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本文使用西北地区95个测站,时间序列为1960-2000年共41年的加密测站资料,运用旋转经验正交函数分解(REOF)以及功率谱分析等方法,对西北地区夏季降水的时空特征及其演变规律进行了诊断分析,结果表明:西北地区夏季降水区域性较强,存在多时空尺度特征,且平均具有准3年和4.8年的周期变化。将西北地区夏季水异常区域划分为4个区后可见,北疆地区夏季降水的周期振荡最接近平均状况,南疆降水异常区次之,相比之下青海异常降水区解释方差最小,同时发现,在近41年来,西北地区夏季降水的平均状况是80年代以前多数年份降水偏少,干旱性强,80年代以后整个西北地区夏季降水则有增多的趋势;分区分析后表现为:北疆区和南疆区从80年代后期开始降水增多,内蒙古西部地区90年代以后降水增多,而东北高原区夏季除70年代到80年代中期降水偏多外,其前和其后降水都偏少。 相似文献
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利用观测资料和MM5中尺度非静力模式产生的客观分析资料, 分析了2004年9月3~5日出现在川东地区大暴雨过程的大尺度环流特征和主要的中尺度天气系统及其结构。分析表明:中纬度低压槽的东移与西伸加强的副热带高压在青藏高原北部地区形成了有利于高原切变线和西南低涡生成发展的环流条件;西南低涡东侧的暖式切变线是对流活动最活跃的区域, 强降水主要出现在暖式切变线上;西南低涡是一个主要出现在对流层中低层的涡旋系统, 与大暴雨区相对应的整层强上升运动是低涡切变线南北两侧的正反向垂直环流共同作用的结果。 相似文献
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Most of the rocks in Precambrian shield areas have experienced a complex structural and metamorphic evolution, processes which have a strong influence on bedrock quality. The properties vary on both a local and a regional scale. It is highly beneficial to know the variations in detail when exploiting the rocks for industrial purposes. The main objective of the investigation was to study the variation of rock mechanical properties in an originally more or less isotropic rock at various stages of ductile deformation. The rocks investigated were Paleoproterozoic and with ages of ca. 1.80-1.88 Ga, and the areas chosen for sampling were situated north-east of Lake Vänern (Kristinehamn; 10 samples), south of the city of Eskilstuna, South Central Sweden (5 samples) and south of Ödeshög, near Lake Vättern in southern Sweden (7 samples). The 12 samples from the latter two areas are described in this investigation, while the 10 samples from Kristinehamn have been published earlier (Göransson et al., 2004). A comprehensive study of various parameters of importance for bedrock quality has been performed on all samples, e.g. studded tyre test (STT) and Los Angeles test (LA), uniaxial compressive strength (UCS), ultrasonic velocity, perimeter measurements of mineral phases, and petrographic and chemical analyses. The weakly deformed and massive (more or less isotropic) rocks show a tendency towards better properties of abrasion (STT) than the strongly deformed rocks and this can also be said for UCS, reflecting the greater ability of rocks to split along foliation planes. This is not entirely unambiguous, as the more deformed rocks, such as the mylonites, may have varying properties. This depends on the combined effects of, e.g. grain size, recrystallisation and foliation. However, the brittleness (LA) shows somewhat better values with increased deformation. This may depend on higher amounts of dark minerals, as their existence does not affect this test as much as in the case of abrasion tests. The perimeter values of the mineral phases display generally higher values, i.e. grain boundaries for the more strongly deformed rocks are more complicated. However, the values for the investigated mylonites may vary between low and high. The lower value may be due to dynamic recrystallisation and the creation of triple points (static recrystallisation) making the rock weaker. Besides, the development of a strong foliation may decrease rock strength despite the usually finer grain size. The results show that it is extremely important to consider all possible variations of bedrock before classification and exploitation, as the bedrock material in fact is highly heterogeneous. 相似文献
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George D. Bouckovalas Achilleas G. Papadimitriou 《Soil Dynamics and Earthquake Engineering》2005,25(7-10):547-558
This paper presents results of numerical analyses for the seismic response of step-like ground slopes in uniform visco-elastic soil, under vertically propagating SV seismic waves. The aim of the analyses is to explore the effects of slope geometry, predominant excitation frequency and duration, as well as of the dynamic soil properties on seismic ground motion in a parametric manner, and provide qualitative as well as quantitative insight to the phenomenon. Among the main conclusions of this study is that this kind of topography may lead to intense amplification or de-amplification variability at neighboring (within a few tens of meters) points behind the crest of the slope, especially for high frequency excitations. Nevertheless, a general trend of amplification near the crest and de-amplification near the toe of the slope seems to hold for the horizontal motion. As a result of these two findings, it becomes evident that reliable field evidence of slope topography aggravation is extremely difficult to establish. Furthermore, this study highlights the generation of a parasitic vertical component of motion in the vicinity of the slope, due to wave reflections at the slope surface, that under certain preconditions may become as large as the horizontal. Criteria are established for deciding on the importance of topography effects, while approximate relations are provided for the preliminary evaluation of the topographic aggravation of seismic ground motion and the width of the affected zone behind the crest. 相似文献
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