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101.
Keisuke Hoshikawa Yoichi Fujihara Sokly Siev Seiya Arai Takashi Nakamura Hideto Fujii Ty Sok Chihiro Yoshimura 《水文研究》2019,33(21):2745-2758
Large, shallow‐water lakes located on floodplains play an important role in creating highly productive ecosystems and are prone to high concentrations of suspended solids due to sediment resuspension. In this study, the aim was to determine the dominant processes governing the total suspended solid (TSS) concentration at the water surface in Tonle Sap Lake, Cambodia, which is a large, shallow‐water lake. Satellite remotely sensed daily reflectance data from 2003 to 2017 were used. Seasonal changes in TSS concentration indicated that bottom sediment resuspension during dry seasons was mostly caused by wind and the TSS concentration was closely correlated with the water depth of the lake. The TSS concentration during flood periods was controlled by both wind and inflow currents from the Tonle Sap River. Additionally, we confirmed that surface/subsurface flow with a low TSS concentration from forests on the floodplain lowered the TSS concentration year round, except during August and September. This fact implied that the floodplain forest area decrease may increase the lake TSS concentration. An analysis of the long‐term changes in TSS indicated that a decrease in the water level during flood periods resulted in the high TSS concentrations observed during the subsequent dry periods. Therefore, climate change and water resource development, which are likely to cause water level reductions in the Mekong River during flood periods, may increase the TSS concentration in Tonle Sap Lake, particularly during the dry season. 相似文献
102.
Iskhaq Iskandar Hideharu Sasaki Yoshikazu Sasai Yukio Masumoto Keisuke Mizuno 《Ocean Dynamics》2010,60(3):731-742
An eddy-resolving coupled physical–biological model is used to study the effect of cyclonic eddy in enhancing offshore chlorophyll-a (Chl-a) bloom in the southeastern tropical Indian Ocean during boreal summer–fall 2006. The results demonstrate that the offshore
Chl-a blooms are markedly coincident with the high eddy kinetic energy. Moreover, the vertical variations in Chl-a, nitrate, temperature, and mixed-layer depth (MLD) strongly imply that the cyclonic eddies induce surface Chl-a bloom through the injection of nutrient-rich water into the upper layer. Interestingly, we found that the surface bloom only
occurs when the deep Chl-a maximum is located within the MLD. On the other hand, the response of subsurface Chl-a to the eddy pumping is remarkable, although it is hardly observable at the surface. 相似文献
103.
Contributions of slab fluid and sediment melt components to magmatism in the Mariana Arc–Trough system: Evidence from geochemical compositions and Sr,Nd, and noble gas isotope systematics
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Yasuo Ikeda Keisuke Nagao Teruaki Ishii Daisuke Matsumoto Robert J. Stern Hiroo Kagami Makoto Arima Sherman H. Bloomer 《Island Arc》2016,25(4):253-273
This study presents new major and trace element, mineral, and Sr, Nd, and noble gas isotope geochemical analyses of basalts, gabbro, and clinopyroxenite from the Mariana Arc (Central Islands and Southern Seamount provinces) including the forearc, and the Mariana Trough (Central Graben and Spreading Ridge). Mantle source compositions beneath the Mariana Arc and the Mariana Trough indicate a mantle source that is depleted in high field strength elements relative to MORB (mid‐oceanic ridge basalt). Samples from the Mariana Arc, characterized by high ratios of Ba/Th, U/Th, 84Kr/4He and 132Xe/4He, are explained by addition of fluid from the subducted slab to the mantle wedge. Correlations of noble gas data, as well as large ion lithophile elements, indicate that heavy noble gases (Ar, Kr, and Xe) provide evidence for fluid fluxing into the mantle wedge. On the other hand, major elements and Sr, Nd, He, and Ne isotopic data of basalts from the Mariana Trough are geochemically indistinguishable from MORB. Correlations of 3He/4He and 40Ar/36Ar in the Mariana Trough samples are explained by mixing between MORB and atmosphere. One sample from the Central Graben indicates extreme enrichment in 20Ne/22Ne and 21Ne/22Ne, suggesting incorporation of solar‐type Ne in the magma source. Excess 129Xe is also observed in this sample suggesting primordial noble gases in the mantle source. The Mariana Trough basalts indicate that both fluid and sediment components contributed to the basalts, with slab‐derived fluids dominating beneath the Spreading Ridge, and that sediment melts, characterized by high La/Sm and relatively low U/Th and Zr/Nb, dominate in the source region of basalts from the Central Graben. 相似文献
104.
T. Tanabé S. Nishida Y. Nakada T. Onaka I.S. Glass M. Sauvage 《Astrophysics and Space Science》1997,255(1-2):407-413
We report preliminary results of our systematic survey for infrared stars in the globular clusters of the Magellanic Clouds.
In the course of an ISOCAM survey for AGB stars in the intermediate-age clusters, we have discovered extremely red AGB stars
in NGC 419 and NGC 1978. From their colours and luminosities, they are thought to be experiencing intense mass-loss and to
be in the final or superwind phase of the AGB evolution. However, they seem to be of somewhat lower luminosity than the corresponding
visible AGB stars when only the mid-infrared data are taken into account. This suggests that hitherto unobserved infrared
excesses may exist at longer wavelengths.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
105.
