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991.
Late Cretaceous rift-related upwelling and melting of the Trindade starting mantle plume head beneath western Brazil 总被引:1,自引:0,他引:1
S. A. Gibson R. N. Thompson R. K. Weska A. P. Dickin O. H. Leonardos 《Contributions to Mineralogy and Petrology》1997,126(3):303-314
High mantle potential temperatures and local extension, associated with the Late-Cretaceous impact of the Trindade mantle plume, produced substantial widespread and voluminous magmatism around the northern half of the Paraná sedimentary basin. Our previous studies have shown that, above the central and eastern portions of the postulated impact zone where lithosphere extension is minimal, heat conducted by the plume caused large-scale melting of the more fusible parts of the subcontinental lithospheric mantle beneath the margin of the São Francisco craton and the surrounding Brasilía mobile belt. Here we combine geochemical data and field evidence from the Poxoreu Igneous Province, western Brazil to show how more intense lithospheric extension above the western margin of the postulated impact zone permitted greater upwelling and melting of the Trindade plume than further east. Laser 40Ar/39Ar age determinations indicate that rift-related basaltic magmas of the Poxoreu Igneous Province were emplaced at ? 84 Ma. Our detailed geochemical study of the mafic magmas shows that the parental melts underwent polybaric crystal fractionation within the crust prior to final emplacement. Furthermore, some magmas (quartz-normative) appear to have assimilated upper crust whereas others (nepheline- and hypersthene-normative) appear to have been unaffected by open-system crustal magma chamber processes. Incompatible trace element ratios (e.g. chondrite-normalised La/Nb?=?1) and isotopic ratios (87Sr/86Sr?=?0.704 and 143Nd/144Nd?=?0.51274) of the Hy-normative basalts resemble those of oceanic islands (OIB). We therefore propose that these “OIB-like” magmas were predominantly derived from convecting-mantle-source melts (i.e. Trindade mantle plume). Inverse modelling of rare-earth element (REE) abundances suggests that the initial melts were predominantly generated within the depth range of ?80–100 km, in mantle with a potential temperature of ?1500 °C. 相似文献
992.
A nitrogen isotope study of soil water and groundwater in southern Indiana, USA, in 1991–1992 demonstrated considerable variations
in nitrate degradation processes compared to an earlier investigation in 1986–1987. Although N-fertilizers were applied in
May 1991, the δ15N values in soil water decreased in February 1992, indicating its delayed release into the system after substantial rainfall.
The δ15N values of groundwater decreased from +12.3‰ in November 1991 to +11.3‰ in February 1992, and to +7.5‰ in March 1992. The
increased residence time of nitrate in the soil resulted in increased denitrification, ammonia volatilization and plant uptake,
and reduced threat to the groundwater quality. The 1986–1987 study in the area reported that excessive rainfall during the
summer rapidly transported the nitrate to deeper horizons and drastically reduced volatilization and microbial reduction of
nitrate, thus increasing the immediate threat to the groundwater quality in the area. The present study demonstrated that
nitrogen isotopic signatures can be used to determine the effects of local soil type, rainfall, and land-use practices on
the fate of nitrate in the subsurface.
Received: 18 February 1997 · Accepted: 17 June 1997 相似文献
993.
Sources of Water Supplying Pumpage from Regional Aquifer Systems of the United States 总被引:1,自引:0,他引:1
Richard H. Johnston 《Hydrogeology Journal》1997,5(2):54-63
994.
K. S. Voudouris N. J. Lambrakis G. Papatheothorou P. Daskalaki 《Mathematical Geology》1997,29(1):43-59
R- andQ-mode factor analysis is applied to 51 groundwater samples collected from wells drilled in the Plio-Pleistocene aquifers of
NW Achaia, Greece. The purpose ofR- andQ-mode factor analysis application is to identify (i) the regional groundwater flow pattern, and (ii) the deterioration of
groundwater quality. Sixteen hydrogeological parameters are used in order to examine their importance and to provide significant
insight into their correlations. In theR-mode factor analysis, a six-factor model is suggested which can explain more than 77.5% of the total variance. The contribution
of each factor at every site (factor scores) also is computed. Maps are constructed showing the geographical distribution
of the factor scores. From these maps, the high salinity areas are delineated (seawater intrusion, possible appearance of
halite layers) and the areas with elevated contribution of karastic-water are defined. Using theQ-mode correspondence analysis the meaning of the electrical conductivity as the most important variable in groundwater quality
characterization is demonstrated. 相似文献
995.
