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961.
Philippe Cte Pierre Degauque Richard Lagabrielle Nathalie Levent 《Geophysical Prospecting》1995,43(8):1083-1107
In order to detect underground cavities, we have designed a geophysical method based on the interaction of a monochromatic electromagnetic wave in the frequency band 100 MHz to 1 GHz with the ground situated between two boreholes. Three different approaches are involved in the design of this EM tomographic method.
- 1 The application of an ‘exact’ theory is used to calculate artificial data, based on an integral equation method. These data are inverted using a tomographic algorithm whose basic assumptions are rather coarse. The results show that, however, cavities can very well be detected and their position recovered.
- 2 Data are obtained with a physical reduced-scale model in a water tank, in which all parameters are known in advance. The inversion process confirms the validity of the method.
- 3 Underground cavities are actually detected.
962.
963.
964.
Francesca Micheletti Pierre Barbey Annamaria Fornelli Giuseppe Piccarreta Etienne Deloule 《International Journal of Earth Sciences》2007,96(5):843-860
In situ U–Pb dating of zircons from five samples of Calabrian augen gneisses shows that their protoliths are Latest Precambrian
to Early Cambrian in age (562 ± 15, 547 ± 7, 540 ± 4, 539 ± 16 and 526 ± 10 Ma), and contain Archaean (3.1 Ga), Palaeoproterozoic
(1.7–2.4 Ga) and Neoproterozoic (0.6–0.9 Ga) inheritance. Geochemical signature of augen gneisses is typical of high-K calc-alkaline
post-collisional magmatism. Their Sr–Nd isotopic compositions [0.7093 < (87Sr/86Sr)i < 0.7139; −3.2 < ɛNd(t) < −5.4; 1.5 < T
DM < 1.7 Ga] indicate the involvement of a crustal component in significant proportions. The Calabrian augen gneisses have,
therefore, to be distinguished from the orthogneisses of Sardinia and northern Algeria, and from the porphyroids of Sicily,
which are Middle Ordovician. By contrast, the Calabrian augen gneisses show a close similarity to the Pan-African post-collisional
granitoids of the northern edge of the West African craton (e.g. the Moroccan Anti-Atlas). This suggests a peri-Gondwana origin
and corroborates previous palaeogeodynamic reconstructions attributing the Alboran microplate to the northern margin of the
West African craton. 相似文献
965.
Sedimentology,clay mineralogy and grain-size as indicators of 65 ka of climate change from El’gygytgyn Crater Lake,Northeastern Siberia 总被引:2,自引:0,他引:2
Celeste A. Asikainen Pierre Francus Julie Brigham-Grette 《Journal of Paleolimnology》2007,37(1):105-122
El’gygytgyn Crater Lake, NE Siberia was investigated for sedimentological proxies for regional climate change with a focus
on the past 65 ka. Sedimentological parameters assessed relative to magnetic susceptibility include stratigraphy, grain size,
clay mineralogy and crystallinity. Earlier work suggests that intervals of high susceptibility in these sediments are coincident
with warmer (interglacial-like) conditions and well-mixed oxygenated bottom waters. In contrast, low susceptibility intervals
correlate with cold (glacial-like) conditions when perennial ice-cover resulted in anoxia and the dissolution of magnetic
carrier minerals.
The core stratigraphy contains both well-laminated to non-laminated sequences. Reduced oxygen and lack of water column mixing
preserved laminated sequences in the core. A bioturbation index based upon these laminated and non- laminated sequences co-varies
with total organic carbon (TOC) and magnetic susceptibility.
Clay mineral assemblages include illite, highly inter-stratified illite/smectite, and chlorite. Under warm or hydrolyzing
conditions on the landscape around the lake, chlorite weathers easily and illite/smectite abundance increase, which produces
an inverse relationship in the relative abundance of these clays. Trends in relative abundance show distinct down-core changes
that correlate with shifts in susceptibility. The mean grain-size (6.92 μm) is in the silt-size fraction, with few grains
larger than 65 μm. Terrigenous input to the lake comes from over 50 streams that are filtered through storm berms, which limits
clastic deposition into the lake system. The sedimentation rate and terrigenous input grain-size is reduced during glacial
intervals. Measurements of particle-size distribution indicate that the magnetic susceptibility fluctuations are not related
to grain size.
