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31.
The problems of large irrigation project are usually presented by water logging and soil salinity. These are associated with geomorphic conditions and type of soils present in the command areas and could be easily identified by use of aerial photographs and remote sensing techniques. The present study aims to make use of satellite data for identification of these problems. The study indicates that satellite information would be useful to locate such problem areas by adopting multistage approach for its interpretation. 相似文献
32.
Geochemical factors controlling the chemical nature of water and sediments in the Gomti River, India 总被引:3,自引:2,他引:1
R-mode factor analysis of the recently acquired data on water and sediment chemistry has been performed. Basic chemical parameters
have been merged together which aid in interpreting a few empirical geochemical factors controlling the chemical nature of
water and sediments of the Gomti River, a major Himalayan tributary of the Ganges drainage basin. Water chemistry seems to
be controlled by three factors: bicarbonate, rainfall and silicate and phosphate factors. Sediment chemistry is largely controlled
by the following four factors: clay, adsorption/desorption, Fe-Mn hydroxide and mercury factors. These factors show spatial
and temporal variability in terms of their R-scores.
Received: 8 September 1997 · Accepted: 15 December 1997 相似文献
33.
Distribution and fractionation of heavy metals in the surface sediments of the Ganges- Brahmaputra-Meghna river system in the Bengal basin 总被引:1,自引:0,他引:1
The lower Ganges-Brahmaputra-Meghna (G-B-M) drainage basin occupies the total Bengal Basin, which is one of the unique basins
of the world because of its location and size, density of population, and catastrophic deposition of sediments. The increased
heavy metal concentration in the 63 m fraction of surface sediments shows similarity among major segments of the G-B-M system
in the basin, which reflects the homogenization of lithologic and chemical diversity of the greater denudation regime by the
river processes. The differences in heavy metal concentation in the lower G-B-M system with that of its upper and middle counterpart
is mainly related to the contrast between Himalayan rivers and the other major South Asian rivers, and may be due to the geological
differences of their denudation regime. Heavy metals in the Lower G-B-M system have an affinity towards the clay fraction
of the sediments. The correlation matix of heavy metals in the lower Brahmaputra and Meghna suggests the importance of Fe-Mn
oxyhydroxides in their accumulations. Iron, Ti and Mn are higher in the Meghna main channel, Zn is higher in the Meghna tributaries,
and Cr is higher in both the Brahmaputra and Meghna compared to the value for standard shale. The enrichment factor is ≤1
for most of the metals except Mn which is relatively higher in the Meghna and lower Ganges main channels. The geoaccumulation
index (Igeo) for most of the heavy metals lies below grade zero, suggesting unpolluted sediment quality. The lower Ganges system shows
relatively higher concentration in the nondetrital fraction of heavy metals, probably due to the presence of petroleum refinery,
industrial and mining effluents, and agricultural runoff in the drainage basin. The relative uniformity in concentration of
heavy metals in vertical profiles may be due to the uniformity in sediment grain size and catastrophic deposition of sediments,
where the time period represented by the vertical sediment column is not enough to reflect the cultural accumulation of heavy
metals. The Bengal basin thus represents a relatively unperturbed alluvial basin with regards to heavy metal pollution.
Received: 21 July 1997 · Accepted: 13 October 1997 相似文献
34.
Kandaswamy Subramanian 《Monthly notices of the Royal Astronomical Society》1998,294(4):718-728
Large-scale magnetic fields in galaxies are thought to be generated by a turbulent dynamo. However, the same turbulence also leads to a small-scale dynamo which generates magnetic noise at a more rapid rate. The efficiency of the large-scale dynamo depends on how this noise saturates. We examine this issue, taking into account ambipolar drift, which obtains in a galaxy with significant neutral gas. We argue as follows.
(i) The small-scale dynamo generated field does not fill the volume, but is concentrated into intermittent rope-like structures. The flux ropes are curved on the turbulent eddy scales. Their thickness is set by the diffusive scale determined by the effective ambipolar diffusion.
(ii) For a largely neutral galactic gas, the small-scale dynamo saturates, as a result of inefficient random stretching, when the peak field in a flux rope has grown to a few times the equipartition value.
(iii) The average energy density in the saturated small-scale field is subequipartition, since it does not fill the volume.
(iv) Such fields neither drain significant energy from the turbulence nor convert eddy motion of the turbulence on the outer scale into wave-like motion. The diffusive effects needed for the large-scale dynamo operation are then preserved until the large-scale field itself grows to near equipartition levels. 相似文献
(i) The small-scale dynamo generated field does not fill the volume, but is concentrated into intermittent rope-like structures. The flux ropes are curved on the turbulent eddy scales. Their thickness is set by the diffusive scale determined by the effective ambipolar diffusion.
(ii) For a largely neutral galactic gas, the small-scale dynamo saturates, as a result of inefficient random stretching, when the peak field in a flux rope has grown to a few times the equipartition value.
(iii) The average energy density in the saturated small-scale field is subequipartition, since it does not fill the volume.
(iv) Such fields neither drain significant energy from the turbulence nor convert eddy motion of the turbulence on the outer scale into wave-like motion. The diffusive effects needed for the large-scale dynamo operation are then preserved until the large-scale field itself grows to near equipartition levels. 相似文献
35.
The Gauribidanur Radioheliograph 总被引:1,自引:0,他引:1
A new radio heliograph for obtaining two-dimensional images of the solar corona sequentially at many frequencies in the range 40–150 MHz has been built by the Indian Institute of Astrophysics at the Gauribidanur Radio Observatory (lat. 13°3612 N and long. 77°2707 E) about 100 km north of Bangalore, India. This paper describes various aspects of the antenna system, receiver front end, digital hardware, the data acquisition and the calibration procedure. The performance of the instrument is illustrated with maps of the continuum emission from the undisturbed corona at different frequencies. 相似文献
36.
