全文获取类型
收费全文 | 759篇 |
免费 | 63篇 |
专业分类
测绘学 | 24篇 |
大气科学 | 54篇 |
地球物理 | 252篇 |
地质学 | 227篇 |
海洋学 | 56篇 |
天文学 | 175篇 |
综合类 | 2篇 |
自然地理 | 32篇 |
出版年
2023年 | 3篇 |
2022年 | 4篇 |
2021年 | 11篇 |
2020年 | 17篇 |
2019年 | 12篇 |
2018年 | 38篇 |
2017年 | 42篇 |
2016年 | 39篇 |
2015年 | 34篇 |
2014年 | 43篇 |
2013年 | 32篇 |
2012年 | 36篇 |
2011年 | 45篇 |
2010年 | 46篇 |
2009年 | 49篇 |
2008年 | 32篇 |
2007年 | 40篇 |
2006年 | 21篇 |
2005年 | 17篇 |
2004年 | 28篇 |
2003年 | 20篇 |
2002年 | 23篇 |
2001年 | 22篇 |
2000年 | 13篇 |
1999年 | 14篇 |
1998年 | 15篇 |
1997年 | 9篇 |
1996年 | 6篇 |
1995年 | 10篇 |
1994年 | 4篇 |
1993年 | 13篇 |
1992年 | 4篇 |
1991年 | 5篇 |
1989年 | 5篇 |
1987年 | 3篇 |
1986年 | 5篇 |
1983年 | 3篇 |
1982年 | 3篇 |
1980年 | 6篇 |
1979年 | 5篇 |
1978年 | 7篇 |
1977年 | 2篇 |
1976年 | 3篇 |
1975年 | 4篇 |
1974年 | 4篇 |
1973年 | 3篇 |
1971年 | 3篇 |
1969年 | 3篇 |
1968年 | 2篇 |
1955年 | 3篇 |
排序方式: 共有822条查询结果,搜索用时 15 毫秒
1.
We discuss the main mechanisms affecting the dynamical evolution of Near-Earth Asteroids (NEAs) by analyzing the results of three numerical integrations over 1 Myr of the NEA (4179) Toutatis. In the first integration the only perturbing planet is the Earth. So the evolution is dominated by close encounters and looks like a random walk in semimajor axis and a correlated random walk in eccentricity, keeping almost constant the perihelion distance and the Tisserand invariant. In the second integration Jupiter and Saturn are present instead of the Earth, and the 3/1 (mean motion) and v
6 (secular) resonances substantially change the eccentricity but not the semimajor axis. The third, most realistic, integration including all the three planets together shows a complex interplay of effects, with close encounters switching the orbit between different resonant states and no approximate conservation of the Tisserand invariant. This shows that simplified 3-body or 4-body models cannot be used to predict the typical evolution patterns and time scales of NEAs, and in particular that resonances provide some fast-track dynamical routes from low-eccentricity to very eccentric, planet-crossing orbits.On leave from the Department of Mathematics, University of Pisa, Via Buonarroti 2, 56127 Pisa, Italy, thanks to the G. Colombo fellowships of the European Space Agency. 相似文献
2.
This paper describes a real-time spectrum analyser with on-line definition of the confidence levels built at Trieste Observatory
for a research program on periodical components of unknown period in optical sources. The system is realized by a photon counting
photometer on-line to a computer that elaborates in real-time the data from the photometer and displays the spectrum of the
source measured. The spectrum is normalized to unity mean value and the display is calibrated by the confidence levels relative
to the
distribution of the spectrum displayed. Some experimental data show the performance of the machine that operates in real-time
at full efficiency up to 160 Hz.
Work supported by Consiglio Nazionale delle Ricerche, Gruppo Nazionale di Astronomia, Settore Tecnologie Astronomiche. Trieste
Observatory — Instruments and Data Processing Group (1/76). 相似文献
3.
