
专业分类
测绘学 | 29篇 |
大气科学 | 95篇 |
地球物理 | 323篇 |
地质学 | 576篇 |
海洋学 | 106篇 |
天文学 | 133篇 |
综合类 | 5篇 |
自然地理 | 92篇 |
出版年
2021年 | 9篇 |
2020年 | 8篇 |
2019年 | 8篇 |
2018年 | 26篇 |
2017年 | 28篇 |
2016年 | 45篇 |
2015年 | 29篇 |
2014年 | 33篇 |
2013年 | 76篇 |
2012年 | 47篇 |
2011年 | 71篇 |
2010年 | 74篇 |
2009年 | 77篇 |
2008年 | 60篇 |
2007年 | 62篇 |
2006年 | 59篇 |
2005年 | 82篇 |
2004年 | 60篇 |
2003年 | 42篇 |
2002年 | 43篇 |
2001年 | 35篇 |
2000年 | 21篇 |
1999年 | 28篇 |
1998年 | 22篇 |
1997年 | 10篇 |
1996年 | 20篇 |
1995年 | 15篇 |
1994年 | 16篇 |
1993年 | 9篇 |
1992年 | 18篇 |
1991年 | 11篇 |
1990年 | 15篇 |
1989年 | 9篇 |
1988年 | 10篇 |
1987年 | 10篇 |
1986年 | 16篇 |
1985年 | 15篇 |
1984年 | 15篇 |
1983年 | 10篇 |
1982年 | 10篇 |
1981年 | 15篇 |
1980年 | 8篇 |
1979年 | 12篇 |
1978年 | 9篇 |
1977年 | 10篇 |
1976年 | 11篇 |
1975年 | 4篇 |
1974年 | 13篇 |
1973年 | 4篇 |
1953年 | 2篇 |
排序方式: 共有1359条查询结果,搜索用时 15 毫秒
991.
A simple filter is developed which transforms VLF-EM real magnetic field transfer functions into apparent resistivities. It is based on the relationship between the horizontal derivative of the surface electric field and the vertical magnetic field at the surface of a two-dimensional earth model. The performance of this simple autoregressive filter is tested for modelled and real survey data. The technique yields profiles of apparent resistivity very similar, both in magnitude and in wavelength, to those which would have been obtained using VLF-EM resistivity measurements or d.c. resistivity profiling. This low-pass filter has the advantage of reducing high-wavenumber noise in the data; therefore only the major features of the VLF-EM profile are displayed. 相似文献
992.
Manual cutting of marsh vegetation contaminated by oil spills is often debated as a clean-up technique. Cutting oiled marsh vegetation may be proposed to prevent the oiling of sensitive wildlife associated with marshes. Less frequently, oiled marshes may be cut to aid vegetation recovery. This paper reviews several studies of oiled marsh cutting, and presents a case history of the Grand Eagle oil spill, where marsh cutting was used as a clean-up technique. The paper focuses on the effects of cutting on oiled vegetation survival and structural recovery. We conclude that cutting is often detrimental. Marshes should not be cut when impacted by light oils, where high flushing rates are present, or where re-oiling would be likely. Cutting should only be considered for areas where oil may persist, significant impacts to wildlife are likely, and less destructive clean-up techniques have proven insufficient. Seasonality should be considered prior to cutting, since vegetation impacts may be less likely during fall and winter. Concurrent stresses, such as unusual or extreme salinity or hydrological conditions, should also be considered prior to cutting, to avoid stressing oiled vegetation beyond recovery. Finally, if vegetation cutting is used in marsh environments, substrate disturbance should be avoided. 相似文献
993.
Michel Colonna Joel Casanova Wolf-Christian Dullo Gilbert Camoin 《Quaternary Research》1996,46(3):335-339
New sea-level and δ18O curves for the past 34,000 yr, based on uranium–thorium chronology, are proposed for the southwestern part of the Indian Ocean. The archives include cores drilled from onshore coral reefs and submersed samples from foreslope corals of Mayotte in the Comoro Islands. The Mayotte sea-level curve shows a lowstand of 145 ± 5 m below the present level during the last glacial maximum dated at 18,400 yr. This lowstand is supported by the maximum18O enrichment in the coral colonies. The residual signal (Δδ18O), controlled by sea-surface temperature changes, indicates that surface waters 18,400 yr ago were approximately 5°C cooler than present. The deglacial sea-level rise is clearly recorded, with a mean rate of about 1.7 cm yr−1between 18,400 and 10,000 yr ago. The deglaciation phase is characterized by a strong18O depletion marked by two pulses related to meltwater discharges into the North Atlantic Ocean but also characterized by responses specific to the tropical Indian Ocean. 相似文献
994.
