全文获取类型
收费全文 | 142篇 |
免费 | 12篇 |
国内免费 | 18篇 |
专业分类
测绘学 | 14篇 |
大气科学 | 11篇 |
地球物理 | 65篇 |
地质学 | 38篇 |
海洋学 | 30篇 |
天文学 | 6篇 |
综合类 | 1篇 |
自然地理 | 7篇 |
出版年
2024年 | 1篇 |
2023年 | 2篇 |
2022年 | 3篇 |
2021年 | 7篇 |
2020年 | 2篇 |
2019年 | 4篇 |
2018年 | 3篇 |
2017年 | 7篇 |
2016年 | 2篇 |
2015年 | 5篇 |
2014年 | 7篇 |
2013年 | 6篇 |
2012年 | 6篇 |
2011年 | 10篇 |
2010年 | 3篇 |
2009年 | 2篇 |
2008年 | 10篇 |
2007年 | 8篇 |
2006年 | 10篇 |
2005年 | 5篇 |
2004年 | 7篇 |
2003年 | 6篇 |
2002年 | 8篇 |
2001年 | 7篇 |
2000年 | 4篇 |
1999年 | 5篇 |
1997年 | 4篇 |
1996年 | 4篇 |
1995年 | 3篇 |
1994年 | 3篇 |
1993年 | 4篇 |
1992年 | 5篇 |
1991年 | 3篇 |
1990年 | 4篇 |
1983年 | 1篇 |
1981年 | 1篇 |
排序方式: 共有172条查询结果,搜索用时 15 毫秒
12.
13.
A comparison of solute-transport solution techniques and their effect on sensitivity analysis and inverse modeling results 总被引:1,自引:0,他引:1
Five common numerical techniques for solving the advection-dispersion equation (finite difference, predictor corrector, total variation diminishing, method of characteristics, and modified method of characteristics) were tested using simulations of a controlled conservative tracer-test experiment through a heterogeneous, two-dimensional sand tank. The experimental facility was constructed using discrete, randomly distributed, homogeneous blocks of five sand types. This experimental model provides an opportunity to compare the solution techniques: the heterogeneous hydraulic-conductivity distribution of known structure can be accurately represented by a numerical model, and detailed measurements can be compared with simulated concentrations and total flow through the tank. The present work uses this opportunity to investigate how three common types of results--simulated breakthrough curves, sensitivity analysis, and calibrated parameter values--change in this heterogeneous situation given the different methods of simulating solute transport. The breakthrough curves show that simulated peak concentrations, even at very fine grid spacings, varied between the techniques because of different amounts of numerical dispersion. Sensitivity-analysis results revealed: (1) a high correlation between hydraulic conductivity and porosity given the concentration and flow observations used, so that both could not be estimated; and (2) that the breakthrough curve data did not provide enough information to estimate individual values of dispersivity for the five sands. This study demonstrates that the choice of assigned dispersivity and the amount of numerical dispersion present in the solution technique influence estimated hydraulic conductivity values to a surprising degree. 相似文献
14.
15.
16.
Ho KT Burgess RM Pelletier MC Serbst JR Ryba SA Cantwell MG Kuhn A Raczelowski P 《Marine pollution bulletin》2002,44(4):286-293
The identification of toxicants affecting aquatic benthic systems is critical to sound assessment and management of our nation's waterways. Identification of toxicants can be useful in designing effective sediment remediation plans and reasonable options for sediment disposal. Knowledge of which contaminants affect benthic systems allows managers to link pollution to specific dischargers and prevent further release of toxicant(s). In addition, identification of major causes of toxicity in sediments may guide programs such as those developing environmental sediment guidelines and registering pesticides, while knowledge of the causes of toxicity which drive ecological changes such as shifts in benthic community structure would be useful in performing ecological risk assessments. To this end, the US Environmental Protection Agency has developed tools (toxicity identification and evaluation (TIE) methods) that allow investigators to characterize and identify chemicals causing acute toxicity in sediments and dredged materials. To date, most sediment TIEs have been performed on interstitial waters. Preliminary evidence from the use of interstitial water TIEs reveals certain patterns in causes of sediment toxicity. First, among all sediments tested, there is no one predominant cause of toxicity; metals, organics, and ammonia play approximately equal roles in causing toxicity. Second, within a single sediment there are multiple causes of toxicity detected; not just one chemical class is active. Third, the role of ammonia is very prominent in these interstitial waters. Finally, if sediments are divided into marine or freshwater, TIEs perforMed on interstitial waters from freshwater sediments indicate a variety of toxicants in fairly equal proportions, while TIEs performed on interstitial waters from marine sediments have identified only ammonia and organics as toxicants, with metals playing a minor role. Preliminary evidence from whole sediment TIEs indicates that organic compounds play a major role in the toxicity of marine sediments, with almost no evidence for either metal or ammonia toxicity. However, interpretation of these results may be skewed because only a small number of interstitial water (n = 13) and whole sediment (n = 5) TIEs have been completed. These trends may change as more data are collected. 相似文献
17.
