全文获取类型
收费全文 | 3432篇 |
免费 | 474篇 |
国内免费 | 731篇 |
专业分类
测绘学 | 884篇 |
大气科学 | 388篇 |
地球物理 | 652篇 |
地质学 | 1515篇 |
海洋学 | 466篇 |
天文学 | 68篇 |
综合类 | 278篇 |
自然地理 | 386篇 |
出版年
2024年 | 12篇 |
2023年 | 39篇 |
2022年 | 98篇 |
2021年 | 125篇 |
2020年 | 151篇 |
2019年 | 192篇 |
2018年 | 107篇 |
2017年 | 161篇 |
2016年 | 153篇 |
2015年 | 184篇 |
2014年 | 256篇 |
2013年 | 265篇 |
2012年 | 260篇 |
2011年 | 241篇 |
2010年 | 183篇 |
2009年 | 186篇 |
2008年 | 164篇 |
2007年 | 210篇 |
2006年 | 201篇 |
2005年 | 190篇 |
2004年 | 162篇 |
2003年 | 136篇 |
2002年 | 126篇 |
2001年 | 118篇 |
2000年 | 99篇 |
1999年 | 81篇 |
1998年 | 72篇 |
1997年 | 89篇 |
1996年 | 68篇 |
1995年 | 53篇 |
1994年 | 60篇 |
1993年 | 40篇 |
1992年 | 35篇 |
1991年 | 17篇 |
1990年 | 17篇 |
1989年 | 19篇 |
1988年 | 23篇 |
1987年 | 14篇 |
1986年 | 7篇 |
1985年 | 8篇 |
1984年 | 5篇 |
1983年 | 2篇 |
1982年 | 2篇 |
1979年 | 4篇 |
1977年 | 1篇 |
1954年 | 1篇 |
排序方式: 共有4637条查询结果,搜索用时 15 毫秒
101.
裂变径迹长度测量标准化研究——锆石蚀刻标准探讨 总被引:1,自引:0,他引:1
通过不同时间蚀刻的锆石径迹长度测量,确定海南迅速冷却火山型锆石的水平封闭径迹(HCT)平均长度L=10.93 μm,标准偏差为0.93 μm,可作为锆石长度测量的参照标准.对锆石的蚀刻标准进行了探讨,确立垂直C轴的表面径迹宽度Dpar为(1.0±0.5)μm的蚀刻标准,要求Dpar>1 μm和Dpar<1 μm的颗粒数应大致相当,以及封闭径迹宽度DHCT<1.0 μm的测量标准. 相似文献
102.
整套重力计算程序包括了除“地改”以外的所有项目的计算,在这篇文章中我们先仅介绍了“测区参数初始化”及“基点网联测”(采用三程小循环方式)的计算方法。“测区参数初始化”功能可生成gdw.csh初始化文件(该套程序的各项功能都要读取该文件中的相关参量);“基点网联测”功能可计算基点网联测的各项精度,对联测进行平差,求出各分基点绝对重力值。 相似文献
103.
论述了用瑞利波检测高速公路软基加固的一些主要技术问题,应用新兴的小波分析技术,改善和提高检测的质量,并通过福宁高速公路的实际检测结果,说明了其思路和方法的正确性、实用性. 相似文献
104.
Estimates of the number of undiscovered deposits on a statewide basis offer a different perspective on the nation's undiscovered resources of gold, silver, copper, lead, and zinc. Mean estimates of the number of undiscovered deposits statewide were extracted from the estimates of undiscovered deposits nationwide. More than 50 undiscovered deposits are estimated to occur in Alaska, Arizona, Nevada, and Wisconsin. Estimating the number of undiscovered deposits statewide serves as a measure of a state's total remaining mineral resources in known conventional deposit types. 相似文献
105.
In a recent issue of Urban Geography (2001) , a number of key players in the 1960s and 1970s school of quantitative urban geography (called Chicago II in this article) set out some of the approach's key methodological premises and assessed its influence in the wider arena of urban studies. At about the same time, the 1920s and 1930s Chicago School of urban sociology (called the Chicago School in this article) was being reassessed in France ( Huet 2000 ), and deconstructed in Los Angeles ( Dear 2001 ). In this article, we outline a selection of basic models of urban space proposed by the Chicago School and further elaborated by Chicago II. We then consider certain aspects of three important critiques: humanist/aesthetic, Marxist, and postmodern. We argue that none of these invalidates the Chicago II approach to the study of urban areas, and we demonstrate its resilience and usefulness by way of the empirical example of Montreal. Though the results are of interest in their own right, the principal purpose of the analysis is to illustrate the type of insight that a structured quantitative approach provides and the way this approach rests on a theoretical understanding of processes at work in cities. We conclude by arguing that the humanist and Marxist critiques shed important light upon the possibilities and limits of the Chicago II approach, but that the postmodern claim that the spatial development of urban areas is not structured by at least some general processes is inaccurate. 相似文献
106.
