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31.
Jan?Pa?avaEmail author Ivan?Vav?ín Ji?í?Fryda Vojtěch?Janou?ek Emil?Jelínek 《Mineralium Deposita》2003,38(3):298-311
The Ransko gabbro-peridotite massif in Eastern Bohemia is a strongly differentiated intrusive complex of Lower Cambrian age. The complex hosts low grade Ni-Cu ores mainly developed close to the contact of olivine-rich rocks with gabbros, in troctolites and, to a much lesser extent, in both pyroxene and olivine gabbros and plagioclase-rich peridotites. The ore zone is characterized by strong serpentinization and uralitization. The total Ni + Cu locally reaches up to 4 wt%. Anomalous concentrations of platinum-group elements (PGE's) (maximum 532 ppb Pd, 182 ppb Pt, 53 ppb Rh, 15 ppb Ru, 41 ppb Ir) were detected in samples of Cu-Ni and Ni-Cu ores (maximum 2.63 wt% Ni and 2.31 wt% Cu) from the Jezírka orebody. The main ore paragenesis includes pyrrhotite, pentlandite, chalcopyrite, cubanite, pyrite, magnetite, mackinawite, valleriite, ilmenite and sphalerite. During this work, michenerite, froodite, sperrylite, gold, native bismuth, altaite, tsumoite, hessite, an unnamed Bi-Ni telluride, cobaltite-gersdorffite and galena were newly identified. The host rocks originated through partial melting of a slightly depleted mantle source with noble metals scavenged from this primitive magma prior to the development of these rocks. 相似文献
32.
Shiveluch Volcano, located in the Central Kamchatka Depression, has experienced multiple flank failures during its lifetime,
most recently in 1964. The overlapping deposits of at least 13 large Holocene debris avalanches cover an area of approximately
200 km2 of the southern sector of the volcano. Deposits of two debris avalanches associated with flank extrusive domes are, in addition,
located on its western slope. The maximum travel distance of individual Holocene avalanches exceeds 20 km, and their volumes
reach ∼3 km3. The deposits of most avalanches typically have a hummocky surface, are poorly sorted and graded, and contain angular heterogeneous
rock fragments of various sizes surrounded by coarse to fine matrix. The deposits differ in color, indicating different sources
on the edifice. Tephrochronological and radiocarbon dating of the avalanches shows that the first large Holocene avalanches
were emplaced approximately 4530–4350 BC. From ∼2490 BC at least 13 avalanches occurred after intervals of 30–900 years. Six
large avalanches were emplaced between 120 and 970 AD, with recurrence intervals of 30–340 years. All the debris avalanches
were followed by eruptions that produced various types of pyroclastic deposits. Features of some surge deposits suggest that
they might have originated as a result of directed blasts triggered by rockslides. Most avalanche deposits are composed of
fresh andesitic rocks of extrusive domes, so the avalanches might have resulted from the high magma supply rate and the repetitive
formation of the domes. No trace of the 1854 summit failure mentioned in historical records has been found beyond 8 km from
the crater; perhaps witnesses exaggerated or misinterpreted the events.
Received: 18 August 1997 / Accepted: 19 December 1997 相似文献
33.
A numerical investigation was made of the relationships between fracture initiation, growth, stress field and boundary conditions. Two-dimensional plane strain continuum models were used in which fractures appeared as zones of strain localization developed through application of a strain softening Mohr–Coulomb constitutive model. R and R′ fractures developed first, followed by Y fractures at larger strains. The models showed that equal development of conjugate R and R′ fractures is easily changed to favor one or the other set by minor variations in model initial conditions. Strength loss in fractures caused stress field rotations in regions bounded by fractures, altering the orientation of subsequent fractures. The amount and sense of stress field rotation is dependent on the strength loss during displacement on the fractures, the orientation of fractures, and on the boundary conditions. Y oriented fractures could be explained on the basis of a Mohr–Coulomb failure criterion provided that stress field rotation is accounted for. Monitoring of fracture slip activity showed that, under conditions of constant boundary velocity, slip was discontinuous in time, alternating on fractures throughout the model. 相似文献
34.
Jennifer Hauxwell James McClelland Peter J. Behr Ivan Valiela 《Estuaries and Coasts》1998,21(2):347-360
Macroalgal biomass and competitive interactions among primary producers in coastal ecosystems may be controlled by bottom-up processes such as nutrient supply and top-down processes such as grazing, as well as other environmental factors. To determine the relative importance of bottom-up and top-down processes under different nutrient loading conditions, we estimated potential amphipod and isopod grazer impact on a dominant macroalgal species in three estuaries in Waquoit Bay, Cape Cod, Massachusetts, that are subject to different nitrogen loading rates. We calculated growth increases and grazing losses in each estuary based on monthly benthic survey data of macrophyte biomass and herbivore abundance, field grazing rates of amphipods (Microdeutopus gryllotalpa andCymadusa compta) and an isopod (Idotea baltica) on the preferred and most abundant macroalga (Cladophora vagabunda) and laboratory grazing rates for the remaining species, and in situ macroalgal growth rates. As nitrogen loading rates increased, macroalgal biomass increased (3×), eelgrass (Zostera marina) was lost, and herbivore abundance decreased (1/4×). Grazing rates increased with relative size of grazer (I. baltica > C. compta > M. gryllotalpa) and, for two of the three species investigated, were faster on algae from the high-nitrogen estuary in comparison to the low-nitrogen estuary, paralleting the increased macroalgal tissue percent nitrogen with nitrogen load. Macroalgal growth rates increased (2×) with increasing nitrogen loading rate. The comparison between estimated growth increases versus losses ofC. vagabunda biomass to grazing suggested first, that grazers could lower macroalgal biomass in midsummer, but only in estuaries subject to lower nitrogen loads. Second, the impact of grazing decreased as nitrogen loading rate increased as a result of the increased macroalgal growth rates and biomass, plus the diminished abundance of grazers. This study suggests the relative impact of top-down and bottom-up controls on primary producers varies depending on rate of nitrogen loading, and specifically, that the impact of herbivory on macroalgal biomass decreases with increasing nitrogen load to estuaries. 相似文献
35.
