全文获取类型
收费全文 | 1367篇 |
免费 | 251篇 |
国内免费 | 366篇 |
专业分类
测绘学 | 79篇 |
大气科学 | 375篇 |
地球物理 | 356篇 |
地质学 | 654篇 |
海洋学 | 141篇 |
天文学 | 31篇 |
综合类 | 198篇 |
自然地理 | 150篇 |
出版年
2024年 | 10篇 |
2023年 | 20篇 |
2022年 | 80篇 |
2021年 | 81篇 |
2020年 | 85篇 |
2019年 | 104篇 |
2018年 | 89篇 |
2017年 | 96篇 |
2016年 | 113篇 |
2015年 | 75篇 |
2014年 | 110篇 |
2013年 | 62篇 |
2012年 | 85篇 |
2011年 | 86篇 |
2010年 | 74篇 |
2009年 | 80篇 |
2008年 | 56篇 |
2007年 | 68篇 |
2006年 | 51篇 |
2005年 | 60篇 |
2004年 | 41篇 |
2003年 | 36篇 |
2002年 | 35篇 |
2001年 | 25篇 |
2000年 | 35篇 |
1999年 | 71篇 |
1998年 | 44篇 |
1997年 | 36篇 |
1996年 | 39篇 |
1995年 | 25篇 |
1994年 | 22篇 |
1993年 | 21篇 |
1992年 | 20篇 |
1991年 | 12篇 |
1990年 | 8篇 |
1989年 | 12篇 |
1988年 | 6篇 |
1987年 | 3篇 |
1986年 | 1篇 |
1985年 | 1篇 |
1984年 | 1篇 |
1983年 | 2篇 |
1982年 | 1篇 |
1980年 | 1篇 |
1958年 | 1篇 |
排序方式: 共有1984条查询结果,搜索用时 15 毫秒
991.
为探讨中高山区土壤及成土母质理化特征及其控制因素,本文以四川省西昌市为例,系统性总结了该地区的地质建造背景,并在基于地质建造背景划分的成土母质类型基础上,讨论了成土母质、土壤的物理化学性质、土壤含水率、pH值、高程等之间的关系,通过对不同成土母质发育的同种土壤的数据分析及营养元素含量评价,我们认为地质建造背景直接制约了成土母质及土壤的物理化学性质,营养元素在成土母质—土壤之间的迁移存在继承性。本文研究成果为中高山区土壤类型和成土母质之间的关系以及成土母质单元划分方案提供理论依据。 相似文献
992.
定量解析污染源对水质影响的贡献是水环境精细化管理的重要基础。目前多通过水质和土地利用类型的关系以解析水体污染源的研究,忽略了空间尺度的差异性,引发景观配置不合理的后果。为此,本研究依据考虑空间尺度效应的污染源解析方法,基于异龙湖流域3条主要入湖河流的入湖口监测断面对雨季和旱季的水质数据进行研究。同时利用绝对主成分—多元线性回归模型(APCS-MLR)和bioenv分析揭示河道不同尺度缓冲区的土地利用变化对水质的影响并解析河流主要污染源。研究结果表明:(1)异龙湖主要入湖河流水质表现出季节性差异,旱季期间3条主要入湖河流的浊度、化学需氧量(CODCr)、氨氮(NH3-N)、总磷(TP)和总氮(TN)浓度平均值相比于雨季减幅分别为39.53%、39.93%、94.48%、38.29%和1.72%。其中,入湖河流水体中的TN在旱季和雨季的超标率分别为58%和74%,成为首要污染物;(2)在旱季,20 m缓冲区尺度内河流水质受耕地和裸地占比影响较大,随着空间尺度的扩大,至50~300 m缓冲区尺度时建设用地、林地及水体占比对水质的影响增加;在雨季,C... 相似文献
993.
994.
995.
碳酸盐岩溶蚀动力学模拟实验 总被引:6,自引:1,他引:5
本文给出了碳酸盐岩溶蚀研究的新方法,即动力学模拟实验方法。这一方法是依据碳酸盐(MeCO3)——二氧化碳(CO2)——水(H2O)体系的两个水化学模型:方解石—CO2-H2O体系模型和白云石—CO2—H2O体系模型—而建立起来的。此二体系的状态和方解石或白云石的瞬时溶解速度仅受两个独立变量如Pco2和pH的制约。因而所设计的动力学实验也仅需要观侧二个独立变量。此外本文还以实验结果为基础讨论了方解石溶解的动力学规律。实验结果表明,动力学模拟实验结果较为可靠,方法较为先进。它为进行碳酸盐岩溶蚀的定量研究提供了一个新方法。 相似文献
996.
利用绵阳机场最近10年逐时能见度资料,对低能见度的气候特征进行了统计,并对造成低能见度障碍的天气现象和影响系统作了普查分折.结果表明:绵阳机场低能见度有明显的季节变化和闩变化:造成低能见度障碍的天气现象主要是雾. 相似文献
997.