The gravity field of the earth is a natural element of the Global Geodetic Observing System (GGOS). Gravity field quantities are like spatial geodetic observations of potential very high accuracy, with measurements, currently at part-per-billion (ppb) accuracy, but gravity field quantities are also unique as they can be globally represented by harmonic functions (long-wavelength geopotential model primarily from satellite gravity field missions), or based on point sampling (airborne and in situ absolute and superconducting gravimetry). From a GGOS global perspective, one of the main challenges is to ensure the consistency of the global and regional geopotential and geoid models, and the temporal changes of the gravity field at large spatial scales. The International Gravity Field Service, an umbrella “level-2” IAG service (incorporating the International Gravity Bureau, International Geoid Service, International Center for Earth Tides, International Center for Global Earth models, and other future new services for, e.g., digital terrain models), would be a natural key element contributing to GGOS. Major parts of the work of the services would, however, remain complementary to the GGOS contributions, which focus on the long-wavelength components of the geopotential and its temporal variations, the consistent procedures for regional data processing in a unified vertical datum and Terrestrial Reference Frame, and the ensuring validations of long-wavelength gravity field data products. 相似文献
107.
108.
This paper presents a critical and reflexive account of using Q methodology in human geography. Q methodology has a long pedigree in psychological, political and sociological research, but is only recently beginning to be used by human geographers. We discuss, in particular, the parts of the process(es) of Q methodology that are often glossed over in the literature, through reflecting on our learning in using Q within a project examining the use and production of environmental science by NGOs. We conclude that Q may be a useful supplement to existing methods in human geography, as long as it is used creatively and reflexively and with full awareness of its interpretative dimensions. 相似文献
109.
Measurement of stream cross section using ground penetration radar with Hilbert–Huang transform
下载免费PDF全文
![点击此处可从《水文研究》网站下载免费的PDF全文](/ch/ext_images/free.gif)
This study presents a new method to measure stream cross section without having contact with water. Compared with conventional measurement methods which apply instruments such as sounding weight, ground penetration radar (GPR), used in this study, is a non‐contact measurement method. This non‐contact measurement method can reduce the risk to hydrologists when they are conducting measurements, particularly in high flow period. However, the original signals obtained by using GPR are very complex, different from studies in the past where the measured data were mostly interpreted by experts with special skill or knowledge of GPR so that the results obtained were less objective. This study employs Hilbert–Huang transform (HHT) to process GPR signals which are difficult to interpret by hydrologists. HHT is a newly developed signal processing method that can not only process the nonlinear and non‐stationary complex signals, but also maintain the physical significance of the signal itself. Using GPR with HHT, this study establishes a non‐contact stream cross‐section measurement method with the ability to measure stream cross‐sectional areas precisely and quickly. Also, in comparison with the conventional method, no significant difference in results is found to exist between the two methods, but the new method can considerably reduce risk, measurement time, and manpower. It is proven that the non‐contact method combining GPR with HHT is applicable to quickly and accurately measure stream cross section. Copyright © 2013 John Wiley & Sons, Ltd. 相似文献
110.
This paper analyses the effect of rain data uncertainty on the performance of two hydrological models with different spatial structures: a semidistributed and a fully distributed model. The study is performed on a small catchment of 19.6 km2 located in the north‐west of Spain, where the arrival of low pressure fronts from the Atlantic Ocean causes highly variable rainfall events. The rainfall fields in this catchment during a series of storm events are estimated using rainfall point measurements. The uncertainty of the estimated fields is quantified using a conditional simulation technique. Discharge and rain data, including the uncertainty of the estimated rainfall fields, are then used to calibrate and validate both hydrological models following the generalized likelihood uncertainty estimation (GLUE) methodology. In the storm events analysed, the two models show similar performance. In all cases, results show that the calibrated distribution of the input parameters narrows when the rain uncertainty is included in the analysis. Otherwise, when rain uncertainty is not considered, the calibration of the input parameters must account for all uncertainty in the rainfall–runoff transformation process. Also, in both models, the uncertainty of the predicted discharges increase in similar magnitude when the uncertainty of rainfall input increase. 相似文献