Other Types of Contamination     

Evan K. Nyer  Eric P. Carman  Raymond M. Flynn 《Ground Water Monitoring & Remediation》1999,19(2):61-64

"Hazardous levels of methane in the subsurface occur at numerous locations across the United States."  相似文献   

Concave accretion discs and X-ray reprocessing     

Eric G. Blackman 《Monthly notices of the Royal Astronomical Society》1999,306(3):L25-L30

Spectra of Seyfert 1s are commonly modelled as emission from an X-ray-illuminated flat accretion disc orbiting a central black hole. This provides both reprocessed and direct components of the X-ray emission, as required by observations of individual objects, and possibly a fraction of the cosmological X-ray background. There is some observational motivation for us to at least consider the role that an effectively concave disc surface might play: (1) a reprocessed fraction ≳1/2 in some Seyferts and possibly in the X-ray background, and (2) the commonality of a sharp iron line peak for Seyferts at 6.4 keV despite a dependence of peak location on inclination angle for flat disc models. Here it is shown that a concave disc may not only provide a larger total fraction of reprocessed photons, but can also reprocess a much larger fraction of photons in its outer regions compared with a flat disc. This reduces the sensitivity of the 6.4-keV peak location to the inner disc inclination angle because the outer regions are less affected by Doppler and gravitational effects. If the X-ray source is isotropic, the reprocessed fraction is directly determined by the concavity. If the X-ray source is anisotropic, the location of iron line peak can still be determined by concavity but the total reflected fraction need not be as large as for the isotropic emitter case. The geometric calculations herein are applicable to general accretion disc systems illuminated from the centre.  相似文献   

Improved mappint of the Lake Agassiz Herman strandline by integrating geological and soil maps     

Eric C. Brevik 《Journal of Paleolimnology》1999,22(3):253-257

Several discontinuous, segmented strandlines that mark the former shores of Lake Agassiz can be found in eastern North Dakota. These strandlines are difficult to map and correlate because of their segmented nature. Better mapping would benefit researchers working on a number of problems involving Lake Agassiz including dating the strandlines, reconstructing the lake's history, and calculation of asthenosphere viscosity beneath the Lake Agassiz basin. Elongated soil delineations representing beach-indicative soils with a north-northwest trend and extending from currently mapped areas of the Herman strandline were identified on Grand Forks County soil maps. This information was combined with the geologic map of Grand Forks County in an attempt to better define the location of the Herman strandline in the southern part of the county. This approach worked well, and it is recommended that similar studies be attempted on other Lake Agassiz strandlines in North Dakota and the surrounding area.  相似文献   

Transatlantic latest Cretaceous mosasaurs (Reptilia,Lacertilia) from the Maastrichtian type area and New Jersey     

Mulder  Eric W.A. 《Geologie en Mijnbouw》1999,78(3-4):281-300

Cranial and postcranial elements of the mosasaurid reptiles Mosasaurus hoffmanni Mantell 1829 and Plioplatecarpus marshi Dollo 1882 from the Maastrichtian type area, and of Mosasaurus maximus Cope 1869 and Plioplatecarpus depressus (Cope 1869) from New Jersey are compared. Views held by previous authors are discussed. It is concluded that these European and North American taxa are conspecific.  相似文献   

Globalizing cities – introduction     

Eric Clark 《Geoforum》2000,31(4)

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DAMÉE-NAB: the base experiments     

Eric P. Chassignet  Hernan Arango  David Dietrich  Tal Ezer  Michael Ghil  Dale B. Haidvogel  C. -C. Ma  Avichal Mehra  Afonso M. Paiva  Ziv Sirkes 《Dynamics of Atmospheres and Oceans》2000,32(3-4)

The results of an intercomparison experiment performed with five numerical ocean models of different architecture are presented. While all models are able to simulate the large-scale characteristics of the North Atlantic circulation with a fair degree of realism, they also exhibit differences that can be attributed to the choices made in vertical coordinates, domain size, and boundary conditions.  相似文献   