Analysis of the karst aquifer structure of the Lamalou area (Hérault, France) with ground penetrating radar 总被引:1,自引:0,他引:1
Walid Al-fares Michel Bakalowicz Roger Gurin Michel Dukhan 《Journal of Applied Geophysics》2002,51(2-4)
The study site at Lamalou karst spring (Hortus karst plateau) is situated 40 km north of Montpellier in France. It consists of a limestone plateau, drained by a karst conduit discharging as a spring. This conduit extends for a few dozen meters in fractured and karstified limestone rocks, 15 to 70 m below the surface. The conduit is accessible from the surface. The main goal of this study is to analyze the surface part of the karst and to highlight the karstic features and among them the conduit, and to test the performances of ground penetrating radar (GPR) in a karstic environment. This method thus appears particularly well adapted to the analysis of the near-surface (<30 m in depth) structure of a karst, especially when clayey coating or soil that absorbs and attenuates the radar is rare and discontinuous. A GPR pulseEKKO 100 (Sensors and Software) was used on the site with a 50-MHz antenna frequency. The results highlight structures characterizing the karstic environment: the epikarst, bedding planes, fractured and karstified zones, compact and massive rock and karrens, a typical karst landform. One of the sections revealed in detail the main conduit located at a depth of 20 m, and made it possible to determine its geometry. This site offers possibilities of validation of the GPR data by giving direct access to the karstic conduits and through two cored boreholes. These direct observations confirm the interpretation of all the GPR sections. 相似文献
995.
Structural analysis of data displaying trends may be performed with the help of generalized increments, the variance of these
increments being a function of a generalized covariance. Generalized covariances are estimated primarily by parametric methods
(i. e., methods searching for the best coefficients of a predetermined function), but also may be computed by one known nonparametric
alternative. In this paper, a new nonparametric method is proposed. It is founded on the following principles: (1) least-squares
residues are generalized increments; and (2) the generalized covariance is not a unique function, but a family of functions
(the system is indeterminate). The method is presented in a general context of a k order trend in Rd, although the full solution is given only fork = I in Ri. In Ri, higher order trends may be developed easily with the equations included in this paper. For higher dimensions in space, the
problem is more complex, but a research approach is proposed. The method is tested on soil pH data and compared to a parametric
and nonparametric method. 相似文献
996.
Geochemistry, genesis, and health implications of fluoriferous groundwaters in the upper regions of Ghana 总被引:9,自引:2,他引:9
The most appropriate and widely used source of drinking water for the populations of the upper regions of Ghana is groundwater.
