全文获取类型
收费全文 | 1249篇 |
免费 | 57篇 |
国内免费 | 10篇 |
专业分类
测绘学 | 47篇 |
大气科学 | 68篇 |
地球物理 | 339篇 |
地质学 | 483篇 |
海洋学 | 132篇 |
天文学 | 163篇 |
综合类 | 6篇 |
自然地理 | 78篇 |
出版年
2023年 | 9篇 |
2022年 | 9篇 |
2021年 | 21篇 |
2020年 | 23篇 |
2019年 | 26篇 |
2018年 | 51篇 |
2017年 | 52篇 |
2016年 | 66篇 |
2015年 | 41篇 |
2014年 | 70篇 |
2013年 | 88篇 |
2012年 | 57篇 |
2011年 | 78篇 |
2010年 | 60篇 |
2009年 | 94篇 |
2008年 | 61篇 |
2007年 | 48篇 |
2006年 | 45篇 |
2005年 | 52篇 |
2004年 | 35篇 |
2003年 | 39篇 |
2002年 | 37篇 |
2001年 | 18篇 |
2000年 | 24篇 |
1999年 | 14篇 |
1998年 | 13篇 |
1997年 | 15篇 |
1996年 | 12篇 |
1995年 | 12篇 |
1994年 | 11篇 |
1993年 | 12篇 |
1992年 | 4篇 |
1991年 | 3篇 |
1990年 | 11篇 |
1989年 | 8篇 |
1988年 | 3篇 |
1987年 | 5篇 |
1986年 | 5篇 |
1985年 | 7篇 |
1984年 | 8篇 |
1983年 | 9篇 |
1982年 | 5篇 |
1981年 | 3篇 |
1979年 | 4篇 |
1975年 | 3篇 |
1957年 | 3篇 |
1956年 | 4篇 |
1955年 | 3篇 |
1953年 | 3篇 |
1950年 | 3篇 |
排序方式: 共有1316条查询结果,搜索用时 288 毫秒
31.
Ray tracing in elliptical anisotropic media using the linear traveltime interpolation (LTI) method applied to traveltime seismic tomography 总被引:3,自引:0,他引:3
The linear traveltime interpolation (LTI) method is a suitable ray‐tracing technique for modelling first‐arrival times in isotropic media. LTI is extended to elliptical anisotropic media and applied to a tomographic inversion procedure. A theoretical formulation is first derived and then LTI implementation is discussed in terms of source–receiver arrays and cell size. The method is then combined with the tomographic inversion procedure adopted. The matching of the ray tracing with inversion in elliptical anisotropic media posed a double non‐linear problem. Thus two assumptions were made: the velocity in each cell is uniform and the main directions of anisotropy are known. To take into account the geometrical characteristics of the area under investigation (depth and velocity of the weathering, and thickness of the inner media), cells of varying size were considered. No hypothesis was made on anisotropy weakness.
The algorithm was first tested on synthetic models and then applied to a field survey. On comparing the results of the synthetic models and the field survey with those obtained with a linear raypath approximation, it was found that there were fewer data misfits. 相似文献
The algorithm was first tested on synthetic models and then applied to a field survey. On comparing the results of the synthetic models and the field survey with those obtained with a linear raypath approximation, it was found that there were fewer data misfits. 相似文献
32.
