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91.
To elucidate the age and origin of seamounts in the eastern North Atlantic, 54 titanite and 10 zircon fractions were dated by the U-Pb chronometer, and initial Pb, Sr, and Hf isotope ratios were measured in feldspars and zircon, respectively. Rocks analyzed are essentially trachy-andesites and trachytes dredged during the “Tore Madeira” cruise of the Atalante in 2001. The ages reveal different pulses of alkaline magmatism occurring at 104.4 ± 1.4 (2σ) Ma and 102.8 ± 0.7 Ma on the Sponge Bob seamount, at 96.3 ± 1.0 Ma on Ashton seamount, at 92.3 ± 3.8 Ma on the Gago Coutinho seamount, at 89.3 ± 2.3 Ma and 86.5 ± 3.4 Ma on the Jo Sister volcanic complex, and at 88.3 ± 3.3 Ma, 88.2 ± 3.9, and 80.5 ± 0.9 Ma on the Tore locality. No space-time correlation is observed for alkaline volcanism in the northern section of the Tore-Madeira Rise, which occurred 20-30 m.y. after opening of the eastern North Atlantic. Initial isotope signatures are: 19.139-19.620 for 206Pb/204Pb, 15.544-15.828 for 207Pb/204Pb, 38.750-39.936 for 208Pb/204Pb, 0.70231-0.70340 for 87Sr/86Sr, and +6.9 to +12.9 for initial epsilon Hf. These signatures are different from Atlantic MORB, the Madeira Archipelago and the Azores, but they lie in the field of worldwide OIB. The Cretaceous seamounts therefore seem to be generated by melts from a OIB-type source that interact with continental lithospheric mantle lying formerly beneath Iberia and presently within the ocean-continent transition zone. Inheritance in zircon and high 207Pb of initial Pb substantiate the presence of very minor amounts of continental material in the lithospheric mantle. A long-lived thermal anomaly is the most plausible explanation for alkaline magmatism since 104 Ma and it could well be that the same anomaly is still the driving force for tertiary and quaternary alkaline magmatism in the eastern North Atlantic region. This hypothesis is agreement with the plate-tectonic position of the region since Cretaceous time, including an about 30° anti-clockwise rotation of Iberia. 相似文献
92.
Felix J. Herrmann Urs Böniger Dirk Jacob Verschuur 《Geophysical Journal International》2007,170(2):781-799
93.
Patrick Henkel Dimitrios Psychas Christoph Günther Urs Hugentobler 《Journal of Geodesy》2018,92(10):1199-1217
Precise point positioning with integer ambiguity resolution requires precise knowledge of satellite position, clock and phase bias corrections. In this paper, a method for the estimation of these parameters with a global network of reference stations is presented. The method processes uncombined and undifferenced measurements of an arbitrary number of frequencies such that the obtained satellite position, clock and bias corrections can be used for any type of differenced and/or combined measurements. We perform a clustering of reference stations. The clustering enables a common satellite visibility within each cluster and an efficient fixing of the double difference ambiguities within each cluster. Additionally, the double difference ambiguities between the reference stations of different clusters are fixed. We use an integer decorrelation for ambiguity fixing in dense global networks. The performance of the proposed method is analysed with both simulated Galileo measurements on E1 and E5a and real GPS measurements of the IGS network. We defined 16 clusters and obtained satellite position, clock and phase bias corrections with a precision of better than 2 cm. 相似文献
94.
95.
