全文获取类型
收费全文 | 593篇 |
免费 | 73篇 |
国内免费 | 57篇 |
专业分类
测绘学 | 46篇 |
大气科学 | 162篇 |
地球物理 | 194篇 |
地质学 | 108篇 |
海洋学 | 133篇 |
天文学 | 23篇 |
综合类 | 25篇 |
自然地理 | 32篇 |
出版年
2024年 | 1篇 |
2023年 | 4篇 |
2022年 | 5篇 |
2021年 | 13篇 |
2020年 | 13篇 |
2019年 | 20篇 |
2018年 | 16篇 |
2017年 | 15篇 |
2016年 | 23篇 |
2015年 | 12篇 |
2014年 | 20篇 |
2013年 | 35篇 |
2012年 | 19篇 |
2011年 | 23篇 |
2010年 | 20篇 |
2009年 | 39篇 |
2008年 | 45篇 |
2007年 | 35篇 |
2006年 | 25篇 |
2005年 | 39篇 |
2004年 | 29篇 |
2003年 | 16篇 |
2002年 | 19篇 |
2001年 | 24篇 |
2000年 | 26篇 |
1999年 | 30篇 |
1998年 | 28篇 |
1997年 | 17篇 |
1996年 | 16篇 |
1995年 | 10篇 |
1994年 | 4篇 |
1993年 | 10篇 |
1992年 | 8篇 |
1991年 | 10篇 |
1990年 | 7篇 |
1989年 | 9篇 |
1988年 | 4篇 |
1987年 | 4篇 |
1986年 | 3篇 |
1985年 | 10篇 |
1984年 | 9篇 |
1983年 | 2篇 |
1982年 | 2篇 |
1981年 | 2篇 |
1980年 | 1篇 |
1978年 | 1篇 |
排序方式: 共有723条查询结果,搜索用时 187 毫秒
71.
Three resection–intersection algorithms were applied to simulated projections and clinical data from radiostereometric patients. On simulated data, the more advanced bundle-adjustment-based algorithms outperformed the classical Selvik algorithm, even if the error reductions were small for some parameters. On clinical data, the results were inconclusive.The two different projection geometries had a much larger influence on the error size and distribution. For the biplanar configuration, the position and motion errors were small and almost isotropic. For the uniplanar configuration, the position errors were comparably high and anisotropic, but still resulted in a high accuracy for some motion parameters at the expense of others.The simplified resection–intersection algorithm by Selvik may still be considered a good and robust algorithm for radiostereometry. More studies will have to be performed to find out how the theoretical advantages of the bundle methods can be utilized in clinical radiostereometry. 相似文献
72.
Sven-Erik Gryning Ekaterina Batchvarova H. A. R. De Bruin 《Boundary-Layer Meteorology》2001,99(3):465-488
Measurements carried out in Northern Finland on radiation and turbulent fluxes over a sparse, sub-arctic boreal forest with snow covered ground were analysed. The measurements represent late winter conditions characterised by low solar elevation angles. During the experiment (12–24 March 1997) day and night were about equally long. At low solar elevation angles the forest shades most of the snow surface. Therefore an important part of the radiation never reaches the snow surface but is absorbed by the forest. The sensible heat flux above the forest was fairly large, reaching more than 100 W m-2. The measurements of sensible heat flux within and above the forest revealed that the sensible heat flux from the snow surface is negligible and the sensible heat flux above the forest stems from warming of the trees. A simple model for the surface energy balance of a sparse forest is presented. The model treats the diffuse and direct shortwave (solar) radiation separately. It introduces a factor that accounts for the shading of the ground at low solar elevation angles, and a parameter that deals with the partial transparency of the forest.Input to the model are the direct and diffuse incoming shortwave radiation.Measurements of the global radiation (direct plus diffuse incoming shortwaveradiation) above the forest revealed a considerable attenuation of the globalradiation at low solar elevation. A relation for the atmospheric turbidity asfunction of the solar elevation angle is suggested. The global radiation wassimulated for a three month period. For conditions with a cloud cover of lessthan 7 oktas good agreement between model predictions and measurementswere found. For cloud cover 7 and 8 oktas a considerable spread can beobserved. To apply the proposed energy balance model, the global radiationmust be separated into its diffuse and direct components. We propose a simpleempirical relationship between diffuse shortwave and global radiation asfunction of cloud cover. 相似文献
73.
