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51.
52.
Amygdaloidal tholeiitic flows, locally pillowed, have been mapped at intervals over a distance of 280 km along the margins of the North Savannas Rift Valley in southern Guyana and Brazil. The combined chemical and petrological evidence suggests that the formation represents an originally uniform continental tholeiitic sequence that has undergone variable secondary alteration involving increasing oxidation and ingress of soda, potash and water. The lavas are both spatially and temporally related to the rift faulting; field observations and K∶Ar age determinations on eleven samples from different localities indicate that the flows were erupted during the Early to Middle Jurassic (180-150 m. y.). It is shown that this tectono-magmatic event is synchronous with the initiation of the southern North Atlantic rift and the pan-Antarctic rift which resulted in fragmentation of the continents according to the scheme ofDietz &Holden (1970). 相似文献
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55.
We make a detailed analysis of cross-correlation and time-lag between monthly data of galactic cosmic rays (GCRs) intensity and different solar activity indices (e.g., sunspot number, sunspot area, green coronal Fe line and 10.7 cm solar radio flux) during 19–23 solar cycles. GCRs time-series data from Kiel neutron monitor station and solar data from the last 50 years period, covering five solar cycles (19–23), and alternating solar polarity states (i.e., five A < 0 and four A > 0) have been investigated. We find a clear asymmetry in the cross-correlation between GCRs and solar activity indicators for both odd and even-numbered solar cycles. The time-lags between GCRs and solar parameters are found different in different solar cycles as well as in the opposite polarity states (A < 0 and A > 0) within the same solar cycle. Possible explanations of the observed results are discussed in light of modulation models, including drift effects. 相似文献
56.
Evaluation of the intermediate-term periodicities in solar and cosmic ray activities during cycle 23
In this paper we investigate the presence and temporal evolution of galactic cosmic rays (GCRs) time-series and three solar
parameters, namely the daily sunspot number, the coronal green line and the 10.7 cm solar radio flux over the period 1996–2003
by the wavelet technique. A number of short- and intermediate-term quasi-periodicities were also detected in both GCR and
solar parameters. For the short-term range, we have identified quasi-periods of 16–30 days, 40–55 days, 60–70 days, 80–90 days
and 80–100 days. In the case of intermediate range, the significant periods were 120–140 days, 150–170 days, 190–210 days,
240–260 days, ≈1.09 yr. and ≈1.23 yr. The wavelet power spectra show that all the above-mentioned periods are intermittent
in nature and occurred in different time-series in different intervals. The result exhibits that the well-known “Rieger period”
of (150–160 days) was prominent in both GCR and solar data sets during the ascending phase of cycle 23. Possible reasons behind
the observed periods were discussed with the help of previous results and existing numerical models. 相似文献
57.
David Thomas Murphy Alan D. Brandon Ray Burgess 《Geochimica et cosmochimica acta》2010,74(2):738-4475
Here we search for evidence of the existence of a sub-chondritic 142Nd/144Nd reservoir that balances the Nd isotope chemistry of the Earth relative to chondrites. If present, it may reside in the source region of deeply sourced mantle plume material. We suggest that lavas from Hawai’i with coupled elevations in 186Os/188Os and 187Os/188Os, from Iceland that represent mixing of upper mantle and lower mantle components, and from Gough with sub-chondritic 143Nd/144Nd and high 207Pb/206Pb, are favorable samples that could reflect mantle sources that have interacted with an Early-Enriched Reservoir (EER) with sub-chondritic 142Nd/144Nd.High-precision Nd isotope analyses of basalts from Hawai’i, Iceland and Gough demonstrate no discernable 142Nd/144Nd deviation from terrestrial standards. These data are consistent with previous high-precision Nd isotope analysis of recent mantle-derived samples and demonstrate that no mantle-derived material to date provides evidence for the existence of an EER in the mantle.We then evaluate mass balance in the Earth with respect to both 142Nd/144Nd and 143Nd/144Nd. The Nd isotope systematics of EERs are modeled for different sizes and timing of formation relative to ε143Nd estimates of the reservoirs in the μ142Nd = 0 Earth, where μ142Nd is ((measured 142Nd/144Nd/terrestrial standard 142Nd/144Nd)−1 * 10−6) and the μ142Nd = 0 Earth is the proportion of the silicate Earth with 142Nd/144Nd indistinguishable from the terrestrial standard. The models indicate that it is not possible to balance the Earth with respect to both 142Nd/144Nd and 143Nd/144Nd unless the μ142Nd = 0 Earth has a ε143Nd within error of the present-day Depleted Mid-ocean ridge basalt Mantle source (DMM). The 4567 Myr age 142Nd-143Nd isochron for the Earth intersects μ142Nd = 0 at ε143Nd of +8 ± 2 providing a minimum ε143Nd for the μ142Nd = 0 Earth. The high ε143Nd of the μ142Nd = 0 Earth is confirmed by the Nd isotope systematics of Archean mantle-derived rocks that consistently have positive ε143Nd.If the EER formed early after solar system formation (0-70 Ma) continental crust and DMM can be complementary reservoirs with respect to Nd isotopes, with no requirement for significant additional reservoirs. If the EER formed after 70 Ma then the μ142Nd = 0 Earth must have a bulk ε143Nd more radiogenic than DMM and additional high ε143Nd material is required to balance the Nd isotope systematics of the Earth. 相似文献
58.