Allen K. Kennedy Jörn-Frederik Wotzlaw James L. Crowley Mark Schmitz Urs Schaltegger Benjamin Wade Laure Martin Cristina Talavera Bryant Ware Thi Hao Bui 《Geostandards and Geoanalytical Research》2023,47(2):373-402
Twelve apatite samples have been tested as secondary ion mass spectrometry (SIMS) reference materials. Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) analysis shows that the SLAP, NUAN and GR40 apatite gems are internally homogeneous, with most trace element mass fractions having 2 standard deviations (2s) ≤ 2.0%. BR2, BR5, OL2, AFG2 and AFB1, which have U > 63 μg g-1, 206Pb/204Pb > 283, and homogeneous SIMS U-Pb data, have respective isotope dilution thermal ionisation mass spectrometry (ID-TIMS) ages of 2053.83 ± 0.21 Ma, 2040.34 ± 0.09 Ma, 868.87 ± 0.25 Ma, 478.71 ± 0.22 Ma and 473.25 ± 0.09 Ma. Minor U-Pb heterogeneity exists and accurate SIMS results require correction with the 3D Concordia-constrained common Pb composition. Among the studied samples, AFG2 and BR5 are the most homogeneous U-Pb reference materials. The SIMS sulfur isotopic compositions of eight of the apatites shows they are homogeneous, with 2s for both 103δ34S and 103δ33S < 0.55‰. One apatite, BR96, has Δ33S = -0.36 ± 0.2‰. The apatite samples have ID-TIMS 87Sr/86Sr between 0.704214 ± 0.000030 and 0.723134 ± 0.000035. 相似文献
96.
Cheinway Hwang Tzu-Pang Tseng Ting-Jung Lin Dražen Švehla Urs Hugentobler Benjamin Fong Chao 《GPS Solutions》2010,14(1):121-131
The precise orbit determination antennas of F3/C and GRACE-A satellites are from the same manufacturer, but are installed
in different configurations. The current orbit accuracy of F3/C is 3 cm at arcs with good GPS data, compared to 1 cm of GRACE,
which has a larger ratio of usable GPS data. This paper compares the qualities of GPS observables from F3/C and GRACE. Using
selected satellites and time spans, the following average values for the satellite F3/C and satellite A of GRACE are obtained:
multipath effect on the pseudorange P1, 0.78 and 0.38 m; multipath effect on the pseudorange P2, 1.03 and 0.69 m; occurrence
frequency of cycle slip, 1/29 and 1/84; standard error of unit weight, 4 and 1 cm; dynamic–kinematic orbit difference, 10
and 2 cm. For gravity determination using F3/C GPS data, a careful selection of GPS data is critical. With six satellites
in orbit, F3/C’s large amount of GPS data will make up the deficiency in data quality. 相似文献
97.
Measurements of surface velocity, ice deformation (at 42 and 89% ice depth) and proglacial stream discharge were made at Haut Glacier d'Arolla, Switzerland, to determine diurnal patterns of variation in each. Data are analysed in order to understand better the relationship between hydraulically induced basal motion and glacier ice deformation over short timescales. The data suggest that hydraulically induced localized basal ‘slippery’ spots are created over diurnal cycles, causing enhanced basal motion and spatially variable glacier speed‐up. Our data indicate that daily glacier speed‐up is associated with reduced internal deformation over areas previously identified as slippery spots and increased deformation in areas located adjacent to or down‐glacier from slippery spots. We interpret this pattern in terms of a transfer of mechanical support for basal shear stress away from slippery spots to adjacent sticky areas, where the resulting stronger ice–bed coupling causes increased ice deformation near the bed. These patterns indicate that basal ice is subjected to stress regimes that are variable at a high spatial and temporal resolution. Such variations may be central to the creation of anomalous vertical velocity profiles measured above and down‐glacier of basal slippery zones, which have shown evidence for ‘plug flow’ and extrusion flow over annual timescales. Copyright © 2007 John Wiley & Sons, Ltd. 相似文献
98.
Post-granulite facies monazite growth and rejuvenation during Permian to Lower Jurassic thermal and fluid events in the Ivrea Zone (Southern Alps) 总被引:11,自引:2,他引:9
U-Pb analyses of single monazite grains from two granulite facies metapelites in the Ivrea Zone (Southern Alps) reveal the
presence, in both samples, of at least three different ages and prove that earlier interpretations of supposedly concordant
monazite data as cooling ages are unwarranted. One group of monazite data defines a subconcordant discordia line with an upper
intercept age of 293.4 ± 5.8 Ma and a lower intercept age of 210 ± 14 Ma. The upper intercept is interpreted as the real cooling
age of the monazites. The lower intercept is interpreted as an episode of fluid-driven Pb-loss, indicated by the presence
of internal and external corrosion structures not only of the monazites but also of the zircons in the same samples that are
also rejuvenated at 210 ± 12 Ma. Another group of monazite data lies above the concordia. The presence of excess 206Pb indicates that these crystals have grown below the monazite blocking temperature, thus after the granulite facies metamorphism.
