全文获取类型
收费全文 | 420篇 |
免费 | 38篇 |
国内免费 | 28篇 |
专业分类
测绘学 | 11篇 |
大气科学 | 28篇 |
地球物理 | 240篇 |
地质学 | 99篇 |
海洋学 | 41篇 |
天文学 | 16篇 |
综合类 | 11篇 |
自然地理 | 40篇 |
出版年
2023年 | 3篇 |
2022年 | 7篇 |
2021年 | 19篇 |
2020年 | 15篇 |
2019年 | 11篇 |
2018年 | 12篇 |
2017年 | 33篇 |
2016年 | 19篇 |
2015年 | 16篇 |
2014年 | 25篇 |
2013年 | 32篇 |
2012年 | 17篇 |
2011年 | 22篇 |
2010年 | 24篇 |
2009年 | 20篇 |
2008年 | 23篇 |
2007年 | 21篇 |
2006年 | 26篇 |
2005年 | 21篇 |
2004年 | 11篇 |
2003年 | 6篇 |
2002年 | 7篇 |
2001年 | 11篇 |
2000年 | 6篇 |
1999年 | 6篇 |
1998年 | 11篇 |
1997年 | 15篇 |
1996年 | 12篇 |
1995年 | 7篇 |
1994年 | 2篇 |
1993年 | 4篇 |
1992年 | 2篇 |
1991年 | 5篇 |
1990年 | 1篇 |
1989年 | 1篇 |
1988年 | 2篇 |
1987年 | 1篇 |
1984年 | 1篇 |
1983年 | 2篇 |
1982年 | 2篇 |
1981年 | 1篇 |
1980年 | 2篇 |
1978年 | 1篇 |
1977年 | 1篇 |
排序方式: 共有486条查询结果,搜索用时 15 毫秒
11.
《Marine Policy》2017
Socio-economic development of small-island fishing communities is greatly dependent on local coastal and marine resources. However, illegal fishing and aggressive practices in insular ecosystems lead to over-exploitation and environmental deterioration. Moreover, a lack of scientific data increases uncertainty and prevents the adequate monitoring of marine resources. This paper focuses on the integration of local fishing communities into decision-making processes with the aim of promoting artisanal fishing on the Island of Tenerife (the Canary Islands), as a way to preserve the marine ecosystem and socio-economic development of traditional cofradias (fishers' organisations). A qualitative methodological framework, based on participatory problem-solution trees and focus groups, was used to identify the main factors impeding the sustainable development of the artisanal fishing sector on the island and to elaborate collective proposals with policy implications. The fishing community involved identified four main issues that are maintaining an unsustainable island fishery: 1) Over-exploitation; 2) Poor self-management of cofradias and commercialisation problems; 3) Fisher individualism and low co-management strategies, and 4) Illegal fishing increase vs. artisanal fishing decline. Results show the required policy enhancements to tackle them and the need to adapt regulations to the local situation. 相似文献
12.
13.
Obesity is a serious public health problem in the United States. It is important to estimate obesity prevalence at the local level to target programmatic and policy interventions. It is challenging, however, to obtain local estimates of obesity prevalence because national health surveys such as the Centers for Disease Control and Prevention (CDC) Behavioral Risk Factor Surveillance System (BRFSS) are not designed to produce direct estimates at the local levels (e.g. census tracts) due to small population samples and the need to preserve individual confidentiality. In this study we address the problem of estimating local obesity prevalence rates by implementing a spatial microsimulation modeling technique to proportionally replicate the demographic characteristics of BRFSS respondents to census tract populations in metropolitan Detroit. Obesity prevalence rates are examined for high and low spatial clusters and studied in relation to the U.S. Department of Agriculture's (USDA) measures of low-income neighborhoods and local food deserts and CDC's measure of healthy and less healthy food environments currently used to target obesity reduction initiatives. This study found that obesity prevalence was largely clustered in the City of Detroit extending north into contiguous suburbs. The spatial patterns of highest obesity prevalence tracts were most similarly aligned with USDA-defined low-income tracts and CDC's less healthy food tracts. The locations of USDA's food desert tracts rarely overlapped with the highest obesity prevalence tracts. This study demonstrated a new methodology by which to assess local areas in need of future obesity interventions. 相似文献
14.
