全文获取类型
收费全文 | 1176篇 |
免费 | 41篇 |
国内免费 | 19篇 |
专业分类
测绘学 | 20篇 |
大气科学 | 104篇 |
地球物理 | 294篇 |
地质学 | 388篇 |
海洋学 | 126篇 |
天文学 | 184篇 |
综合类 | 7篇 |
自然地理 | 113篇 |
出版年
2021年 | 14篇 |
2020年 | 13篇 |
2019年 | 21篇 |
2018年 | 28篇 |
2017年 | 20篇 |
2016年 | 29篇 |
2015年 | 27篇 |
2014年 | 40篇 |
2013年 | 60篇 |
2012年 | 43篇 |
2011年 | 39篇 |
2010年 | 40篇 |
2009年 | 56篇 |
2008年 | 51篇 |
2007年 | 39篇 |
2006年 | 43篇 |
2005年 | 36篇 |
2004年 | 24篇 |
2003年 | 41篇 |
2002年 | 24篇 |
2001年 | 32篇 |
2000年 | 28篇 |
1999年 | 19篇 |
1998年 | 20篇 |
1997年 | 17篇 |
1996年 | 9篇 |
1995年 | 14篇 |
1994年 | 17篇 |
1993年 | 20篇 |
1992年 | 9篇 |
1991年 | 13篇 |
1990年 | 19篇 |
1989年 | 15篇 |
1987年 | 12篇 |
1986年 | 7篇 |
1985年 | 20篇 |
1984年 | 18篇 |
1983年 | 19篇 |
1982年 | 30篇 |
1981年 | 24篇 |
1980年 | 22篇 |
1979年 | 20篇 |
1978年 | 18篇 |
1977年 | 21篇 |
1976年 | 18篇 |
1975年 | 13篇 |
1974年 | 16篇 |
1973年 | 18篇 |
1972年 | 6篇 |
1971年 | 8篇 |
排序方式: 共有1236条查询结果,搜索用时 15 毫秒
131.
The existence of detachment surfaces or décollement zones beneath folded rocks of the Valley and Ridge and Plateau provinces of the Appalachians has been recognized as an important condition of folding. Large folds at the border between the two provinces resulted primarily from repetition of strata by thrusting of blocks over ramp faults that connect detachement surfaces at different horizons. Some investigators have suggested that folds in the Plateau province of Pennsylvania were produced by splay faults arising from detachment surfaces, but field observations and theoretical analyses by Sherwin and by Wiltschko & Chapple suggest that the folds are a result of buckling of multilayered rocks above a décollement. An exception may be the Burning Springs anticline in West Virginia, which appears to have formed at the termination of a detachment surface.Investigation of the translation of an homogenous, viscous material above a flat detachment surface that terminates laterally indicates that the termination produces a broad, low-amplitude anticline in passive layering as a result of thickening induced by a gradient of shear stresses in the vertical direction. This thickening above the termination of a detachment is a mechanism of folding. If the viscous fluid contained mechanical layering, the fold would become amplified by buckling. Computations of stresses in the material indicate that minor faults should be generated first near the termination of the flat detachment surface. The Burning Springs anticline probably was initiated by termination of a detachment surface and subsequently amplified by buckling. 相似文献
132.
Kenneth S. Johnson 《Environmental Geology》1991,17(1):3-7
The Oklahoma Geological Survey has developed several maps and reports for preliminary screening of the state of Oklahoma to identify areas that are generally acceptable or unacceptable for disposal of a wide variety of waste materials. These maps and reports focus on the geologic and hydrogeologic parameters that must be evaluated in the screening process. One map (and report) shows the outcrop distribution of 35 thick shale or clay units that are generally suitable for use as host rocks for surface disposal of wastes. A second map shows the distribution of unconsolidated alluvial and terrace-deposit aquifers, and a third map shows the distribution and hydrologic character of bedrock aquifers and their recharge areas. These latter two maps show the areas in the state where special attention must be exercised in permitting storage or disposal of waste materials that could degrade the quality of groundwater. State regulatory agencies and industry are using these maps and reports in preliminary screening of the state to identify potential disposal sites. These maps in no way replace the need for site-specific investigations to prove (or disprove) the adequacy of a site to safely contain waste materials. 相似文献
133.