Antonio Longinelli Paola IacuminMichele Ramigni 《Earth and Planetary Science Letters》2002,203(1):445-459
Belemnite guards of Cretaceous and Jurassic age were found to contain varying amounts of quartz deposited both on the external surface and inside the rostrum. The oxygen isotopic composition of coexisting carbonate, quartz and phosphate from the same rostrum was measured according to well-established techniques. None of these compounds showed isotopic values in equilibrium with one another. Assuming δ18O values of the diagenetic water within the range of meteoric waters, the δ18O(SiO2) yield temperatures in agreement with the apparent secondary origin of this phase. The δ18O(CO32−) range, with a certain continuity, between −10.8 and +0.97 PDB-1 with most of the intermediate values being within the range of the carbonate isotopic values of Mesozoic fossils. The most positive isotopic results obtained from phosphate are close to +23/+24‰ (V-SMOW). They can hardly be related to a secondary origin of the phosphate, or to the presence of diagenetic effects, since these results are among the most positive ever measured on phosphate. As far as we know there is no widespread diagenetic process determining an 18O enrichment of phosphate. The very low concentration of phosphate did not allow the determination of its mineralogical composition. All the available δ18O(PO43−) values from belemnite and non-belemnite fossils of marine origin of Tertiary and Mesozoic age are reported along with the newly measured belemnites. The following conclusions may be drawn from the data reported: (1) the pristine oxygen isotope composition of fossil marine organisms (either carbonate or phosphate) may easily undergo fairly large changes because of oxygen isotope exchange processes with diagenetic water; this process is apparent even in the case of geologically recent fossils; (2) the δ18O(PO43−) of belemnite rostra seems to be, at least in the case of the most positive results, in isotopic equilibrium with environmental water because of the similarity between the results from Cretaceous belemnites and the results from Cretaceous and Lower Tertiary pelecypods and fish teeth; 3) if so, the only feasible interpretation that can be suggested for the 18O enriched data is the possibility of a relatively large variation of the oxygen isotopic composition of ocean paleowater from Jurassic to recent time. 相似文献
33.
34.
The Serra Gelada sea cliffs are carved in Mesozoic carbonate rocks belonging to the External Zones of the eastern Betic Cordillera (Alicante, SE Spain). Several normal faults with vertical slips of more than a hundred metres have played an important role in the origin of this coastline. Some previous studies propose that the present cliff morphology was mainly originated by Quaternary fault activity. However, the integration of geomorphological features, stratigraphical and sedimentological data, together with the results of the tectonic analysis of fractures occurring in Serra Gelada, and a detailed study of seismic reflection profiles carried out in the adjacent continental shelf, indicate that these normal faults were active mainly during the late Miocene. Therefore, the Serra Gelada sea cliffs represent a tectonically controlled long-term landscape. Thus, normal faults have not significantly modified the Serra Gelada relief since then. Furthermore, the northern part of the Serra Gelada cliff may be considered as an inherited pre-Quaternary relict palaeocliff since it has only undergone very little erosive recession. 相似文献
35.
The columnar cactus Polaskia chende, endemic to Central Mexico, occurs in the wild and in silviculturally managed populations where artificial selection for better edible fruits operates by sparing and favouring desirable phenotypes during vegetation clearance. Wild and managed populations were compared in morphology and reproductive biology to analyse if human selection has influenced phenotypic divergence, if it has modified breeding systems and if reproductive isolation exists between them. Fruits from managed populations were larger (6·36±0·17 cm3), heavier (19·62±1·32), sweeter (9·07±0·25 °Brix), and with thinner peel (2·09±0·13 mm) than those from wild populations (5·77±0·18 cm3, 15·73±0·85 g, 8·23±0·31 °Brix, and 2·58±0·14 mm, respectively). Both population types bloom synchronically, show diurnal anthesis, with bees as the most likely pollinators, and have breeding system predominantly self-incompatible. Spatial and temporal barriers to pollen exchange between wild and managed populations are unlikely, and morphological differences would be related with an ongoing process of artificial selection. 相似文献
36.