Shaukat Goderya 《Astrophysics and Space Science》2002,282(1):121-130
Astronomical research with a small telescope (20 cm – 40 cm) has always been a challenging problem. The invention of CCD cameras
and personal computers has now put this question to past as small telescopes can do good and practicle science. This paper
describes the use of small telescope in the study of eclipsing binary stars. Binary stars play an important role as astrophysical
laboratories in our quest to understand the evolution and structure of stars. The most useful aspects of research with a small
telescope in binary star research is; 1) as a viable teaching laboratory for begining students 2) to teach and learn the fundamental
observational techniques that are common to many types of astronomical research areas 3) as a starting point to initiate research
programs in observational astronomy, optics, instrumentation, computational astrophysics and 4) as a foundation to develop
an infrastructure and technical know how for larger telescope facility.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
15.
16.
In the paper, we present newly developed hydro-mechanical hypoplastic model for partially saturated soils predicting small strain stiffness. Hysteretic void ratio dependent water retention model has been incorporated into the existing hypoplastic model. This required thorough revision of the model structure to allow for the hydro-mechanical coupling dependencies. The model is formulated in terms of degree of saturation, rather than of suction. Subsequently, the small strain stiffness effects were incorporated using the intergranular strain concept modified for unsaturated conditions. New features included degree of saturation-dependent size of the elastic range and an updated evolution equation for the intergranular strain. The model has been evaluated using two comprehensive data sets on completely decomposed tuff from Hong-Kong and Zenos Kaolin from Iran. It has been shown that the modified intergranular strain formulation coupled with the hysteretic water retention model correctly reproduces the effects of both the stress and suction histories on small strain stiffness evolution. The model can correctly predict also different other aspects of partially saturated soil behaviour, starting from the very small strain range up to the asymptotic large-strain response. 相似文献
17.
Investigating the effect of soil models on deformations caused by braced excavations through an inverse-analysis technique 总被引:1,自引:0,他引:1
In this study, a series of inverse-analysis numerical experiments was performed to investigate the effect of soil models on the deformations caused by excavation by using the finite element method. The nonlinear optimization technique that was incorporated into the finite element code was used for the inverse-analysis numerical experiments. Three soil models (the hyperbolic model, pseudo-plasticity model, and modified pseudo-plasticity model) were employed in the intended numerical experiments on a well-documented excavation case history. The results indicate that wall deflection due to excavation can be accurately back-figured by each of the three soil models, while the ground surface settlement can be reasonably optimized only by the pseudo-plasticity model and the modified pseudo-plasticity model. Importantly, the modified pseudo-plasticity model can yield more reasonable simulations when the wall deflection and the ground surface settlement are simultaneously back-figured. The results show that selection of an adequate soil model that is capable of adequately describing the stress–strain-strength characteristics of the soils is essentially crucial when predicting the excavation-induced ground response. 相似文献
18.
某工程设计采用轻质砼小型空心砌块墙体,在施工过程中,从各个建筑细部上很下工夫,精益求精,使施工质量得到保证,本文介绍其施工技术与细部做法。 相似文献
19.
P. D. Katsabanis A. Tawadrous C. Braun C. Kennedy 《Fragblast: International Journal for Blasting and Fragmentation》2006,10(1):83-93
A series of small scale tests, simulating multi-hole blasts have been performed to establish the effect of delays on blast fragmentation. The blasts were performed in high quality granodiorite blocks, which were cut from stone prepared by dimensional stone quarry operations. The pattern used was equilateral triangular, with a distance of 10.2 cm between boreholes, which had a diameter of 11 mm, were loaded with detonating cord and the coupling medium was water. The delays used were achieved using different lengths of detonating cord for the cases of delays between 0 and 100 μs between holes and a sequential blasting machine firing seismic detonators for larger delays up to 4 ms. All fragments were collected and screened. The experiments showed that the worst fragmentation was achieved with simultaneous initiation of all charges. Fragmentation improved with the delay time between holes up to 1 ms between holes. If the experiments are scaled up, the results show that in granodiorite, fragmentation optimization requires delays of few milliseconds per metre of burden. The findings, agree with previously published work, involving larger scale experiments and other rock types. 相似文献
20.
W. L. Fourney U. Leiste R. Bonenberger D. J. Goodings 《Fragblast: International Journal for Blasting and Fragmentation》2005,9(4):205-217
This paper investigates the various mechanisms and parameters that are responsible for delivering impulse to a vehicle that is unfortunate enough to detonate a buried mine. Small scale tests are used to examine the effects of air blast or ejected sand in imparting impulse to a plate that is located above the surface of the saturated soil that contains the explosive. Parameters such as confinement, stand off distance, depth of burial of the explosive, density of the soil, and saturation level of the soil are also examined. 相似文献