In this study, turbulent heat flux data from two sites within the Baltic Sea are compared with estimates from two models. The main focus is on the latent heat flux. The measuring sites are located on small islands close to the islands of Bornholm and Gotland. Both sites have a wide wind direction sector with undisturbed over-water fetch. Mean parameters and direct fluxes were measured on masts during May to December 1998.The two models used in this study are the regional-scale atmospheric model HIRLAM and the ocean model PROBE-Baltic. It is shown that both models overestimate the sensible and latent heat fluxes. The overestimation can, to a large extent, be explained by errors in the air-water temperature and humidity differences. From comparing observed and modelled data, the estimated 8-month mean errors in temperature and humidity are up to 1 °C and 1 g kg-1, respectively. The mean errors in the sensible and latent heat fluxes for the same period are approximately 15 and 30 W m-2, respectively.Bulk transfer coefficients used for calculating heat and humidity fluxes at the surface were shown to agree rather well with the measurements, at least for the unstable data. For stable stratification, the scatter in data is generally large, and it appears that the bulk formulation chosen overestimates turbulent heat fluxes. 相似文献
74.
Absorption cross-section measurements of NO2 performed in our laboratory have been extended to the 200–300 nm region at ambient temperature. Low pressures have been used, limiting the effects of the dimer N2O4 which has an absorption cross-section from one to two orders of magnitude larger than that of NO2 in this region. The results have been compared to those of previous authors and are now available for atmospheric purposes at 0.01 nm intervals.Unité de Recherche Associée au CNRS. 相似文献
75.
钻孔应变,应力测量区域特征初探 总被引:1,自引:0,他引:1
我国钻孔应变、应力测量依不同时段区域变化形态有明显差异;短期(一月内)、中期(一年内)区域变化特征不显著。正常形态显示平稳、应变测量显示固体潮形态、应力测量显示年变形态。震前异常多呈现突升、突降式的应变阶跃、固体潮畸变、年变形态改变。长期(10年内)与其构造部位、地震活动性及区域构造运动,矢量速率有一定的相关性,因而存在着明显的区域变化特征。识别不同区域的曲线变化特征,对判定震兆信息有一定的指导作用,从而提高了应变、应力测量资料的应用价值。 相似文献
76.
You YI Zhaonan CAI Yi LIU Shuangxi FANG Yuli ZHANG Dongxu YANG Yong WANG Miao LIANG Maohua WANG 《大气科学进展》2020,37(6):608-616
The balloon-borne Aircore campaign was conducted in Inner Mongolia,China,on June 13 and 14 2018,which detected carbon dioxide(CO_2) and carbon monoxide(CO) profiles from surface to 24 km,showing strong positive and negative correlations between 8 km and 10 km on 13 and 14 June,respectively.Backward trajectories,meteorological analyses,and CO_2 horizontal distributions were combined to interpret this phenomenon.The results indicated that the source region experienced a stratospheric intrusion and exhibited a large horizontal CO_2 gradient;namely,lower CO concentrations corresponded to higher CO_2 concentrations and vice versa.The laminar structure with multiple origins resulted in the highly negative correlation between CO_2 and CO in the upper troposphere on 14 June.The contribution of stratospheric air mass to the upper troposphere and that of tropospheric air mass to the lower stratosphere were 26.7% and24.3%,respectively,based on a mass balance approach.Another interesting phenomenon is that CO_2 and CO concentrations increased substantially at approximately 8 km on 13 June.An analysis based on the backward trajectory implied that the air mass possibly came from anthropogenic sources.The slope of CO_2/CO representing the anthropogenic sources was 87.3 ppm ppm~(-1).In addition,the CO_2 profile showed that there was a large CO_2 gradient of 4 ppm km-1 within the boundary layer on 13 June,and this gradient disappeared on 14 June. 相似文献
77.
78.
Roughness control on hydraulic conductivity in fractured rocks 总被引:1,自引:0,他引:1
The influence of joint roughness on the typologies of fluid flow inside fractures is well known and, thanks to experiences
in the field of hydraulics, it has been studied from both a physical and mathematical point of view. Nevertheless, the formulations
adopted by traditional hydraulic models are hardly applicable in the geological field, because of the difficulty encountered
in the roughness parameter estimation. Normally this parameter can be estimated using the joint roughness coefficient (JRC),
which considers both the asperity height and its regularity and directional trend. The main advantage in using the JRC arises
from the fact that it can easily be obtained from geological-technical surveys and from comparison with the standard Barton
profiles. Some relationships have been built up that allow for the estimation of the hydraulic conductivity tensor (an essential
parameter for understanding water flow in fractured rock masses), not only as a function of traditional parameters like aperture,
spacing, dip and dip direction, etc., but also of joint roughness, precisely expressed in terms of the roughness coefficient.
These relationships have been studied initially from a theoretical point of view and then practically, through laboratory
investigations.
