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61.
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
62.
Jean-François Crétaux Stephane Calmant Vladimir Romanovski Anton Shabunin Florent Lyard Muriel Bergé-Nguyen Anny Cazenave Fabrice Hernandez Felix Perosanz 《Journal of Geodesy》2009,83(8):723-735
Altimetry missions such as Topex/Poseidon, Jason-1, GFO and ENVISAT have been widely used in the continental domain over lakes,
rivers and wetland although they were mostly dedicated to oceanic studies. Knowledge of the instrumental biases is a key issue.
Numerous sites have been dedicated to calibration purposes, either in the oceanic domain (Harvest offshore platform in California,
Corsica, Bass Strait in Australia) or over lakes (Lake Erie in United States). A new site (Lake Issykkul in Kirghizstan) is
proposed for calibration in the continental domain. This lake is covered by past (T/P) and current radar altimetry satellites
(Jason-1, T/P, GFO, and ENVISAT). Several in situ water levels and local meteorological variables are available at the site.
Located in a mountainous area, it offers an opportunity for calibration far away from all other existing sites and very different
environment contexts. Two GPS campaigns have been conducted on the lake in 2004 and in 2005. They consisted of cruises with
stations installed onboard a boat following the satellite ground tracks, and onshore settings. This enabled estimating a bias
for each altimeter and each tracking algorithm available. Biases obtained for Envisat, GFO, T/P and Jason-1 using the default
ocean tracker (respectively, 48.1 ± 6.6, 7.5 ± 4.0, 0 ± 4.3 and 7.0 ± 5.5 cm) agree with biases published at the other calibration
sites. For Jason-1, there is a significant disagreement with results obtained in the ocean field (7 cm instead of 13 cm) but
is coherent with bias obtained on the Lake Erie site. Erroneous estimates of the sea state bias correction from non-oceanic-like
waveforms is discussed as a possible explanation. Errors in the ionospheric, wet and dry tropospheric corrections for the
continental domain are also highlighted and quantified. 相似文献
63.
Etienne Balan Marc Blanchard Jean-François Hochepied Michele Lazzeri 《Physics and Chemistry of Minerals》2008,35(5):279-285
The theoretical infrared (IR) and Raman spectra of bayerite (β-Al(OH)3) are computed in the density functional theory framework, using the linear response theory. The results are consistent with
the occurrence of six non-equivalent OH groups in a bayerite structure with space group P21/n. Similar to gibbsite, the transmission powder IR spectrum of bayerite in the region of the OH stretching bands is found to
depend on the shape of particles. In particular, the broadening of the strong band observed at about 3,460 cm−1 in the spectrum of Al hydroxides is related to the electrostatic charges occurring at the surface of the polarized dielectric
particles. The experimental correlation observed between the shape of this band and morphological parameters has therefore
a physical, instead of chemical, origin. 相似文献
64.
65.
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
66.
67.
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
68.
69.
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
70.
The amount of useful scientific data that a space-borne telescope produces is often limited by the available telemetry of
the platform. General purpose image compression schemes are usually used to compress the image either lossy or losslessly.
These schemes do not take into account the fact that pixel values of typical solar images are only known to within a certain
uncertainty range. We present a preprocessing method to enhance the performance (compression ratio) of any subsequent image
compression scheme. The method uses estimates of the photon shot and thermal noises to compute a recoding look-up table that
maps the initial data into uncertainty intervals. The recoding method is lossy in a mathematical sense but lossless in a physical
sense, since the image alterations are guaranteed to be smaller than the Poisson noise. The performance of any further compression
algorithm is enhanced while achieving a known guaranteed maximum absolute error for each pixel in the case of lossless compression.
This revised version was published online in October 2005 with corrections to one of the authors' names. 相似文献