全文获取类型
收费全文 | 1123篇 |
免费 | 30篇 |
国内免费 | 10篇 |
专业分类
测绘学 | 67篇 |
大气科学 | 78篇 |
地球物理 | 260篇 |
地质学 | 363篇 |
海洋学 | 60篇 |
天文学 | 251篇 |
综合类 | 4篇 |
自然地理 | 80篇 |
出版年
2021年 | 13篇 |
2020年 | 11篇 |
2019年 | 14篇 |
2018年 | 25篇 |
2017年 | 27篇 |
2016年 | 28篇 |
2015年 | 29篇 |
2014年 | 35篇 |
2013年 | 51篇 |
2012年 | 32篇 |
2011年 | 44篇 |
2010年 | 46篇 |
2009年 | 62篇 |
2008年 | 50篇 |
2007年 | 41篇 |
2006年 | 52篇 |
2005年 | 37篇 |
2004年 | 42篇 |
2003年 | 26篇 |
2002年 | 42篇 |
2001年 | 29篇 |
2000年 | 35篇 |
1999年 | 22篇 |
1998年 | 10篇 |
1997年 | 19篇 |
1996年 | 14篇 |
1995年 | 19篇 |
1994年 | 23篇 |
1993年 | 11篇 |
1991年 | 14篇 |
1990年 | 11篇 |
1989年 | 15篇 |
1988年 | 6篇 |
1987年 | 9篇 |
1986年 | 6篇 |
1985年 | 11篇 |
1984年 | 18篇 |
1983年 | 12篇 |
1982年 | 14篇 |
1981年 | 12篇 |
1980年 | 17篇 |
1979年 | 20篇 |
1978年 | 14篇 |
1977年 | 11篇 |
1976年 | 13篇 |
1975年 | 9篇 |
1974年 | 8篇 |
1973年 | 10篇 |
1972年 | 11篇 |
1970年 | 7篇 |
排序方式: 共有1163条查询结果,搜索用时 15 毫秒
51.
52.
53.
54.
55.
Cr-spinel bearing wehrlite rocks of Bangriposi are found within the multiply deformed metasedimentary rocks of Singhbhum Group belonging to North Singhbhum Mobile Belt of eastern India. Detailed mineralogical and geochemical studies reveal that these rocks have suffered a two-stage alteration involving a deeper level modal and cryptic metasomatism and a subsequent shallower depth pervasive hydrothermal alteration. Cryptic metasomatism is defined by elevated LREE contents of the wehrlite and its clinopyroxne grains. Metasomatism induced changes in the modal mineralogy of the rocks include the absence of primary orthopyroxene grains, presence of secondary diopside-phlogopite(now present as vermiculite) defining disequilibrium reaction textures and secondary orthopyroxene rims around serpentinized olivine. The mineralogical and geochemical changes due to the metasomatic event present a contrasting picture in regard to the metasomatic history of the rocks. Possible scenarios involving a single stage or multiple stage metasomatism events have been discussed while explaining the metasomatic reactions that took place. An attempt has been made to estimate the REE concentrations of the final equilibrating melt from REE contents of clinopyroxene grains of the wehrlite. The possibility of the LREE-enriched equilibrating melt of the wehrlite rocks(the deeper level metasomatic agent) being similar to residual melts from the E-MORB type parental melts of nearby gabbro suite has been ruled out by geochemical modeling. REE abundance patterns of several natural enriched melts have been compared with REE pattern of calculated LREE-enriched equilibrating melt of the wehrlite and most resemblance has been observed with calcic and potassic melts. It is therefore suggested that the Cr-spinel bearing wehrlite rocks of Bangriposi has been affected by a calcio-potassic melt in deeper level, prior to the shallow level serpentinization event. 相似文献
56.
University faculty partners from the Departments of Geography and Instruction and Teacher Education at a large, public university collaborated with K-12 teachers and the leadership of a rural school district in order to investigate the crosscutting content of science, mathematics, and geography through the integration of web-based GIS technologies. The project explored the critical connections among technology, pedagogy, and content with a particular emphasis on developing technology-enhanced, inquiry-based lessons in which the teachers and their students used GIS technologies to analyze, visualize, and present data in real-world contexts. The findings highlight the importance of well-structured professional development that builds community, integrates diverse content and pedagogical expertise, provides feedback and coaching, and is of sufficient duration to effect change. 相似文献
57.
