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61.
Jean-François Oehler Jean-François Lénat Philippe Labazuy 《Bulletin of Volcanology》2008,70(6):717-742
This work presents the first exhaustive study of the entire surface of the Reunion Island volcanic system. The focus is on
the submarine part, for which a compilation of all multibeam data collected during the last 20 years has been made. Different
types of submarine features have been identified: a coastal shelf, debris avalanches and sedimentary deposits, erosion canyons,
volcanic constructions near the coast, and seamounts offshore. Criteria have been defined to differentiate the types of surfaces
and to establish their relative chronology where possible. Debris avalanche deposits are by far the most extensive and voluminous
formations in the submarine domain. They have built four huge Submarine Bulges to the east, north, west, and south of the
island. They form fans 20–30 km wide at the coastline and 100–150 km wide at their ends, 70–80 km offshore. They were built
gradually by the superimposition and/or juxtaposition of products moved during landslide episodes, involving up to several
hundred cubic kilometers of material. About 50 individual events deposits can be recognized at the surface. The landslides
have recurrently dismantled Piton des Neiges, Les Alizés, and Piton de La Fournaise volcanoes since 2 Ma. About one third
are interpreted as secondary landslides, affecting previously emplaced debris avalanche deposits. On land, landslide deposits
are observed in the extensively eroded central area of Piton des Neiges and in its coastal areas. Analysis of the present-day
topography and of geology allows us to identify presumed faults and scars of previous large landslides. The Submarine Bulges
are dissected and bound by canyons up to 200 m deep and 40 km long, filled with coarse-grained sediments, and generally connected
to streams onshore. A large zone of sedimentary accumulation exists to the north–east of the island. It covers a zone 20 km
in width, extending up to 15 km offshore. Volcanic constructions are observed near the coast on both Piton des Neiges and
Piton de la Fournaise volcanoes and are continuations of subaerial structures. Individual seamounts are present on the submarine
flanks and the surrounding ocean floor. A few seem to be young volcanoes, but the majority are probably old, eroded seamounts.
This study suggests a larger scale and frequency of mass-wasting events on Reunion Island compared to similar islands. The
virtual absence of downward flexure of the lithosphere beneath the island probably contributes to this feature. The increased
number of known flank–failure events has to be taken into consideration when assessing hazards from future landslides, in
particular, the probability of landslide-generated tsunamis.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献
62.
Geoelectrical structure of the central zone of Piton de la Fournaise volcano (Réunion) 总被引:2,自引:2,他引:0
Jean-François Lénat David Fitterman Dallas B. Jackson Philippe Labazuy 《Bulletin of Volcanology》2000,62(2):75-89
A study of the geoelectrical structure of the central part of Piton de la Fournaise volcano (Réunion, Indian Ocean) was made
using direct current electrical (DC) and transient electromagnetic soundings (TEM). Piton de la Fournaise is a highly active
oceanic basaltic shield and has been active for more than half a million years. Joint interpretation of the DC and TEM data
allows us to obtain reliable 1D models of the resistivity distribution. The depth of investigation is of the order of 1.5 km
but varies with the resistivity pattern encountered at each sounding. Two-dimensional resistivity cross sections were constructed
by interpolation between the soundings of the 1D interpreted models. Conductors with resistivities less than 100 ohm-m are
present at depth beneath all of the soundings and are located high in the volcanic edifice at elevations between 2000 and
1200 m. The deepest conductor has a resistivity less than 20 ohm-m for soundings located inside the Enclos and less than 60–100
ohm-m for soundings outside the Enclos. From the resistivity distributions, two zones are distinguished: (a) the central zone
of the Enclos; and (b) the outer zone beyond the Enclos. Beneath the highly active summit area, the conductor rises to within
a few hundred meters of the surface. This bulge coincides with a 2000-mV self-potential anomaly. Low-resistivity zones are
inferred to show the presence of a hydrothermal system where alteration by steam and hot water has lowered the resistivity
of the rocks. Farther from the summit, but inside the Enclos, the depth to the conductive layers increases to approximately
1 km and is inferred to be a deepening of the hydrothermally altered zone. Outside of the Enclos, the nature of the deep,