The sedimentary succession of piston core RC26-16, dated by 14C accelerator mass spectrometry, provides a nearly continuous palaeoceanographic record of the northeastern South China Sea for the last 15000 yr. Planktic foraminiferal assemblages indicate that winter sea-surface temperatures (SSTs) rose from 18°C to about 24°C from the last glacial to the Holocene. A short-lived cooling of 1°C in winter temperature centred at about 11000 14C yr ago may reflect the Younger Dryas cooling event in this area. Summer SSTs have remained between 27°C and 29°C throughout the record. The temperature difference between summer and winter was about ca. 9°C during the last glacial, much higher than the Holocene value of ca. 5°C. During the late Holocene a short-lived cooling event occurred at about 4000 14C yr ago. Oxygen and carbon isotopic gradients between surface (0–50 m) and subsurface (50–100 m) waters were smaller during the last glacial than those in the Holocene. The fluctuation in the isotopic gradients are caused most likely by changes in upwelling intensity. Smaller gradients indicate stronger upwelling during the glacial winter monsoon. The fauna-derived estimates of nutrient content of the surface waters indicate that the upwelling induced higher fertility and biological productivity during the glacial. The winter monsoon became weaker during the Holocene. The carbonate compensation depth and foraminiferal lysocline were shallower during the Holocene, except for a short-lived deepening at about 5000 14C yr ago. A preservation peak of planktic foraminifera and calcium carbonate occurred between 13400 and 12000 14C yr ago, synchronous to the global preservation event of Termination I. 相似文献
998.
Chonglong Gao Youliang Ji Chenliang Wu Jun Jin Ying Ren Zhao Yang Dawei Liu Zhijun Huan Xiaobing Duan Yuqi Zhou 《Sedimentology》2020,67(4):1750-1796
Alluvial fans are usually constructed through episodic flood events. Despite the significance of these ephemeral floods on the morphodynamics of alluvial fans, depositional responses to the variations in flood conditions are still poorly documented. This greatly limits the ability to interpret ancient sedimentary successions of fans and the associated flood hydrodynamics. The Quaternary Poplar Fan from endorheic Heshituoluogai Basin provides an optimal case for addressing this issue. Based on the variations in facies associations and flood conditions, three depositional stages – namely; lobe building stage, channel building stage and the abandonment stage – are identified. During the lobe building stage the Poplar Fan is predominately constructed through incised channel flood, sheetflood and unconfined streamflood, with coeval development of distal surficial ephemeral ponds. The channel building stage is characterized by the development of gravelly braided rivers. However, only scour pool fill deposits are preferentially preserved in the Poplar Fan. During the abandonment stage, erosional lags and aeolian sands randomly occur throughout the fan, while gully deposits can only be found in the distal fan. The distinctive facies architecture of the Poplar Fan is likely to be the result of periodicity of climate fluctuations between wetter and drier conditions during the Late Pleistocene to Holocene. The ephemeral floods formed under wetter conditions usually show high discharge and sediment concentrations which facilitate the lobe building processes. During the drier periods, only gravelly braided rivers can be developed through ephemeral floods as the intensity and frequency in precipitation, discharge and sediment concentrations of the flood flows significantly decrease. The abandonment stage of the fan may occur between recurring flood episodes or during the driest periods. Furthermore, the long-term (105 to 106 year) geomorphic evolution of the Poplar Fan shows the influence of tectonic activities. The ongoing thrust uplift tectonic activities have caused destruction of the fan but can also facilitate the fan-head trench/incision of the fan, which in turn facilitate the progradation of the fan. This study proposes a new depositional model for alluvial fans constructed through episodic flood events, which shows the character of both sheet-flood dominated and stream-flow dominated end members of alluvial fans. These findings supplement the understanding of the variability of the alluvial fans and provide means to characterize rock record of alluvial fans and their associated flood and climate conditions. 相似文献
999.
ABSTRACT Subduction-related basaltic rocks in active continental margins should record information about the lithospheric mantle. Mafic rocks from the Qimantag region of the East Kunlun Orogenic Belt (EKOB), NW China, can be used to constrain the evolution of mantle sources. The Heishan basalts (445 Ma) and Xiarihamu gabbros (427 Ma) display distinct geochemical and isotopic features, with basalts yielding relatively lower Na2O+K2O (1.48–4.16 wt.%) and Mg# (0.50–0.57) than gabbros (Na2O+K2O = 2.96–4.07 wt.%, Mg# = 0.65–0.81). Although the basalts and gabbros show similar enrichment of LILE and depletion of HFSE, the gabbros have higher Th/Y and lower Sm/Th and Nb/U ratios than the basalts, indicative of derivation from a more enriched mantle source. The Heishan basalts have relatively positive εNd(t) values (+4.7 to +5.8) whereas the Xiarihamu gabbros have negative εNd(t) values ranging from ?5.5 to ?3.8. Crustal contamination played an insignificant role in the formation of the basalts and gabbros. Our data suggest that the basalts originated from a depleted mantle source, slightly enriched by subduction-related fluids, whereas the gabbros originated from an enriched mantle source. These findings support a subduction-related progressive lithospheric mantle enrichment model over ~20 Ma beneath the Qimantag region in the Early Palaeozoic. 相似文献
1000.
高精度曲面模型解算改进的Gauss-Seidel法 总被引:1,自引:1,他引:0
为了降低HASM的时间复杂度,采用一种改进Gauss-Seidel(GS)算法(MGS)解算HASM方程组。首先,从理论上分析了MGS算法收敛速度快于GS算法,然后以高斯合成曲面作为研究对象,用四组模拟试验表明,相同的网格数、达到相同的计算精度,MGS算法计算时间小于GS算法,且两种算法时间差与模拟区域网格数呈二次线性相关;固定网格数,使用相同的内迭代或者外迭代次数,MGS算法精度高于GS算法,但增加内迭代或者外迭代次数,GS算法同样收敛;MGS算法计算时间与网格数呈线性相关。MGS算法能够有效解决HASM模拟大区域的计算时间瓶颈,提高HASM运算速度。以甘肃省董志塬某测区SRTM3作为研究对象,基于MGS的HASM用于模拟DEM表明,HASM精度要高于传统的插值方法。 相似文献