In general, groundwater quality is acceptable except for some parts of the Bolgatanga and Bongo Districts, where there are
occurrences of elevated levels of natural groundwater fluoride. Concentrations of groundwater fluoride in excess of the World
Health Organization (WHO) maximum guideline value (1.5 mg/l) in the Bongo area have been known since 1978. However, the effect
of fluoride on people ingesting the water did not receive public and medical attention until October 1993, when health personnel
were asked to investigate the cause of stained teeth in school children. The investigation established that 62% of the total
population of school children in the Bongo area had dental fluorosis. Against this background, a study was initiated to understand
the geochemistry, genesis, and distribution of fluoride in relation to the geology of the area. Groundwater fluoride in the
upper regions ranges from 0.11 to 4.60 ppm, with the highest concentrations associated with the fluorine-enriched Bongo coarse-grained
hornblende granite and syenite suite. The source of groundwater fluoride within the Bongo granitoids is dissolution of the
mineral fluorite and dissolution of and anion exchange with micaceous minerals and their clay products. Applying the WHO recommended
guideline values for fluoride in drinking water reveals that 49% of wells in the area deliver water below the optimum level
of 0.5 mg/l F–; these populations are thus prone to dental caries. Twenty-eight percent of the wells fall within the optimum interval for
good dental health (0.5–1.5 mg/l F–). Twenty-three percent of the wells have concentrations above the recommended maximum guideline limit of 1.5 mg/l F–; this population is susceptible to dental and possibly skeletal fluorosis. Climatic conditions of the area suggest that the
individual water consumption is in the order of 3 to 4 l which is higher than the WHO estimate of 2 l/adult/day. In addition,
dietary intake for the upper region population is probably higher than WHO baseline values (0.2–0.5 mg/day). This implies
that a much higher population is susceptible to developing dental and skeletal fluorosis than originally suspected. Geochemical
symbol plot maps help geochemists understand factors controlling the distribution and uptake of fluoride in the upper regions,
but they are of minimal value to health officials responsible for planning epidemiological studies and dental health education
programs in the region. By casting fluoride data into contoured 'geochemical health-risk maps' using intake interval guidelines
more closely aligned to regional climatic and dietary conditions, health officals can better judge the impacts (regional and
population based) of fluoride on segments of the population, such as various sex and age groups.
Received: 11 March 1997 · Accepted: 17 June 1997 相似文献
997.
The level fluctuations of Lake Ilay, Jura (France) during the last three millennia are reconstructed from sedimentological and malacological analyses of a core that is well-dated by tree-ring, radiocarbon and pollen datings. Changes in sediment facies, in carbonate concretion assemblages and in mollusc assemblages highlight a major lowering phase atc. 1550 BP and minor lowering phases atc. 2800 BP and shortly before AD 1394. Rises in lake level developed during the early Subatlantic and betweenc. 1500 and 1000 BP. These data are in good agreement with other proxy data from higher European and American latitudes. These correlations support the climatic origin of the level fluctuations of the Lake Ilay during the late Holocene. They suggest that the mediaeval climatic optimum is centred rather in the early than the late Middle Age. 相似文献
998.
999.
Karst groundwater resources in the countries of eastern Mediterranean: the example of Lebanon 总被引:1,自引:0,他引:1
The hydrogeological study of some karst systems in Lebanon shows important storage capacity, up to 27 billions m3 for Zarka system, the spring of Orontes River. Their geological and morphological settings, as well as their hydrodynamic
function, show structures developed below the level of present springs, probably up to several hundreds meters at depth. Since
Miocene, those carbonate formations were subject to a remarkable instability of their base levels. Variations of sea level,
of which the Messinian salinity crisis is the major phase, and effects of the alpine orogenesis, combined with climate variations
to develop complex, multiphased karst systems. Karstification occurs frequently in the whole formation, often deeply below
sea level or underneath sediment filling of continental basins. The surface karst landscape is often intensely eroded. Those
multiple, superimposed karst phases give the carbonate aquifers their large extension and storage capacity. These karst aquifers
show the typical easy restoration of storage. They have the ability to bear large interannual recharge variations and support
exploitation under high pumping rates. However, these aquifers have some disadvantages for sustainable management. In some
regions, successful boreholes, which allow high pumping rates also induce uncontrolled exploitation. Another consequence is
natural seawater intrusion, as well as the submarine discharge of fresh groundwater, in coastal aquifers. Those carbonate
aquifers are subject to important economical stress which jeopardizes their durability. The systematic study and understanding
of carbonate aquifers in Eastern Mediterranean is a preliminary condition to any integrated and sustainable management of
water resources. Studies in progress in Lebanon may serve as examples to the whole region. 相似文献
1000.
A model of population synthesis for our Galaxy is used to estimate the constraints imposed by photometric and astrometric star counts. We construct a statistical method to compute the degree of compatibility of the model with the data in the form of a multidimensional functionf(V, B-V, U-B, ). We apply the method to the determination of the star formation history in the Galaxy from Schmidt plates towards the anticentre. Although the presently available data do not allow us to draw firm conclusions, we show that the same kind of data with a slightly better photometric accuracy and with proper motions would give new constraints on the past star formation rate of the Galaxy. 相似文献