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251.
252.
A continuous transition from early Friedmann-like radiation era through to late time cosmic acceleration passing through a long Friedmann-like matter dominated era followed by a second phase of radiation era has been realized in modified theory of gravity containing a combination of curvature squared term, a linear term, a three-half term and an ideal fluid. Thus the history of cosmic evolution is explained by modified theory of gravity singlehandedly. The second phase of radiation-like era might provide an explanation to the hydrogen and helium reionization at low redshift. 相似文献
253.
K.L. Kaila P.R.K. Murty D.M. Mall M.M. Dixit D. Sarkar 《Geophysical Journal International》1987,89(1):399-404
Summary. The crustal depth section along Hirapur-Mandla profile has been computed in two steps from Deep Seismic Sounding (DSS) data. The shallow section up to the crystalline basement is derived by inverting first arrival refraction travel times. The upper Vindhyan sediments (velocity 4.5 km s−1 ) have a maximum thickness of about 1.5 km at Bakshaho. The lower Vindhyan sediments (velocity 5.4 km s−1 ) were deposited north of Narmada-Son lineament between Katangi and Narsinghgarh in a graben developed in crystalline basement. The thickness of the lower Vindhyans increases from north to south towards Katangi and the depth to the basement reaches 5.5 km near Jabera. The depth to the Moho boundary varies from 39.5 km near Tikaria to 45 km at Narsinghgarh. The narrow block between Katangi and Jabalpur forms a horst feature which represents the Narmada-Son lineament forming the southern boundary of the Vindhyan basin. Two-dimensional ray tracing was performed generating travel time curves from various shot points which were matched with observed travel time data. 相似文献
254.
255.
Peak flux spectra of solar radio bursts in a wide frequency band have been statistically determined for different morphological types of bursts, for various ranges of magnetic field of the burst-associated sunspots and also for the bursts occurring in the central and limb region of the solar disk. Important results obtained are: (i) The generalised spectra have two peaks, one near to meter-wave and the other in the centimeter-wave region, the former peak being more pronounced than the latter; (ii) identical spectral shape is observed for the great and impulsive types and also for GRF and PBI types of bursts; (iii) the radio emission intensity is relatively higher in the central part than that in the limb part of the solar disk for frequencies 1–10 GHz, while the reverse is true for frequencies 0.245–1 GHz and 10–35 GHz; (iv) the optical depth of the absorbing layer above the source of a burst is found to be the same for meter to centimeter-wavelength bursts, implying that the radio sources in this wide band have uniform characteristics with respect to optical thickness; (v) in case of simultaneous emission in the dekameter to X-ray band, most of the decimetric bursts are seen to be very prompt and coincident with the associated flare's starting time. The interpretations of the obtained spectra give an insight into the possible generation mechanisms, pointing to the location of the source region in the solar atmosphere. 相似文献
256.
Rock Mass Classification and Slope Stability Assessment of Road Cut Slopes in Garhwal Himalaya, India 总被引:1,自引:0,他引:1
There are many rock mass classification schemes which are frequently used for different purposes such as estimation of strength and deformability of rock masses, stability assessment of rock slopes, tunneling and underground mining operations etc. The rock mass classification includes some inputs obtained from intact rock and discontinuity properties which have major influence on assessment of engineering behaviour of rock mass. In the present study, detail measurements were employed on road cuts slope faces in Garhwal Himalayas to collect required data to be used for rock mass classification of Rock Mass Rating (RMR) and Geological Strength Index (GSI). The stability assessment of rock slopes were also done by using Slope Mass Rating. In addition the relation between RMR and GSI were also evaluated using 50 data pairs. 相似文献
257.
An Empirical Correlation of Index Geomechanical Parameters with the Compressional Wave Velocity 总被引:2,自引:0,他引:2
The geomechanical strength of rockmass plays a key role in planning and design of mining and civil construction projects.
Determination of geomechanical properties in the field as well as laboratory is time consuming, tedious and a costly affair.
In this study, density, slake durability index, uniaxial compressive strength (UCS) and P-wave velocity tests were conducted
on four igneous, six sedimentary and three metamorphic rock varieties. These properties are crucial and used extensively in
geotechnical engineering to understand the stability of the structures. The main aim of this study is to determine the various
mechanical properties of 13 different rock types in the laboratory and establish a possible and acceptable correlation with
P-wave velocity which can be determined in the field as well as laboratory with ease and accuracy. Empirical equations were
developed to calculate the density, slake durability index and UCS from P-wave velocities. Strong correlations among P-wave
velocity with the physical properties of different rock were established. The relations mainly follow a linear trend. Student’s
‘t’ test and ‘F’ test were performed to ensure proper analysis and validation of the proposed correlations. These correlations can save time
and reduce cost during design and planning process as they represent a reliable engineering tool. 相似文献
258.