Hydrodynamic model of cells for designing systems of urban groundwater drainage     

Eric D. Zimmermann  Gerardo A. Riccardi 《Hydrogeology Journal》2000,8(5):538-548

An improved mathematical hydrodynamic quasi-two-dimensional model of cells, CELSUB3, is presented for simulating drainage systems that consist of pumping well fields or subsurface drains. The CELSUB3 model is composed of an assemblage of algorithms that have been developed and tested previously and that simulate saturated flow in porous media, closed conduit flow, and flow through pumping stations. A new type of link between aquifer cells and drainage conduits is proposed. This link is verified in simple problems with well known analytical solutions. The correlation between results from analytical and mathematical solutions was considered satisfactory in all cases. To simulate more complex situations, the new proposed version, CELSUB3, was applied in a project designed to control the water-table level within a sewer system in Cha?ar Ladeado Town, Santa Fe Province, Argentina. Alternative drainage designs, which were evaluated under conditions of dynamic recharge caused by rainfall in a critical year (wettest year for the period of record) and a typical year, are briefly described. After analyzing ten alternative designs, the best technical–economic solution is a subsurface drainage system of closed conduits with pumping stations and evacuation channels. Electronic Publication  相似文献   

The star formation histories of low surface brightness galaxies     

Eric F. Bell  †  David Barnaby  Richard G. Bower  Roelof S. de Jong  ‡  Doyal A. Harper  Jr  Mark Hereld  Robert F. Loewenstein  Bernard J. Rauscher 《Monthly notices of the Royal Astronomical Society》2000,312(3):470-496

We have performed deep imaging of a diverse sample of 26 low surface brightness galaxies (LSBGs) in the optical and the near-infrared. Using stellar population synthesis models, we find that it is possible to place constraints on the ratio of young to old stars (which we parametrize in terms of the average age of the galaxy), as well as the metallicity of the galaxy, using optical and near-infrared colours. LSBGs have a wide range of morphologies and stellar populations, ranging from older, high-metallicity earlier types to much younger and lower-metallicity late-type galaxies. Despite this wide range of star formation histories, we find that colour gradients are common in LSBGs. These are most naturally interpreted as gradients in mean stellar age, with the outer regions of LSBGs having lower ages than their inner regions. In an attempt to understand what drives the differences in LSBG stellar populations, we compare LSBG average ages and metallicities with their physical parameters. Strong correlations are seen between an LSBG's star formation history and its K -band surface brightness, K -band absolute magnitude and gas fraction. These correlations are consistent with a scenario in which the star formation history of an LSBG primarily correlates with its surface density and its metallicity correlates with both its mass and its surface density.  相似文献   

Coronal activity from dynamos in astrophysical rotators     

Eric G. Blackman  George B. Field 《Monthly notices of the Royal Astronomical Society》2000,318(3):724-732

We show that a steady mean-field dynamo in astrophysical rotators leads to an outflow of relative magnetic helicity and thus magnetic energy available for particle and wind acceleration in a corona. The connection between energy and magnetic helicity arises because mean-field generation is linked to an inverse cascade of magnetic helicity. To maintain a steady state in large magnetic Reynolds number rotators, there must then be an escape of relative magnetic helicity associated with the mean field, accompanied by an equal and opposite contribution from the fluctuating field. From the helicity flow, a lower limit on the magnetic energy deposited in the corona can be estimated. Steady coronal activity including the dissipation of magnetic energy, and formation of multi-scale helical structures therefore necessarily accompanies an internal dynamo. This highlights the importance of boundary conditions which allow this to occur for non-linear astrophysical dynamo simulations. Our theoretical estimate of the power delivered by a mean-field dynamo is consistent with that inferred from observations to be delivered to the solar corona, the Galactic corona, and Seyfert 1 AGN coronae.  相似文献   

Simulating flood events at the Twin Ports of Duluth-Superior using a linked hydrologic-hydrodynamic framework     

Fitzpatrick  Lindsay  Titze  Daniel  Anderson  Eric J.  Beletsky  Dmitry  Kelley  John G. W. 《Ocean Dynamics》2023,73(7):433-447

Ocean Dynamics - Generally, ports in the North American Great Lakes are not supported with navigational guidance (water level, water temperature, currents, ice) by NOAA’s Great Lakes...  相似文献