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531.
Mu Ramkumar Doris Stüben Zsolt Berner Jens Schneider 《Journal of Earth System Science》2010,119(1):1-17
The Ariyalur-Pondicherry sub-basin of the Cauvery basin comprises a near complete stratigraphic record of Upper Cretaceous-Lower
Tertiary periods. Earlier studies have documented variations of clay mineral assemblages, change in microtexture of siliciclasts
and many geochemical and stable isotopic anomalies far below the Cretaceous-Tertiary boundary (KTB) in these strata. This
paper documents the occurrences of two positive 87Sr/86Sr anomalies preceding K-T boundary in this basin and discusses plausible causes. Analysis of trace elemental and stable isotopic
profiles, sedimentation history, petrography and mineralogy of the rocks reveal that while both the anomalies may be due to
increased detrital influx caused by sea level and climatic changes, the second anomaly might have been influenced by Deccan
volcanism which in turn predated KTB. Record of such anomalies preceding K-T boundary supports the view of multi-causal step-wise
extinction of biota across KTB. 相似文献
532.
Vincent Sastre Jean-Luc Loizeau Jens Greinert Lieven Naudts Philippe Arpagaus Flavio Anselmetti Walter Wildi 《Swiss Journal of Geoscience》2010,103(1):33-42
The current topographic maps of the Rhone Delta—and of Lake Geneva in general—are mainly based on hydrographic data that were
acquired during the time of F.-A. Forel at the end of the nineteenth century. In this paper we present results of a new bathymetric
survey, based on single- and multi-beam echosounder data. The new data, presented as a digital terrain model, show a well-structured
lake bottom morphology, reflecting depositional and erosional processes that shape the lake floor. As a major geomorphologic
element, the sub-aquatic Rhone Delta extends from the coastal platform to the depositional fans of the central plain of the
lake at 310 m depth. 9 canyons cut the platform edge of the delta. These are sinuous (“meandering”) channels formed by erosional
and depositional processes, as indicated by the steep erosional canyon walls and the depositional levees on the canyon shoulders.
Ripples or dune-like morphologies wrinkle the canyon bottoms and some slope areas. Subaquatic mass movements are apparently
missing on the delta and are of minor importance on the lateral lake slopes. Morphologies of the underlying bedrock and small
local river deltas are located along the lateral slopes of Lake Geneva. Based on historical maps, the recent history of the
Rhone River connection to the sub-aquatic delta and the canyons is reconstructed. The transition from three to two river branches
dates to 1830–1840, when the river branch to the Le Bouveret lake bay was cut. The transition from two to one river branch
corresponds to the achievement of the correction and dam construction work on the modern Rhone River channel between 1870
and 1880. 相似文献
533.
Beyene G.Haile Urszula Czarniecka Kelai Xi Aleksandra Smyrak-Sikora Jens Jahren Alvar Braathen Helge Hellevang 《地学前缘(英文版)》2019,10(2):629-649
Sedimentary basins containing igneous intrusions within sedimentary reservoir units represent an important risk in petroleum exploration. The Upper Triassic to Lower Jurassic sediments at Wilhelm?ya(Svalbard) contains reservoir heterogeneity as a result of sill emplacement and represent a unique case study to better understand the effect of magmatic intrusions on the general burial diagenesis of siliciclastic sediments. Sills develop contact metamorphic aureoles by conduction as presented in many earlier studies. However, there is significant impact of localized hydrothermal circulation systems affecting reservoir sediments at considerable distance from the sill intrusions. Dolerite sill intrusions in the studied area are of limited vertical extent(~12 m thick), but created localized hydrothermal convection cells affecting sediments at considerable distance(more than five times the thickness of the sill)from the intrusions. We present evidence that the sedimentary sequence can be divided into two units:(1) the bulk poorly lithified sediment with a maximum burial temperature much lower than 60-70 ℃,and(2) thinner intervals outside the contact zone that have experienced hydrothermal temperatures(around 140 ℃). The main diagenetic alteration associated with normal burial diagenesis is minor mechanical plastic deformation of ductile grains such as mica. Mineral grain contacts show no evidence of pressure dissolution and the vitrinite reflectance suggests a maximum temperature of ~40 ℃. Contrary to this, part of the sediment, preferentially along calcite cemented flow baffles, show evidence of hydrothermal alteration. These hydrothermally altered sediment sections are characterized by recrystallized carbonate cemented intervals. Further, the hydrothermal solutions have resulted in localized sericitization(illitization) of feldspars, albitization of both K-feldspar and plagioclase and the formation of fibrous illite nucleated on kaolinite. These observations suggest hydrothermal alteration at T 120-140 ℃ at distances considerably further away than expected from sill heat dissipation by conduction only, which commonly affect sediments about twice the thickness of the sill intrusion. We propose that carbonate-cemented sections acted as flow baffles already during the hydrothermal fluid mobility and controlled the migration pathways of the buoyant hot fluids. Significant hydrothermally induced diagenetic alterations affecting the porosity and hence reservoir quality was not noted in the noncarbonate-cemented reservoir intervals. 相似文献
534.