The National Capital Region (NCR) of India is exposed to high seismic hazard and risk due to a great earthquake in the central
seismic gap of Himalaya and/or due to moderate-size earthquake within NCR. The high population density, rapid growth of infrastructure,
and old engineering structures in the region make it more vulnerable to the human as well as economic loss due to moderate-size
earthquakes also. The evaluation of seismic hazard is the first step to prepare a proper mitigation plan for the region. The
aim of this paper is to evaluate the seismic hazard and risk due to moderate-size earthquakes in the vicinity of NCR. For
this purpose, a suit of accelerograms have been generated from hypothetical moderate-size earthquakes (M 5.5 and 6.0) in the region. A basic fault-plane solution is assumed for this purpose. The ranges of the different parameters
like depth of focus and stress drop values have been used in order to examine the effect of these parameters on hazard. The
accelerograms have been synthesized using two basic velocity models, one representing a hard site and the other a site with
a significant low-velocity cover. These two velocity models represent the ridge area and trans-Yamuna river area in the NCR.
The decay of peak ground acceleration (pga) values with distance is fast in trans-Yamuna region (with low-velocity surface
layer of 100 m) as compared to that of ridge area (with low-velocity surface layer of 1 m). Also, the decay of pga becomes
slower if we increase the depth of focus from 10 to 20 km. The response spectra (5% damping) of the synthetic accelerograms
for the three periods T = 0.4s, 0.75s, and 1.25s have been estimated and presented in the form of decay curves. The amplifications as a function
of epicentral distance and stress drop have also been estimated. We note that the amplifications in 100-m layer case do not
occur uniformly at all the distances, rather it is dependent on the angle of incidence of energy into the layers. The pga
values are generally amplified by more than twice with increase in stress drop from 100 to 400 bars. The seismic exposure
of the population in Delhi city has been presented. The results presented in this study may serve as an important input in
the planning of mitigation and disaster management programs in the National Capital Region. 相似文献
259.
Saibal Ghosh C. J. van Westen E. J. M. Carranza T. B. Ghoshal N. K. Sarkar M. Surendranath 《Journal of the Geological Society of India》2009,74(5):625-638
In India, the Bureau of Indian Standards (BIS) recommends a heuristic method for medium-scale (1:25,000/1:50,000) landslide
susceptibility mapping. This is based on fixed ratings of geofactors, without the inclusion of landslide inventory information.
In BIS method, the pre-defined ratings of geofactors are applied over diverse areas, irrespective of the terrain-specific
spatial inter-dependence of geofactors and landslide types, which leads to rather moderate prediction. In this paper, we evaluate
the effectiveness of the existing BIS method in Darjeeling Himalaya through a quantitative method adapting weights of evidence
(WofE) modeling. The quantified spatial associations between specific geofactors for different landslide types and failure
mechanisms that were generated, using this method showed improved prediction rates as compared to the BIS method of fixed
ratings of geofactors. We therefore recommend adjusting the existing BIS guidelines by inclusions of weights, derived locally
through quantitative spatial analysis of landslide inventories and geofactor maps. 相似文献
260.
Narendra Kumar Meena M. Prakasam Ravi Bhushan Sudipta Sarkar Pranaya Diwate Upasana Banerji 《Environmental Earth Sciences》2017,76(1):39
The lakes of the Himalaya are degrading due to increase in toxic heavy metal loading. This study reports the last 50-year heavy metal pollution loading in the Rewalsar Lake, Himachal Pradesh, India. Sediment cores were recovered to study the pollution loading in the lake sediments. The 137Cs and 210Pb isotope-based sedimentation rate suggest rapid sedimentation in the lake during the last ~50 years. The concentrations of Mn, Cu, Zn, Cd, Pb, Co, Ni, Cr metals in the lake sediments owe its contributions both to the natural and anthropogenic sources. Prior to ca 1990 AD, metal loading was dominated by the lithogenic input, whereas post ca 1990 AD the metal loading was controlled by the anthropogenic factors. The Pb concentration in the lake gradually increased during 1990–2004 and then decreased significantly till present. The higher concentration of Pb seems to be derived from the fossil fuel burning, while the Cr concentration in the lake indicates the use of fertilizer in the catchment area. The lowest concentrations of elements around ca 1990 AD seem to have occurred due to channelization of the lake feeding system. 相似文献