David A.Wood 《地学前缘(英文版)》2021,12(6):444-457
The capability of accurately predicting mineralogical brittleness index(BI)from basic suites of well logs is desir-able as it provides a useful indicator of the fracability of tight formations.Measuring mineralogical components in rocks is expensive and time consuming.However,the basic well log curves are not well correlated with BI so correlation-based,machine-learning methods are not able to derive highly accurate BI predictions using such data.A correlation-free,optimized data-matching algorithm is configured to predict BI on a supervised basis from well log and core data available from two published wells in the Lower Barnett Shale Formation(Texas).This transparent open box(TOB)algorithm matches data records by calculating the sum of squared errors be-tween their variables and selecting the best matches as those with the minimum squared errors.It then applies optimizers to adjust weights applied to individual variable errors to minimize the root mean square error(RMSE)between calculated and predicted(BI).The prediction accuracy achieved by TOB using just five well logs(Gr,pb,Ns,Rs,Dt)to predict BI is dependent on the density of data records sampled.At a sampling density of about one sample per 0.5 ft BI is predicted with RMSE~0.056 and R2~0.790.At a sampling density of about one sample per 0.1 ft BI is predicted with RMSE~0.008 and R2~0.995.Adding a stratigraphic height index as an additional(sixth)input variable method improves BI prediction accuracy to RMSE~0.003 and R2~0.999 for the two wells with only 1 record in 10,000 yielding a BI prediction error of>±0.1.The model has the potential to be applied in an unsupervised basis to predict BI from basic well log data in surrounding wells lacking mineralogical measure-ments but with similar lithofacies and burial histories.The method could also be extended to predict elastic rock properties in and seismic attributes from wells and seismic data to improve the precision of brittleness index and fracability mapping spatially. 相似文献
58.
59.
Barry Smit Michael Brklacich Robert B. Stewart Ray McBride Murray Brown Deborah Bond 《Climatic change》1989,14(2):153-174
Increasing concentrations of atmospheric CO2 and other greenhouse gases are expected to contribute to a global warming. This paper examines the potential implications of a climatic change corresponding to a doubling of atmospheric concentrations of CO2 on crop production opportunities throughout Ontario, a major food producing region in Canada. The climate is projected to become warmer and drier, but the extent of these shifts are expected to vary from region to region within Ontario. The effect of this altered climate on crop yields and the area of land capable of supporting specific crops varies according to region, soil quality and crop type. Most notable are the enhanced opportunities for grains and oilseeds in the northern regions, and the diminished production prospects for most crops in the most southerly parts of Ontario. 相似文献
60.
The partitioning of Fe and Mg between olivine and carbonate and the stability of carbonate under mantle conditions 总被引:4,自引:1,他引:4
We have investigated the effect of Fe on the stabilities of carbonate (carb) in lherzolite assemblages by determining the partitioning of Fe and Mg between silicate (olivine; ol) and carbonates (magnesite, dolomite, magnesian calcite) at high pressures and temperatures. Fe enters olivine preferentially relative to magnesite and ordered dolomite, but Fe and Mg partition almost equally between disordered calcic carbonate and olivine. Measurement of K
d
(X
Fe
carb
X
Mg
ol
/X
Fe
ol
X
Mg
carb
) as a function of Fe/ Mg ratio indicates that Fe–Mg carbonates deviate only slightly from ideality. Using the regular solution parameter for olivine W
FeMg
ol
of 3.7±0.8 kJ/mol (Wiser and Wood 1991) we obtain for (FeMg)CO3 a W
FeMg
carb
of 3.05±1.50 kJ/mol. The effect of Ca–Mg–Fe disordering is to raise K
d
substantially enabling us to calculate W
CaMg
carb
-W
CaFe
carb
of 5.3±2.2 kJ/mol. The activity-composition relationships and partitioning data have been used to calculate the effect of Fe/Mg ratio on mantle decarbonation and exchange reactions. We find that carbonate (dolomite and magnesian calcite) is stable to slightly lower pressures (by 1 kbar) in mantle lherzolitic assemblages than in the CaO–MgO–SiO2(CMS)–CO2 system. The high pressure breakdown of dolomite + orthopyroxene to magnesite + clinopyroxene is displaced to higher pressures (by 2 kbar) in natural compositions relative to CMS. CO2. We also find a stability field of magnesian calcite in lherzolite at 15–25 kbar and 750–1000°C. 相似文献