Yuanming Guo Chase Holton Hong Luo Paul Dahlen Paul C. Johnson 《Ground Water Monitoring & Remediation》2019,39(2):43-52
Groundwater elevation fluctuation has been recognized as one mechanism causing temporal indoor air volatile organic chemical (VOC) impacts in vapor intrusion risk assessment guidance. For dissolved VOC sources, groundwater table fluctuation shortens/lengthens the transport pathway, and delivers dissolved contaminants to soils that are alternating between water saturated and variably saturated conditions, thereby enhancing volatilization potential. To date, this mechanism has not been assessed with field data, but enhanced VOC emission flux has been observed in lab-scale and modeling studies. This work evaluates the impact of groundwater elevation changes on VOC emission flux from a dissolved VOC plume into a house, supplemented with modeling results for cyclic groundwater elevation changes. Indoor air concentrations, air exchange rates, and depth to groundwater (DTW) were collected at the study house during an 86-d constant building underpressurization test. These data were used to calculate changes in trichloroethylene (TCE) emission flux to indoor air, during a period when DTW varied daily and seasonally from about 3.1 to 3.4 m below the building foundation (BF). Overall, TCE flux to indoor air varied by about 50% of the average, without any clear correlation to changes in DTW or its change rate. To complement the field study, TCE surface emission fluxes were simulated using a one-dimensional model (HYDRUS 1D) for conditions similar to the field site. Simulation results showed time-averaged surface TCE fluxes for cyclic water-table elevations were greater than for stationary water-table conditions at an equivalent time-averaged water-table position. The magnitudes of temporal TCE emission flux changes were generally less than 50% of the time-averaged flux, consistent with the field site observations. Simulation results also suggested that TCE emission flux changes due to groundwater fluctuation are likely to be significant at sites with shallow groundwater (e.g., < 0.5 m BF) and permeable soil types (e.g., sand). 相似文献
134.
P.J. Hamilton R.W. Johnson D.E. Mackenzie R.K. O'Nions 《Journal of Volcanology and Geothermal Research》1983,18(1-4)
RB-Sr and Sm-Nd isotopic and trace-element-abundance values have been determined for 15 mafic and intermediate rocks from six Pleistocene volcanic centres of the Fly-Highlands province. 87Sr/86Sr and N d values range from 0.70362 to 0.70540, and +1.9 to +5.9, respectively. These new data can be accounted for by contamination of mantle-derived magmas by the continental crust through which the magmas have risen. They do not, however, preclude derivation of some of the Sr and Nd from subducted crust, nor are they inconsistent with Sr and Nd enrichments having taken place by means of mantle metasomatic events. Nevertheless, there is no Benioff zone beneath the Fly-Highlands province (although there is geological evidence for Cretaceous subduction). A preferred interpretation is that uncontaminated, mantle-derived magmas are related to the Pliocene crustal uplift that caused the development of the highlands and which formed in response to a mid-Tertiary continent/island-arc collision. 相似文献
135.
Four periods of loess deposition in the Lower Mississippi Valley can be identified on the basis of geochemical and mineralogical criteria, radiocarbon dating, and thermoluminescence dating. These are designated Loess Units 1,2,3, and 4 in order of increasing age. Carbonate-rich Unit 1 loess comprises more than 70 per cent of the thickness of the loess profiles. 14C and TL dates indicate this loess was deposited between 9 000 and 20 000 years ago. A maximum sedimentation rate of 2.17 mm yr?1 has been recorded near Vicksburg just after the last Laurentide glacial maximum, between 17 190 and 15 580 years ago. The Unit 2 loess, which is thin and partly decalcified, was deposited slowly between about 25 000 and 20 000 years ago. The Unit 3 and Unit 4 loess formations, which are both highly weathered, have yielded TL ages of 76 000–85 000 years and 119 000- > 132 000 years, suggesting they were deposited during the Altonian Substage of the early Wisconsinan and the Illinoian glacial stage respectively. The four loess units are stratigraphically equivalent to the Peoria, Farmdale, Roxana, and Loveland loess formations previously recognized in Illinois. The source of dust in both areas was glacial outwash in the Mississippi Valley. During interglacials and interstadials, when the supply of glacial debris was reduced and the Mississippi River changed from a braided to a meandering regime, dust sedimentation in southern Mississippi virtually ceased, allowing weathering and pedogenesis to proceed. 相似文献
136.
Using X-ray Energy Spectroscopy trace metal concentrations including Pb, Cu and Zn were studied in the common blue mussel, Mytilus edulis. These organisms, sampled at intervals of only 1 km and less for a line transect study of the port of Vancouver, Canada, exhibited dramatic concentration variations. In particular, mussels only 30 m from a point source of lead displayed essentially base-line concentrations, representing a decline of two orders of magnitude. 相似文献
137.