The age of growth of the new monazite crystals is approached by their 207Pb/235U ages that range between 273 and 244 Ma. The two groups of post-cooling age (post-293.4 ± 5.8 Ma) monazite data correspond
to two distinct late- and post-Variscan geotectonic regimes that affected the Southern Alps, (1) Permian transtension with
decompression and anatectic melting; (2) Upper Triassic to Lower Jurassic rifting with geographically dispersed hydrothermal
activity and alkaline magmatism.
Received: 7 July 1998 / Accepted: 4 November 1998 相似文献
99.
Indigenous knowledge related to climate variability and change: insights from droughts in semi-arid areas of former Makueni District,Kenya 总被引:1,自引:1,他引:0
Chinwe Ifejika Speranza Boniface Kiteme Peter Ambenje Urs Wiesmann Samuel Makali 《Climatic change》2010,100(2):295-315
This article describes the indigenous knowledge (IK) that agro-pastoralists in larger Makueni District, Kenya hold and how
they use it to monitor, mitigate and adapt to drought. It examines ways of integrating IK into formal monitoring, how to enhance
its value and acceptability. Data was collected through target interviews, group discussions and questionnaires covering 127
households in eight villages. Daily rainfall data from 1961–2003 were analysed. Results show that agro-pastoralists hold IK
on indicators of rainfall variability; they believe in IK efficacy and they rely on them. Because agro-pastoralists consult
additional sources, the authors interpret that IK forms a basic knowledge frame within which agro-pastoralists position and
interpret meteorological forecasts. Only a few agro-pastoralists adapt their practices in anticipation of IK-based forecasts
partly due to the conditioning of the actors to the high rainfall variability characteristic of the area and partly due to
lack of resources. Non-drought factors such as poverty, inadequate resources and lack of preparedness expose agro-pastoralists
to drought impacts and limit their adaptive capacity. These factors need to be understood and effectively addressed to increase
agro-pastoralists’ decision options and the influence of IK-based forecasts on their decision-making patterns. The limited
intergenerational transfer of IK currently threatens its existence in the longer term. One way to ensure its continued existence
and use is to integrate IK into the education curriculum and to link IK with formal climate change research through the participation
of the local people. However, further studies are necessary to address the reliability and validity of the identified IK indicators
of climate variability and change. 相似文献
100.
Elemental characterization and source identification of PM2.5 using Positive Matrix Factorization: The Malraux road tunnel, Nice, France 总被引:1,自引:0,他引:1
Jean-Franois Fabretti Nathalie Sauret Jean-Franois Gal Pierre-Charles Maria Urs Schrer 《Atmospheric Research》2009,94(2):320-329
This work was dedicated to the characterization of the elemental composition of atmospheric particles associated with urban traffic. PM2.5 sampling campaigns were conducted in an urban road tunnel within the area of Nice: the Malraux tunnel. Samples were analyzed by ICP-MS-DRC for the determination of 16 metallic trace elements: Na, Ti, V, Mn, Fe, Co, Ni, Cu, Zn, As, Sr, Rb, Cd, Sn, Sb and Pb. An in depth study was carried out on the PM2.5 dataset collected with specific characterisation tools (enrichment factors, elemental ratio) and a receptor model: the Positive Matrix Factorization (PMF). The model showed that different profiles of emission could influence the ambient air of the tunnel and their contribution to the metal concentration in the air. Elemental fingerprints were used to attribute to each profile a specific emission source linked to road traffic. Finally, results showed that road traffic emissions could be divided into 3 main “subcategory” sources of emission: “soil resuspension” by automobile movement, “vehicular wear” and “fuel combustion”. 相似文献