全文获取类型
收费全文 | 957篇 |
免费 | 60篇 |
国内免费 | 14篇 |
专业分类
测绘学 | 33篇 |
大气科学 | 65篇 |
地球物理 | 312篇 |
地质学 | 336篇 |
海洋学 | 63篇 |
天文学 | 155篇 |
综合类 | 5篇 |
自然地理 | 62篇 |
出版年
2023年 | 5篇 |
2022年 | 23篇 |
2021年 | 14篇 |
2020年 | 27篇 |
2019年 | 22篇 |
2018年 | 47篇 |
2017年 | 48篇 |
2016年 | 51篇 |
2015年 | 52篇 |
2014年 | 65篇 |
2013年 | 64篇 |
2012年 | 63篇 |
2011年 | 62篇 |
2010年 | 61篇 |
2009年 | 80篇 |
2008年 | 57篇 |
2007年 | 43篇 |
2006年 | 30篇 |
2005年 | 38篇 |
2004年 | 25篇 |
2003年 | 28篇 |
2002年 | 26篇 |
2001年 | 13篇 |
2000年 | 7篇 |
1999年 | 13篇 |
1998年 | 10篇 |
1997年 | 7篇 |
1996年 | 7篇 |
1995年 | 2篇 |
1994年 | 3篇 |
1992年 | 1篇 |
1990年 | 3篇 |
1989年 | 1篇 |
1988年 | 2篇 |
1987年 | 2篇 |
1986年 | 2篇 |
1985年 | 1篇 |
1984年 | 4篇 |
1983年 | 3篇 |
1982年 | 4篇 |
1981年 | 2篇 |
1980年 | 2篇 |
1979年 | 1篇 |
1978年 | 1篇 |
1977年 | 1篇 |
1976年 | 1篇 |
1975年 | 1篇 |
1974年 | 1篇 |
1971年 | 1篇 |
1969年 | 3篇 |
排序方式: 共有1031条查询结果,搜索用时 31 毫秒
151.
This paper addresses oil spill detection from remotely sensed optical images. In particular, it focuses on the automatic classification
of regions of interest (ROIs) in two classes, namely oil spills or look-alikes. Candidate regions and the corresponding boundaries
have been manually identified from full resolution Moderate Resolution Imaging Spectroradiometer images, related to the Mediterranean
Sea over the years 2008 and 2009. Then, a set of features has been extracted from each ROI, allowing to formulate the oil
spill detection problem as a two-class classification task on the provided regions (i.e. using a supervised learning strategy).
Since ROI classification is challenging, some desired characteristics for the classification algorithm are first identified,
such as accuracy, robustness, etc. Then, a solution (called SVME) is provided: it is based on an ensemble of incremental/decremental
cost-oriented Support Vector Machines, aggregated with the Receiving Operating Characteristic (ROC) convex hull method in
the ROC space. Such a solution addresses all the desired characteristics. Finally, the results obtained on the collected dataset
are shown. The importance of this study is the devising of a powerful classification technique that may have an impact on
optical oil spill detection from space, especially if fused with satellite synthetic aperture radar data. Moreover, it is
shown how the proposed system can be used as a decision support tool, to help a junior operator in making more reliable detections. 相似文献
152.
Rita Cannas Flavio Sacco Maria Cristina Follesa Andrea Sabatini Marco Arculeo Sabrina Lo Brutto Teresa Maggio Anna Maria Deiana Angelo Cau 《Marine Ecology》2012,33(3):350-363
Genetic variation at eight microsatellite loci was studied in nine populations of the blue and red shrimp Aristeus antennatus to investigate whether distinct stocks are present in the Western Mediterranean Sea. A high level of gene flow and no evidence of genetic partitioning were discovered. No significant variation was found (FST = 0.00673, P-value = 0.067) even when shrimps from exploited and those from deep-water unexploited grounds were compared. No evidence of reduction or expansion of population size in the recent past was found, as indicated by the bottleneck and interlocus g-tests. Our results are consistent with previous studies using mitochondrial gene methods and allozymes, indicating that, for this species, extensive pelagic larval dispersal and adult migration are probably responsible for the genetic homogeneity observed. In particular, due to a different bathymetric distribution of males and females, reported to be associated with different water masses and hence with possible differential dispersal capacity between sexes, the hypothesis of sex-biased dispersal was tested. Mean values of corrected assignment indices and mean relatedness values were higher for males, suggesting that females are the more widely dispersing sex. Molecular assessment of A. antennatus from the Western Mediterranean provides data of biological and evolutionary interest for the successful management of such a highly valuable fishery resource. 相似文献
153.