conductive layers is not established. The observed resistivities suggest the presence of hydrated minerals, which could be
found in landslide breccias, in hydrothermally altered zones, or in thick pyroclastic layers. Such formations often create
perched water tables. The known occurrence of large eastward-moving landslides in the evolution of Piton de la Fournaise strongly
suggests that large volumes of breccias should exist in the interior of the volcano; however, extensive breccia deposits are
not observed at the bottom of the deep valleys that incise the volcano to elevations lower than those determined for the top
of the conductors. The presence of the center of Piton de la Fournaise beneath the Plaine des Sables area during earlier volcanic
stages (ca. 0.5 to 0.150 Ma) may have resulted in broad hydrothermal alteration of this zone. However, this interpretation
cannot account for the low resistivities in peripheral zones. It is not presently possible to discriminate between these general
interpretations. In addition, the nature of the deep conductors may be different in each zone. Whatever the geologic nature
of these conductive layers, their presence indicates a major change of lithology at depth, unexpected for a shield volcano
such as Piton de la Fournaise.
Received: 3 November 1999 / Accepted: 15 September 1999 相似文献
63.
Imene Meriem Benbakhti Said Maouche Djelloul Belhai Assia Harbi Jean-François Ritz Ghilles Rabai Ahmed Rezouk Fawzi Doumaz 《Journal of Seismology》2018,22(6):1549-1561
In this work, we reappraise the seismogenic potential of the geologic structures in the western Tell Atlas of Algeria, considered active host to moderate to low magnitude earthquakes. The direct identification of active faults is generally a difficult task in northern Algeria. The active tectonics in the Oran Plio-Quaternary age basin (Northwestern Algeria) is analyzed and characterized through a morpho-structural study combining topographic, geomorphologic, geological, and neotectonic data. Folds and fault scarps affecting Quaternary deposits show that the region is affected by compressional deformation still active nowadays, as shown by the recorded seismic activity. Our new observations enable a better understanding of the present seismotectonic context of the Oran region, particularly with regard to the magnitude and source of the 1790 Oran damaging event. The obtained result helps to shed some light on the elusive active tectonics characterizing this coastal area, and to assess regional seismic hazard, particularly in coastal zones where large seismogenic areas straddle the onshore–offshore zones. 相似文献
64.
Paul-Emile Bernard Nicolas Chevaugeon Vincent Legat Eric Deleersnijder Jean-François Remacle 《Ocean Dynamics》2007,57(2):109-121
In this paper, we present an h-adaptive discontinuous Galerkin formulation of the shallow water equations. For a discontinuous Galerkin scheme using polynomials
up to order , the spatial error of discretization of the method can be shown to be of the order of , where is the mesh spacing. It can be shown by rigorous error analysis that the discontinuous Galerkin method discretization error
can be related to the amplitude of the inter-element jumps. Therefore, we use the information contained in jumps to build
error metrics and size field. Results are presented for ocean modelling problems. A first experiment shows that the theoretical
convergence rate is reached with the discontinuous Galerkin high-order h-adaptive method applied to the Stommel wind-driven gyre. A second experiment shows the propagation of an anticyclonic eddy
in the Gulf of Mexico.
An erratum to this article can be found at 相似文献
65.
Roger Guérin Jean-Michel Baltassat Marie Boucher Konstantinos Chalikakis Pierre-Yves Galibert Jean-François Girard Valérie Plagnes Rémi Valois 《Comptes Rendus Geoscience》2009,341(10-11):810-817
In the framework of the management of karstic aquifers, geophysical reconnaissance can be used to locate conduits and caves, and to characterise the surrounding limestone matrix. Suitable characterisation of heterogeneities in the karstic environment is, however, challenging for ground-based geophysical methods. The present article describes the results, and evaluates the response and accuracy of combined geophysical measurements carried out at the Poumeyssen test site in France, involving electrical resistivity imaging (ERI), magnetic resonance sounding (MRS), “mise-à-la-masse” electrical mapping, and seismic tomography. This site provides the opportunity to study a relatively wide, shallow, water-filled conduit whose location and shape are known from topographic work carried out by cave divers. Seismic and MRS provided the exact location and width of the conduit, to within a few meters. The seismic and electrical data suggest that the limestone medium surrounding the conduit is not homogeneous. 相似文献
66.