Agusa T Kunito T Yasunaga G Iwata H Subramanian A Ismail A Tanabe S 《Marine pollution bulletin》2005,51(8-12):896-911
Concentrations of trace elements (V, Cr, Mn, Co, Cu, Zn, Ga, Se, Rb, Sr, Mo, Ag, Cd, Sn, Sb, Cs, Ba, Hg, Tl, Pb and Bi) were determined in muscle and liver of 12 species of marine fish collected from coastal areas in Malaysia. Levels of V, Cr, Mn, Co, Cu, Zn, Ga, Sr, Mo, Ag, Cd, Sn, Ba and Pb in liver were higher than those in muscle, whereas Rb and Cs concentrations showed the opposite trend. Positive correlations between concentrations in liver and muscle were observed for all the trace elements except Cu and Sn. Copper, Zn, Se, Ag, Cd, Cs and Hg concentrations in bigeye scads from the east coast of the Peninsular Malaysia were higher than those from the west, whereas V showed the opposite trend. The high concentration of V in the west coast might indicate oil contamination in the Strait of Malacca. To evaluate the health risk to Malaysian population through consumption of fish, intake rates of trace elements were estimated on the basis of the concentrations of trace elements in muscle of fish and daily fish consumption. Some specimens of the marine fish had Hg levels higher than the guideline value by US Environmental Protection Agency (EPA), indicating that consumption of these fish at the present rate may be hazardous to Malaysian people. To our knowledge, this is the first study on multielemental accumulation in marine fish from the Malaysian coast. 相似文献
37.
The dissolved fluoride (F−) in the Lower Ganges-Brahmaputra-Meghna (GBM) river system, Bengal basin, Bangladesh, was studied during 1991–1993 to determine
its distribution and source in the basin, and its annual flux to the Bay of Bengal. The concentration of dissolved F− varied between 2 and 11 μmol l−1 with statistically significant variations both spatially and temporally in the basin. Such variations are attributable to
the geology of the individual subbasins (Ganges, Brahmaputra and Meghna), dilution by rainwater during monsoon and groundwater
contribution to the river systems during dry season. Correlation coefficients among F− and major cations and anions suggest diverse inorganic processes responsible for regulating the concentration of F− in these river systems. However, fluorite seems to be one of the major sources of dissolved F−. The concentration of F− in the Lower GBM river system is low compared to the rivers draining Deccan Plateau and arid regions of the subcontinent,
for example, Yamuna and its tributaries. However, it is within the range of most of the other Peninsular and Himalayan rivers.
The GBM system contributes about 115×103 tonnes year−1 of dissolved F− into the Bay of Bengal, and thus accounts for about 3% of the global F− flux to the oceans annually.
Received: 19 May 1999 · Accepted: 11 October 1999 相似文献
38.
Using simulations of isotropically forced helical turbulence the contributions to kinetic and magnetic alpha effects are computed. It is shown that for the parameter regimes considered in an earlier publication (Brandenburg & Subramanian 2005), the expressions for isotropic and anisotropic alpha effects give quantitatively similar results. Both kinetic and magnetic alpha effects are proportional to a relaxation time whose value, in units of the turnover time, is shown to be approximately unity and independent of the magnetic Reynolds number. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
39.
Several one and two dimensional mean field models are analyzed where the effects of current helicity fluxes and boundaries are included within the framework of the dynamical quenching model. In contrast to the case with periodic boundary conditions, the final saturation energy of the mean field decreases inversely proportional to the magnetic Reynolds number. If a nondimensional scaling factor in the current helicity flux exceeds a certain critical value, the dynamo can operate even without kinetic helicity, i.e. it is based only on shear and current helicity fluxes, as first suggested by Vishniac & Cho (2001, ApJ 550, 752). Only above this threshold is the current helicity flux also able to alleviate catastrophic quenching. The fact that certain turbulence simulations have now shown apparently non‐resistively limited mean field saturation amplitudes may be suggestive of the current helicity flux having exceeded this critical value. Even below this critical value the field still reaches appreciable strength at the end of the kinematic phase, which is in qualitative agreement with dynamos in periodic domains. However, for large magnetic Reynolds numbers the field undergoes subsequent variations on a resistive time scale when, for long periods, the field can be extremely weak. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
40.
M. R. Kundu S. M. White V. I. Garaimov P. Subramanian S. Ananthakrishnan P. Janardhan 《Solar physics》2006,236(2):369-387
Observations of a solar flare at 617 MHz with the Giant Meter-wave Radio Telescope (GMRT) are used to study the morphology
of flare radio emission at decimetric wavelengths. There has been very little imaging in the 500 – 1000 MHz frequency range,
but it is of great interest, since it corresponds to densities at which energy is believed to be released in solar flares.
This event has a very distinctive morphology at 617 MHz: the radio emission is clearly resolved by the 30″ beam into arc-shaped
sources seeming to lie at the tops of long loops, anchored at one end in the active region in which the flare occurs, with
the other end lying some 200 000 km away in a region of quiet solar atmosphere. Microwave images show fairly conventional
behaviour for the flare in the active region: it consists of two compact sources overlying regions of opposite magnetic polarity
in the photosphere. The decimetric emission is confined to the period leading up to the impulsive phase of the flare, and
does not extend over a wide frequency range. This fact suggests a flare mechanism in which the magnetic field at considerable
height in the corona is destabilized a few minutes prior to the main energy release lower in the corona. The radio morphology
also suggests that the radiating electrons are trapped near the tops of magnetic loops, and therefore may have pitch angles
near 90˚. 相似文献