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161.
N. A. Chapman 《Contributions to Mineralogy and Petrology》1975,51(3):223-230
Xenoliths of coarse-grained spinel-clinopyroxenite up to 15 cm in size occur in tuff in an isolated Permian vent on the Caithness coast at Duncansby Ness. Highly altered fragments of chrome-spinel lherzolite and wehrlite are also found in the tuff and in a body of monchiquite within the vent. The spinel-clinopyroxenites consist of aluminous augite and aluminous pleonaste spinel (FeO/MgO = 0.9) and their texture suggests the spinel to have exsolved from the augite. Experiments on representative natural xenolith compositions at 18 kb (dry) indicate that all the spinel in the estimated average bulk composition (Sp4.9Px95.1) could have exsolved from an original homogeneous pyroxene. Initial fractionation of such a pyroxene from an alkali basaltic magma at P≥18 kb, 1450-1350 °C, would be followed by spinel exsolution at T< 1290 ° C. A similar origin by fractionation of a highly aluminous augite (± aluminous spinel) at high pressure, with subsequent spinel exsolution is proposed for spinel-clinopyroxenites from alkali basalts elsewhere in the world. The similarity of these xenoliths suggests that such a process may form an important stage in the evolution of some undersaturated basaltic rocks. 相似文献
162.
C. H. Chapman 《Pure and Applied Geophysics》1972,94(1):233-247
Summary In a recent paper,Gupta [5]2) re-examined the significance of leaking modes in Lamb's problem (Lamb [7]). In this paper, we present a brief review of the exact Cagniard-de Hoop solution to this problem, and use these results to examine the question of the leaking mode in more detail. The leaking mode may either cause a separate arrival,P, or influence the shape of other arrivals e.g.SpS. We have attempted to clarify and extend previous results and correct misconceptions which have appeared elsewhere and, therefore, most of this discussion is tutorial in nature. 相似文献
163.
Two acid mine drainage streams have been investigated by detailed analysis of their sediments and waters, to obtain an understanding of the dominant processes which control the transport and attenuation of heavy metals under conditions of chronic high-level pollutant input. One of the water-courses has a thick hydrous iron oxide crust on its bed, where biotically mediated oxidation of ferrous iron resulted in precipitation of amorphous ferric hydroxide, along with substantial quantities of adsorbed silica, sulphate and Al and lesser quantities of As. Small amounts of K and Pb (and possibly hydronium) jarosites were also present in the sediments. Changes in pH and in the concentrations of Cu, Zn, and Cd appear to be mainly the result of dilution by seeps and tributaries.Although no sediment was recovered during collection of water samples from the second stream, saturation index calculations imply that precipitation should have been occurring. The observed down-stream loss of a number of elements supported this conclusion. The solids predicted to be precipitating were A1(OH)3, Cu2(OH)2CO3, and Fe(OH)3. Observed decreases in the concentrations of Cd, Zn and Mn can be accounted for on the basis of dilution alone. However, the additional mechanism of neutralization by higher pH inflows is required to account for the decrease in hydrogen ion concentration downstream.The basis for a potentially useful new technique (congruent element analysis) which enables the identification of conservative components in streams is presented. Comparison of logarithmic concentration versus distance plots delineates the point where chemical removal mechanisms become important for each element. 相似文献
164.