Recently, detection of discrete features in the X-ray afterglow spectra of GRB 970508 and GRB 970828 was reported. The most natural interpretation of these features is that they are redshifted Fe K emission complexes. The identification of the line emission mechanism has drastic implications for the inferred mass of radiating material and hence the nature of the burst site. X-ray spectroscopy provides a direct observational constraint on these properties of gamma-ray bursters. We briefly discuss how these constraints arise in the context of an application to the spectrum of GRB 970508. 相似文献
18.
Many methods can be used to test alternative ground water models. Of concern in this work are methods able to (1) rank alternative models (also called model discrimination) and (2) identify observations important to parameter estimates and predictions (equivalent to the purpose served by some types of sensitivity analysis). Some of the measures investigated are computationally efficient; others are computationally demanding. The latter are generally needed to account for model nonlinearity. The efficient model discrimination methods investigated include the information criteria: the corrected Akaike information criterion, Bayesian information criterion, and generalized cross-validation. The efficient sensitivity analysis measures used are dimensionless scaled sensitivity (DSS), composite scaled sensitivity, and parameter correlation coefficient (PCC); the other statistics are DFBETAS, Cook's D, and observation-prediction statistic. Acronyms are explained in the introduction. Cross-validation (CV) is a computationally intensive nonlinear method that is used for both model discrimination and sensitivity analysis. The methods are tested using up to five alternative parsimoniously constructed models of the ground water system of the Maggia Valley in southern Switzerland. The alternative models differ in their representation of hydraulic conductivity. A new method for graphically representing CV and sensitivity analysis results for complex models is presented and used to evaluate the utility of the efficient statistics. The results indicate that for model selection, the information criteria produce similar results at much smaller computational cost than CV. For identifying important observations, the only obviously inferior linear measure is DSS; the poor performance was expected because DSS does not include the effects of parameter correlation and PCC reveals large parameter correlations. 相似文献
19.
Macroalgal survival in ballast water tanks 总被引:1,自引:0,他引:1
Despite a large amount of research into invasive species and their introductions, there have been no studies focused on macroalgal transport in ballast water. To address this, we collected replicate samples of ballast water from 12 ships in two Mediterranean harbours (Naples and Salerno). Filtered samples were kept in culture for a month at Mediterranean mean conditions (18 degrees C, 12:12h LD, 60micromol photons m(-2)s(-1)). Fifteen macroalgal taxa were cultured and differed according to the geographic origin of the ballast water. Most of the cultured algae were widely distributed species (e.g. Ulva spp. and Acinetospora-phase). However, Ulva ohnoi Hiraoka and Shimada, described from Japan, was hitherto unknown in the Mediterranean Sea. We show for the first time that ballast water can be an important vector for the transport of microscopic stages of macroalgae and that this can be a vector for the introduction of alien species. 相似文献
20.
为探究新疆准东基地水源地地下水开采量可实行方案与地下水水位下降状况,采用Processing MODFLOW软件,对项目区进行地下水数值模拟研究。经过模拟流场形态与观测值以及对地下水动态进行拟合,提取地下水梯度场、降速场与实测资料进行对比分析。总结得出符合该地区地下水变化的地下水开采量方案。研究表明该水源地地下水开采实施方案符合该地区水资源长期发展。 相似文献