全文获取类型
收费全文 | 107035篇 |
免费 | 2020篇 |
国内免费 | 949篇 |
专业分类
测绘学 | 2845篇 |
大气科学 | 8022篇 |
地球物理 | 21339篇 |
地质学 | 37018篇 |
海洋学 | 9537篇 |
天文学 | 24287篇 |
综合类 | 309篇 |
自然地理 | 6647篇 |
出版年
2021年 | 883篇 |
2020年 | 1047篇 |
2019年 | 1110篇 |
2018年 | 2288篇 |
2017年 | 2220篇 |
2016年 | 2936篇 |
2015年 | 1782篇 |
2014年 | 2877篇 |
2013年 | 5561篇 |
2012年 | 2995篇 |
2011年 | 4106篇 |
2010年 | 3618篇 |
2009年 | 4900篇 |
2008年 | 4340篇 |
2007年 | 4153篇 |
2006年 | 4073篇 |
2005年 | 3223篇 |
2004年 | 3342篇 |
2003年 | 3148篇 |
2002年 | 2987篇 |
2001年 | 2656篇 |
2000年 | 2590篇 |
1999年 | 2200篇 |
1998年 | 2232篇 |
1997年 | 2121篇 |
1996年 | 1892篇 |
1995年 | 1765篇 |
1994年 | 1585篇 |
1993年 | 1454篇 |
1992年 | 1395篇 |
1991年 | 1275篇 |
1990年 | 1467篇 |
1989年 | 1233篇 |
1988年 | 1157篇 |
1987年 | 1351篇 |
1986年 | 1194篇 |
1985年 | 1490篇 |
1984年 | 1698篇 |
1983年 | 1552篇 |
1982年 | 1457篇 |
1981年 | 1363篇 |
1980年 | 1210篇 |
1979年 | 1193篇 |
1978年 | 1239篇 |
1977年 | 1079篇 |
1976年 | 1033篇 |
1975年 | 987篇 |
1974年 | 945篇 |
1973年 | 988篇 |
1971年 | 609篇 |
排序方式: 共有10000条查询结果,搜索用时 46 毫秒
951.
A numerical analysis of cyclotron instabilities is carried out by computing the dispersion relation for a three component cold plasma-beam system. Rates of growth and damping for various values of the stream density are calculated from the dispersion relation. The rates of growth and damping increase monotonically as the number density of the proton stream increases. It is found that the frequencies at the rates of maximum growth and the damping decrease slightly to lower frequencies and a sharp peak at these frequencies becomes blunt. The minimum e-folding times of an ion cyclotron wave for (a) σs = 10−4, σi = 10−2 and (b) σs = 10−1, σi = 10−2 are about 3·84 and 0·16 sec respectively in the vicinity of the equatorial plane at 6 Re, where σs and σi are the ratios of the beam density Ns and the helium ion (H6+) density Ni to the total positive ions in the plasma-beam system. 相似文献
952.
P. N. S. O'BRIEN 《Geophysical Prospecting》1969,17(4):511-547
Recordings were made with three types of detector of the primary compressional (P) and shear (S) wave pulses generated by explosions in boreholes. Charge weights varied from 0.08 kg to 9.5 kg and detector distances varied from about 3 m to about 80 m. Scaling by the simple factor W1/3 where W is the charge weight, enabled observations from different sized charges to be fitted to a single expression. Experiments were carried out in the Bunter sandstone and the London clay and both fluid and solid tamping were used. This variation in tamping had no significant effect on the P-waves but it may have affected the generation of SV-waves. In both media the P-wave energy carried at 30 m from the shot by frequencies less than 100 Hz decreased rapidly with depth and was usually 1–2 % of the available chemical energy for a shot depth of 15 m. The S-wave energy was much less than this, but was highly directional. The P-wave pulse had the appearance of a damped sinusoid in very good agreement with the predictions of the ‘equivalent radiator’ hypothesis. However, the surface of this radiator should be identified not with the blown cavity but with the surface at which the tensile stresses associated with the stress wave become less than the tensile strength of the rock. The predominant frequency for a 1 kg charge at a depth of 15 m was 24 Hz in the clay and 52 Hz in the sandstone. In these and similar media, therefore, an effort should be made to keep individual charges less than 1 kg in reflection shooting and less than 10 kg in refraction shooting. The value of Q was about 50 in clay and about 25 in the sandstone. These estimates are rather uncertain because of the small distances over which the pulses were observed. The Z-transforms of the sampled pulses indicated that they were all of minimum phase, or very near to it. 相似文献
953.