36.
Dana Procházková Vít Kárník Ivan Brouček Reviewer A. Zátopek 《Studia Geophysica et Geodaetica》1980,24(1):102-103
an¶rt;u am mu 15- an 1979. a auu a mumuu auu. 相似文献
37.
mam u¶rt;m uu aumuu a m n uamu ¶rt;u nmu umu ¶rt;ua a maum ¶rt;a amu u. a um naa nu nu naama umuu , au a um aumu m n aau umuu mau aum n. aam, m mum au ¶rt;-amu a ma mu aum u u n a aumu m n. 相似文献
38.
Zusammenfassung Die Arbeit beschreibt die stratigraphische Einteilung der Paläogen-Serie des Magura-Flysches in Ost-Mähren und der West-Slowakei. Die lithologischen Änderungen der stratigraphischen Glieder des Magura-Flysches werden charakterisiert. Auf Grund der qualitativen und quantitativen Analysen wurden die lithofaziellen Zonen in der unteren und oberen Abteilung des Paläogens festgestellt (V. Pesl).Die sedimentologische Untersuchung befaßte sich mit den Texturen der Sandsteine, mit Messungen von Strömungsmarken, mit Schwermineral-Assoziationen, um die Entstehung der Flyschsedimente dieses Gebietes zu erklären (I. Krystek).
The paper gives survey and the results of the lithological and facial division and of the sedimentological studies in the paleogene of the Magura flysch group in Eastern Moravia and Western Slovakia.
Résumé Cet essai donne les resultats de la division stratigraphique et des recherches sédimentaires au paléogène du groupe Magura flysch à l'Est de la Moravie et à l'ouest de la Slovaquie.
- .相似文献
39.
Very Long Baseline Interferometry (VLBI) is one of the new techniques which will probably dominate geodesy and geophysics
in the near future. Its main advantage lies in the fact that it brings the accuracy of direction measurements to a level previously
possible only for range measurements. This closes the gap between powerful range determination techniques such as laser ranging
and the much less accurate determination of directions through photographic tracking of artificial earth satellites.
The technique is geometric in the sense that the relevant observations are independent of the gravity field of the earth.
However, the “orbits” of the observed extragalactic radio sources with respect to an earth-fixed system are dominated and
perturbed by the rotation of the earth with respect to inertial frame. This allows the determination of polar motion, precession-nutation
and length-of-the-day variations, and the technique becomes also “dynamic” in this respect.
The capability of determining the geometry of a network of stations within a short time interval and with a centimeter level
accuracy also allows the study of the variation of network geometry with time caused by earth tides and other periodic or
secular station drifts.
The main objective of the present work is the exploration of the capabilities of VLBI for the recovery of earth rotation and
network geometry parameters. For this purpose, a number of characteristic experimental designs based on present and candidate
for the near future station locations is chosen. The results from the analysis of simulated observations for each particular
design are presented in the paper.
Presented at IAG International Symposium on “Optimization of Design and Computation of Control Networks”, Sopron, Hungary,
4–10 July 1977. 相似文献
40.
The spaceborne laser ranging (or lasering) system provides a method of precise positioning of a large number of points on
the earth's surface in a short period of time. That is, a measure of the relative location of geodetic markers from a space
platform can maintain horizontal and vertical control to 2 to 5 cm. At this level of control, small earth surface crustal
motions should be detectable. Development of a model for the strain field can be constructed. Furthermore, the spaceborne
lasering system can survey an area in a very short period of time (one to two weeks) and resurvey the area as required.
System design parameters are now being established by NASA for a possible test flight aboard the Shuttle in 1982. These include
design specifications of economical corner cubes for ground retroreflectors coupled with the evolution of engineering model
to flight model development. If the experiment of the Shuttle proves to be successful, it is hoped to put the laser in a free
flight satellite. This paper presents the results of a simulated analysis for this latter case.
The system is conceived as an orbiting ranging device with a ground base grid of reflectors or transponders (spacing 10 to
30 km), which are projected to be of low cost (maintenance-free and unattended) and which will permit the saturation of a
local area to obtain data useful to monitor crustal movements. The test network includes 75 stations with roughly half of
them on either side of the San Andreas fault zone. Critical study comparatively evaluates various observational schemes and
statistically analyzed crustal motion recovery.
The study considers laser radar as the main ranging system pending final selection from many possible candidates. The satellite
orbit is inclined at 110° and slightly eccentric (e=0.04) with orbital altitudes varying from 370 km to 930 km.
The results indicate that the geometric mode (simultaneous ranging) with a minimum of five grid and three distant (fundamental)
stations and mixed ranging to satellite and airplane seems to be most promising. The fundamental stations are distinguished
from the grid station in their location and this location should be “distant” enough from the area of crustal movement so
that they can be considered stationary over the time span of the motion involved.
Presented at the 1977 I.A.G. International Symposia on Satellite Geodesy, Budapest, Hungary, June 28–July 1, and on Recent
Crustal Movements, Palo Alto, California, USA, July 25–30. 相似文献