Different methods to determine the height of the convective boundary layer from lidar measurements are described and compared.
The differences in either aerosol backscatter or in humidity between the boundary layer and the free troposphere are used,
and either the variance or the gradient profile of the parameter under study is evaluated. On average the different methods
are in very good agreement. Temporal resolution of the gradient methods is very high, on the order of seconds, but often there
is an ambiguity in the choice of the “relevant” minimum in the gradient that corresponds to the boundary-layer height. This
is avoided by combining the variance and the gradient methods, using the result of the variance analysis as an indicator for
the region where the minimum of the gradient is sought. The combined method is useful for automated determination of the boundary-layer
height at least under convective conditions. Aerosol backscatter is found to be as good an indicator for boundary-layer air
as humidity, so a relatively simple backscatter lidar is sufficient for determination of the boundary-layer height. 相似文献
535.
The mineralogy of the managanese ores of the giant Paleoproterozoic Kalahari manganese deposit of the Transvaal Supergroup has been subject of many studies and up to now 135 different ore and gangue minerals have been described, of which eight represent new mineral species. Through correlation of different mineral assemblages with specific geological events and by determining relative ages of minerals in outcrop, hand specimen and under the microscope an attempt is made in this publication to construct a paragenetic sequence for this complex mineral assemblage. Sedimentation and early diagenesis of the Hotazel Formation, composed of interbedded iron formation and braunite lutite, was followed by low-grade metamorphism and associated stratabound metasomatism. Braunite lutite of sedimentary origin, comprising 97% of the total ore reserve, is composed of braunite, hematite and kutnahorite, and abundant early diagenetic kutnahorite and manganoan calcite forming laminae and ovoids. Fluid flow during late diagenesis or lower greenschist facies metamorphism led to stratabound metasomatic oxidation of Mn-bearing carbonates to hausmannite and Mn-poor calcite. Three structurally controlled hydrothermal alteration events succeed metamorphism. These events are referred to as Wessels, Mamatwan and Smartt events. The Wessels alteration event is the oldest of the three events and it is of great economic importance because virtually all of the high-grade ore (> 42% Mn), 3% of the total ore reserve, formed during this event through alteration of carbonate-rich low-grade Mamatwan-type ore (braunite lutite) to high-grade, carbonate-poor Wessels-type manganese ore. This Wessels hydrothermal alteration event took place in the northwestern part of the Kalahari deposit, associated with a system of major north-south- and minor east-west-striking normal faults. The Mamatwan alteration event is observed throughout the Kalahari manganese deposit but alteration is very localized. Reduction halos and discolouration of braunite lutite around fracture or joint-hosted sulphide-carbonate mineralization are typical of the Mamatwan alteration event. In contrast, the Smartt alteration event is characterized by oxidation of braunite lutite and the formation of todorokite and manganomelane. Syn- and Post-Kalahari supergene alteration has taken place below the suboutcrop of the Hotazel Formation against the calcretized sediments of the Cenozoic Kalahari Formation. Cryptomelane and pyrolusite are the predominant products of surficial weathering. 相似文献