Permanent Scatterers for landslide investigations: outcomes from the ESA-SLAM project 总被引:3,自引:0,他引:3
Paolo Farina Davide Colombo Alfio Fumagalli Florian Marks Sandro Moretti 《Engineering Geology》2006,88(3-4):200-217
Within the SLAM project (Service for Landslide Monitoring), launched in 2003 by the European Space Agency (ESA) the Permanent Scatterers (PS) technique, a multi-image interferometric approach, coupled with the interpretation of aerial-photos and optical satellite images, was carried out for landslide investigations. The PS analysis was applied at a regional scale as support for landslide inventory mapping and at local scale for the monitoring of single well-known slope movements. For the integration of the PS measurements within a landslide inventory the Arno river basin (Italy) was chosen as test site for the presence of a high number of mass movements (to date about 300 areas at high landslide risk and more than 27,000 individual landslides mapped by the institutional authorities). About 350 SAR images have been interferometrically processed by means of the PS technique, with the detection of about 600,000 PS. The use of optical images contributed spatial meaning to the point-wise information provided by the PS, making it easier to identify terrain features related to slope instability and the landslide boundaries. Here we describe the employed methodology and its impact in the updating of a preexisting landslide inventory. 6.8% of the total number of landslides were characterized by ground displacement measurements from the PS: 6.1% of already mapped landslides and 0.8% of new unstable areas detected through the PS analysis. Moreover, most of the PS are located in urban areas, showing that the proposed methodology is suitable for landslide mapping in areas with a quite high density of urbanization, but that over vegetated areas it still suffers from the limitations induced by the current space-borne SAR missions (e.g. temporal de-correlation). On the other hand, the use of InSAR for the monitoring of single slow landslides threatening built-up areas has provided satisfactory results, allowing the measurement of superficial deformations with high accuracy on the landslide sectors characterized by a good radar reflectivity and coherence. 相似文献
4.
5.
Paolo Fabbri 《Ground water》1997,35(5):881-887
6.
Richard Herrington Valeriy Maslennikov Victor Zaykov Igor Seravkin Alexander Kosarev Bernd Buschmann Jean-Jacques Orgeval Nicola Holland Svetlana Tesalina Paolo Nimis Robin Armstrong 《Ore Geology Reviews》2005,27(1-4):203
VMS deposits of the South Urals developed within the evolving Urals palaeo-ocean between Silurian and Late Devonian times. Arc-continent collision between Baltica and the Magnitogorsk Zone (arc) in the south-western Urals effectively terminated submarine volcanism in the Magnitogorsk Zone with which the bulk of the VMS deposits are associated. The majority of the Urals VMS deposits formed within volcanic-dominated sequences in deep seawater settings. Preservation of macro and micro vent fauna in the sulphide bodies is both testament to the seafloor setting for much of the sulphides but also the exceptional degree of preservation and lack of metamorphic overprint of the deposits and host rocks. The deposits in the Urals have previously been classified in terms of tectonic setting, host rock associations and metal ratios in line with recent tectono-stratigraphic classifications. In addition to these broad classes, it is clear that in a number of the Urals settings, an evolution of the host volcanic stratigraphy is accompanied by an associated change in the metal ratios of the VMS deposits, a situation previously discussed, for example, in the Noranda district of Canada.Two key structural settings are implicated in the South Urals. The first is seen in a preserved marginal allochthon west of the Main Urals Fault where early arc tholeiites host Cu–Zn mineralization in deposits including Yaman Kasy, which is host to the oldest macro vent fauna assembly known to science. The second tectonic setting for the South Urals VMS is the Magnitogorsk arc where study has highlighted the presence of a preserved early forearc assemblage, arc tholeiite to calc-alkaline sequences and rifted arc bimodal tholeiite sequences. The boninitc rocks of the forearc host Cu–(Zn) and Cu–Co VMS deposits, the latter hosted in fragments within the Main Urals Fault Zone (MUFZ) which marks the line of arc-continent collision in Late Devonian times. The arc tholeiites host Cu–Zn deposits with an evolution to more calc-alkaline felsic volcanic sequences matched with a change to Zn–Pb–Cu polymetallic deposits, often gold-rich. Large rifts in the arc sequence are filled by thick bimodal tholeiite sequences, themselves often showing an evolution to a more calc-alkaline nature. These thick bimodal sequences are host to the largest of the Cu–Zn VMS deposits.The exceptional degree of preservation in the Urals has permitted the identification of early seafloor clastic and hydrolytic modification (here termed halmyrolysis sensu lato) to the sulphide assemblages prior to diagenesis and this results in large-scale modification to the primary VMS body, resulting in distinctive morphological and mineralogical sub-types of sulphide body superimposed upon the tectonic association classification.It is proposed that a better classification of seafloor VMS systems is thus achievable using a three stage classification based on (a) tectonic (hence bulk volcanic chemistry) association, (b) local volcanic chemical evolution within a single edifice and (c) seafloor reworking and halmyrolysis. 相似文献
7.