Inversion of DC resistivity data using neural networks 总被引:9,自引:0,他引:9
The inversion of geoelectrical resistivity data is a difficult task due to its non-linear nature. In this work, the neural network (NN) approach is studied to solve both 1D and 2D resistivity inverse problems. The efficiency of a widespread, supervised training network, the back-propagation technique and its applicability to the resistivity problem, is investigated. Several NN paradigms have been tried on a basis of trial-and-error for two types of data set. In the 1D problem, the batch back-propagation paradigm was efficient while another paradigm, called resilient propagation, was used in the 2D problem. The network was trained with synthetic examples and tested on another set of synthetic data as well as on the field data. The neural network gave a result highly correlated with that of conventional serial algorithms. It proved to be a fast, accurate and objective method for depth and resistivity estimation of both 1D and 2D DC resistivity data. The main advantage of using NN for resistivity inversion is that once the network has been trained it can perform the inversion of any vertical electrical sounding data set very rapidly. 相似文献
106.
Bottom mud was collected from the upper and lower reaches of 92 rivers in Japan to determine the distribution of 0.5N-HCI-soluble
heavy metals. The average concentrations of Cu, Zn, Pb, and Cd in the mud from the lower reaches were from two to three times
higher than those in the upper reaches. Japanese rivers were classified according to the degree of heavy metal pollution by
using the values of 0.5N-HCI-soluble metals in the lower reaches of rivers. The content of 0.5N-HCI-soluble heavy metals in
the mud of the upper reaches was used as the standard value. The classification of rivers was made by a cluster analysis according
to Euclidian distance. As a result, rivers in Japan could be classified into three groups based on the degree of metal pollution,
and the classification was in agreement with the pollution index. 相似文献
107.
Keisuke Ito 《Earth and Planetary Science Letters》1980,51(2):451-456
The chaotic dynamics of the Rikitake two-disc dynamo system is studied for a wide range of parameters and results are compared with the sequence of geomagnetic polarity reversals. The chaos of the Rikitake system belongs to the Lorenz type, which is characterized by irregular travelling of an orbit between two unstable fixed points. Travelling corresponds to the polarity reversal. The frequency of the polarity reversals depends strongly on the parameter μ representing the resistive dissipation in the core. In the center of the chaotic regime, there is a parameter region in which reversals seldom occur and the dynamics is less disordered. The Markov entropy of the Lorenz map for the system has a sharp minimum in this parameter region named as the minimum entropy regime (M.E.R.). The average frequency of reversals in the M.E.R. is less than 0.1 per cycle of oscillation, but is not uniform even seen through a 10-m.y.-long window. The non-uniformity as well as the low rate of reversals in the M.E.R. markedly resemble the behavior of geomagnetic reversals over the past 153 m.y. It is suggested that the Earth selected the minimum entropy regime on the principle of the minimum entropy production, as is known to be the case for other dissipative dynamical systems. 相似文献
108.
Keisuke Nagao Nobuo Takaoka Osamu Matsubayashi 《Earth and Planetary Science Letters》1981,53(2):175-188
Isotopic and elemental compositions of rare gases in various types of gas samples collected in the Japanese Islands were investigated. Excess3He was found in most samples. Many samples showed a regionally uniform high3He/4He ratio of about 7 times the atmospheric ratio. The He concentrations varied from 0.6 to 1800 ppm, and they were low in CO2-rich gases and high in N2-rich gases. Ne isotopic deviations from the atmospheric Ne were detected in most volcanic gases. The deviations and the elemental abundance patterns in volcanic gases can be explained by a mixing between two components, one is mass fractionated rare gases and the other is isotopically atmospheric and is enriched in heavy rare gas elements. Ar was a mixture of mass fractionated Ar, atmospheric Ar and radiogenic Ar, and the contribution of radiogenic40Ar was small in all samples. Except for He, elemental abundance patterns were progressively enriched in the heavier rare gases relative to the atmosphere. Several samples were highly enriched in Kr and Xe relative to the abundance pattern of dissolution equilibrium of atmospheric rare gases in water. The component which is highly enriched in heavy rare gases may be released from sedimentary materials in the crust. 相似文献
109.
Aya Shimizu Hirochika Sumino Keisuke Nagao Kenji Notsu Panagiotis Mitropoulos 《Journal of Volcanology and Geothermal Research》2005,140(4):118-339
In contrast to most other arcs with oceanic plate subduction, the Aegean arc is characterized by continent–continent subduction. Noble gas abundances and isotopic compositions of 45 gas samples have been determined from 6 volcanoes along the arc, 2 islands in the back-arc region and 7 sites in the surrounding areas. The 3He/4He ratios of the samples ranged from 0.027RA to 6.2RA (RA denotes the atmospheric 3He/4He ratio of 1.4×10−6), demonstrating that even the maximum 3He/4He ratio in the region is significantly lower than the maximum ratios of most oceanic subduction systems, which are equal to the MORB value of 8±1 RA. Regional variations in the 3He/4He ratio were observed both along and across the arc. The maximum 3He/4He ratio was obtained from Nisyros volcano located in the eastern end of the arc, and the ratio decreased westward possibly reflecting the difference in potential degree of crustal assimilation or the present magmatic activity in each volcano. Across the volcanic arc, the 3He/4He ratio decreased with an increasing distance from the arc front, reaching a low ratio of 0.063RA in Macedonia, which suggested a major contribution of radiogenic helium derived from the continental crust. At Nisyros, a temporal increase in 3He/4He ratio due to ascending subsurface magma was observed after the seismic crisis of 1995–1998 and mantle neon was possibly detected. The maximum 3He/4He ratio (6.2RA) in the Aegean region, which is significantly lower than the MORB value, is not probably due to crustal assimilation at shallow depth or addition of slab-derived helium to MORB-like mantle wedge, but inherent characteristics of the subcontinental lithospheric mantle (SCLM) beneath the Aegean arc. 相似文献
110.