Thomas Backhaus Rolf Altenburger sa Arrhenius Hans Blanck Michael Faust Antonio Finizio Paola Gramatica Matthias Grote Marion Junghans Wiebke Meyer Manuela Pavan Tobias Porsbring Martin Scholze Roberto Todeschini Marco Vighi Helge Walter L. Horst Grimme 《Continental Shelf Research》2003,23(17-19):1757
Freshwater and marine ecosystems are exposed to various multi-component mixtures of pollutants. Nevertheless, most ecotoxicological research and chemicals regulation focus on hazard and exposure assessment of individual substances only, the problem of chemical mixtures in the environment is ignored to a large extent. In contrast, the assessment of combination effects has a long tradition in pharmacology, where mixtures of chemicals are specifically designed to develop new products, e.g. human and veterinary drugs or agricultural and non-agricultural pesticides. In this area, two concepts are frequently used and are thought to describe fundamental relationships between single substance and mixture effects: Independent Action (Response Addition) and Concentration Addition. The question, to what extent these concepts may also be applied in an ecotoxicological and regulatory context may be considered a research topic of major importance, as the concepts would allow to make use of already existing single substance toxicity data for the predictive assessment of mixture toxicities. Two critical knowledge gaps are identified: (a) There is a lack of environmental realism, as a huge part of our current knowledge about the applicability of the concepts is restricted to artificial situations with respect to mixture composition or biological effect assessment. (b) The knowledge on what exactly is needed for using the concepts as tools for the predictive mixture toxicity assessment is insufficient. Both gaps seriously hamper the necessary, scientifically sound consideration of mixture toxicities in a regulatory context.In this paper, the two concepts will be briefly introduced, the necessity of considering the toxicities of chemical mixtures in the environment will be demonstrated and the applicability of Independent Action and Concentration Addition as tools for the prediction and assessment of mixture toxicities will be discussed. An overview of the specific aims and approaches of the BEAM project to fill in the identified knowledge gaps is given and first results are outlined. 相似文献
37.
Is Prediction of Future Landslides Possible with a GIS? 总被引:5,自引:0,他引:5
Fabbri Andrea G. Chung Chang-Jo F. Cendrero Antonio Remondo Juan 《Natural Hazards》2003,30(3):487-503
This contribution explores a strategy for landslide hazard zonation inwhich layers of spatial data are used to represent typical settings inwhich given dynamic types of landslides are likely to occur. Theconcepts of assessment and prediction are defined to focus on therepresentation of future hazardous events and in particular on themyths that often provide obstacles in the application of quantitativemethods. The prediction rate curves for different applications describethe support provided by the different data layers in experiments inwhich the typical setting of hazardous events is approximated bystatistically integrating the spatial information. 相似文献
38.
39.
Validation of Landslide Susceptibility Maps; Examples and Applications from a Case Study in Northern Spain 总被引:9,自引:3,他引:9
Remondo Juan González Alberto De Terán José Ramón Díaz Cendrero Antonio Fabbri Andrea Chung Chang-Jo F. 《Natural Hazards》2003,30(3):437-449
A procedure for validating landslide susceptibility maps wasapplied in a study area in northern Spain and the results obtained compared. Validationwas used to carry out sensitivity analysis for individual variables and combinationsof variables. The validity of different map-making methods was tested, as well as theutility of different types of Favourability Functions. The results obtained show thatvalidation is essential to determine the predictive value of susceptibility maps. Italso helps to better select the most suitable function and significant variables, thus improving the efficiency of the mapping process. Validation based on a temporal strategy makes it possible to derive hazard maps from susceptibility maps. 相似文献
40.
Concepción L. Alados Fernando G. Barroso Antonio Aguirre Juan Escós 《Journal of Arid Environments》1996,34(4):455-463
To determine if defoliation of a woody plant affects foraging by folivorous insects, we examined the infection rate (number of leaves damaged per total number of leaves sampled on marked stems) ofAnthyllis cytisoidesunder three experimental treatments: 10, 50 and 90% plant defoliation. Observations were made for three age classes, established by trunk base perimeter (equal to or lower than 11 cm, between 11 and 20 cm, larger than 20 cm). Plants respond positively to artificial defoliation by increasing total vegetative length of the stem and total inflorescence length. This response is most evident in young individuals.Response to herbivory was measured as overall infection rate and also as infection rate by different feeding guilds—chewing, mining, or sucking insects. We found that increased defoliation elicited increased resistance of leaves to insect attack. This was particularly evident in young plants. Different insect guilds respond in different ways. Attack by chewing insects declines with defoliation for all plant age classes; only sucking insects which feed on the oldest plants reduce feeding rate with plant defoliation. Finally, mining insects present the opposite trend in young and senescent plants. 相似文献