Resumen Se conoce muy bien la influencia de la rugosidad de las grietas en las tipologías del flujo de fluidos a lo interior de las fracturas y gracias a las experiencias en el campo de hidráulica ha sido posible estudiarla desde puntos de vista matemáticos y físicos. Sin embargo, las formulaciones adoptadas por los modelos hidráulicos tradicionales tienen poca aplicabilidad en el campo geológico debido a la dificultad relacionada con la estimación del parámetro de rugosidad. Normalmente este parámetro puede estimarse usando el coeficiente de rugosidad de grieta (JRC) el cual considera tanto la altura de la aspereza como su regularidad y tendencia direccional. La principal ventaja de utilizar el JRC se deriva del hecho que puede obtenerse fácilmente de levantamientos técnico-geológicos y de la comparación con los perfiles Standard Barton. Se han construido algunas relaciones que permiten la estimación del tensor de conductividad hidráulica (un parámetro esencial para el entendimiento del flujo de agua en masas de roca fracturadas), no solo en función de parámetros tradicionales como apertura, espaciado, buzamiento y dirección de buzamiento, etc., sino también en función de la rugosidad de la grieta estimada con precisión en términos del coeficiente de rugosidad. Estas relaciones se han estudiado inicialmente desde un punto de vista teórico y luego de modo práctico a través de investigaciones de laboratorio.
Résumé L’influence de la rugosité des joints sur les types d’écoulement de fluide dans les fractures est bien connue et a été étudiée aussi bien du point de vue physique que mathématique grace à des expériences menées dans le domaine de l’hydraulique. Cependant les formulations adoptées dans les modèles hydrauliques traditionnels sont difficilement applicables dans le domaine de la géologie à cause de la difficulté rencontrée pour estimer la rugosité. Ce paramètre peut normalement être apprécié grace au coefficient de rugosité du joint (JRC), lequel prend en compte à la fois la hauteur de l’aspérité ainsi que sa régularité et sa direction. Le principal avantage dans l’utilisation du JRC réside dans le fait qu’il peut facilement être obtenu à partir d’études techniques-géologiques et par comparaison avec la classification de Barton. Des relations qui permettent une estimation du tenseur de conductivité hydraulique (un paramètre essentiel pour comprendre l’écoulement de l’eau dans les masses rocheuses fracturées) ont été élaborées, pas seulement en fonction de paramètres traditionnels tels que l’ouverture, l’espacement, l’inclinaison et la direction d’inclinaison, etc , mais aussi en prenant en compte la rugosité des joints à travers le coefficient de rugosité. Ces relations ont initialement été étudiées d’un point de vue théorique puis expérimentalement à travers des recherches en laboratoire.相似文献
79.
On the use of microtremor recordings in seismic microzonation 总被引:4,自引:0,他引:4
M. Bour D. Fouissac P. Dominique C. Martin 《Soil Dynamics and Earthquake Engineering》1998,17(7-8):465-474
Experimental methods involving microtremor recordings are useful for determining site effects in regions of moderate seismic activity where ground motion records are few, and in urban or industrial contexts where the noise level is high. The aim of this study is to establish a microzonation by using the Nogoshi–Nakamura method,[1, 2] a simple experimental technique based on microtremor recordings. Since the physical phenomena underlying the method are only partially understood, the spectral responses obtained cannot be used alone. We, therefore, complete our experimental results by comparing them with the solutions of a one-dimensional numerical simulation (SHAKE91).[3, 4] The experimental programme was carried out on a plain near the Rhone Delta (South of France). H/V spectral ratios were calculated at 137 noise measurement points. In addition, we were able to compute the numerical transfer functions from soil columns defined by geotechnical characteristics inside the studied region. A comparison of the results obtained by the experimental and numerical methods showed that the fundamental frequencies are in good agreement, but that the amplitudes obtained by the two techniques are sometimes different. The analysis of H/V spectral ratios enabled us to establish maps to characterize the region: a resonance frequency map and maps of amplification levels as a function of frequency range, leading to a seismic microzonation for the whole of the region. 相似文献
80.
A comparison of estimated and calculated effective porosity 总被引:1,自引:1,他引:0
Daniel B. Stephens Kuo-Chin Hsu Mark A. Prieksat Mark D. Ankeny Neil Blandford Tracy L. Roth James A. Kelsey Julia R. Whitworth 《Hydrogeology Journal》1998,6(1):156-165
Effective porosity in solute-transport analyses is usually estimated rather than calculated from tracer tests in the field
or laboratory. Calculated values of effective porosity in the laboratory on three different textured samples were compared
to estimates derived from particle-size distributions and soil–water characteristic curves. The agreement was poor and it
seems that no clear relationships exist between effective porosity calculated from laboratory tracer tests and effective porosity
estimated from particle-size distributions and soil–water characteristic curves. A field tracer test in a sand-and-gravel
aquifer produced a calculated effective porosity of approximately 0.17. By comparison, estimates of effective porosity from
textural data, moisture retention, and published values were approximately 50–90% greater than the field calibrated value.
Thus, estimation of effective porosity for chemical transport is highly dependent on the chosen transport model and is best
obtained by laboratory or field tracer tests.
Received, March 1997 · Revised, August 1997 · Accepted, August 1997 相似文献