全文获取类型
收费全文 | 256篇 |
免费 | 10篇 |
国内免费 | 4篇 |
专业分类
测绘学 | 48篇 |
大气科学 | 17篇 |
地球物理 | 59篇 |
地质学 | 47篇 |
海洋学 | 8篇 |
天文学 | 62篇 |
自然地理 | 29篇 |
出版年
2023年 | 3篇 |
2022年 | 3篇 |
2021年 | 5篇 |
2020年 | 6篇 |
2019年 | 8篇 |
2018年 | 5篇 |
2017年 | 2篇 |
2016年 | 6篇 |
2015年 | 5篇 |
2014年 | 9篇 |
2013年 | 43篇 |
2012年 | 11篇 |
2011年 | 10篇 |
2010年 | 15篇 |
2009年 | 13篇 |
2008年 | 18篇 |
2007年 | 15篇 |
2006年 | 12篇 |
2005年 | 7篇 |
2004年 | 13篇 |
2003年 | 6篇 |
2002年 | 12篇 |
2001年 | 5篇 |
2000年 | 6篇 |
1999年 | 4篇 |
1998年 | 4篇 |
1997年 | 2篇 |
1994年 | 2篇 |
1992年 | 2篇 |
1990年 | 2篇 |
1987年 | 2篇 |
1986年 | 1篇 |
1984年 | 1篇 |
1981年 | 1篇 |
1979年 | 1篇 |
1978年 | 1篇 |
1977年 | 1篇 |
1975年 | 1篇 |
1974年 | 1篇 |
1973年 | 2篇 |
1972年 | 1篇 |
1971年 | 2篇 |
1905年 | 1篇 |
排序方式: 共有270条查询结果,搜索用时 62 毫秒
131.
132.
133.
Granular activated charcoal (GAC) acts as a general adsorptive medium for organic molecules, and is widely used to capture fluorescent tracer dyes. To investigate the capability of GAC in contaminant screening and tracer adsorption, a number of detectors were deployed for 2-6 days in a range of urban surface waters, and the adsorbed compounds eluted in an alkaline alcohol solution. Simultaneous water samples showed a diverse range of fluorescence environments, ranging from relatively clean, steady groundwater discharge, to highly concentrated and variable treated municipal sewage. A wide variety of organic compounds and dyes were found in the waters, as chronic and acute contaminants. The relationship between charcoal and water spectra depended on exposure time and loading. Short exposure times emphasized short wavelengths, longer exposure times emphasized longer wavelengths at the expense of shorter wavelengths. The magnitude of the effect depended on loading, being greater in enriched waters. In general, charcoal eluent shows a significant gain in fluorescence intensity over water. However, there may be an apparent loss at shorter wavelengths for samples with long exposure times and high loading. A similar bias was also discovered with the elution time of activated charcoal. Short wavelength fluorescence intensity peaked after a few minutes of elution; longer wavelength fluorescence increased over many days. These results show that charcoal is a reasonably effective material for adsorption of longer wavelength compounds. However, the ubiquity of many fluorescent dyes in the environment, and the complex relationship between the waters and the eluent spectrum, suggest that considerable care is required in the interpretation of eluent spectra. 相似文献
134.
The Quaternary System is under threat of extinction as a chronostratigraphic unit in the international Geological Time Scale. In its place, the Neogene System (previously comprising Miocene and Pliocene Series), is to be extended to include the Pleistocene and Holocene Series. It may be possible to “save” the Quaternary by defining it as a Subsystem of the Neogene. In doing so, it would be an opportune time to extend the base of the Quaternary to 2.6 Ma to encompass the time during which (1) Earth's climate has been strongly influenced by bi-polar glaciation, and (2) the genus Homo first appeared and evolved. 相似文献
135.
Evidence and Implications of Recent Climate Change in Northern Alaska and Other Arctic Regions 总被引:19,自引:0,他引:19
Larry D. Hinzman Neil D. Bettez W. Robert Bolton F. Stuart Chapin Mark B. Dyurgerov Chris L. Fastie Brad Griffith Robert D. Hollister Allen Hope Henry P. Huntington Anne M. Jensen Gensuo J. Jia Torre Jorgenson Douglas L. Kane David R. Klein Gary Kofinas Amanda H. Lynch Andrea H. Lloyd A. David McGuire Frederick E. Nelson Walter C. Oechel Thomas E. Osterkamp Charles H. Racine Vladimir E. Romanovsky Robert S. Stone Douglas A. Stow Matthew Sturm Craig E. Tweedie George L. Vourlitis Marilyn D. Walker Donald A. Walker Patrick J. Webber Jeffrey M. Welker Kevin S. Winker Kenji Yoshikawa 《Climatic change》2005,72(3):251-298
The Arctic climate is changing. Permafrost is warming, hydrological processes are changing and biological and social systems
are also evolving in response to these changing conditions. Knowing how the structure and function of arctic terrestrial ecosystems
are responding to recent and persistent climate change is paramount to understanding the future state of the Earth system
and how humans will need to adapt. Our holistic review presents a broad array of evidence that illustrates convincingly; the
Arctic is undergoing a system-wide response to an altered climatic state. New extreme and seasonal surface climatic conditions
are being experienced, a range of biophysical states and processes influenced by the threshold and phase change of freezing
point are being altered, hydrological and biogeochemical cycles are shifting, and more regularly human sub-systems are being
affected. Importantly, the patterns, magnitude and mechanisms of change have sometimes been unpredictable or difficult to
isolate due to compounding factors. In almost every discipline represented, we show how the biocomplexity of the Arctic system
has highlighted and challenged a paucity of integrated scientific knowledge, the lack of sustained observational and experimental
time series, and the technical and logistic constraints of researching the Arctic environment. This study supports ongoing
efforts to strengthen the interdisciplinarity of arctic system science and improve the coupling of large scale experimental
manipulation with sustained time series observations by incorporating and integrating novel technologies, remote sensing and
modeling. 相似文献
136.