Relative contributions of silicate and carbonate rocks to riverine Sr fluxes in the headwaters of the Ganges 总被引:1,自引:0,他引:1
Mike J. Bickle Hazel J. Chapman Nigel B.W. Harris Talat Ahmad 《Geochimica et cosmochimica acta》2005,69(9):2221-2240
Exhumation of the Himalayan-Tibetan orogen is implicated in the marked rise in seawater 87Sr/86Sr ratios since 40 Ma. However both silicate and carbonate rocks in the Himalaya have elevated 87Sr/86Sr ratios and there is disagreement as to how much of the 87Sr flux is derived from silicate weathering. Most previous studies have used element ratios from bedrock to constrain the proportions of silicate- and carbonate-derived Sr in river waters. Here we use arrays of water compositions sampled from the head waters of the Ganges in the Indian and Nepalese Himalaya to constrain the end-member element ratios. The compositions of tributaries draining catchments restricted to a limited range of geological units can be described by two-component mixing of silicate and carbonate-derived components and lie on a plane in multicomponent composition space. Key elemental ratios of the carbonate and silicate components are determined by the intersection of the tributary mixing plane with the planes Na = 0 for carbonate and constant Ca/Na for silicate. The fractions of Sr derived from silicate and carbonate sources are then calculated by mass-balance in Sr-Ca-Mg-Na composition space. Comparison of end-member compositions with bedrock implies that secondary calcite deposition may be important in some catchments and that dissolution of low-Mg trace calcite in silicate rocks may explain discrepancies in Sr-Ca-Na-Mg covariation. Alternatively, composition-dependent precipitation or incongruent dissolution reactions may rotate mixing trends on cation-ratio diagrams. However the calculations are not sensitive to transformations of the compositions by incongruent dissolution or precipitation processes provided that the transformed silicate and carbonate component vectors are constrained. Silicates are calculated to provide ∼50% of the dissolved Sr flux from the head waters of the Ganges assuming that discrepancies between Ca-Mg-Na covariation and the silicate rock compositions arise from addition of trace calcite. If the Ca-Mg-Na mixing plane is rotated by composition-dependent secondary calcite deposition, this estimate would be increased. Moreover, when 87Sr/86Sr ratios of the Sr inputs are considered, silicate Sr is responsible for 70 ± 16% (1σ) of the 87Sr flux forcing changes in seawater Sr-isotopic composition. Since earlier studies predict that silicate weathering generates as little as 20% of the total Sr flux in Himalayan river systems, this study demonstrates that the significance of silicate weathering can be greatly underestimated if the processes that decouple the water cation ratios from those of the source rocks are not properly evaluated. 相似文献
165.
Pore water testing and analysis: the good,the bad,and the ugly 总被引:8,自引:0,他引:8
The increasingly common practice of collecting and assessing sediment pore water as a primary measure of sediment quality is reviewed. Good features of this practice include: pore water is a key exposure route for some organisms associated with sediments; pore water testing eliminates particle size effects; pore water analyses and tests can provide useful information regarding contamination and pollution. Bad features include: pore water is not the only exposure route; pore water tests lack chemical or biological realism: their "sensitivity" relative to other tests may be meaningless due to manipulation and laboratory artifacts; many sediment and surface dwelling organisms are not directly influenced by pore water. Bad features can become ugly if: other exposure pathways are not considered (for toxicity or bioaccumulation); manipulation techniques are not appropriate; pore water tests are inappropriately linked to population-level effects. Pore water testing and analyses can be effective tools provided their limitations are well understood by researchers and managers. 相似文献
166.
内蒙古土地荒漠化定性定量研究 总被引:3,自引:0,他引:3
通过对阴山北麓四旗为期5年的家计的问卷调查,和不同草地的连续样方数据等样本进行分析,对影响草地生物多样性、草地景观、草地生态功能及其生态系统的食物链、能流和物流的干扰因素进行分析,得出阴山北麓四旗土地退化的人文因素。运用现代统计模型计算出各因素所占的比重,并就土地退化的人文因素进行成因分析,得出农业文化入侵是造成土地退化的文化根源,制度安排不合理是其土地退化的制度原因。 相似文献
167.