Hong Luo Paul Dahlen Paul C. Johnson Tom Peargin Todd Creamer 《Ground Water Monitoring & Remediation》2009,29(1):81-91
Data requirements for assessing the significance of the soil vapor intrusion pathway are evolving, and the collection and interpretation of subslab and near-slab soil-gas samples are under discussion. The potential for different assessment paradigms for aerobically biodegradable and recalcitrant chemicals is also frequently debated. In this work, the soil-gas distribution beneath and around a slab-on-grade building overlying shallow (0.5 to >1.5 m below ground surface) petroleum hydrocarbon–impacted coarse alluvial soils was studied. The study spanned about 12 months, including the sampling of soil-gas hydrocarbon and oxygen concentrations, subslab soil vs. building pressure differentials and included weather conditions. Three-dimensional soil-gas concentration "snapshots" using samples from 79 soil-gas sampling points are presented here. Significant spatial variability was observed with hydrocarbon and oxygen concentrations ranging from about <0.01 to 200 mg/L and 0 to 21% v/v, respectively. The presence of oxygen and the depth to petroleum-impacted soils appeared to be the dominant factors in controlling the soil-gas distribution; the depletion of hydrocarbons over short lateral and vertical distances (<2 m) was observed in the well-oxygenated regions. Composition data suggest preferential biodegradation of lighter compounds at some points, as reflected in the ratio of the masses of chemicals eluting on the gas chromatography between methane and pentane (C1 and C5) and all others after pentane (>C5). 相似文献
138.
by Tom Clemo Warren Barrash Edward C. Reboulet Timothy C. Johnson Carsten Leven 《Ground water》2009,47(4):515-525
This paper describes a combined field, laboratory, and numerical study of electromagnetic borehole flowmeter measurements acquired without the use of a packer or skirt to block bypass flow around the flowmeter. The most significant finding is that inflow through the wellbore screen changes the ratio of flow through the flowmeter to wellbore flow. Experiments reveal up to a factor of two differences in this ratio for conditions with and without inflow through the wellbore screen. Standard practice is to assume the ratio is constant. A numerical model has been developed to simulate the effect of inflow on the flowmeter. The model is formulated using momentum conservation within the borehole and around the flowmeter. The model is embedded in the MODFLOW-2000 ground water flow code. 相似文献
139.
Iron and Manganese in Groundwater: Using Kriging and GIS to Locate High Concentrations in Buncombe County,North Carolina 下载免费PDF全文
For health, economic, and aesthetic reasons, allowable concentrations (as suggested by the United States Environmental Protection Agency) of the secondary contaminants iron (Fe) and manganese (Mn) found present in drinking water are 0.3 and 0.05 mg/L, respectively. Water samples taken from private drinking wells in rural communities within Buncombe County, North Carolina contain concentrations of these metals that exceed secondary water quality criteria. This study predicted the spatial distribution of Fe and Mn in the county, and evaluated the effect of site environmental factors (bedrock geology, ground elevation, saprolite thickness, and drinking water well depth) in controlling the variability of Fe and Mn in groundwater. A statistically significant correlation between Fe and Mn concentrations, attributable to bedrock geology, was identified. Prediction models were created using ordinary kriging and cokriging interpolation techniques to estimate the presence of Fe and Mn in groundwater where direct measurements are not possible. This same procedure can be used to estimate the trend of other contaminants in the groundwater in different areas with similar hydrogeological settings. 相似文献
140.
David R. Burris Paul C. Johnson Robert E. Hinchee Paul R. Dahlen 《Ground Water Monitoring & Remediation》2018,38(2):49-56
1,4‐Dioxane is a volatile organic compound that is fully miscible in water, allowing it to sequester in vadose zone pore water and serve as a long‐term source of groundwater contamination. Conventional soil vapor extraction (SVE) removes 1,4‐dioxane; however, substantial 1,4‐dioxane can remain even after other colocated chlorinated solvents have been remediated. A field demonstration of “enhanced SVE” (XSVE) with focused extraction and heated injection was conducted at former McClellan AFB, CA, achieving 94% reduction in soil concentrations. A screening‐level tool, HypeVent XSVE, was created to assist in system design and data reduction and to anticipate how operating factors affect XSVE performance (e.g., cleanup level, remediation time, etc.). It assumes well‐mixed conditions, and combines an energy balance, mass balances for water and contaminant, and a temperature‐dependent 1,4‐dioxane Henry's Law constant. User inputs include the target treatment zone size, initial 1,4‐dioxane and soil moisture concentrations, and ambient site and injection/extraction conditions (temperature, humidity). Projections based on inputs representative of demonstration site conditions adequately anticipated the observed macroscopic field results. Sensitivity analyses show that removal increases with increasing heated air injection temperature and relative humidity and decreasing initial soil moisture content. 相似文献