Abhishek Saha Avik Dhang Jyotisankar Ray Suvankar Chakraborty David Moecher 《Journal of Earth System Science》2010,119(3):365-381
Field studies supplemented by petrographic analyses clearly reveal complete preservation of ophiolite suite from Port Blair
(11°39′N: 92°45′E) to Chiriyatapu (11°30′24″N: 92°42′30″E) stretch of South Andaman. The ophiolite suite reveals serpentinite
at the base which is overlain unconformably by cumulate ultramafic-mafic members with discernible cumulus texture and igneous
layering. Basaltic dykes are found to cut across the cumulate ultramafic-mafic members. The succession is capped by well exposed
pillow basalts interlayered with arkosic sediments. Olivine from the basal serpentinite unit are highly magnesian (Fo80.1–86.2). All clinopyroxene analyses from cumulate pyroxenite, cumulate gabbro and basaltic dyke are discriminated to be ‘Quad’ and
are uniformly restricted to the diopside field. Composition of plagioclase in different lithomembers is systematically varying
from calcic to sodic endmembers progressively from cumulate pyroxenite to pillow basalt through cumulate gabbro and basaltic
dyke. Plagioclase phenocrysts from basaltic dyke are found to be distinctly zoned (An60.7-An35.3) whereas groundmass plagioclase are relatively sodic (An33-An23.5). Deduced thermobarometric data from different lithomembers clearly correspond to the observed preservation of complete ophiolite
suite. 相似文献
59.
David Ray Abernathy 《Transactions in GIS》2023,27(5):1467-1478
Open-source software and open data are becoming increasingly popular in the teaching and learning of geographic information science. The cost savings that are derived from using free software over proprietary software are one driving factor, yet the move from “closed” to “open” represents much more than financial austerity—it signifies a broader shift in educational philosophy. This article documents the gradual transition of an introductory undergraduate course in geographic information systems from an entirely closed course to one that has become increasingly open. Having completely adopted the first three layers of open—software, data, and educational resources—the course is now turning toward the next layer: embracing the philosophy of open pedagogy. 相似文献
60.
A dealiasing algorithm for radar radial velocity observed by C-band Doppler radars is presented as an extension of an existing S-band dealiasing algorithm. This has operational significance in that many portable and many commercial broadcast radars, as well as approximately one half of the Chinese weather radar network (CINRAD), are C-band radars. With a wavelength of about 5 cm, the Nyquist interval of C-band radars is just about one half that of S-band radars (wavelength of about 10 cm) and thus has more velocity folding. The proposed algorithm includes seven modules to remove noisy data, find the starting radials, dealias velocities, and apply least squares error checking in both the radial and azimuth directions. The proposed velocity dealiasing method was applied to one widespread rain case and three strong convective cases from radars operating in China. It was found that, on average, 92.95% of the aliased radial velocity data could be correctly de-aliased by the algorithm, resulting in 96.65% of the data being valid. 相似文献