Lake El’gygytgyn’s magnetic susceptibility and clay mineralogy preserves regional shifts in climate including many globally
recognized␣events like the Younger Dryas and Bolling/Allerod. The sedimentary deposits reflect the climatic transitions starting
with MIS4 through the Holocene transition. This work represents the first extensive sedimentological study of limnic sediment
proxies of this age from Chukotka (Fig. 1).
This is the tenth in a series of eleven papers published in this special issue dedicated to initial studies of El'gygytgyn Crater Lake and
its catchment in NE Russia. Julie Brigham-Grette, Martin Melles, Pavel Minyuk were guest editors of this special issue. 相似文献
966.
Gérard Gleizes Gérard Crevon Asfawossen Asrat Pierre Barbey 《International Journal of Earth Sciences》2006,95(6):1039-1052
The Hercynian Bordères-Louron pluton (20 km2) in the Central Pyrenees intrudes Devonian and Carboniferous metasediments. It shows a concentric zoning and consists of a significant proportion of (quartz) gabbros in its periphery, and of granodiorites, biotite monzogranites and biotite-muscovite monzogranites in its core. AMS study shows that the pluton corresponds to an elongated dome with a N100°E-trending axis. The anisotropy intensity Ppara% is high in the south and in the core of the pluton, whereas it is low in the north. The shape parameter T indicates that the fabric is strongly planar in a large central band oriented NW-SE, whereas it is strongly linear in the western and eastern tips of the pluton. These characteristics suggest that the Bordères-Louron pluton emplaced in two episodes: (1) intrusion of mafic magmas along a N100°E sill parallel to the regional foliation of the host metasediments; and (2) injection of three successive silicic batches (granodiorite, biotite monzogranite, two-micas monzogranite) which pushed aside the early mafic injections. In situ U-Pb dating of zircon grains indicates that the emplacement age of the biotite monzogranite is 309 ± 4 Ma, synchronous with the D
2 dextral transpressive event and close to the ages of the eastern Pyrenean plutons, may be slightly older. 相似文献
967.
The present paper provides C- and N-stable isotope characteristics, N-contents and N-aggregation states for alluvial diamonds of known paragenesis from placers along the Namibian coast. The sample set includes diamonds with typical peridotitic and eclogitic inclusions and the recently reported “undetermined” suite of Leost et al. [Contrib. Mineral. Petrol. 145 (2003) 15] which resulted from infiltration of high temperature, carbonate-rich melts. δ13C-values range from −20.3‰ to −0.5‰ (n=48) for peridotitic diamonds and from −38.5‰ to −1.6‰ (n=45) for eclogitic diamonds. Diamonds belonging to the “undetermined” suite span a narrower range in δ13C from −8.5‰ to −2.7‰ (n=13). When compared with previous studies, diamonds from Namibia are characterised by unusually low proportions of N-free (i.e. Type II) peridotitic and eclogitic diamonds (3% and 2%, respectively) and an unprecedented high proportion of N-rich diamonds (15% and 73%, respectively, have N-contents >600 ppm). δ15N-values for diamonds of the peridotitic, eclogitic and “undetermined” suites range from −10‰ to +13‰ without correlations with either N-content or δ13C. The similarity in N-isotopic composition and the N-rich character of diamonds belonging to the eclogitic, peridotitic and “undetermined” suites is striking and suggests a close genetic relationship. We propose that a large part of the diamonds mined in Namibia formed during metasomatic events of similar style that introduced carbon and nitrogen into a range of different host lithologies. 相似文献
968.