954.
The petrology of the East Otago Volcanic Province (late Miocene),including the Dunedin Complex, is dominated by undersaturatedsodic lavas and shallow intmsives whose compositions range fromalkali basalt, basanite, and nephelinite to trachyte and phonolite.A range of relatively potassic volcanic rocks is also developedin the Province. The degree of undersaturation and ratios suchas (FeO+Fe2O3):MgO and K2O:Na2O vary widely in rocks with comparabledifferentiation indices. Chemical and mineralogical data onwhole rock-glass pairs and variants of shallow intrusives inthe Province, assessed in conjunction with similar data fromother volcanic provinces, demonstrate the production of trachyticliquids from alkali basaltic parents and of phonolitic liquidsfrom basanitic parents. Fractionation trends in both sodic andpotassic series, defined by variation in differentiation indexand normative nepheline, indicate that the degree of undersaturationof the derivative salic liquids is dependent on that of theparent; the slope of the fractionation trend lines is influencedby additional factors, including Po2. The unequivocal productionof phonolite from trachyte in the East Otago Province has notbeen demonstrated. The mafic variants of theralites from Waihola and a basanite-pegmatoidassociation from Omimi are similar in composition, but the mostsalic differentiates from these differentiation sequences displaypronounced differences in their KO:Na2O ratios, tending towardsleuco-theralitic (cf. lugarite) and malignitic compositionsrespectively. A multicomponent spectrum of fractionation lineages is suggestedfor individual differentiated bodies and for lava series. Sodiclineages include: (I) alkali basalthawaiitemugearitebenmoreitetrachyte;(2) a more undersaturated series from basanite through nephelinehawaiite, nepheline mugearite, and nepheline benmoreite, tophonolite; (3) a nephelinite series, more restricted in compositionalrange. The relatively high Fe: Mg ratios characterizing themugearitic variants are sometimes exhibited by members of themore potassic series in which, however, Fe:Mg ratios tend tobe characteristically lower. This feature is correlated withthe frequent occurrence of kaersutitic amphibole or its resorptionproducts, indicative of intratelluric fractionation under relativelyhydrous conditions and probably relatively high Po2. Certainnepheline trachyandesites from East Otago are described in detail.Trachy-basalt-trachyandesite-tristanite-trachyte and sanidinebasanite-nepheline trachyandesite-nepheline tristanite-phonolitelineages are also proposed, and the nomenclature of these moreseries discussed. 相似文献
955.
956.
J. W. Norman 《The Photogrammetric Record》1968,6(32):133-149
The geologist's use of air photographs and the characteristics studied by photogeologists are briefly described. The influence of photographic factors such as scale, season, time of day, film type, processing, overlap and dimensional accuracy are considered from the geologist's viewpoint. 相似文献
957.
Photogrammetric methods using sequential aerial photography can provide the geomorphologist with comparative measurements of rapidly changing glacial landforms. Examples are given of eskers, kame and kettle areas, an ice-dammed lake and coastal features in south-east Iceland. 相似文献
958.
959.
960.
The historical development of analytical aerial triangulation block adjustment is mentioned by way of introduction. The advantages of separating the plan and height adjustments are given and the observation equations of a typical adjustment section are analysed to show that this convenient practice is justified in normal circumstances. The principles of a direct solution of the normal equations suitable for a moderate-sized computer, are explained and developed in detail for the height adjustment. The advantages of tie-strips in the height adjustment are demonstrated and the difficulties which then arise are discussed. An approximate method which overcomes these difficulties is developed. Details of the computer programmes used are given in Appendix B. 相似文献