67.
Camps M Briand JF Guentas-Dombrowsky L Culioli G Bazire A Blache Y 《Marine pollution bulletin》2011,62(5):1032-6755
Biofilm formation is a key step during marine biofouling, the natural colonization of immersed substrata, leading to major economic and ecological consequences. Consequently, bacteria have been used for the screening of new non-toxic antifoulants: the adhesion of five strains isolated on three French locations was monitored using a fluorescence-based assay and toxicity was also evaluated. Nine biocides including commercial, natural and natural-derived products were tested. The commercial antifoulants, TBTO and Sea Nine showed low EC50 but high toxicity. The non-commercial products TFA-Z showed significant anti-adhesion activities and appeared to be non-toxic, suggesting a specific anti-adhesion mechanism. In addition, the strains could be classified depending on their sensitivity to the molecules used even if strain sensitivity also depended on the molecules tested. In conclusion, TFA-Z would be a promising candidate as non-toxic antifoulant and our results strengthen the need to perform antifouling bioassays with a panel of strains showing different response profiles. 相似文献
68.
The first cosmic mirage was discovered approximately 20 years ago as the double optical counterpart of a radio source. This
phenomenon had been predicted some 70 years earlier as a consequence of General Relativity. We present here a summary of what
we have learnt since. The applications are so numerous that we had to concentrate on a few selected aspects of this new field
of research.
This review is focused on strong gravitational lensing, i.e. the formation of multiple images, in QSO samples. It is intended
to give the reader an up-to-date status of the observations and to present an overview of its most interesting potential applications
in cosmology and astrophysics, as well as numerous important results achieved so far.
The first section follows an intuitive approach to the basics of gravitational lensing and is developed in view of our interest
in multiply imaged quasars. The astrophysical and cosmological applications of gravitational lensing are outlined in Sect.
2 and the most important results are presented in Sect. 5. Sections 3 and 4 are devoted to the observations. Finally, conclusions
are summarized in the last section.
We have tried to avoid duplication with existing (and excellent) introductions to the field of gravitational lensing. For
this reason, we did not concentrate on the individual properties of specific lens models, as these are already well presented
in Narayan and Bartelmann (1996) and on a more intuitive ground in Refsdal and Surdej (1994). Wambsganss (1998) proposes a
broad view on gravitational lensing in astronomy; the reviews by Fort and Mellier (1994) and Hattori et al. (1999) deal with
lensing by galaxy clusters; microlensing in the Galaxy and the local group is reviewed by Paczyński (1996) and a general panorama
on weak lensing is given by Bartelmann and Schneider (1999) and Mellier (1999). The monograph on the theory of gravitational
lensing by Schneider, Ehlers and Falco (1992) also remains a reference in the field.
Received 4 April 2000 / Published online 9 August 2000 相似文献
69.
70.
The SARAL/AltiKa project is based on a single Ka band altimeter (35.75 GHz), which is the first oceanography altimeter to operate at such a high frequency. Ka band offers reduced radar footprint in comparison to traditional Ku band altimeters and negligible ionospheric effects. In this paper we present and evaluate benefits of AltiKa altimeter applied in the study of lakes in Andean chain in South America. Water levels time series obtained with Envisat/RA-2 and SARAL/AltiKa altimeters over 17 lakes of various sizes are calculated and compared to in situ observations. SARAL/AltiKa measurements tend to be extremely well correlated with in situ measurements and offer significant improvements compared to the Envisat mission. 相似文献