Effects of trace metal and exposure to air on serotonin and dopamine levels in tissues of the mussel Perna perna 总被引:1,自引:0,他引:1
We evaluated levels of serotonin (5HT) and dopamine (DOPA) in muscle and digestive glands of the mussel, Perna perna, collected at different times of day; exposed to air for 24 h, followed by re-submersion; and after exposure to different metals. Mussels collected at different periods of day showed little oscillation in 5HT and DOPA levels. Mussels exposed to metals showed significant changes in 5HT and DOPA levels in digestive gland and muscle, as did mussels exposed to air. Our data suggest that analyses of 5HT and DOPA in tissues of mussels could serve as a tool to evaluate the presence and effects of heavy metal contamination in mussels. Care in data interpretation is required, however, since other environmental factors such as exposure of mussels to air (i.e. at low tides) can also cause changes in DOPA and 5HT levels. Additional research is necessary to separate such natural environmental effects from effects of contaminants. 相似文献
8.
A pollen record from a sediment core drilled underneath the Acheulian site of Fontana Ranuccio provides new data on the vegetation characters in central Italy during the late Early Pleistocene, in a forest phase possibly corresponding to a substage of Marine Isotope Stage 21. A number of tree taxa nowadays extinct in Europe (Cathaya, Tsuga, Taxodium type, Carya and Pterocarya) are present at Fontana Ranuccio in a temperate phase characterised by dense forest conditions, dominated by Alnus and Pterocarya. The comparison of the Fontana Ranuccio record with other dated sites distributed along the Italian peninsula indicates that the floristic and vegetational characters of the Mid Pleistocene revolution present a rather distinct succession of biostratigraphic events. Compared to other Early Pleistocene pollen records from southern Europe, the Italian Peninsula shows greater floristic similarities with the Eastern Mediterranean regions than with the Iberian Peninsula. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
9.
10.
Manuela Lasagna Domenico Antonio De Luca Laura Debernardi Paolo Clemente 《Environmental Earth Sciences》2013,70(6):2767-2784
This paper discusses the effects of the dilution process on levels of aquifer contamination attenuation, developing analytical equations for application in theoretical and practical cases. The key finding is that, in aquifers, the dilution process causes a reduction of the contaminant concentration. Analytical equations for the assessment of the contaminant attenuation capacity, by means of the dilution process in an aquifer, are put forward. In particular, the attenuation of the contaminant concentration in an aquifer, and thus the dilution, is described by the volumetric flow rate per unit width perpendicular to the flow direction (q u). Moreover, a classification of q u was developed, considering six classes of attenuation capacity due to dilution. For the equations validation, nitrate contamination attenuation due to dilution was analysed in two sample areas in Piedmont (North-West Italy). The results confirmed the hypothesis, evidencing high nitrate concentrations in areas with low q u values, connected to low dilution rate of the contaminant in the groundwater; vice versa, low nitrate concentrations were evidenced in areas with high q u values, due to a high nitrate dilution rate. 相似文献