In the northwestern part of the Kalahari manganese field low-grade carbonate-rich Mamatwan-type ore is altered to high-grade
oxide-rich Wessels-type ore in association with normal faults. Mass balance calculations, based on the assumption that manganese
was geochemically immobile, suggest that upgrading of the manganese ore can be attributed to leaching of Mg, Ca, CO2 and SiO2 from the sedimentary ore with residual enrichment of Mn. Hydrothermal alteration resulted in development of about 10 to 20%
of secondary porosity in the ores and the orebed was compacted to two thirds of its original stratigraphic thickness.
Received: 28 May 1996 / Accepted: 22 January 1997 相似文献
996.
Conservation tillage and input use 总被引:1,自引:0,他引:1
N. D. Uri 《Environmental Geology》1997,29(3-4):188-201
There continues to be a question as to the overall effectiveness of conservation tillage practices in reducing the impact
of agricultural production on the environment. While it is generally recognized that water runoff and soil erosion will decline
further, as tillage and mulch tillage systems are not used more extensively on cropland, what will happen to pesticide and
fertilizer use remains uncertain. To gain some insight into this, the conservation tillage adoption decision is modelled.
On the assumption that the decision to adopt conservation tillage is a two-step procedure, the first decision is whether or
not to adopt a conservation tillage production system and the second concerns the extent to which conservation tillage should
be used – appropriate models of the Cragg and Heckman (dominance) type are estimated. Based on farm-level data on corn production
in the United States for 1987, the profile of a farm on which conservation tillage was adopted is that the cropland had above-average
slope and experienced above-average rainfall, the farm was a cash grain enterprise, and it had an above-average expenditure
on pesticides and a below-average expenditure on fuel and custom pesticide applications. Additionally, for a farm adopting
a no-tillage production practice, an above-average expenditure was made on fertilizer.
Received: 18 September 1995 · Accepted: 6 December 1995 相似文献
997.
Effect of tunnel depth on modulus of deformation of rock mass 总被引:1,自引:2,他引:1
M. Verman B. Singh M. N. Viladkar J. L. Jethwa 《Rock Mechanics and Rock Engineering》1997,30(3):121-127
Summary Deformability of rock mass significantly influences its behaviour and is, therefore, an important consideration for the design of underground openings. The modulus of deformation of rock mass is, however, normally obtained from expensive and time-consuming uniaxial jacking tests, whose results often have a large scatter. An empirical correlation has, therefore, been proposed for a quick and inexpensive preliminary estimation of the modulus of deformation of rock mass on the basis of field instrumentation carried out in tunnels in India. 相似文献
998.
999.
—The Nikko region, in northern Kanto, central Japan, which is a high seismicity region, is located near the volcanic front. Many seismic observations have been carried out around the Nikko region because the seismograms of the shallow earthquakes occurring in this region show an S-wave reflected phase (SxS) and an S-to-P mode-converted and reflected phase (SxP) from a mid-crustal seismic velocity discontinuity. The depth of the mid-crustal reflector was estimated to be about 9–16 km. From the amplitude analyses of SxS and SxP phases, the material of the mid-crustal layer just beneath the reflector was expected to be a low rigidity layer, which was interpreted as partially molten rocks. No evidence has been shown for the internal properties of the material of the reflector. The Vp/Vs ratio is a good measure to distinguish between the two candidates for seismic reflectors; i.e., partially molten rocks and rocks containing water. We investigate the Vp/Vs ratio of the reflector using the spectrum analyses of the data observed by a seismic array. From spectrum analysis of the data, we indicate that the Vp/Vs ratio of 2.5 is better than that of 1.8 to explain the observed data, which shows that the material beneath the reflector consists of partially molten rocks. 相似文献
1000.