Brad F. Lyles 《Ground Water Monitoring & Remediation》1994,14(4):191-199
Measurement accuracy was increased by nearly one order of magnitude by outfitting the thermal-pulse flowmeter (TFM) with an inflatable packer. To accurately measure slow water velocities in boreholes greater than 15 cm diameter, it is necessary to divert borehole fluids through the TFM by inflating a packer. During calibration it was noted that the TFM's accuracy decreased as the borehole diameter increased. With Lhe packer inflated the TFM has a useful flow measurement range of 0.08 to 15 L/min (with flow velocities of 0.24 ± 0.012 cm/inin to 45.7 ± 0.61 cm/min, respectively, in 20-cm-diameter pipe), compared to 0.8 to 57 L/min for a packcrless TFM. A computer interlace was added to the TFM to provide a real-time graphical display of the differential voltage output from the TFM, a running mean and standard deviation of the pulse-response time, and a mean flow rate and velocity based on calibration curve fits. 相似文献
137.
A novel cryogenic sampling method combining the matrix isolation technique with FTIR spectroscopy has been developed for atmospheric trace gas analysis. It is applicable to a wide range of molecules with detection limits typically in the 10–50 ppt range. The method is described along with some measurements of N2O, CFCl3, CF2Cl2, OCS, CS2, SO2 and PAN from samples collected at ground level and from an aircraft between 9 and 14 km. 相似文献
138.
Dozens of references recognizing pediment landforms in widely varying lithologic, climatic, and tectonic settings suggest a ubiquity in pediment forming processes on mountain piedmonts worldwide. Previous modeling work illustrates the development of a unique range in arid/semiarid piedmont slope (< 0.2 or 11.3°) and regolith thickness (2–4 m) that defines pediments, despite varying the initial conditions and domain characteristics (initial regolith thickness, slope, distance from basin to crest, topographic perturbations, and boundary conditions) and process rates (fluvial sediment transport efficiency and weathering rates). This paper expands upon the sensitivity analysis through numerical simulation of pediment development in the presence of spatially varying rock type, various base level histories, various styles of sediment transport, and various rainfall rates to determine how pediment development might be restricted in certain environments. This work suggests that in landscapes characterized by soil and vegetation types that favor incisive fluvial sediment transport styles coupled with incisive base level conditions, pediment development will be disrupted by the roughening of sediment mantled surfaces, thereby creating spatial variability in topography, regolith thickness, and bedrock weathering rates. Base level incision rates that exceed the integrated sediment flux along a hillslope derived from upslope weathering and sediment transport on the order of 10− 3 m y− 1 restrict pediment development by fostering piedmont incision and/or wholesale removal (stripping) of regolith mantles prior to footslope pediment development. Simulations illustrate an insensitivity to alternating layers of sandstone and shale 3–15 m thick oriented in various geometric configurations (vertical, horizontal, and dip-slope) and generating different regolith hydrologic properties and exhibiting weathering rate variations up to 3-fold. Higher fluxes and residence times of subsurface groundwater in more humid environments, as well as dissolution-type weathering, lead to a thickening of regolith mantles on erosional piedmonts on the order of 101 m and an elimination of pediment morphology. An initial test of the model sensitivity analysis in arid/semiarid environments, for which field reconnaissance and detailed geomorphic mapping indicate the presence of pediments controlled by climatic conditions (soil hydrologic properties, vegetation characteristics, and bedrock weathering style) that are known and constant, supports our modeling results that pediments are more prevalent in hydrologically-open basins. 相似文献
139.
140.
The soft uppermost sediments in lakes and wetlands contain important records of past environmental change. Although the technology
for coring these sediments is relatively well established, there has been a continuing effort to improve the design of equipment
that can provide the precise volumetric samples required for high-resolution dating and elemental analysis. Soft-sediment
cores are usually sectioned near a coring site under a wide range of field conditions creating a challenge for both field
crews and equipment. A new field extruder was therefore designed to simplify the process of fine-sectioning these cores in
the field while improving volumetric-sampling accuracy. The new extruder is relatively light, stable, and easy to use, but
sufficiently strong to extrude long continuous cores of stiff sediment if required. The key features of this extruder minimize
sampling errors by enabling an operator to automate the measurement and extrusion of a pre-determined volume of sediment.
The extruder was successfully field tested on a variety of sediment types in a large wetland from southern Florida, USA and
proved easy to master by even inexperienced field crews. 相似文献