Viara Minkovska Bernard Peybernès Pierre Cugny Todor Nikolov 《Comptes Rendus Geoscience》2004,336(10):931-938
Within the Emerici zone–Barremense zone biostratigraphic interval, the Barremian deposits of Central Fore-Balkan (Lovech–Veliko Tarnovo shelf) consist of a succession of several formations where alternate terrigenous argillaceous/sandy-dominated facies (Kormjansko Fm., Balgarene Fm.) and carbonate-dominated (‘Urgonian’) facies (Krushevo Fm., Emen Fm.). The qualitative and, particularly, quantitative facies analysis of the carbonate successions observed along 13 detailed cross-sections and in one drill hole show the stacking of about 40 fifth-order T–R cycles induced by numerous eustatic jerks contributing to the progressive settlement of this shelf. These high-frequency cycles of about 100 000 years must be regarded as valuable correlation tools for the subsurface hydrocarbon research. To cite this article: V. Minkovska et al., C. R. Geoscience 336 (2004). 相似文献
969.
Driss Khattach Pierre Keating El Mostafa Mili Touria Chennouf Pierre Andrieux Abdellah Milhi 《Comptes Rendus Geoscience》2004,336(16):1427-1432
Various techniques (horizontal and vertical derivatives, upward continuation, Euler deconvolution) have been applied to the gravity data from the Triffa's plain and the north flank of the Beni-Snassen massif to delineate various major geological structures such as faults and basins. These results allow the production of a structural map showing the fault systems for the survey area. This map forms the basis for planning future hydrogeological research in this region. To cite this article: D. Khattach et al., C. R. Geoscience 336 (2004). 相似文献
970.
Pierre Routhier 《Mineralium Deposita》1969,4(2):213-218
Des publications récentes conduisent à insister sur trois démarches, ou trois principes fondamentaux sans lesquels la Métallogénie (comme la Pétrologie) et la recherche minérale ne peuvent progresser. 1. Sortons des «types» connus et posons en principe que tout élément chimique peut se concentrer à diverses étapes du cycle géologique et géochimique; recherchons donc ces concentrations et traquons les par tous les moyens minéralogiques et géochimiques à notre disposition. 2. Récusons la confusion des concepts chronologiques et des concepts spatiaux, et en particulier la superposition totalement erronée d'«épigénétique» et d'«étranger». 3. Tout est hérité d'un passé plus ou moins lointain; principe qui rejoint le premier et nous conduira, dans certains cas favorables, vers des concentrations antérieures.
Recent publications lead to three steps without which metallogenic and thus petrologic investigations are impossible: 1. We should depart from known types in ore genesis and assume that any element can be enriched during various stages of the geological and geochemical cicle. These enrichments should thus be investigated with all possible methods of mineralogy and geochemistry. 2. We should reject the confusion of terms referring to time and space of ore genesis, especially the completely confused superposition of the terms epigenetic and introduced from the outside. 3. All deposits show signs of inheritance; this leads us back to point one which involves economic accumulations under certain favorable conditions.
Neuere Veröffentlichungen zeigen die Notwendigkeit, drei Umstände anzuerkennen, ohne die eine petrologische Forschung nicht mehr möglich ist: 1. Wir müssen davon Abstand nehmen, von den bekannten Lagerstätten-Typen zu reden. Richtig ist vielmehr, daß jedes chemische Element in den verschiedenen Etappen des geologischen und geochemischen Geschehens angereichert werden kann; forschen wir also nach diesen Anreicherungen und verfolgen wir sie mit allen mineralogischen und geochemischen Mitteln, die uns zur Verfügung stehen. 2. Wir müssen die Verwirrung der chronologischen und räumlichen Vorstellungen vermeiden, vor allem die vollkommen falsche Überlagerung von epigenetisch und fremd. 3. Alles was wir beobachten, ist das Erbe einer mehr oder weniger fernen Vergangenheit. Nur wenn wir uns ständig über dieses Faktum klar sind, dann können wir unter günstigen Umständen zu frühen Anreicherungen hingeführt werden.相似文献