We consider the problem of determining and predicting how the wave speeds in particular directions for a transversely isotropic (TI) medium depend on particular combinations of the density-normalized moduli Aij. The expressions for the qP and qSV velocities are known to depend on four moduli. Normally, we can only determine three independent parameters from qP data, or two from qSZ data, as the others have much lower sensitivity. The resolvable parameters are conveniently described by axial and off-axis parameters: for qP rays, P0°= A11, P90°= A33 and P45°=(A11+ A33)/4 + (A13+2A55)/2; and for qSV rays, S0°= S90°=A55 and S 45°= (A11+ A 33)/4- A13/2. These parameters control the magnitude of the squared-velocities on the axes and at approximately 45°. For an arbitrary TI medium, if the medium is perturbed in a way that preserves a particular parameter, then slowness points in the associated direction and mode witl be approximately preserved in the new medium. we refer to these parameters as ‘push-pins’, i.e. if a parameter is fixed, the associated part of the slowness surface is pinned in place. Because, these five push-pins only contain four independent moduli, we can only fix at most three push-pins. Perturbing one of the other parameters inevitably perturbs the other. Numerical results illustrating the linkage between two push-pins, when three are fixed, are presented. So-called anomalous TI media occur when the roles of the qP and qSV waves are reversed: in some directions the faster ray has transverse polarization. That, in turn, requires anomalous velocities at the push-pins, i.e. S0° > P0°, S45° > P45° and/or S90° > P90° (equivalent to the usual anomalous conditions A11 < A55, < 0 and/or A33 < A55). In the Appendix, we confirm that anomalous sensitivities of the velocities at the five push-pins only occur in such media, although the push-pins still apply if interpreted appropriately. Truly anomalous sensitivities, in which push-pins play no role, only occur in media near the boundary between normal and anomalous. 相似文献
168.
Rajendra Singh D P Chapman K B Atkinson 《Journal of the Indian Society of Remote Sensing》1997,25(1):47-59
Automatic close range photogrammetric measurements are of immense importance for a hazardous industry like mining. Unfortunately, available stereomatching techniques fail for high resolution close range images due to the large variation of object depth. In this paper a diffraction grating based laser dot-matrix projector along with a CCD camera is used for automatic close range measurement of a textureless and featureless object like massive sand stone strata. Targeting of object’s surface with large number of laser dots simultaneously solved two major problems: identification of conjugate points and precise image co-ordinate measurement. Least squares based template matching is used for centroid location of images of laser dots which provided 0.03 (mean) pixels accuracy. Using analytical techniques, camera model of the projector was developed by placing it rigidly on the teleseope of a geodimeter Bundle adjustment procedure is adopted for accurate estimation of interior orientation parameters of the projector which resulted precise co-ordinates of the object space during a test scan by the developed system. 相似文献
169.
A quantitative study of observations of the ionospheric signatures of magnetospheric ultra low frequency (ULF) waves by a high-latitude (geographic: 69.6°N 19.2°E) high-frequency Doppler sounder has been undertaken. The signatures, which are clearly correlated with pulsations in ground magnetometer data, exhibit periods in the range 100–400 s and have azimuthal wave numbers in the range 3–8. They are interpreted here as local field line resonances. Phase information provided by O- and X-mode Doppler data support the view that these are associated with field line resonances having large azimuthal scale sizes. The relative phases and amplitudes of the signatures in the Doppler and ground magnetometer data are compared with a model for the generation of Doppler signatures from incident ULF waves. The outcome suggests that the dominant mechanism involved in producing the Doppler signature is the vertical component of an E × B bulk motion of the local plasma caused by the electric field perturbation of the ULF wave. 相似文献
170.
P. F. Karrow T. W. Anderson L. D. Delorme B. B. Miller L. J. Chapman 《Journal of Paleolimnology》1995,14(3):297-309
Excavation below the Lake Algonquin gravel beach bar near Clarksburg, Ontario, exposed mollusc-bearing clay over a lens of plant debris. This is the northernmost and most deeply buried Lake Algonquin fossil site found thus far in Ontario. It is the first site to provide dates from directly below the Algonquin beach bar. Two radiocarbon dates of about 11 200 years confirm the age of isostatically transgressing Lake Algonquin. Plant macrofossils (21 taxa), pollen (39 taxa), molluscs (12 taxa), and ostracodes (18 taxa) indicate that the climate was colder than present by several degrees and the forest-tundra ecotone was nearby initially but retreated northward rather quickly. Upward increases in abundances and diversity of molluscs and ostracodes suggest it was a time of rapid migration and colonization of species.Deceased, 1 November 1994This is the 7th in a series of papers published in this special AMQUA issue. These papers were presented at the 1994 meeting of the American Quaternary Association held 19–22 June, 1994, at the University of Minnesota, Minneapolis, Minnesota, USA. Dr Linda C. K. Shane served as guest editor for these papers. 相似文献