154.
Marco Benvenuti Marco Bonini Giovanna Moratti Marianna Ricci Chiara Tanini 《Geomorphology》2008,100(3-4):269-284
Integration of geomorphology, stratigraphy, sedimentology and morphotectonics in the analysis of the lower Cecina River reach, coastal Tuscany, reveals an undocumented historical channel avulsion. Geomorphological evidence and radiocarbon dating support that, from the Last Glacial Maximum until the end of the 16th century, the Cecina River flowed north of the present course and formed a well-developed cuspate delta. Two concurrent factors, active tectonics as a preparing factor and discharge regime as an activation factor, are thus inferred to have favored the avulsion of Cecina River. Fragmentary archaeological and historical records indicate that the late Holocene Cecina River plain was virtually unpopulated until the latest 16th century. This seems the main reason why high-magnitude hydrological events and prominent river channel avulsions were not reported in historical chronicles. From this perspective, geomorphological data may provide important knowledge and understanding of recent dynamics of environmental change when historical record is lacking or missing. 相似文献
155.
Marisa?Giuffrida Francois?Holtz Francesco?Vetere Marco?ViccaroEmail author 《Contributions to Mineralogy and Petrology》2017,172(11-12):90
Changes in magmatic assemblages and crystal stability as a response of CO2-flushing in basaltic systems have rarely been directly addressed experimentally, making the role of CO2 in magma dynamics still controversial and object of scientific debate. We conducted a series of experiments to understand the response of magmas from Etna volcano to CO2 flushing. We performed a first experiment at 300 MPa to synthesize a starting material composed of crystals of some hundreds of µm and melt pools. This material is representative of an initial magmatic assemblage composed of plagioclase, clinopyroxene and a water-undersaturated melt with 1.6 wt% H2O. In a second step, the initial assemblage was equilibrated at 300 and 100 MPa with fluids having different XCO 2 fl (CO2/(H2O + CO2)). At low XCO 2 fl (< 0.2 to 0.4), plagioclase is completely dissolved and clinopyroxene show dissolution textures. For relatively high XCO 2 fl (0.9 at 300 MPa), the flushing of a CO2-rich fluid phase leads to an increase of the amount of clinopyroxene and a decrease of the abundance of plagioclase at 300 MPa. This decrease of plagioclase proportion is associated with a change in An content. Our experiments demonstrate that flushing basaltic systems with fluids may drastically affect crystal textures and phase equilibria depending on proportions of H2O and CO2 in the fluid phase. Since texture and crystal proportions are among the most important parameters governing the rheology of magmas, fluid flushing will also influence magma ascent to the Earth’s surface. The experimental results open new perspectives to decipher the textural and compositional record of minerals observed in volcanic rocks from Mt. Etna, and at the same time offer the basis for interpreting the information preserved in minerals from other basaltic volcanoes erupting magmas enriched in CO2. 相似文献
156.
Modeling ground deformations of Panarea volcano hydrothermal/geothermal system (Aeolian Islands, Italy) from GPS data 总被引:1,自引:0,他引:1
Alessandra Esposito Marco Anzidei Simone Atzori Roberto Devoti Guido Giordano Grazia Pietrantonio 《Bulletin of Volcanology》2010,72(5):609-621
Panarea volcano (Aeolian Islands, Italy) was considered extinct until November 3, 2002, when a submarine gas eruption began
in the area of the islets of Lisca Bianca, Bottaro, Lisca Nera, Dattilo, and Panarelli, about 2.5 km east of Panarea Island.
The gas eruption decreased to a state of low degassing by July 2003. Before 2002, the activity of Panarea volcano was characterized
by mild degassing of hydrothermal fluid. The compositions of the 2002 gases and their isotopic signatures suggested that the
emissions originated from a hydrothermal/geothermal reservoir fed by magmatic fluids. We investigate crustal deformation of
Panarea volcano using the global positioning system (GPS) velocity field obtained by the combination of continuous and episodic
site observations of the Panarea GPS network in the time span 1995–2007. We present a combined model of Okada sources, which
explains the GPS results acquired in the area from December 2002. The kinematics of Panarea volcano show two distinct active
crustal domains characterized by different styles of horizontal deformation, supported also by volcanological and structural
evidence. Subsidence on order of several millimeters/year is affecting the entire Panarea volcano, and a shortening of 10−6 year−1 has been estimated in the Islets area. Our model reveals that the degassing intensity and distribution are strongly influenced
by geophysical-geochemical changes within the hydrothermal/geothermal system. These variations may be triggered by changes
in the regional stress field as suggested by the geophysical and volcanological events which occurred in 2002 in the Southern
Tyrrhenian area. 相似文献
157.
Paola Morasca Marco Massa Enrica Laprocina Kevin Mayeda Scott Phillips Luca Malagnini Daniele Spallarossa Giovanni Costa Paolo Augliera 《Journal of Seismology》2010,14(4):727-738
A merged, high-quality waveform dataset from different seismic networks has been used to improve our understanding of lateral
seismic attenuation for Northern Italy. In a previous study on the same region, Morasca et al. (Bull Seismol Soc Am 98:1936–1946,
2008) were able to resolve only a small area due to limited data coverage. For this reason, the interpretation of the attenuation
anomalies was difficult given the complexity of the region and the poor resolution of the available data. In order to better
understand the lateral changes in the crustal structure and thickness of this region, we selected 770 earthquakes recorded
by 54 stations for a total of almost 16,000 waveforms derived from seismic networks operating totally or partially in Northern
Italy. Direct S-wave and coda attenuation images were obtained using an amplitude ratio technique that eliminates source terms
from the formulation. Both direct and early-coda amplitudes are used as input for the inversions, and the results are compared.
Results were obtained for various frequency bands ranging between 0.3 and 25.0 Hz and in all cases show significant improvement
with respect to the previous study since the resolved area has been extended and more crossing paths have been used to image
smaller scale anomalies. Quality-factor estimates are consistent with the regional tectonic structure exhibiting a general
trend of low attenuation under the Po Plain basin and higher values for the Western Alps and Northern Apennines. The interpretation
of the results for the Eastern Alps is not simple, possibly because our resolution for this area is still not adequate to
resolve small-scale structures. 相似文献
158.
Correlations of seismic noise are commonly used to monitor temporal variations of relative seismic velocity in period ranges from 1 s up to 100 s. Of particular interest is the detection of small changes in the order of 0.01–0.1 % in propagation speeds. Measuring such small differences can, however, be significantly biased by temporal variations in the properties of the noise sources within the corresponding frequency band. Using synthetic data, we show that apparent relative velocity variations might appear only due to changes in the amplitude and frequency content caused by source variations. Removing such unwanted effects by applying narrow bandpass filters in the preprocessing restricts the high-resolution analysis of any signal due to Gabor’s uncertainty limit, i.e., the correlation function suffers a limited resolution to time delay estimates for small correlation times, low-frequency ranges, and in narrow frequency bands. Better understanding of spatiotemporal noise source properties and the theoretical limitations of time–frequency analysis is critical for accurate and reliable passive monitoring. 相似文献
159.
Giovanni Tecchio Marco Donà Francesca da Porto 《Bulletin of Earthquake Engineering》2016,14(11):3099-3124
Masonry arch bridges are crucial elements in the railway transportation network throughout Europe. Although significant advances in seismic risk assessment of various bridge types have been made by developing fragility curves of generalized classes of structures, there are no comparable tools for masonry arch structures. In this context, this paper presents the construction of seismic fragility curves of single-span masonry bridges according to the limit analysis method. An iterative procedure is implemented to define the capacity curve of the equivalent single degree of freedom system through non-linear kinematic analysis. The process involves determination of the collapse mechanism, calculation of the limit load multiplier, and definition of the thrust line. The intrinsic variability of the seismic action is incorporated with the use of different sets of elastic spectra compatible with EC 8 Type-1 spectrum for various types of soil, with peak ground acceleration varying over the range 0.05–1.5 g. The fragility curves of the generalized classes of single-span masonry bridges are finally obtained from the effective ranges of the main geometric and material parameters affecting arch bridge capacity. 相似文献
160.
Earlier observations on the deformation of rock-forming quartz during Alpine metamorphism that were made by Voll (1976) along
a traverse following the Reuss valley (Aar massif, Central Swiss Alps) are still relevant but can be explained in more modern
term. Voll found that a few kilometres south of the contact between the Aar massif and its sedimentary cover quartz has partly
undergone a combination of dynamic recrystallization and nucleation along grain boundaries and fractures at the expense of
the deformed parent grains. Further to the south, increasing grain growth of quartz under nearly static conditions is observed,
with polygonal grain shapes typical of annealed microfabrics. Comparable stages of recrystallization could be found in several
N–S traverses through the Aar massif so that “points of first occurrence” of newly formed quartz could be connected to define
a quartz recrystallization isograd. This isograd extents over 90 km through the northern Aar massif, following a course parallel
to the sanidine/microcline isograd but at a distance of 10–15 km further to the north. The evolution of the quartz microfabric
is discussed with respect to recent estimates of the T-t path of the Aar massif. 相似文献